Best glide — the key descent speed
In case of engine failure, the best glide speed (Vbg) is the speed at which the aircraft glides the furthest distance through still air. It corresponds typically to the speed for best L/D (minimum drag).
Best glide is published in the AFM/POH — memorise! Example values:
- Cessna 172: ~ 65 KIAS (type-specific)
- Piper PA-28: ~ 73 KIAS (type-specific)
- Aquila A210: ~ 70 KIAS (type-specific)
Glide ratio
The L/D ratio equals glide distance per unit altitude in still air:
| L/D | Glide ratio | Example |
|---|---|---|
| 9 : 1 | From 6 000 ft AGL → ~9 nm in still air | Typical SEP trainer (C172, PA-28) |
| 17 : 1 | From 6 000 ft AGL → ~17 nm | High-performance trainer (Diamond DA40 type) |
| 30–60 : 1 | From 6 000 ft AGL → 30–60 nm | Sailplanes |
SEP rule of thumb: glide distance (nm) ≈ altitude (kft AGL) × L/D / 6; for typical 9
this reduces to approximately altitude × 1.5.Wind effect
| Wind | Ground-track glide range |
|---|---|
| Still air | Maximum at Vbg |
| Headwind | Reduced; optimal glide speed slightly higher than Vbg (steeper glide, less time in wind) |
| Tailwind | Extended; optimal speed slightly lower than Vbg |
In practice: keep Vbg constant is usually the best strategy — wind optimisation yields only a few percent.
Power-off vs power-idle
- Power-off glide (engine completely shut down, prop windmilling) — matches AFM performance data.
- Power-idle (engine at idle, no thrust but prop rotating) — slightly shallower glide angle.