Multi-Engine Oral Exam Prep

Vmc is the minimum control speed with the critical engine inoperative. It is the minimum speed at which you can maintain directional control (heading within 20 degrees) with one engine inoperative and the other at max takeoff power. Below Vmc, the rudder does not have enough authority to overcome the asymmetric thrust and the aircraft will yaw uncontrollably toward the dead engine.

SMACFUM: Standard day (density altitude — lower DA increases Vmc because the operating engine produces more power), Max takeoff power (more power = more asymmetric thrust = higher Vmc), Aft CG (shorter rudder moment arm = less rudder effectiveness = higher Vmc), Critical engine inoperative (the engine whose failure produces the most adverse yaw), Flaps takeoff position, Up landing gear (gear acts as a keel, helping directional stability — gear down reduces Vmc), Max gross weight (more weight means more inertia helping resist yaw — lighter = higher Vmc relative to stall).

On a conventional twin with both engines rotating clockwise (viewed from the cockpit), the left engine is the critical engine. P-factor causes the descending blade (right side of each propeller disk) to produce more thrust. On the right engine, this thrust is farther from the aircraft centerline (longer moment arm), producing more yaw when the left engine fails. Therefore, failure of the left engine produces the most adverse effect — making it the critical engine.

Dead foot, dead engine — the foot with no rudder pressure tells you which engine failed. Identify by closing the throttle on the suspected engine. Verify by slowly retarding the throttle — if there is no change in performance, you have the correct engine. Then feather: mixture idle cut-off, propeller feather, throttle closed, fuel selector off, boost pump off, alternator off, cowl flap closed. Never rush — misidentifying and feathering the good engine is far worse than a delayed response.

Vyse (blue line speed) is the best rate of climb speed with one engine inoperative — it gives you the most altitude gain per unit of time. Vxse is the best angle of climb speed with one engine inoperative — it gives you the most altitude gain per unit of distance (for clearing obstacles). In many light twins, single-engine climb performance is marginal at best, and blue line speed is your survival speed after an engine failure.

If you are below Vmc with one engine at full power, you will not be able to maintain directional control. The aircraft will yaw and roll toward the dead engine, potentially entering a spin. If an engine fails below Vmc (such as during initial climb), you must immediately reduce power on the operating engine to regain control, then decide whether to continue or abort based on remaining runway and altitude.

Zero sideslip (also called Vyse configuration) produces the best single-engine climb performance by minimizing drag. Technique: bank 3-5 degrees into the operating engine and use enough rudder to center the ball or slip it slightly toward the operating engine. This eliminates the sideslip that would otherwise result from asymmetric thrust. The inclined lift component helps offset the yawing tendency, reducing the rudder deflection needed and therefore reducing rudder-induced drag.

Accelerate-stop distance is the total runway required to accelerate to a specified speed (liftoff speed or Vmc) and then stop on the remaining runway if an engine fails. If your runway is shorter than the accelerate-stop distance, an engine failure after the decision point means you cannot stop on the runway. You must know this distance for your aircraft and factor it into your takeoff planning. In many light twins, if you lose an engine before reaching Vyse and a safe altitude, the safest option may be to close both throttles and land straight ahead.