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A light twin piston-engined aeroplane is cruising at 2000 ft when one of its engines fails. Whilst the pilot is performing the necessary actions to secure the engine, they observe that the aircraft is in a very slow descent, even with their operative engine at maximum continuous power. What is the reason for this?
  • A
    The power required has reduced because drag is increased by the stationary propeller and the required control inputs to maintain direction, and the remaining engine has sufficient excess power.
  • B
    The power available has reduced because the feathered propeller only produces a small amount of thrust compared to normal and therefore the power required is less than the power available.
  • C
    The pilot must NOT have feathered the propeller of the failed engine, because ALL twin-engined aeroplanes that are certified in accordance with CS-23 must be able to climb within their environmental envelope.
  • D

    The increased drag from the propeller of the failed engine and the required control inputs to maintain direction mean that the power from the remaining engine is insufficient to maintain altitude.

Refer to figure.
When a multi-engined aircraft loses an engine, not only does thrust (or power) reduce, but the overall drag is increased, so more thrust is required. The drag increases due to the dead engine, for both a jet or a propeller engine, however, propellers can be feathered to reduce that drag to a minimum. Drag also increases due to the control deflections that are required to keep the aircraft flying straight, and by the sideslip induced when maintaining wings level.

In this case, it is a propeller aircraft, so produces power. This means that when the engine fails, the power required will increase, due to the increased drag mentioned above, whilst the power available is decreased by 50%, due to the loss of one engine's power output. This has put the aircraft into a "power deficit", where the power required is higher than the power available. This means that there is not enough thrust to overcome the drag, and the aircraft has begun to descend. This is often called a drift-down, where an aircraft descends into the denser air until the remaining engine is powerful enough to maintain altitude on its own.


To go through the available options:

  • "The power required has reduced because drag is increased by the stationary propeller and the required control inputs to maintain direction, and the remaining engine has sufficient excess power." INCORRECT.
    • The remaining engine does NOT have sufficient excess power, and the power required has increased.
  • "The power available has reduced because the feathered propeller only produces a small amount of thrust compared to normal and therefore the power required is less than the power available." INCORRECT.
    • A feathered propeller does not provide any thrust, just less drag, and the power required is MORE than the power available.
  • "The pilot must NOT have feathered the propeller of the failed engine, because ALL twin-engined aeroplanes that are certified in accordance with CS-23 must be able to climb within their environmental envelope." INCORRECT.
    • Whilst there are requirements for certain climb gradients (after take-off or go-around) for some operations, no aircraft is expected to have a positive climb gradient with an engine failed throughout their whole environmental envelope, which we are assuming to mean up to their maximum operating altitude.
  • "The increased drag from the propeller of the failed engine and the required control inputs to maintain direction mean that the power from the remaining engine is insufficient to maintain altitude." CORRECT.
    • Yes, nothing wrong with that option. Drag increased so power required increased, and power available was cut in half.

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