Advanced Pilot Training: TM 1-205 Air Navigation - Section V Time, Speed and Distance

39. Time, speed, and distance. - a. Relationship. - There is a definite relationship between time, speed, and distance. Speed is a rate of motion or a ratio between time and distance.

Speed = distance / time

Distance = time x speed

These formulas may be used as shown above, but care should be taken to use the correct units. If speed is given in miles per hour, then distance should be in miles, and time should be in hours.

b. Example.-(1) Using the formula

Speed =. distance / time

and substituting numerical values might result in

180 m. p. h. = 45 miles / .25 hours

This formula is correct numerically. If the time is changed to 15 minutes it would appear to be

180 = 45/15

which would be incorrect. It could be changed to read

3 miles per minute= 45 miles/15 minutes

and it would again be correct. As a rule it is better to use miles and hours (m. p. h.) as the units.

(2) If any two of the above values are known, the third one may be found by using the suitable formula. Thus, if the time is required when the distance is known to be 134 miles and the speed to be 170 m. p. h., the formula would be

TIme = distance / speed

or

Time = 134 /170

= .788 hours

= .788 x 60 minutes

= 47.28 minutes

40. Knots and miles per hour.-a. Definition.-A knot is a unit of speed equal to I nautical mile per hour. (It is equivalent to 1.15 statute miles per hour.) It is frequently used by navigators of airplanes which have a crew member designated as such. The word "knot" is complete without the additional words "per hour"; for example, 120 knots is the same as 120 nautical miles per hour.

b. Use.-The formulas given in paragraph 14 for statute miles also apply to nautical miles. For example, if distances and speeds are given in nautical miles and knots, the same formulas would be used. (Par. 14 gives the conversion factors for changing one of these units to the other.) No attempt should be made to work a problem without insuring that the distances and rates of travel are in the same units.

41. Air speed and ground speed.-The air speed of an airplane depends on its rate of movement in the air, while the ground speed is its speed in relation to the ground. (See sec. VI.) The difference between the two is caused by the movement of the air or the wind.

(...)