Asymmetric Flight

Question 1

Normal non-asymmetric flight

Answer 1

In normal flight of a multi engine a/c the line of total thrust and drag act through and are balanced about the normal axis, with all engines at the same power setting there will be no residual yawing moments resulting from the offset engine thrust line and in steady flight total thrust will equal total drag

Question 2

What causes asymmetric thrust

Answer 2

Any multi-engine a/c that suffers a failure of one of its engines has a potential asymmetric flight problem (the exception being if all engines share the same thrust line)

Question 3

The result of asymmetric thrust

Answer 3

• The line of total thrust will be offset from the normal axis causing a yawing moment toward the failed engine
• The line of total drag moves toward the failed engine (which adds to the asymmetric yawing moment)
• There’s a reduction of total thrust available which leads to a deterioration in performance

Question 4

What is the immediate concern following a failed engine

Answer 4

Is to control the yaw and any roll that results while also maintaining a safe flight path, then with appropriate drills completed and the failed engine secured the aim is to fly in such a way as to achieve optimum performance with the asymmetric thrust and reduced power available

Question 5

Critical engine

Answer 5

Is the one whose failure would result in the most adverse effects of the a/c handling + performance (this is the left engine in a/c that rotate clockwise)

Question 6

Factors affecting the yawing moment (asymmetric flight)

Answer 6

• Thrust from live engine
• Distance of the thrust-line from the CG
• Directional stability
• Rate of thrust decay
• Drag of the failed engine

Question 7

Thrust from live engine - effect on yawing moment

Answer 7

Higher thrust = Greater yawing moment

Question 8

Distance of the thrust-line from the CG - effect on yawing moment

Answer 8

Increased distance = larger arm through which the thrust from the live engine acts = greater yawing moment

Question 9

Directional stability - effect on yawing moment

Answer 9

To obtain directional stability the a/c is constructed with a greater proportion of fin and rudder SA behind the CG than ahead of it this provides a weathercocking effect which acts to keep the fuselage aligned with the direction of flight. The weathercocking moment resists the asymmetric thrust yawing moment and it increases with airspeed. Directional stability is decreased with rearward movement of the CG and the weathercocking moment is therefore less effective when the a/c has aft loading

Question 10

Rate of thrust decay - effect on yawing moment

Answer 10

If the engine failure is gradual the onset of the yawing moment will also be gradual

Question 11

Drag of the failed engine - effect on yawing moment

Answer 11

The drag of the failed engine will be higher than one that is operating normally particularly if the propeller is windmilling. The greater the increase in drag on the dead engine the stronger the yawing moment toward that side

Question 12

Rolling moment

Answer 12

Failure of an engine also causes a rolling moment to develop mainly through the further effect of yaw and the loss of slipstream over the wing resulting in a loss of lift on the side of the failed engine. Roll is also in the direction of the failed engine