One-engine-inoperative aerodynamics is one of the major areas students struggle with when first learning how to fly multiengine airplanes. In this article I will address the principles of flying a multi engine airplane when one of its engines fails.
Side-slip Vs Zero side-slip
The second concept I would like to discuss is the critical engine. The critical engine is the engine whose failure would most adversely affect the performance or handling qualities of the airplane. (FAR 1.1). On conventional light twins both propellers rotate clockwise (from pilot’s point of view) making the left engine critical. Other twins overcome the problem, of having a critical engine, by having counter-rotating engines (right engine rotates counter-clockwise) and the effect of loosing either one of the engines would be the same.
Four factors are responsible for making the left engine critical on a conventional twin:
P-Factor (asymmetric thrust)
On high angles of attack, the descending blade (right blade) produces more thrust then the ascending blade (left blade). The descending, right, blade on the right engine has a longer arm from the CG than the descending (right) blade of the left engine, creating a yaw force to the left.
|P- Factor causes a conventional twin to yaw to the left. Failure of the left engine will cause more loss of directional than loss of right engine because of the longer arm of the right engine’s thrust from the CG.|
|P- Factor counter-rotating engines, no yaw produced. Failure of either left or right engine will cause the same amount of directional control loss.|
|Accelerated slipstream (roll and pitch)
As a result of p-factor, stronger induced lift is produced on the right side of the right engine than on the left side of the left engine by the prop wash.
|Accelerated slipstream. Conventional twin. In case of a left engine failure, there would be a strong moment rolling the plane to the left. Also on a failure of the left engine, less negative lift will be produced by the tail, resulting in a pitch down.|
|Accelerated slipstream. Counter-rotating engines. Failure of either engines will result in the same loss of control. The arms from the CG are much closer than they are in case of a left engine failure on a conventional twin.|
|Spiraling SlipstreamThe spiraling slipstream from the left engine hits the tail from the left. In case of a right engine failure on a conventional twin, this tail force will counteract the yaw towards the left dead engine; but in case of a left engine failure, the slipstream does not hit the tail to counteract the yaw, so there is more loss of directional control.|
|Spiraling slipstream – Conventional twin|
|Spiraling slipstream – Counter rotating engines|
|TorqueFor every action there is an opposite an equal reaction (Newton’s 3rd law of motion). When the propeller spins clockwise torque will cause the airplane to roll counter-clockwise.|
|Torque – Conventional twin
As a result of the propellers turning clockwise on a conventional twin, there is a left rolling tendency of the airplane. If the right engine fails, this left roll tendency will help us maintain control and resist the right roll towards the right, dead engine, caused by asymmetric thrust; but if the left engine fails, the left roll tendency by torque will add to the left turning force caused by asymmetric thrust, making it much more difficult to maintain directional control. This makes the left engine critical.
|Torque – Counter rotating engines
On a counter-rotating twin, No matter which engine fails, torquewill oppose the roll created by asymmetric thrust.
VMC is the calibrated airspeed at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane with that engine still inoperative, and thereafter maintain straight flight at the same speed with an angle of bank of not more than 5 degrees. The method used to simulate critical engine failure must represent the most critical mode of powerplant failure expected in service with respect to controllability. (FAR 23.149)
- Published Vmc is marked as a red line on the airspeed indicator.
- Actual Vmc changes with different factors, while published Vmc remains the same.
- Published Vmc is close to the worst case scenario, actual Vmc may be lower, especially after feathering the inoperative engine’s propeller. Don’t bet your life on that fact, Vmc may be higher than you assume it is.
- Vmc, as defined by 23.149 must not exceed 1.2 Vs1.
- Vsse is the Single engine safety speed. This speed is slightly higher than published Vmc and creates a safety buffer from Vmc for intentional engine out operations. We should never fly the airplane below Vmc or Vsse, if published, under single-engine operations.
- Why is directional control affected by airspeed?
The faster the airspeed the more force the rudder can produce to resist the yawing tendency caused by asymmetrical thrust.
|Conditions by which Vmc for takeoff is determined by the manufacturer for certification of the airplane: (FAR 23.149, Airplane Flying Handbook p. 12-28)|
Recommended further reading:
|Multi-Engine Oral Exam Guide: The Comprehensive Guide to Prepare You for the FAA Oral Exam (Oral Exam Guide series)
Author: Michael D. Hayes
Author: Paul A. Craig