FAA Flight Instructor Airplane (FIA)Performance, Weight and BalanceHard
An airplane's POH lists a best glide speed of 68 KIAS at a gross weight of 2,300 lb. After burning fuel in flight, the airplane's weight is now 1,900 lb. What best glide speed should the pilot use at this reduced weight?
- A75 KIAS
- B62 KIAS
- C56 KIAS
- D68 KIAS
Show answer & explanationAnswer & explanation
Correct answer: B. 62 KIAS
Best glide speed varies with the square root of the weight ratio: new speed = 68 × √(1,900/2,300) = 68 × √0.826 = 68 × 0.909 ≈ 62 KIAS. As weight decreases, the airplane can glide efficiently at a lower airspeed.
Why the other options are wrong
- A. Incorrectly increases the speed instead of decreasing it for lower weight.
- C. Underestimates the new speed, likely from an arithmetic error in the ratio.
- D. Assumes best glide speed doesn't change with weight, which is incorrect.
Best Glide Speed vs Weight
Best glide speed, like stall speed, varies with the square root of the weight ratio; lower weight requires a lower best glide speed for maximum glide distance.
- New speed = old speed × √(new weight ÷ old weight)
- Applies the same relationship used for stall speed changes with weight
- Using the wrong glide speed reduces glide ratio and range during an emergency
Memory trick: Lighter plane, slower glide — square root the weight to find your ride.