Engine inoperative principles in a twin airplane

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

  • With a multiengine airplane with all its engines operating, sideslip is eliminated by keeping the ball in the inclinometer centered, just the same you are used to do with a single-engine airplane. This is called zero sideslip and is the condition where the airplane is presenting the smallest profile to the relative wind, creating minimum drag.
  • Because of the asymetrical thrust, A centered ball is no longer an indication for zero sideslip if one engine fails.
  • Even with the ball centered, on a multiengine airplane producing asymmetrical thrust, we are still flying in a side slip due to the lateral force created by the rudder.
  • The only equipment that can indicate slip in an asymmetrical thrust condition is a yaw string attached to the windshield.
  • To achieve zero side slip with one engine inoperative we should use both rudder and ailerons. Place 1/2-1/3 of the ball out of its cage towards the operative engine and use about 2° of bank towards the operative engine (“raise the dead” engine). The opposing forces of horizontal component of lift and rudder side force will eliminate the sideslip.
  • Zero side slip will give us best performance and directional control.
  • Remember that loss of directional controlled is caused by asymmetrical thrust. The reduction of power will restore directional control, but will also decrease performance.
  • When a light twin loses an engine, it loses 80% of its climb performance, due to the increased drag and decrease in excess power required for a climb.

 


Critical Engine

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)
Accelerated slipstream
Spiraling slipstream
Torque

P-Factor(yaw)
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. image
P- Factor counter-rotating engines, no yaw produced. Failure of either left or right engine will cause the same amount of directional control loss. image

 


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. image
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. image

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 image
Spiraling slipstream – Counter rotating engines image

 


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.
image 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.
image

VMC

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)
  1. Standard atmosphere. (FAR 23.45)
  2. Most unfavorable CG and weight.
  3. Out of ground effect.
  4. Critical engine INOP
  5. Bank no more than 5° towards operating engine.
  6. Max available takeoff power on each engine initially
  7. Trimmed for takeoff.
  8. Wing flaps set to takeoff position.
  9. Cowl flaps set to takeoff position.
  10. Landing gear retracted.
  11. All propeller controls in takeoff position. (INOP engine windmilling)
  12. Rudder force required by the pilot to maintain control must not exceed 150 pounds.
  13. It must be possible to maintain heading ±°20.
Factor VMC performance
Increase in density altitude Decreases (good) Decreases (bad)
Increase in weight Decreases (good) Decreases (bad)
Windmilling prop (vs. feathered) Increases (bad) Decreases (bad)
Aft CG Increases (bad) Increases (good)
Flaps extended Decreases (good) Decreases (bad)
Gear retracted Increases (bad) Increases (good)
Up to 5° Bank towards good Decreases (good) increases (good)

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
Multi-Engine Flying
Author: Paul A. Craig

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