Base-to-Final Stall in a T-tail Twin
A twin turboprop with a T-tail, MTOW 5 700 kg, is on a visual approach at 4 000 ft. Landing mass is 5 100 kg with the CG at 34% MAC; the aft limit is 35%. The clean 1 g stall is 88 kt IAS. The aircraft is turned from base to final in a 30° bank at 90 kt IAS with idle power, and full flap is selected during the turn. The slip indicator shows a slight skid, buffet begins, the elevator feels light and the nose tends to pitch up. The pilot responds with back pressure and a touch of aileron into the turn. The aircraft rolls further and the nose drops; height is about 1 200 ft AGL, surface wind 20 kt, turbulence light.
1. Show, with the numbers, that the aircraft was probably stalled before the buffet was recognised as such.
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The published 88 kt is a 1 g stall in level flight. In a bank the wing must produce n times the weight, and the stall speed rises with sqrt(n):
30° bank: n = 1/cos30° = 1.155; sqrt(n) = 1.075; Vs = 88 × 1.075 ≈ 94.6 kt IAS.
Even though the turn was descending and the actual n was below 1.155, any 30° bank puts n above 1, so the accelerated stall speed was at least about 92–95 kt. At 90 kt the wing was at or beyond the critical angle of attack. Buffet onset normally appears at 1.05–1.10 Vs, i.e. around 99–104 kt in that turn, so buffet at 90 kt is consistent with a stalled wing rather than a warning.
Three other factors made it worse: full flap selected in the turn changes CLmax and pitching moment as the flaps run; the skid means the aircraft was yawed, so the outer wing had a higher angle of attack than the inner one, the classic pre-spin condition; and the CG at 34% MAC, only 1% forward of the aft limit, reduces the static margin and the natural pitch-down at the stall, so the airframe gives less warning than the book figure suggests.
2. The pilot applied back pressure and aileron into the turn. Explain why both inputs worsened the situation, and what the light elevator and slight pitch-up indicate.
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Back pressure directly increases the angle of attack, taking the wing further past the critical angle. The instinct is understandable because the nose had dropped, but when stalled the elevator is not an angle-of-attack control, it is an angle-of-attack demand.
Aileron into the turn is worse. Deflecting the aileron down on the outer wing increases its camber, its CL and its local angle of attack, and at the same time produces adverse yaw that yaws the nose away from the turn. The outer, descending wing is then both at a higher angle of attack and effectively slower in the yaw, and it reaches the critical angle before the inner wing - autorotation. That is how a wing drop becomes an incipient spin.
The light elevator and the slight pitch-up are the signature of a T-tail at a high angle of attack: the wing wake has reached the tailplane, so the stabiliser sits in disturbed, low-energy flow and loses effectiveness, while the loss of tail downforce lets the nose rise. It is a warning of deep-stall risk, not a cue to pull. Recovery must therefore be driven by reducing the angle of attack with whatever pitch authority remains, and by yaw control.
3. Give the recovery sequence, justifying each step, and state why the aft CG matters for recovery and for spin entry.
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Sequence:
- Reduce the angle of attack - push the column firmly forward. This is the only way to get the wing below the critical angle and it must come first.
- Power - apply as required; on a turboprop the slipstream raises elevator and rudder effectiveness and reduces the descent rate, but AoA reduction remains the priority.
- Stop the yaw - opposite rudder to the skid, firmly, without inducing a yaw the other way.
- Level the wings - aileron only once the stall is broken and the yaw stopped, and coordinated with rudder.
- Recover the descent, then restore power and configuration; do not retract flap until a positive rate and speed are established.
Why the aft CG matters: at 34% MAC the static margin is small. The pitch-down moment at the stall is weak, so the aircraft does not self-recover and can sit at a high angle of attack; the elevator has less moment arm, so the recovery push is less effective and the height loss is greater. Once a spin develops, an aft CG increases the chance of a flat, oscillatory spin and lengthens recovery. Practically, an approach at 34% MAC with a 35% limit leaves no margin for a configuration change in a turn - select flap before the turn or fly the turn faster.