Drift illusions at low level
Low flying in strong wind: drift over the ground creates false impressions of slipping or skidding in a turn.
- Most critical: forced landing, precautionary landing, deteriorating weather, any low flying.
- Cause: ground movement looks slow at cruising altitude even at high airspeed, much faster close to the ground; in strong wind this creates the illusion.
- Lower height and airspeed: stronger skid/slip illusion for a given wind.
Straight legs:
| Leg | Appearance |
|---|---|
| Upwind | Ground speed looks reduced |
| Downwind | Ground speed looks high; tempts airspeed reduction, too far risks a stall |
| Crosswind | Sideways drift obvious, especially lined up with a straight road or section line |
Turning through the wind (turn coordinated, ball centred, no real slip; drift real):
| Turn | Seems | Allow room for obstacles |
|---|---|---|
| Upwind to downwind | Slip inward, speed up | Inside of turn |
| Downwind to upwind | Skid outward, slow down | Outside of turn |
- No rudder correction: centred ball confirms no slip.
Bank technique: ground-reference circle, correct drift by varying bank angle (shallower one side, steeper the other), holding altitude and airspeed constant.
Altimeter caution:
- Reads height above sea level, not ground.
- Watch ground contours to estimate height above ground.
- Judge height looking well ahead, not straight down.
Instruments: little attention available close to ground; keep continuous lookout for other aircraft and high obstacles.
Points to remember, flying close to the ground (TP 1102):
- Maintain a safe airspeed.
- Turn accurately despite the illusory effect of drift.
- Maintain a safe height above ground contours.
- Keep a good look-out.
- Do not turn too steeply.
- Do not annoy others or frighten livestock.
Trap: using rudder to "correct" apparent slip or skid in a near-ground turn. Ball centred: only drift is real.
References
- TP 1102 Ex.20 Illusions Created by Drift
- TP 1102 Fig.2-55