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.

DEW POINT
Subtracting the dew point from the surface temperature, dividing the difference by 4.4 and then moving the decimal point three places to the right can approximate the base of a cumulus cloud. Surface dewpoint and temperature is commonly part of every ATIS.

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. Multiply your present altitude times three and the product is the distance from your destination altitude which you should start down.

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.

Distance to Initiate Descent
Distant to descend divided by 1000 (knock off zeros) and multiply by three will give distance.

Distance for 500 fpm Descent
Altitude to lose expressed in thousands of feet times groundspeed in miles per minute x 2
5000' to lose at 120 knots gives 5 X 2 X 2 = 20 as the miles needed at 500 fpm descent.

Using ground speed to get rate of descent
Divide ground speed by two and add a zero.

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.

Pivotal Altitude for Turns on Point
V in knots squared and divided by 14.3

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.

PERFORMANCE
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%.
. Fuel Consumption
At 5000 feet you only get 75% horsepower. POH numbers are minimums.
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
--+15 F or 8.5C density altitude changes by 1000 feet.
--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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