The Critical Engine

An overview of the critical engine discussing what the critical engine is, which engine is the critical engine, and the reasons why it is the critical engine.


THE BASICS

1. WHAT IS THE CRITICAL ENGINE?

The critical engine is the engine that if failed will have the most adverse affect on the CONTROL and PERFORMANCE of the aircraft.

2. WHICH ENGINE IS THE CRITICAL ENGINE?

In a conventional twin (clockwise prop rotation), the LEFT ENGINE is the critical engine.
In a counter rotating twin, there is no critical engine.

3. WHY IS THE LEFT ENGINE CRITICAL?

There are 4 reasons the left engine is the critical engine (REMEMBER - PAST):

  • P-Factor
  • Accelerated Slipstream
  • Spiraling Slipstream
  • Torque

P-FACTOR

  • The descending blade of each propeller produces more thrust than the ascending blade
  • Therefore, in a conventional twin, the center of thrust is offset to the right of each engine (see red thrust arrows in the graphic)
  • There is a greater distance, or arm, between the center of thrust and the longitudinal axis on the right engine than there is on the left engine
  • The greater the distance, or arm, the greater the leverage
  • If the right engine fails, the leverage associated with P-Factor is not as great as if the left engine were to fail
  • If the left engine fails, the yaw from P-Factor is most adverse, therefore the left engine is critical
P-Factor

ACCELERATED SLIPSTREAM

Accelerated Slipstream
  • As mentioned above, the descending propeller blade produces more thrust than the ascending blade
  • This results in greater airflow (more lift) over the wings on the right side of each engine (the blue arrows in the diagram)
  • Just like P-Factor, there is a greater distance, or arm, between the slipstream and the longitudinal axis on the right engine than there is on the left engine
  • The greater the distance, or arm, the greater the leverage
  • If the right engine fails, the leverage associated with the accelerated slipstream is not as great as if the left engine were to fail
  • If the left engine fails, the resulting roll from the accelerated slipstream is most adverse, therefore the left engine is critical

SPIRALING SLIPSTREAM

  • As each propeller rotates, it produces a clockwise spinning, three dimensional slipstream of air behind it
  • The left engine’s slipstream strikes the rudder on the left side creating a left yawing tendency
  • The right engine’s slipstream, on the other hand, does not strike the rudder and has no affect on aircraft control
  • If the right engine were to fail, the left engine’s slipstream will counteract some of the yaw toward the dead engine, assisting in aircraft control
  • If the left engine were to fail, the airplane will yaw uninhibited toward the dead engine
  • Therefore, the left engine has the most adverse affect on control and performance, and is the critical engine
Spiraling Slipstream

TORQUE

Torque
  • Torque is based on Newton’s 3rd law: For every action there is an equal and opposite reaction
  • As the propeller spins clockwise (right, from the pilot's perspective), torque is the equal and opposite reaction rolling the aircraft counterclockwise (left)
  • If the right engine were to fail, the aircraft yaws and rolls right, toward the dead engine, but torque will offset some of the rolling action, lessening the effect of the engine failure
  • If, on the other hand, 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 
  • A failed left engine has the most adverse effect on control of the aircraft and is therefore the critical engine