“Dead Reckoning” from Navigators’ Information File, Headquarters Army Air Forces, War Department, 1944:
DEAD RECKONING
Importance
The successful termination of any flight depends on dead reckoning. Navigators returning from all over the world–from the Aleutians, where weather is always a problem; from the Marianas, where long over-water flights are made constantly; from China, the land of no maps–stress this fact: Dead reckoning is the basis of all navigation. Use it.
Celestial, pilotage, radio, and Loran all are aids to dead reckoning. Use them only as aids.
Dead reckoning is based upon the solution of the time-speed-distance problem, and you are primarily a dead-reckoning navigator. Pilots dead reckon on every flight, though they are not always aware of this fact. Your work must be more exact, of course, than a pilot’s mental calculations. And you must know and use every form of dead reckoning available to you on every flight you make.
If you make but one resolve as a navigator, it should be, “I’ll dead reckon on every flight from the time we take off until the wheels are back on the ground.” If you do less than this you are not doing your job–and that can easily prove fatal.
Computations
To do accurate work you must be able to recognize all possible errors in your computations and know how to compensate for them. Furthermore, you must understand the navigation problems you are likely to encounter and plan your solutions of them before you leave the ground. Constant air practice then gives you needed confidence.
Prepare adequately on the ground to make your work as easy as possible in the air. The distractions of flight conditions such as combat, weather, and lack of oxygen, and the inconveniences of fatigue and cramped quarters unavoidably complicate your job. Don’t make it more difficult for yourself.
True Airspeed
An error that is not always recognized is the error in the free-air temperature gage. Although the free-air temperature gage is mechanically perfect, it measures only the temperature of the air immediately surrounding the airplane. Because of the effects of compressibility and friction, this air invariably is warmer than the free air. Obtain accurate free-air temperature by decreasing indicated temperatures by the following amounts:
TAS
(knots)Decrease
(C°)60 0.4 80 0.7 100 1.1 120 1.5 140 2.1 160 2.7 180 3.4 200 4.2 220 5.1 240 6.1
TAS
(knots)Decrease
(C°)260 7.2 280 8.3 300 9.5 320 10.9 340 12.3 360 13.7 380 15.3 400 17.0 420 18.7 440 20.5 1. Set indicated temperature and altitude on your computer and read approximate true airspeed above calibrated airspeed.
2. Enter the table with approximate true airspeed and find the temperature correction.
3. Use the corrected temperature and altitude to obtain accurate true airspeed.
Record corrected free-air temperatures on weather reports for Weather Intelligence, or note on the report that you have not made the correction.
REFERENCE: Technical Order 05-40-15.
Turns
Depending upon the type of operation and whether your airplane is flying alone or in a formation, it may be a good idea to calculate the distance you travel in a turn. That distance depends upon the rate of turn and the speed at which you are flying.
For example, a formation of B-17’s (approximately 36 airplanes), indicating 150 mph, makes a 90° turn in this manner:
The lead airplane starts to turn one minute before the turning point and straightens out on course one minute past the turning point.
You can gain or lose time by increasing or decreasing the rate of turn.
