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Mountain flying and Turbulence
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Contents
Mountain Flying…Flying Mountains; ...General Advice; ....Flying the
Plane; ...Climb; ...Descent;
...Airports; ...Mountain
Airports; ...Speed; ...Altitude;
Turns at High Altitudes; ...Performance;
...Location; ...Radios;
...Mountain Decisions; …The
Solution; ...Do Your Own Preflight;
…Wind; ...Mountain Winds; …Turbulence; …You and Turbulence; …Wake
turbulence; …Five Stages of a Vortex;
…Vortices; …Wake
Avoidance Checklist; ...Density Altitude;
...Density Altitude Operations; …Finding Density Altitude…Bay
Area to Reno/Tahoe in a C-172; …Running
out of Power; …Saving Yourself
from Hypoxia; …Mountain/Route
Checkout; ... When Terrain
Raises, Know when to Fold; …Mountain
Flying Problems; …Mountain Flying
Revisited; Downdraft; ...
Mountain Flying
"How do I become a good mountain pilot?"
"Make good decisions."
"How do I learn to make good decisions?"
"Experience"
"How do I get experience?"
"By making bad decisions."
"What's wrong with bad decisions?"
"When I get away with them, I may try again."
Flying
Mountains
I have never seen a mountain fly but it behooves the inexperienced
to believe they do fly. Be extra careful not to fly mountains
when the conditions will limit your aircraft's ability to climb
above terrain and turbulence. Bad weather and high terrain breed
flying problems like your wife meeting your girl friend.
The altitude that will avoid terrain will not provide protection from the effects of terrain on the weather/wind. Terrain/weather turbulence can extend to several times the height of the terrain. The first warning is when your aircraft begins a climb with pilot input. If this should happen take all the altitude you can get while making a 180-degree return-turn. The 180 that your were able to make at sea level is not possible at altitude. The aerodynamic characteristics of you aircraft may differ significantly.
The thin air changes the way an airplane flies. The reduced weight of the air has less effect on control surfaces. Lift is reduced. Drag is reduced giving a higher true airspeed even though indicated airspeed remains constant. Horsepower is reduced because of fuel burn being a 1 to 16 factor by weight the air in the engine. At absolute altitude an aircraft becomes uncontrollable to hand flying and sluggish. Rate of climb decreases with altitude. All high altitude flights are on autopilot.
The existence of a low or front makes mountain weather and wind certain. Moisture will give low ceilings and visibilities. Such summer weather gives thunderstorms and winter gives snow and ice. When flying westward you want to fly away from the prevailing jet stream. Flying eastward you should do what you can to make use of any jet stream benefits. Making ground speed checks is a practical consideration. GPS is taking the 'fun' out of flying. Flew to Oshkosh just before the Reagan controllers strike. Never flew any higher than I had to. Had a great time until my wife said to come home. If she hadn't I'd still be out there.
General
Advice
--Every mountain flight will be a lesson.
--The 'good judgment' of the experienced pilot is acquired by
mistakes, caution and good luck over many years of flying.
-- Get and take any local advice that may be available.
--The hotter it is the earlier you want to arrive at your destination.
--Don't let others (passengers) interfere with safe decisions.
-- Night mountain flying lowers all available options.
--Don't compromise safety for the enjoyment of mountain flying.
--The safest route is not always the shortest. Safety has no
price.
--Certain flight operations have inherent risks and do not get
safer or less risky by counting the times flown.
--When you need advanced mountain flying and survival skills
they are really needed.
--Don't make your first long cross-country in an aircraft newly
checked out. Be comfortable and proficient before you go. You
don't need speed you need hours.
--Plan a very early morning departure and arrival by 11 a.m.
Any later, any time except winter, will likely result in a most
unpleasant arrival.
-- You will get a thousand foot a minute up or down draft
for every ten knots of wind over a ridge..
Flying
the Plane
--Mark your MC (magnetic course) for each leg on the sectional
and ETE (estimated time en route) as well.
--Fly small i small f small r. i follow roads. Fly an airport
vicinity route or at least within gliding distance of flat ground.
--Procure a set of en route low-altitude charts (IFR) for your
flight as well as approach plates where you might land. These
have frequencies and valuable altitudes not readily available
to the VFR pilot.
--Fly with WAC charts. They may not have the frequencies but
they do have all the airports. I have flown off the sectional
chart edge only once and it was not a happy experience.
--Flying high is not always the most efficient. The basic rule
should be to fly fast when you are heavy; fly slow when you are
light. The savings in miles-per-gallon could be 30%. If the fuel
weight is a small part of the gross it is better for fly POH
range data. The data are for no wind and may not include reserve,
taxi, takeoff, climb and descent allowances. Maximum range is
not greatly affected by altitude unless favorable winds can be
reached.
--Use the 'spot' landing technique of getting your constants
of speed, power, trim and flaps into order so that the landing
spot stays in one place.
--Avoid flying down the middle of a valley due to wind shear
and turbulence. If you can't get an updraft on one side, try
the other.
--If you must fly a canyon, fly to the highest end and fly downhill
not uphill.
--When following along a ridge or valley fly the upwind side
if it is on the right. This gives you both the potential speed
increase available from the wind uplift and obeys the mountain/road
flying rule of flying to the right side.
--Approach ridges at a 45 degree angle and 2000' higher to maximize
your ability to turn away from the ridge prior to crossing.
-- After crossing the ridge fly at 90 degrees from it to maximize
your distance from possible down draft effects.
--Maintain high wind-cross wind landing skills.
--Fly the valleys when given a choice. This reduces density altitude
and gives improved aircraft performance. Valleys tend to have
less wind and friendlier terrain.
--Always keep the back door open to lower terrain.
-- If you need to make a steep turn get as close to the edge
of your turning space as you can. Slow to Vy with some flaps,
make a 45 degree bank and maintain level or a slight descent
during the turn. This will get you around in both the least time
and space.
Climb
--The use of flaps may assist the liftoff but not the climb.
You can determine the flap setting by lowering them to the same
angle as can be achieved with full down deflection of the ailerons.
This is the angle the manufacturer found to be the most lift
for the least drag.
--Don't press your climb rate. Take what you can get and leave
a back door open.
--Take advantage of all rising thermals. The altitude you gain
may be just enough but don't plan on it.
Descent
--Fly through downdrafts at cruise or higher turbulence permitting.
--Leave yourself room to turn and descend without power.
Airports
--Overfly airports to determine terrain, wind, and go-around
options.
--Never waste an inch of runway in the mountains. Hit the threshold
on landing and use any overrun for takeoff. Full power runup
with leaning for takeoff.
--Lean for takeoff with full power to max EGT + 100 degrees
--If you are going to a 'one-way airport. Try to land uphill
and takeoff downhill. Get some local advice before going. Each
degree of slope adds 10% to your takeoff/landing requirement.
A 10-kt adverse wind doubles your takeoff and landing requirement.
Mountain
Airports
Due to increased ground speeds you will be covering more
ground per minute at mountain airports. This means you will be
running out of runway much sooner than you would at sea level.
Not only will your ground roll be longer so will your climb be
shallower with a gain of fewer feet per unit of time and distance.
Mountain airports tend to always have a slope. Either their approach or departure end will face higher terrain. It is best to take off downhill and land uphill. Uphill landings cause an illusion that makes you think you are too high on final. Get all the help from gravity you can this way. Where possible, consult with locals to find out how to do what. Usually you add ten percent per degree of upslope to the takeoff distance. When a runway has three degrees of upslope, takeoff downslope. A downslope shortens your takeoff by about 5% per degree.
If you need to get extra lift on takeoff, consider putting the flaps down to the maximum deflection angle of the ailerons. This will be close to the maximum lift/minimum drag flap angle for a given aircraft. On takeoff, if you haven't reached 75% of your liftoff speed by the runway half-way point, abort the takeoff.
Speed
1. Figure the 70% of speed required for takeoff. Mark the
half-way point of the runway. If you don't reach 70% of required
speed by the halfway point of the runway, abort. Use this method
regardless of the surface involved.
2. Abort your takeoff if you have not reached 40-kts by the half-way
point of the runway. Many short runways have this spot marked.
3. Know your best-rate-of-climb speed for weight and altitude
then fly it.
4. Never carry excess speed for landing.
5. True airspeed will be considerably faster than indicated airspeed
at high density altitudes.
6. Always fly indicated airspeeds as though at sea level. Accept
the fact that every thing will move faster on the ground when
at mountain airports.
7. Uphill landings give the illusion of being high on approach.
8. Use all the runway for both takeoff and landing.
Altitude
Regardless of the altitude or mode of flight, FAR 91.113(b)
places the avoidance of traffic on the pilot. FAR 91.159 tells
you the altitudes you should fly above 3000' according to the
hemispheric rule. Below 3000' this rule does not apply. FAR 91.121
says that you must have your sensitive altimeter set to the nearest
available setting source.
--Above 5000' do not takeoff or land with a full rich mixture.
--Due to the pressure gradients existing at valley passes plan
your passage so as to be 2,000' above the terrain.
--Since the venturi effect of wind through passes can cause lower
pressure, assume that your altimeter will be reading higher ......than
you actually are.
--You can tell if you are higher than terrain in front of you
if you see more and more terrain by looking over the top.
--You can gain altitude while flying to your destination by making
stretched S - turns with the horizontal part of the S parallel
to ....ridge or mountains.
Turns
at High Altitudes
When making turns at altitudes close to the service ceiling,
you must be careful to keep your banks shallow. Otherwise, you
stand a probability of getting behind the 'banked excess power
curve'. This is a condition where on making the turn you begin
descending and in trying to prevent the descent you pull back
on the yoke instead of leveling the wings. Without the wings
level the resulting loss of altitude will be far more than you
have ever before experienced.
Performance
1. Use maximum performance techniques for takeoff.
2. No flaps until reaching 50 knots, then 10 degrees
3. Pre-plan an abort speed and point. Use it.
4. Adjust mixture for every 2000' of climb or descent.
5. Fly at 90% of gross weight to improve performance-you will
need it.
6. Keep the aircraft as light as you can. Minimum fuel plus reserve.
Item
You can always find the density altitude by seeing how the
airplane performs.
Location
--Keep your checkpoints close(er) together.
--Select easy-to recognize checkpoints, fly to them and identify.
--Unexpected weather may make reliance on your pilotage skills
essential. In mountains you won't know what the weather is ....until
you get there.
--Mark your chart with pattern altitudes of airports along the
way, Put in runway numbers, pattern direction and CTAF ....frequencies.
Radios
--Set up your communications options. This should include
'what if'' both at your departure point, your destination, an
alternate, a filed and opened flight plan and ATC coverage. Get
advice from locals. On your chart mark the points where you plan
to make position reports. Put them on your flight plan, too.
--Monitor flight watch, EFAWS, on 122.0. Give reports to help
other pilots.
--Use VORs or NDBs for cross-checking checkpoints if available
but don't base your flight on either availability or usability.
--AWOS (Automated Weather Observing Systems) exist at an ever
increasing number of mountain airports. They have both radio
and telephone capability. Use the latest A/FD or call an FSS
to get the latest information. ASOS gives precipitation information.
--Be sure to write down the ARTCC frequencies for your flight.
You may be too low to communicate but you will be able to listen
in on other pilots. If you should ever go down, knowing the frequency
will give you a direct line to invaluable assistance. Your cellular
phone may not have a cell you can reach.
Mountain
Decisions
You cannot learn mountain flying by reading about it. Flying
in density altitude is different than reading about it. You must
get training and instruction. Several times!! Personally, I did
not cross the Sierras as PIC until I had three hundred hours.
Prior to that I made four or five trips with instructors or more
experienced pilots. Mountain flying is different because mountains
limit your flight options. The effects of route, wind, weather,
density altitude, emergency preparation, and aircraft performance
are different and require a different pilot perspective. A pilot
who views mountain flying as a routine flight is heading for
a trap. There is always an alternative to making a dangerous
flight, no matter how inconvenient. Have an alternate plan; be
flexible. The ultimate alternate plan is cancellation.
Pilots control their judgment and decision making. The dangerous
transition from VFR to MVFR to IMC requires decisions that allow
little room for error. Entering a situation where conditions
are controlled by the weather means that the VFR pilot is beyond
his ability, skill and knowledge level.
The
Solution:
--Exercise cautious judgment.
--Upgrade your capability.
--Improve your preparation.
--Adjust your attitude.
An instrument rating and an aircraft with altitude performance
do not make all mountain flying either safe or practical. 300-fpm
climb capability is a minimum of required performance after reaching
cruise altitude. Mountain flying becomes relatively safer and
more practical when preparation determines that a flight can
be made. Weather is the primary deterrent. Every pilot should
have personal weather and wind conditions under which a flight
will or will not be made. Allow the real possibility of rapidly
changing or unstable weather. A C-172 is not a mountain aircraft.
I have had pilots return and thank me for not checking them out
in a C-172 for a family trip over the Sierras.
If your aircraft lacks the performance to out climb a downdraft
at Vy, don't hesitate to increase your airspeed. The stronger
the downdraft greater the airspeed needed to reduce the angle
of descent. Va is the recommended speed to use for downdraft
penetration in turbulence. This is counter-intuitive but must
be done. Again, if the aircraft is being flown at Vy and is sinking
faster than it should be climbing, accelerate to maximum-cruise
airspeed. Do not attempt to out-climb a downdraft. but speed
up in a downdraft. Flying at cruise speed through a downdraft
will give a lower net loss of altitude over distance than will
any attempt to climb. Downdrafts can extend from 1 to 12 miles
to the lee side of mountains. Under such conditions you could
lose 65% more altitude per nautical mile at Vy than at cruise
speed.
As a mountain pilot you must continuously position your aircraft
to give the best selection of options available. If the wind
is within 30 degrees of perpendicular at more than 15 knots increasing
with altitude with a stable air mass or inversion below 15,000
you can expect a mountain wave to exist. Orthographic lifting
forces air up the windward side and will form a mountain cap
if instability exists the rise continues to form cumulus and
no mountain wave. Altocumulus, rotor clouds or standing lenticular
(ACSL) clouds are indicative of a mountain wave but if moisture
is not present the turbulence may be there without the visible
warning. Winds aloft weather information doesn't always apply
to mountains. In mountains fly whatever wind correction needed
to maintain course.
Density altitude is pressure altitude corrected for non-standard
temperature and humidity. The most serious negative effects on
aircraft performance occur when landing and takeoff occurs in
conjunction with a high-density altitude. Higher altitude, hot
air, and humidity reduce power, thrust, and lift. There is a
3-factor times 3-factor reduction in the ability of the aircraft
to perform.
Due to these factors I have taken up to thirty minutes with two
people in a four passenger aircraft to climb to a safe crossing
altitude of local terrain. I have chosen to take the time to
climb when the preceding aircraft has requested a straight out
departure. The next day I get to read about the other aircraft
in the newspaper. It seems that high density can affect brain
operations as well as aircraft operation.
Once you opt to climb for altitude before crossing, you should
put into practice any glider experience you may (read 'should')
have acquired. If you note any wind prior to takeoff you can
use that knowledge to locate a local mountain that may offer
ridge lift. A brief conversation with a local pilot may be helpful.
The best rate of climb speed decreases with density altitude.
Even after a successful takeoff the three factors times three
factors work (3 to the third power = 27) against the airplane's
ability to gain altitude. Use the POH to figure the new-plane
performance figures. Figure in a 1-% safety factor every year
of your aircraft's age. Average age of U. S. aircraft is 28 years.
POH figures on rate of climb are figured as feet per minute.
In the mountains you are moving further per minutes than you
would at sea level because the ground speed is necessarily faster
to acquire the needed lift. Early morning or late afternoon takeoffs
is one way around much of the problem.
The pilot who has developed a sense of when the aircraft is prepared
to takeoff at sea level is in for a surprise when this 'sense'
fails him at a mountain airport. Speed over the ground at sea
level in no wind conditions usually agrees with indicated airspeed.
At high altitudes the ground speed will be considerably greater
before the indicated airspeed required to takeoff is reached.
The illusion is likely to cause the inexperienced pilot to rotate
too soon and too much. Once out of ground effect the aircraft,
behind the power curve, will either stall or fly into the ground
in a nose high attitude. How many times have you head of an aircraft
crashing at Lake Tahoe two or three miles from the runway during
takeoff. It happens nearly every summer on the first really warm
weekend.
For much the same reason, the winter landing techniques that
made for near perfect landings will result in 'carrier-like'
controlled crashes. The cold dense air, even in the mountains,
deceives the pilot into believing that the same density exists
in the warmer air of spring. It doesn't. The high flare in winter
conditions will not be cushioned by the warmer air of spring
and the aircraft will fall right through any existing ground
effect. Watch the big twins fall at CCR on the first really hot
day to see what I mean. It happens to the best of us.
Gross weight performance from the POH is less than indicated
at high-density altitudes. A 20% reduction in weight will at
best result in only a 10% improvement in performance. It is usually
easier to leave luggage and fuel behind than passengers. An intermediate
fuel stop is never a waste of time.
Do your
own preflight
--Personal restrictions should be related to your fatigue
factor, weather limitations, eating requirements, and kidney
limits.
--Takeoff restrictions should be related to the length of the
runway, density altitude, aircraft capability, visibility, and
enroute ceilings.
-- T-storm avoidance must be guaranteed. Icing avoidance must
be guaranteed, destination weather must be above personal minimums
with a nearby VFR alternate.
-- No night circling approaches, no contact approaches at unfamiliar
airports, stabilized within 500' AGL or call go-around to missed.
--60' minimum runway width and runway figures 50% over POH figures.
--Vectors through a localizer are likely to be disorienting as
is a tight close in vector. Advise ATC that you would prefer
another option.
--A crew-member will always observe fueling. Discrepancies to
be recorded.
--The more professional you are the more closely you will adhere
to your personal limits.
Wind
Headwinds, tailwinds, heading corrections, runway selection,
pattern adjustments, weather-vaning, light, variable, strong,
light, swirling, turbulence, gusts, wake, relative, calm, variable
all of these exert a powerful and fundamental influence on our
flying.
--Learn to read the winds and their signs of high velocity.
Lenticular, rotor, and cap clouds advise against flying.
-- Headwinds can greatly alter "no wind" figures from
the POH.
--Consider it a "rule" that you will always be flying
into headwinds.
--Keep wings level and ride the altitude wave at Va speed to
avoid bending the airplane. Avoid turns in turbulence.
--Plan to land more often for fuel. Keep a two hour reserve so
you can get back when you can't go on. In the mountains you will
have head winds no matter which way you fly. Be conservative.
Mountain Winds
Wind is a flying variable that remains constant in its variability.
As you descend both velocity and direction may change. The amount
of change may be a matter of degree but often it is significant.
A headwind can become a tailwind. A steady wind ceases or gusts.
Proximity to mountains, buildings, and terrain cause orthographic wind changes in direction, speed and drafts. Virga is indicative of violent wind shifts. Dramatic wind changes occur where thunderstorms exist. The most likely wind change will be due to surface friction, which will reduce wind velocity and change its direction. Holding a tight yoke grip during gusty conditions reduces your ability to react to wind changes. In high wind conditions add at least 1/3 of anticipated gust velocity to your approach speed.
Against a headwind, speed increase should be sooner than later. With a tailwind do not give up on Vy unless the sink exceeds the Vy climb capability by three times. Headwinds increase descent angles relative to the ground; tailwinds decrease the descent angle. This effect can be best noticed by making practice downwind landings before going to a strange place where the skills acquired will be essential for survival.
Wind speeds can easily double through mountain passes. The
Bernoulli effect of lower pressures in a pass can give altimeter
errors of a 1000' or more. Occasionally the pressure gradient
through a pass can cause a reverse flow of the wind. Mountain
winds can become overwhelmingly strong in very short order. With
the strength will come turbulence and runway cross winds. Waiting
twenty minutes one way or the other can make a difference. The
wind at ground level is sure to be stronger higher up. Mountain
surface winds are accompanied by up and down drafts, which make
holding altitude impossible. Accept the changes and try to fly
in areas of upslope winds along upwind ridges. Turbulence is
an unavoidable function of strong winds that can only be reduced
by getting as high as possible.
If you can determine that a forecast headwind is stronger than
predicted, keep careful record of time and fuel. If in doubt,
land and refuel. Many isolated airports have 24-hour credit card
automated fuel pumps.
Cross-country flying into single runway airports requires that
the pilot be proficient in crosswind landing procedures and taxiing
techniques. Proficiency in reading winds and ground reference
airport patterns is an additional requirement. Always fly so
that you are in positing to turn toward lower terrain.
Mountain Winds
Allow one thousand feet of ridge clearance for every ten knots
of wind speed
In a sink, go to full power with nose slightly down below Vne
Use GPS to get highest ground speed
In lifting air slow down, fly into the wind and go for the ride.
Turbulence
Airplanes dislike stress as much as humans. Like humans stress
can cause an airplane to break. Stress for airplanes is defined
as load factor. Load factor is the ration of the total air load
acting on the gross weight. Level flight produces one times the
force of gravity or 1 G. The aircraft is designed to carry 3.8
G's positive load before stress causes folding, spindling, or
mutilation. Excess loads may be causes by flight maneuvers, turbulence,
wind, or excess weight.
Aircraft stall instead of breaking. As the load factor increases
so do the stall speeds. What were previously safe flying speeds
now use load factor to create stalling speeds. A stall is a type
of aerodynamic safety valve. The aircraft has an airspeed called
Va or maneuvering speed. At this speed in rough air and level
flight conditions an aircraft is able to withstand the excess
stress. Additionally the aircraft is designed to withstand full
control defections at this speed without breaking. These structural
speeds are determined in power off conditions. Any use of power
becomes an experiment when maneuvering above Va. To avoid becoming
a pilot of an experimental aircraft you should begin by taking
ten knots off the Va and two additional knots for every 100 pounds
of weight below gross allowable. Oddly, a lighter aircraft has
a lower Va than does a heavy aircraft. Repeated stress above
design load can cause structural failures. Structures likely
to fail are tail surfaces and wing ribs. Failure can occur during
normal operations when prior operations have exceeded design
capability.
Sometimes turbulence is a nuisance. Less often it is a hazard.
The pilot's first option is to slow down. Quickly get to Va -10
knots. Structural breakup is most likely with an abrupt pull-up
effort to regain altitude lost. Keeping a light touch will prevent
the two-for-one bumps you get by holding tight. Do not slow below
the previous recommendations since you will be likely to stall
when a vertical gust strikes. 180-degree turns are not recommended
since they increase the load factor and risk of exceeding structural
limits. Ride the altitude changes of turbulence without striving
to hold altitude. The use of flaps reduce the amount of stress
wing structures are capable of withstanding. Turn off the autopilot.
Any mountain flying in strong winds or after 10 a.m. should be
flown in anticipation of turbulence. Cumulus clouds mean some
turbulence exists. Turbulence at lower levels is caused by hot
air thermals or by wind in motion. Clear air turbulence (CAT)
is usually a high altitude phenomena but in cold weather can
occur as low as 5000'. Wind shear is caused by two adjoining
airflows moving in different directions and speeds. The most
dangerous wind shear is a decreasing headwind on approach. Winds
usually lose velocity at lower altitudes. Full power is the only
correction. This is an emergency.
Know your Va speed before reaching turbulence. Prepare the aircraft
and passengers. Turning adds to structural stress. Stay level
and accept altitude changes. Change power only to remain at Va.
Turn off autopilot. Keep a light touch; accept any altitude gain
you get.
You and
Turbulence
Turbulence affects you physically by adding stress to your
body. Of greater import will be your emotional stress. As the
pilot you do have some ability to control or reduce the effects
of turbulence. Use the rudder to counter yaw. Rudder will raise
the low wing and steady the nose. Avoid reactive aileron and
elevator movements. Gentle and smooth will average out the gyrations
into a less stressful flight.
Instructor Opinion on Turbulence
Two aspects about turbulence (and stalls, for that matter)
are particularly disconcerting to many pilots:
--First, we often cannot "see" the turbulence coming
(unless you've got standing lenticulars, strong surface
winds, and other clear tell-tale signs);
--Second, we often feel a sense of "loss of control"
over the situation as it's happening -- pilots are, after all,
control
freaks ;)
As many have replied already, experience does help with all of
this. But so too does a better understanding of weather, aerodynamics,
and the design of the airplane (all of which I hope will become
clearer to you as you gain experience and advance in your training
with your instructor).
One particular case I had was a private pilot who routinely flew
through the Gorman Pass (connects SoCal with the San Joaquin
valley), which typically can have lumpy-to-down-right-nasty air
roiling in the vicinity. The pilot was very nervous about flying
his 172 through there -- so much so that it was becoming incapacitating
to him (not to mention squelching his desire to fly).
So he and I planned a one-hour sortie flying 'round and 'round
the rim of the valley surrounding Gorman. But here was the catch:
I had him trim the airplane for a comfortable, slow-cruise setting.
Then I made him sit on his hands. The point was to get him to
relax and learn how to absorb the turbulence with his feet, using
small, quick rudder inputs to maintain a general heading and
approximately wings-level.
We found one particularly lumpy patch of sky, so I had him go
hands on, bank to 30 degrees, trim hands off, and sit on his
hands again. He maintained the turn hands-off, just using quick
rudder actions to cancel turbulence-induced bank excursions from
the established 30-degree bank.
I think this was quite instructive for him. The other aspect
of turbulence, flying in wind, and stalls is that many pilots
approach these with a defeatist attitude from the start ("oh
know, the wind is blowing," or "oh no, stalls.")
Instead, I advocate a different approach -- treat these as a
game, a contest! Pilot on one side, wind or stall break on the
other. Will you let the wind or the stall kick your butt, or
will you kick back? Go in looking to "win" the battle
-- don't resign yourself to losing before the contest ever begins!
Of course, we all have to know our limitations, too -- some days
the wind IS clearly the superior force. On such days, it's best
not to take the field in the first place...
Rich Stowell
Wake
turbulence
Every aircraft generates wake turbulence when producing lift.
As air flows over the wing it creates low pressure. Higher pressure
air below the wing tries to fill the vacuum. The low pressure
of the vortex is formed above the wing; the high pressure below
rises over the wing tip and rolls the vortex inward while the
rest of the wing flow holds it away from the aircraft path about
a wingspan apart. This filling is easiest at the wing tips so
a spiraling swirl of air forms at the tips much like water down
a drain. Wing tip vortices are horizontal with the left tip forming
a clockwise twist and the right wing a counterclockwise twist.
The center of the core is very low pressure which maintains the
life of the whole by speeding up the winds of the outer edges.
The longer the low pressure lasts the longer the existence of
the vortex.
This swirling caused by differing relative pressure creates a pair of counter-rotating vortices that in good conditions will carry for up to ten wingspans behind and below the aircraft. Strong wake turbulence dissipates less and sinks further. When the vortices are blown close together they tend to destroy each other. Wakes far apart are the longest lasting. Anything that keeps the wake from sinking will destroy it such as the ground. 200 mph turbulence peels off a B-757 as a 12 inch horizontal tornado. The wake strength is weaker if the aircraft is going fast and has a long wing. Wake strength is greater at altitude (high density) slow speeds and short wings. Helicopters can create strong wakes of short life span while being nearly immune to wake effects.
The vortex diameter may reach up to 40 feet for large aircraft.
The wind velocity may exceed 130 knots. The two vortices will
remain a wingspan apart and not dissipate until other forces
such as friction or turbulence has an effect. The vortices sink
about 4-500' per minutes for two minutes before breaking up.
On reaching the ground the vortices will spread apart and may
reach parallel runways.
Wake turbulence is insidious. It will strike when you least expect
it and will not exist where you think it should. Wake vortices
are not as simple as the AIM makes them seem. They are a hazard
any time the aircraft in front is the larger aircraft. Turbulence
can extend significantly greater than FAA standards would indicate
and persist longer than would be expected. FAA minimum standards
if extended would decrease the risk and possibly the severity.
It is not until the last 10% of a vortex's life span that the
power of the vortex disperses abruptly. Wake turbulence causes
nearly one accident a month and a fatality once a year usually
to small aircraft and their passengers.
We are still learning about the dissipation of wakes as causes by ground friction, their blending together at a distance of six wingspans, and a bursting of the vortex tube. Perpendicular flight into a vortex is a strong abrupt bank as though by a sledge hammer. These are less dangerous than where the vortex roll exceeds the aircraft control authority. Unstable air will cause a wake to dissipate. An inversion that prevents sinking will cause a wake to dissipate. Neutral air stability will prolong vortex life. Certain wind velocities at right angle to the vortices can cause one to dissipate while the other gains power. IFR separation seems to provide adequate takeoff and landing safety. VFR separation seems to rely upon a pilot's judgment and concern.
Over 90-percent of all wake turbulence occurrences is evenly
divided between two places in aviation airspace:
1. 200 feet to ground level on approach to landings
2. 1500 feet to 5000 feet when leveling off at final approach
fix.
--Chicago delays cost 20 million dollars a year with
an average of 10 minutes per flight.
--1993 had 51 accidents with 27 killed, 8 injured and 40 aircraft
destroyed.
--50% of all wake turbulence events occurred between aircarriers.
--Separation only decreases the rate of occurrences not severity.
Wake Turbulence
Takeoff early, land late
Turn early and fly above to avoid
Five
Stages of a Vortex.
--First, is the formation of the vortex, which grows over
the wing as a series of vertices. An aircraft has a dominating
pair of vortices that roll up other vortices into a trailing
edge vortex sheet. This roll-up occurs two to four wing spans
behind the aircraft. The dominant vortices are at 80% of the
wing span from the fuselage.
-- The second stage is the mutual effect the dominant vortices
begin to have one on the other. A vortex has a wind flow velocity
field causes the other vortex to descend. Simplified, the vortices
would descend but this does not happen every time. The vortex
cores have an axial movement parallel to the flight path but
spin in the opposite direction.
-- The third stage is when atmospheric turbulence and temperatures,
which lead to their dissipation, are influencing the vortices.
--The fourth stage is an enlargement of the core and a change
of orientation. The fourth stage is not well understood, yet.
--The fifth stage is composed of vortex rings and are considered
non-hazardous.
While vortices usually descend, near the ground they may rise
or even bounce. Wind has a critical effect; the downwind vortex
tends to climb. The stronger the winds and unstable the air,
the shorter the life of a vortex. Certain combinations of wind
and stable temperature can cause a vortex to remain stationary.
Atmospheric effects determine the vortex strength. The strength
of a 'heavy's' vortex can extend for five miles.
Improved instrumentation has shown that vortices retain 90% of
their power through 85% of its duration. Aircraft speed, angle
of attack, small wing size, clean configuration, and weight all
produce strong vertices. Gear down, flaps and spoilers change
the spanwise lift and turbulence to reduce vortex strength. Newer
aircraft such as the large 757 are 'slicker' than older aircraft.
They have fewer protrusions to disrupt the vortex so the vortices
tend to last longer. The 757 has an exceptionally steep climb,
and produces 50% more vortex for its size than other aircraft.
A light aircraft will be unable to climb above the flight path
of a departing 757 as it might other aircraft. The wake turbulence
of the 757 is greater than many heavier aircraft.
The heavier the aircraft and the slower it is flying, the stronger
the vortex. Vortices last about 80 seconds and decay suddenly.
The roll forces of a vortex from a heavy aircraft will exceed
the power of your ailerons. Entry at right angles will cause
pitch and airspeed displacement. Years ago I flew through a pair
of such vortices and it felt as though a sledgehammer had hit
the bottom of the aircraft. An oblique entry will have symptoms
of both. The vortices move apart at 4-5 knots. A breeze of 4-5
knots is capable of keeping a vortex stationary for its life
of 80 seconds. Pilots of small aircraft should avoid operating
within three-rotor diameter of any helicopter in slow hover taxi
or stationary hover.
The 757 is an aircraft that in a clean configuration can produce
wake turbulence in excess to that produced when dirty. (Gear
and flaps extended) The 757 does not produce the multiple vortices
typical of other aircraft. 757 vortices are very focused and
are at least twice that of similar aircraft in its class.
Vortices
-are invisible
--last longer in calm or light winds
--- are most dangerous close to the ground
----are stronger when made by heavy aircraft
-----affect light aircraft the most
------can be avoided by flying above and upwind
-------of large helicopters are deadly...avoid
--------Climb rates of new jets make it necessary to wait.
You will experience wake turbulence something in your flying
life. You may never hit the center but you will never avoid it
completely. Read all you can about wake turbulence. If you are
on approach following a larger aircraft, plan that your touchdown
will be well past the large aircraft's touchdown. If a larger
aircraft is departing prior to your landing make your landing
point well short of his rotation. A take off behind a larger
aircraft must break ground before his rotation and climb at a
steeper rate or a turn upwind away from his flight path.
The latest information suggests that you stay at least 1000'
above and below the possible turbulence path. Although stronger
at slower speeds the wake turbulence will exist at all speeds.
The more turbulence in the atmosphere the more quickly will any
turbulence be dispersed. With ATC separation standards both old
and new, there have been no reported wake turbulence accidents
in nearly thirty years.
Wake
Avoidance Checklist
--When aircraft is close to your altitude avoid flight below.
Execute 360 and advise ATC.
--Allow three minutes space before takeoff. Rotate and climb
to avoid prior flight path and headings. Windy conditions can
reduce time considerably.
--Make landing approach above prior approach path and land beyond
touchdown point.
-- prior aircraft is on parallel or intersecting runway consider
your ability to estimate location of wake. Better part of valor
may be a go-around.
Density
altitude
Hot, high, humid weather will change mountain operations
proportionately more than flatland operations. Under density
altitude conditions the engine, propeller and wing become less
efficient and effective. Even a turbo engine flies with a less
efficient and effective propeller and wing. Add an additional
10% to your operational parameters under humid conditions. Engine
power can be cut up to 12% when humidity is high. It does this
by displacing air with water in the engine.
The decreased efficiency of the aircraft at density altitude results in longer takeoffs, reduced climb, higher landing speeds/roll and longer takeoff distance and ground roll. Leaning (not turbos) the mixture to adjust the weight of the fuel in proportion to the weight of the air will improve the engine operation but power will be affected by lack of oxygen. Any temperatures above standard will affect all parameters of operation and performance negatively from the pilot's viewpoint. A 6000' airport such as Tahoe has a standard temperature of 31F. At 90 degrees the density altitude is almost 12,000'. At 80 degrees it is over 11,000'.
Density altitude goes up about 100 ft per degree of C rise. You can estimate density altitude by knowing "standard temperatures". Standard at 5000' = 5-degrees C. For any temperature increase or decrease from standard add/subtract 100 feet per degree of change.
Factors
--Temperature
--Pressure altitude
--Humidity
Humidity may not be used because of unimportance. Error less
than 200 feet.
--Not a height reference
--Used as an index of aircraft performance
--An approximate value of density altitude is all that is needed
--Density altitude can rise from sea level to 3000 feet at 100-degrees
F.
--High density altitude reduces:
--Engine power
--Propeller thrust
--Wing lift
--High density altitude increases:
--Takeoff roll
--Time to climb
--Actual required ground speeds to takeoff, landing and flight
--Landing rollout
Density
Altitude Operations
1. Always check density altitude.
2. Lean the mixture for takeoff.
3. Taking two trips to nearby airport with longer runway is always
an option.
4. Know your service ceiling. (100 fpm climb altitude)
Most density altitude accidents do not occur because the aircraft was too heavy, out of C.G. limits, on too short a runway, winds, runway surface or because the computed density altitude exceeded the performance capability of the aircraft. The accidents happen because of improper aircraft operation. Tire inflation, flaps, and leaning are pilot induced difficulties.
In a density altitude situation it is advisable to adjust the mixture at full cruise on the downwind pattern altitude and leave it there. This is especially true if you are planning to make an immediate departure. This is the only time when I can truly recommend using the mag key to stop the engine. Having the mixture set will make a hot start relatively easy. If the aircraft is going to be down for a day, use the mixture to kill the engine.
In an 'engine cold' high-density start keep the mixture in idle cut off. Set in a little throttle. Give a minimal prime and slowly advance the mixture while cranking the propeller. Don't try a full rich start. You will probably flood the engine. When you do your runup for takeoff. Do it at full power and adjust the mixture for best operation. Remember if you takeoff at full rich the engine will become even more rich as you climb to the point of choking itself 'dead'. Watch the EGT during takeoff and climb and make appropriate adjustments.
Don't assume that an airplane can fly anywhere and at any time. The temperature of the runway environment (asphalt pavement is likely to be far different (higher/worse) than that of the grass parking area. The difference can be as much as 20 degrees. A short walk might make a difference in your density altitude computations and your departure plans.
To stay out of trouble, learn all you can about local conditions
and the weather. Always have an escape route. If in doubt, stay
on the ground. Don't fly into convective clouds. You can only
avoid all thunderstorms by maintaining VFR at all times. When
winter comes it will rain on your airplane.
Finding
Density Altitude
--Over twice as many per hour accidents occur in the mountains
--The ground in the mountains is closer and the slope of the
terrain is important
--Aircraft performance is less
--The 'sea-level mind-set' results in the poor planning that
ends in an accident.
--The climb capability can cease to exist
--True airspeed increases 2 percent per 1000 feet
--Density altitude increases 2 percent per 1000 feet
--Engine power decreases 3 percent per 1000 feet
--temperature drops 2 degrees Celsius per 1000 feet.
--By the time you reach 10,000 feet your engine power has decreased
by 30 percent
--At altitude water vapor takes the place of oxygen
Bay
Area to Reno/Tahoe in a C-172
After SAC just stay to the right side of the freeway.
Travis Approach 119.9
Sac Approach 125.25 Handoff to 127.4 or 119.1
Oakland Center 127.95
Reno: get ATIS for Approach frequency abeam Truckee and contact Approach.
Expect right base entry...request the left, taxi to Mercury unless you know a better place.
FSS is close to Mercury.
Mercury will give you a ride to the other side of the airport. Catch a shuttle...cheaper than cab.
Flying the Route
Use a cruise climb of about 90/95 knots until Auburn.
Lean even in climb.
At 10,000+ you can see the freeway on the other side of the hills
just past Blue Canyon Airport. Take the shortcut. Fly about 090
degrees.
I would recommend that you plan to arrive at either airport before 10 o'clock a.m. In fully loaded 172 fly as high as comfortable for your oldest person. Don't go if winds are close to 20 knots unless you can get extra altitude and enjoy bumps. Approach Verdi ridge at an angle so you can turn away, if you must. Stay out of the pass if windy.
It is hot and the least bit windy you will not be able to out climb even the least of downdrafts. VERY uncomfortable. Always leave yourself an escape route. Let down after crossing ridge. You have lots of altitude to lose.
Lake Tahoe,
Direct to Placerville VOR and follow highway. Getting in
is the easy part. Getting out in a loaded C-172 will be a problem.
Do a full power leaned check for best power before takeoff. Do
not climb directly toward the West. You'll never make it.
Depart either early morning (best) or at dusk. Expect the flight home to take a while if into headwinds. Check fuel. Flying into a setting sun hurts. Just plan to be out of the mountains before dark.
I hope you have done some gliding. There is a golf course to the right. Go over there and do switchback turns to stay as close to the nearby ridge as you can. If you get ridge lift keep climbing until you are well above 9000' In a C-172 it is hard to have too much altitude. It is not a good mountain plane.
Alternate climb is to stay in the pattern until you get altitude sufficient to head west. Rental car may be best transportation. Tiedown is expensive. Don't take any more gas than you need to get to SAC or such.
War stories:
Departed Tahoe in mid-afternoon in PA-28 180 with two aboard
and half tanks. Took nearly 30 minutes to get to safe crossing
altitude
Departed Tahoe in PA 28 181 and went to golf course and gained altitude. Aircraft before me departed straight out to the pass. Read about his fatal accident in C-182 the next day.
Had pilot ask to be checked out in C-172 for trip over to Nevada. Refused and suggested PA-28. After making flight in PA-28 the pilot called to thank me.
I have made the trip to Nevada on average 5 times a year for
thirty years. I even flew in a C-150 once. I few back at night
'once'. I flew to Reno for the first time in my own C-172
last month with just my wife. That is my first trip in C-172.
By picking time and weather carefully it is no problem BUT conditions
must be right.
Running
Out of Power
--The one time we deliberately run out of power is in the
flare.
There are four places where you can expect to run out of power:
--When approaching to land you have mistakenly added small bits
of power in an effort to stretch your approach
to the runway only to go lower and lower and slower and slower.
--When slowing behind slower traffic you have gradually added
power in order to fly slower and slower while maintaining altitude
until you are so slow and without additional power so that the
only option is to lose altitude.
--You are in a high-density altitude situation and have applied
full power but are still unable to out-climb the rising terrain.
Your option is to turn away to lower terrain. The loss of altitude
is the only remaining option.
--You are in a slow-flight and have slowed to a speed that requires
full power just to maintain altitude. You
would like to either gain more altitude or more airspeed. Your
only option is to sacrifice altitude first.
--Whenever you use power to fly slower you are in the region
of reverse command. Once you are using full power a descent in
flying attitude will commence and continue unless you lower the
nose or find a way to get
more power.
--Induced drag will vary inversely to the square of the airspeed.
For every 2-knots lost in airspeed, the induced drag will increase
the effective loss by four-knots with additional drag.
--Now the elevator works backwards. Raising the nose with the
elevator will cause the aircraft to descend if power is constant
or unavailable. Lowering the nose will cause the aircraft to
climb.
--Reducing the power will increase the airspeed and increasing
power will reduce airspeed.
--Every aircraft must be retrimmed for ANY change in airspeed,
power or flap.
--We can safely fly behind the power curve in the flare because
we benefit from ground effect.
--Light aircraft have a very narrow range of operation as determined
by power, weight, loading, lift, altitude, and pilot performance
as outlined in the POH.
Saving
Yourself from Hypoxia
--Hypoxia is insidious in its combination of functional impairment
and denial.
--Four hours at 8,000 are the same as 1/-hour at 16,000 feet.
--17-hours without sleep gives you the same functional impairment
and mental capability as a drunk
--Hypoxia is a form of stupidity.
--Symptoms are headache, nausea, tingling skin, fatigue, dizziness,
visual impairment and euphoria
--the sequence and degree of these symptoms varies with the individual
and within the individual.
--Effects of hypoxia are accumulative. Brain cells die from lack
of oxygen.
--TUC means 'time of useful consciousness'. At 25,000'it is less
than three minutes.
--Night vision begins to deteriorate at 4000 feet.
--Hypoxia is made worse, much worse, in the presence of carbon
monoxide.
Mountain/Route
Checkout
Took a pilot to Lake Tahoe from the North Bay Area and gave a
series of 'lessons' that might be suitable for your situation.
Pilot had planned flight via VOR which was o.k. but the weather
was too good. I talked to him that this was a perfect day to
get a 'read' of what the route would need to be in marginal conditions.
As soon as we left the Class D we dropped down to 600' AGL since
the beginning route was a 700' transition area. Along the route
were some 400+ power poles adjacent to the river. Just when the
transition area lifted to 1200' we were crossing relatively close
to some 2000' TV antennae. At 900' our departure VOR became useless.
We tuned in Hangtown VOR and got a good signal.
As we approached the Sierras we could see some cloud buildups.
We initiated a climb and with some deviations found that 500-fpm
was never going to catch up with the rising clouds. At 13,000
we opted to descend and have a go underneath. We were in continuous
contact with radar facilities and overheard a PA 28 making initial
contact with ZOA (Oakland Center) leaving Tahoe westbound. He
was at 8500 so we stayed at 9000 (legal because we were within
3000' AGL) Flight below clouds was a bit bumpy, as it usually
is, but gave pilot a taste of how bad it could be.
Descent and landing into Tahoe required loss of several thousand
feet in less than five miles. Pilot experienced his first landing
on a runway with a 2000' displaced threshold. Told him to ask
for a 180 on the runway to save taxi time. Most interesting part,
to me, was the difference in pilot attitude and confidence that
occurred when we flew
between Hangtown and Tahoe. He had been driving to Tahoe since
age four and knew every hamlet and place
along the highway. Flying low over such familiar territory became
an enjoyable sightseeing tour.
The initial departure, over the central valley, was off road
and filled with unfamiliar obstacles and landmarks.
Used the density altitude of Tahoe to show how to lean for climb,
descent, arrival, and departure. We climbed
in the pattern to 9000' before heading over Echo Summit. When
unable to contact RNO FSS by radio I had him have the tower open
our flight plan. He also learned the safety aspect of flying
on the right side of valleys and roads. I took a very straight
forward VFR cross-country day and turned it into a learning experience
based upon learning the options, limits, procedures and frequencies
that would be required or tried in less than favorable conditions.
When
Terrain Raises, Know when to Fold.
--A gradual slope can out climb an aircraft at high density
altitudes
--When climb rate is low make S-turns and 8's along a ridge to
get a thousand or more feet above highest terrain.
--Do not rely on POH numbers, make flight test to determine Vref
climb performance at density altitudes.
--Use GPS, LORAN or DME to simulate flight to cross a pre-selected
altitude (obstacle).
--Above 10,000 feet there is little excess power to accelerate
or climb in light aircraft.
--A high angle of attack the flight path is likely to have a
very slow rate of climb.
--If you are unable to climb at Vx for the altitude and Vref
you will not climb at Vy either. Turn back.
--Fly on the right side of valleys and turn downhill before running
out of room to turn
--Don't takeoff into a situation where you will be unable to
climb.
--Take some ridge-soaring lessons in a sailplane to learn how
to use ridge thermals for climb.
--Approach mountains and ridges at an angle that will allow you
turn away in a downdraft.
--Weight reduction improves performance in airplanes much as
it does in people.
--A ten knot wind can produce a 1000-fpm downdraft over a ridge.
Mountain
Flying Problems
--There are no longer any new causes of mountain accidents.
--Lessons not learned are doomed to be repeated
--Mountain takeoffs should be short field but not the rolling
short field.
--Abort the takeoff if ever you should over-rotate.
--A l2 knot tailwind increases required distance l10 percent.
--There are13 common causes of takeoff difficulty beginning with
tire inflation, slope, maintenance, leaning, prop damage, airspeed
calibration, and MORE
--By unloading 300 pounds off the aircraft you reduce needed
runway by 1000 feet.
--Use boost pump and make two power checks before releasing brakes.
--Essential that a normal rotation take place.
--No night IFR in the mountains because of high accident rate
--No night VFR in the mountains.
Mountain Flying Revisited
--Mountain flying is not always fun:
--Low performance aircraft
--High peaks and low ceilings
--Poor weather reports and under forecast wind velocities.
--Have a plan and know where you are. (Nothing muddles the brain as much as
being lost)
--Always keep track of where the low land lies.
--Even IFR pilots are required to have current sectionals
--Know the terrain altitudes and your planned safe altitude
--Night mountain flying reduces your emergency options to zero
--Terrain Awareness Warning Systems now exist for small aircraft.
--Turbulence causes about 10% of accidents
--10% of mountain accidents occur during mountain checkouts
--Don't cross ridges having winds above 25 knots.
--Mountain winds, as forecast are inaccurate insofar as ridge winds are
concerned.
--Downwind side of mountains are always most turbulent
--Absolutely worst turbulence is indicated by roll clouds on downwind side of
mountains
--Cold front passage means that there will be severe up and downdrafts.
--Wait several hours till cold front is past before flying the mountain route
--In summer, early morning flying is always best
--It is the upper level air that causes the weather east of the Rockies
--Eastern lows differ from Western lows in that there is more moisture
available.
--Additional moisture causes weather to remain bad longer.
--You need instruction to fly the mountains.
--Careful route selection and good navigation makes mountain flying safer.
Downdraft
--The downdraft occurs as strong winds cross a ridge crest as an updraft
that becomes a turbulent downdraft.
--Flying directly into a headwind, an airspeed indicator will not tell the
difference wind speed and airspeed.
--Fly a downdraft at Va since the higher speed will get you out of the
downdraft sooner with less loss.
--Use maximum speed required clearing the highest point and still remaining
above descending terrain.
--Cross a ridge at a 45-degree angle to reduce the amount you need to turn to
escape a downdraft.
--When crossing a ridge in a downwind direction the 45-degree crossing angle
does not apply.
--On regaining climb capability, ASAP gain 2-3000' agl before attempting to
cross ridge.
--A downdraft at night where an MEA altitude exists can become a crisis
situation.
--The lighter you are below gross weight the slower you want to fly in
turbulence
--These conditions have occurred at altitudes as high a flight level 40.
--In turbulence reduce to the Va maneuvering speed for your weight.
--Use full throttle and maximum rpm and lean for maximum power.
--Fly the attitude required and ignore the loss of altitude,
--Once in a downdraft head for lower terrain.
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