What Is the Critical Engine in a Multi-Engine Airplane?
The critical engine is the engine whose failure would most adversely affect the performance or handling qualities of a multi-engine airplane. In many conventional twins with both propellers rotating the same direction, the left engine is critical because losing it leaves the right engine operating with a less favorable thrust geometry.
Why one engine can be more critical
At high power and high angle of attack, P-factor shifts the effective thrust of each propeller disc. On a conventional twin with both propellers turning clockwise as viewed from the cockpit, the descending blade of the right engine is farther from the airplane centerline than the descending blade of the left engine. If the left engine fails, the right engine remains and its effective thrust line has a longer arm from the centerline. That creates a larger yawing moment for the rudder to oppose.
Why the left engine is often critical
In the common same-direction configuration, failure of the left engine leaves the right engine producing the more unfavorable asymmetric thrust. The airplane needs more rudder to oppose the yaw, so losing the left engine is considered the more adverse condition. That is why the left engine is commonly called the critical engine in many traditional light twins.
Counter-rotating propellers
Counter-rotating twins are designed so the descending propeller blades are arranged symmetrically closer to the fuselage. This removes the usual P-factor disadvantage of one side. The Piper PA-44 Seminole is a common training example: its counter-rotating propellers mean there is no single aerodynamically critical engine from the normal asymmetric-thrust standpoint.
Critical engine versus important accessories
For oral-exam purposes, keep the aerodynamic definition separate from equipment considerations. An airplane may not have a critical engine aerodynamically, yet losing one side could still remove a particular alternator, pump, vacuum source, heater, or other accessory. Those system consequences matter operationally, but they are a different question from the aerodynamic critical-engine definition.
How critical engine connects to VMC
Published VMC is established using the critical-engine concept and specified certification conditions. The more adverse asymmetric-thrust condition requires more rudder authority, which is why critical-engine geometry matters when discussing minimum control speed. Review the full VMC factors guide for power, CG, weight, bank, propeller condition and density-altitude effects.
Common multi-engine oral questions
What is the critical engine? The engine whose failure most adversely affects airplane performance or handling qualities.
Why is the left engine often critical? With both propellers rotating clockwise, P-factor makes the operating right engine's effective thrust line farther from the centerline after the left engine fails.
Does the Piper Seminole have a critical engine? From the normal aerodynamic P-factor standpoint, its counter-rotating propellers eliminate a single critical engine.
Does counter-rotation mean an engine failure is harmless? No. You still lose thrust and single-engine performance, and airplane-specific systems may differ from side to side.
FAA source
This guide follows the FAA Airplane Flying Handbook discussion of multi-engine aerodynamics, asymmetric thrust and critical engine concepts. Open the current FAA Airplane Flying Handbook.
Related study
Back to the full path: Multi-engine checkride study guide.
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