Page 5.83 ( 1949)

**RULES
OF THUMB**

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Contents

**Basic Rule of Thumb**; The
Flying Rule of Three; ...TEMPERATURE;
...Dew
Point; ... LANDING
PERFORMANCE; ... DESCENT; …WEIGHT; ...
RECIPROCALS;
...COMPASS TURNS; ...Knots
to Miles per Hour; ...WINDS; ... Wind Correction Angle;
...Rule of Thumb for
Crosswinds; ... TURNS;
... DENSITY ALTITUDE; ... PERFORMANCE;
...TRUE AIRSPEED; …Survival; …Takeoff;**
**...Finding True Airspeed; ...

**RULES OF THUMB
**Before thermometers were
invented, brewers would dip a thumb or finger into the mix to
find the right temperature for adding yeast. Too cold, and the
yeast wouldn't grow. Too hot, and the yeast would die. This thumb
in the beer is where we get the phrase "rule of thumb."

**Basic
Rule of Thumb**:

You won't remember a rule of thumb unless you use it.

**The Flying Rule of Three**

If three things go wrong on the ground before departure…don't
depart.

If three things go wrong in the air…get on the ground.

**TEMPERATURE
**Double the degrees Celsius,
subtract 10% (round up), then add 32

15deg;C = 15 + 30 -3 = 27 + 32 =59

Rule of thumb:

To find standard temperature Celsius for a given altitude you should double the altitude, subtract 15 and place a - sign in front.

**LANDING PERFORMANCE**

Actual touchdown speed answer x itself (squared)

Proper touchdown speed

Use the number greater than 1 as a % . This is the % greater
the landing distance will be.

--Use of this formula is a good reason to believe in the importance
of precise speed control in landing.

--Standard distance between runway lights is 200'

--Have passenger count lights to determine your aircraft performance.

--Takeoff and landing distances in the book are of maximum performance
type. For normal operations double the book figures.

--Ground speed at which hydroplaning will occur is seven times
the square root of the tire pressure.

**DESCENT
**1.

2. The rate of descent is based on indicated airspeed. Divide ias by two and add a zero. This is the rate of descent required for the distance to be covered.

(150kt range of ground speeds)

3. Distance to Descend

Two times the ground speed in miles-per-minute times the altitude
to lose in thousands of feet.

Example:

Ground speed in descent 120 kts (2 miles per minute). To lose
5000' from 8000 to 3000 you must start your descent 20 miles
out from your 3000' mark. 2 x 2 = 4 x 5 = 20 miles out

4. --Take three times your height above destination to the nearest
1000'. Multiply by four. Gives descent distance in miles to begin
3-degree descent.

5. --To get a 3 degree descent profile multiply your ground speed
by 5 to get a fpm descent.

6. -- Take 1/3 of altitude-to-lose to get distance, descent at
1/2 ground speed.

7. --Take half of ground speed and add a zero for descent rate.

8. Ground speed times 5 to get VSI rate of descent giving 3degree
slope.

9. -- For establishing a 3 degree slope descent is to multiply
your ground speed by 5 and add 50. Not exact but good enough

** Engine Failure Descent**You have a minute in the air for every 1000' of altitude

You can expect to glide one mile for every 1000' of altitude.

A 360 will lose 1000' of altitude

Land into the wind with the rows.

5000' to lose at 120 knots gives 5 X 2 X 2 = 20 as the miles needed at 500 fpm descent.

**Navigation Error**

The "1 in 60 rule" may come in handy too. The 1 in
60 rule says that for every 1 degree off course you are, you
are 1 mile off course at 60 miles.

**WEIGHT**

Rule of Thumb:

--Figure weight and balance when seating is over half filled
or luggage carried.

--A 2% increase in weight will cause a 1% change in stall speed.

**RECIPROCALS
**Rule of Thumb

The 22 rule for Reciprocals

Take any heading and the first digit, 0, 1, 2, or 3. Add or subtract 2 to the first digit to get a 0, 1, 2, or 3 for an answer. If you add then subtract 2 from the second digit, if you subtract then add 2 to the second digit. Third digit remains the same.

**COMPASS
TURNS**

Rules of Thumb compass turns:

--Compass turns can be made by time with a standard rate of 3-degrees
per second.

--For recovery from a bank to a given heading is to lead by half
the angle of bank + any other requirement below.

--When turning Northerly, undershoot the heading by the latitude
in degrees plus half of the bank angle.

--When turning Southerly, overshoot the heading by the latitude
minus half the bank angle.

--When turning East or West from the South roll out 5 degrees
early.

--When turning East or West from the North roll out 10 degrees
early.

**KNOTS
TO MILES PER HOUR**

The conversion from Knots to MPH is about 1.15. This means that
MPH are Knots with a 15 percent tip. If you aren't a big tipper
the conversion can be done by whatever method you use to figure
how much to leave with a 15 percent tip.

Simple way:

Take 10% and then add half that.

The method I use is to mentally
drop a zero off of the number to divide by ten, then take half
what I get, add those two figures and add them to the original
amount.

120 Knots 10% = 12 and half that = 6

12 + 6 = 18 and 120 + 18 = 138 MPH

Even going with 150 Knots 10%
= 15 and half that = 7.5

15 +7.5 = 22.5 and 150 + 22.5 = 172.5 and I doubt it's necessary
to get down to that point five. So round up or down and it either
comes out as 172 or 173.

Miles to knots and back.

A quick and dirty technique that seems to work

The difference between knots and MPH is about fifteen percent.

Move the decimal point one place to the left. That divides by ten. Add half of that to get fifteen percent. Going from knots to mph, add it. Going from mph to knots, subtract it.

**WINDS**

Rule of Thumb--trees

Calm smoke is vertical

1-3 kts smoke drifts with wind

4-6 kts leaves rustle, you feel wind

7-10 kts leaves in motion all the time

11-16 kts Dust & paper move

17-21 kts Small trees move, waves have crests

28-33 kts Trees sway, wires whistle

34-40 kts Branches break, difficulty walking

41-47 kts Signs fall

48-55 kts trees fall, damage

56-71 kts DAMAGE

**Wind Correction Angle**

Divide crosswind velocity component as flown by your TAS in miles
per minute.

Wind component

--A 45-degree wind velocity should be multiplied by .7 to find
its effect on your course. Every 15 degrees of change from the
45 is adjusted by .2 .

Example:

Heading of 360; wind 150 at 24kts. Wind is 30 degrees off tail.
Tailwind multiplier is .9 (.7 + .2) and crosswind multiplier
is .5 (.7 -.2) Tailwind component is 22kts, crosswind component
is 12 kts.

**Rule of Thumb for Crosswinds**

A wind 30 degrees off the nose has only half the wind velocity
as a crosswind component. At 50 degrees; off the nose the component
is 75% of the wind speed. At 70 degrees; the component is 90%.
You can effectively reduce the component by angling across the
runway.

Take the wind angle from the runway drop trailing zero add the
number 2 move decimal place one place to the left multply by
the wind speed

runway 35

wind 320 @ 20

350 - 320 = 30

30 = 3

3 + 2 = 5

5 = .5

.5 X 20 = 10

10 kt crosswind

**TURNS**

1. Bank Angle for Standard Rate Turn

Airspeed in knots divided by 10 and add half of that value.

Example:

IAS 100kts divided by 10 = 10 + 5 = 15 degree angle of bank

2. Standard rate turn bank angle
varies with speed. The airspeed in MILES PER HOUR less the last
zero and plus 5 will give about the required angle of bank on
the attitude indicator.

** Standard Turn Rate**Divide knots of airspeed by 10 and then add back 1/2 of result.

Example: 100 knots divided by 10 = 10 and add 5 - 15degree bank for standard rate.

**DENSITY
ALTITUDE**

Density Altitude (DA): temperature deviation (in deg Celsius)
from ISA at THE ALTITUDE you're interested in, multiplied by
120 ft + or - depending on whether its ISA + or -. you then add
this to the pressure altitude and you have an approximate DA.
So for the above problem normal ISA temp at 5000' = +5 deg C
(15deg at MSL minus 2 deg/1000 ft)

50 deg F = approx 10 deg C

10 DEG C = ISA +5 @ 5000 FT

5*120=600 FT

5000 FT + 600 FT= 5600 FT DA

** Density Altitude**Add 1000 feet of density altitude for every 8.3 degrees Celsius
above standard for that pressure altitude.

Rule of Thumb:

Use the POH to figure aircraft performance + 50% any time the...

--Takeoff or landing runway is higher than 3000'

--Temperatures are over 90 degrees

--Takeoff or landing runway is less than 3000' long.

--Best glide speed can be quickly determined by placing the wingtip level with the horizon.

--Minimum sink speed has the leading edge slightly above the trailing edge relative to the horizon.

--The more near the throttle is to full power the more even leaning will be among the cylinders.

--If you don't know best glide speed use best climb. (close)

--Cruise speed for planing purposes should be increased by one knot for every 100 pounds below gross weight.

--You get a two-percent increase in your true airspeed for every thousand feet of higher altitude.

--For every 2% of weight reduction reduce the gross weight POH maneuvering speed by 1%.

Carbureted engines:

Multiply the horsepower available times .08

Fuel Injection engines:

Multiply the horsepower available times .077

**TRUE AIRSPEED**

Increase indicated airspeed by 2% per thousand feet of altitude.
Wind effect on speed can be determined by finding difference
between ground speed from DME and your calculated true airspeed.

The TAS = IAS * the square root
of (standard sea level air density /current air density). For
the usual standard atmosphere, this can be written:

TAS = IAS*[square root (T/288)] * [(1 - h/144000)^-2.75] where
T = 273 + OAT (C), and h is the pressure altitude. read ^-2.75
as "raised to the minus 2.75 power".

**Survival
Rule of Thumb:
**You have a minute in
the air for every 1000' of altitude

You can expect to glide one mile for every 1000' of altitude.

A 360 will lose 1000' of altitude

Land into the wind with the rows.

**Takeoff Rules of
Thumb
**--

--Increase takeoff requirement by 10% for each two knots of tailwind.

--Increase POH figures by 7% for takeoff on turf.

--Increase POH figures by 10% for short grass.

--Increase POH figures by 25% for soft surface or tall grass.

**Finding True Airspeed
** useful rule of thumb is that true airspeed increases relative to
calibrated air speed by about 1% for each 600 foot increase in density altitude.

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