DETERMINING THE CRITICAL ENGINE

Determining the Critical Engine Lesson by wifiCFI


New Airspeeds

Vxse: Best Angle of Climb with a Single Engine

Vyse: Best Rate of Climb with a Single Engine/Least Rate of Descent

Vsse: Safe Single Engine speed 

Vmc: Minimum controllable airspeed (more information later)

Definitions

Vmc: 

Minimum control speed with the critical engine inoperative. 

Critical Engine:

The engine, that when failed, most adversely affects the handling and performance characteristics of the airplane.

Conventional Twin: 

Both aircraft propellers rotate clockwise as seen from the cockpit.

On Conventional Twins the Critical Engine is the left engine (reasons explained on the following slides). 

Counter Rotating Twin:

The propellers rotate opposite of each other (they both rotate inward).

On Counter Rotating Twins, there is no Critical Engine. It is not worse to lose one engine over another. They are both equally bad.

Determining the Critical Engine

To determine which engine is the Critical Engine, we will use the aerodynamic principles form the acronym PAST.

P – P-Factor

A – Accelerated Slipstream

S – Spiraling Slipstream

T – Torque

P-Factor

P-Factor explains that the descending blade on a propeller produces more thrust than the ascending blade (as we know from previous lessons).

Because the propellers rotate clockwise (as seen from the cockpit) we have the situation depicted on wifiCFI.

As can be seen, the center of thrust is further from the longitudinal axis (or Center of Gravity) of the airplane on the right engine.

Thus, giving it a longer arm.

The longer arm from the right engine’s thrust makes it more effective at yawing the aircraft into an inoperative left engine.

Thus, making the left engine critical.

Remember, P-Factor = Yaw.

Accelerated Slipstream

Accelerated Slipstream is the air that is accelerated by the propeller.

The descending blade produces more accelerated slipstream than the ascending blade.

Because the propellers rotate clockwise (as seen from the cockpit) we have the situation depicted on wifiCFI.

The red circles indicate the areas of the wing that are receiving more accelerated airflow.

Because there is more airflow in these areas, the wing’s Center of Lift shifts toward the circles.

The Center of Lift on the right wing is further from the longitudinal axis of the aircraft.

The longer arm from the right engine’s Center of Thrust makes it more effective at rolling the aircraft into an inoperative left engine.

Thus, making the left engine critical.

Accelerated Slipstream also deals with Pitch.

The accelerated airflow from the left engine flows over the elevator/stabilator. 

This increased airflow helps keep the nose of the airplane up and makes the elevator/stabilator more effective.

The accelerated airflow from the left engine flows over the elevator/stabilator making it more effective.

Where the accelerated slipstream from the right engine does not.

Thus, making the left engine critical.

Remember, Accelerated Slipstream = Roll and Pitch.

Spiraling Slipstream

The slipstream off the propeller doesn’t flow back in a straight line.

The pressure differential between the faster and slower moving air causes a pressure differential.

This pressure differential causes the airflow to spiral as pictured on wifiCFI.

The spiraling slipstream from the left engine flows over the rudder and vertical stabilizer making them more effective.

The spiraling effect from the right engine does not have the same effect.

When the left engine is failed the aircraft yaws to the left.

The Spiraling Slipstream from the right engine does not aid in counteracting this yawing effect.

When the right engine is failed the aircraft yaws to the right.

The Spiraling Slipstream from the left engine aids in counteracting this yawing effect.

Thus, making the left engine critical.

Remember, Spiraling Slipstream = Yaw.

Torque

Newton’s 3rd Law of Motion states: “For every action there is an equal and opposite reaction.” 

Because the propellers rotate clockwise, the airplane has a natural tendency to roll counterclockwise as pictured on wifiCFI.

When the left engine is failed the aircraft rolls to the left.

This rolling moment is added to the effect of Torque.

When the right engine is failed the aircraft rolls to the right.

This rolling moment counteracts the effect of Torque. 

Thus, making the left engine critical.

Remember, Torque = Roll.

FAA Sources Used for This Lesson

Airplane Flying Handbook (AFH) Chapter 12


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