Descent Below Vmca in a Single-Engine Configuration
A pilot of a light twin is practising asymmetric flight at 4,000 ft. The aircraft has a published Vmca of 78 kt IAS in the take-off configuration and a single-engine best-rate-of-climb speed of 92 kt. The pilot shuts down the right engine (simulated, with the propeller feathered) and holds 85 kt with about 5° of bank toward the live engine. Airspeed then decays to 76 kt as the pilot raises the nose to hold altitude. The yaw damper is serviceable. The pilot notices increasing rudder force, a mild buffet, and that the slip ball is displaced toward the dead engine. The pilot's response is to increase bank toward the live engine to 15° to stop the yaw.
1. Assess whether increasing bank is an appropriate response to the yaw at 76 kt, and explain the relationship between bank, sideslip and Vmca.
Read the worked answer
Increasing bank is not the right primary response, and the situation is already outside a safe envelope.
Vmca is the minimum control speed in the air: the speed below which full rudder (with the aircraft at no more than 5° bank) cannot hold the yaw from the asymmetric thrust. At 76 kt the aircraft is below the published 78 kt, so directional control is already marginal, which is exactly what the rising rudder force and buffet indicate. The ball displaced toward the dead engine confirms the aircraft is slipping toward the failed engine, so the fin is already working hard.
Banking toward the live engine does help, because the horizontal component of lift opposes the yawing moment and reduces the rudder force needed. That is why the certification condition allows up to 5° of bank. But bank is a secondary aid, not a substitute for rudder, and using large bank to mask a speed deficiency is dangerous: it increases the load factor, raises the stall speed, and reduces the vertical component of lift, so the aircraft sinks while the pilot believes control is restored.
The correct response is to lower the nose, accelerate back above Vmca, reduce bank to the small amount that minimises rudder force, and only then consider the single-engine climb schedule at 92 kt.
2. Explain why the pilot should not simply hold altitude at 76 kt, and describe the sequence the pilot should fly to regain a safe single-engine climb.
Read the worked answer
Holding altitude at 76 kt means accepting flight below Vmca, where the available rudder cannot fully counter the yaw from the live engine. The aircraft is slipping, which adds drag and further degrades performance, and any gust, power change or distraction can push it into a departure from controlled flight. The buffet is a warning that the wing is near its critical angle of attack at this low speed and high power setting. Trading height for speed is the only way out.
The sequence is: reduce power on the live engine if necessary to reduce the yawing moment, lower the nose to accelerate, hold only the small bank (about 5°) that minimises rudder force, and keep the ball centred with rudder. Once above Vmca with a margin, restore power and accelerate to the single-engine best-rate-of-climb speed of 92 kt. At 92 kt the aircraft has both the rudder authority and the excess power to climb; below it, the pilot is trading control margin for altitude and losing both. The pilot should also brief that a real engine failure at low speed may require accepting a descent until Vmca is regained.