FAA Sport Pilot Airplane (SPA)Aircraft Performance and SystemsMedium
An airplane's best glide speed is published as 65 knots at a gross weight of 1,320 pounds. If the airplane is flown at a reduced weight of 1,100 pounds, which best glide speed should the pilot use to maintain the same glide ratio?
- AAbout 72 knots
- BAbout 54 knots
- C65 knots (unchanged)
- DAbout 59 knots
Show answer & explanationAnswer & explanation
Correct answer: D. About 59 knots
Best glide speed varies with the square root of the weight ratio: 65 × √(1,100/1,320) = 65 × √0.833 = 65 × 0.913 ≈ 59 knots. Lower weight requires a slightly lower airspeed to maintain the same optimum glide angle.
Why the other options are wrong
- A. This increases speed, but reduced weight requires lower, not higher, best glide speed.
- B. This underestimates the calculated adjusted speed.
- C. Best glide speed does change with weight, contrary to this answer.
Best Glide Speed and Weight
Best glide speed varies with the square root of the weight ratio; lighter weight yields a lower best glide airspeed while maintaining the same glide ratio.
- Formula: new speed = published speed × √(new weight/reference weight)
- Glide ratio (distance per altitude lost) stays constant with weight changes
- Heavier airplane requires higher glide speed for same glide angle
Memory trick: Lighter plane, slower glide, same angle down.