Approach to the Stall in a Turn with Ice Accretion
You are practising steep turns at 5,000 ft in a single-engine trainer (clean stall speed 52 KCAS at 1,000 kg). During the sortie the aircraft picks up a thin but continuous rime ice on the leading edges and the tailplane after a climb through a stratus layer; the ice is not shedding. You are now at 3,500 ft in a 45° banked turn, holding 70 KIAS, and the stall warning horn sounds intermittently as you add back pressure. The turn coordinator shows a slight slip. Outside air temperature is −2°C. The aircraft has a published clean stall speed and a note that ice increases stall speed by an unspecified amount. The pilot's instinct is to lower the nose and increase power.
1. Explain why the stall warning is sounding at 70 KIAS in a 45° bank, using the load factor and the effect of the ice.
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In a level 45° bank the load factor is 1/cos45° = 1.41, so the stall speed rises by the square root of 1.41, about 1.19 times, taking the clean stall speed from 52 to about 62 KCAS. Add the ice: leading-edge roughness reduces maximum lift coefficient and tailplane contamination reduces the nose-down pitching moment available at the stall, so the effective stall speed may rise by a further 5–15%, to perhaps 66–71 KCAS. At 70 KIAS you are therefore flying with almost no margin above the stall, and the intermittent horn is telling you the angle of attack is at the limit. The 45° bank is not the problem by itself; the combination of load factor and degraded wing is. The correct response is to reduce bank, lower the nose to restore margin, and add power, not to pull harder to hold altitude.
2. The turn coordinator shows a slight slip. How does that change the stall picture, and what is the correct recovery sequence if the horn becomes continuous?
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A slip means the aircraft is yawed relative to the airflow, so one wing is at a higher angle of attack than the other. In a slip the inner wing in the turn is typically at the higher angle, and it will stall first, producing a wing drop and a tendency to roll into the turn. With ice this asymmetry is worse because the contaminated wing's critical angle is lower. If the horn becomes continuous, the sequence is: reduce angle of attack by relaxing back pressure and lowering the nose, reduce bank to level, apply power smoothly, and keep the ball centred with coordinated rudder. Do not attempt to hold altitude; accept the descent. If a wing drops, use rudder to oppose the yaw, not aileron, because aileron input on a stalled wing can aggravate the spin. With ice on the tailplane, the recovery must be gentle: a sudden nose-down input can produce a tailplane stall, which manifests as an abrupt nose-down pitch and an inability to raise the nose.
3. Assess the risk of continuing the sortie and state the criteria for abandoning it.
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The ice is not shedding and the temperature is −2°C, which is in the worst range for accretion: supercooled water is likely and the layer is continuous. The aircraft is already exhibiting a degraded stall margin at a modest bank angle. Continuing to practise steep turns or stalls in this state is not acceptable. The criteria for abandoning are: any unexplained buffet or horn at normal operating speeds, any change in trim or stick force, any visible accumulation on the tailplane, or any inability to maintain 70 KIAS in level flight without exceeding a sensible power setting. The correct action is to reduce bank and angle of attack, descend to a temperature where the ice will shed (typically below −10°C or into above-freezing air), and land at the nearest suitable aerodrome. Report the icing to ATC. The sortie is over; the remaining exercise is a diversion and an approach with a higher-than-normal approach speed and no flap until the ice is gone.