Breaking Down the Critical Factors of One Engine Inoperative Flight
- Joshua Diehl
- May 22, 2024
- 2 min read
You're flying along in your multi engine airplane. The flight has been without event, but suddenly you experience an engine failure. What exactly happens to the handling characteristics when you experience an engine failure in a multi engine airplane?
There are two main considerations when an engine fails:
Control
Performance
We will focus on the aspect of controllability. To help better recall the factors effecting controllability during an engine failure, we will use the acronym PAST, which stands for:
P-factor
Accelerated Slipstream
Spiraling Slipstream
Torque
P-Factor is a yawing moment, about the center of gravity, which occurs as a result of the descending propeller blade of each engine. The descedning propeller blade will produce a greater thrust than the ascending blade due to angle of the attack (AOA). You may have heard your flight instructor say the that descedning propeller blade takes a "bigger bite of air," this is P-factor in a nutshell. In a conventional rotating multi engine airplane, failure of the left engine will result in the most asymmetrical thrust (adverse yaw) due to the right engines descending propeller having a farther arm (moment) from the center of gravity then that of the left descending propeller.

Accelerated Slipstream, commonly referred to as induced airflow, is a rolling moment. The descending propeller blade produces more thrust than the ascending blade. This results in greater airflow over the wings on the right side of each engine. The descending propeller has a great distance (arm) than the left descending propeller, thus lifting the right wing, causing a turn to the left.

Spiraling Slipstream is a yawing moment. Each propeller produces a clockwise spinning, three dimensional slipstream of air behind it. This is due to the higher pressure airflow of the ascedning blade wanting to move into the lower pressure airflow from the descedning propeller blade. Remember the rule of thumb, higher pressure will always want to fill an area of lower pressure. The left engine’s airstream strikes the rudder on the left side creating a left yawing tendency about the vertical axis. The right engine’s slipstream, does not strike the rudder and, during an engine failure on a conventially rotating multi engine airplane, this has a positive effect on aircraft control.

Torque is a rolling moment. It is based on the principle of Newtons Third Law: For every action there is an equal and opposite reaction. As the propeller spins clockwise (right, from the pilot's perspective), torque is equal and opposite, rolling the aircraft to the left. If the left engine were to fail, the aircraft would yaw and roll toward the dead engine on the left, but in this case torque amplifies the left roll, making aircraft control more difficult.

Comments