081 Principles of Flight topic guide
VMCA and Asymmetric Flight
VMCA, the minimum control speed in the air, is the lowest speed at which directional control can still be maintained immediately after the sudden failure of the critical engine, with the remaining engine or engines at take off thrust, using no more than 5 degrees of bank toward the live engine and within defined control force and heading limits. It is a certified airspeed, not a rule of thumb, and it exists because losing an engine on one side of the aeroplane creates a yawing and rolling couple that the rudder alone must be able to balance.
Below VMCA, the rudder simply runs out of authority before it can stop the yaw, however hard it is applied, so the aeroplane departs from controlled flight. Everything the exam asks about VMCA comes back to that one balance: the yawing moment the failed engine creates, against the moment the rudder, helped by a little bank, can produce.
The yawing and rolling couple, and why a little bank helps
When one engine fails, the operating engine's thrust no longer has an opposite number, and the resulting asymmetric thrust yaws the aeroplane toward the dead engine. The rudder opposes that yaw with a sideforce of its own, and the further that sideforce has to reach, at a lower speed, the less of it the rudder can produce. Banking a few degrees toward the live engine adds a small sideways component of weight that assists the rudder, which is why the certification standard permits up to 5 degrees of bank: it genuinely lowers the speed at which control can be held, compared with insisting on wings level.
More bank than that trades away other things, principally climb performance and stall margin, for only a small further reduction in VMCA, which is why 5 degrees is the certified limit rather than an open invitation to bank further.
Why the critical engine is critical
On aeroplanes with propellers that rotate clockwise as seen from behind, the asymmetric blade effect at the high angle of attack typical of the climb shifts each engine's effective thrust line slightly to the right of that engine's own centreline. For the left engine, mounted left of the aeroplane's centreline, that rightward shift moves its effective thrust point toward the aeroplane's centreline, shortening its yawing arm. For the right engine, mounted right of the centreline, the same rightward shift moves its effective thrust point further from the aeroplane's centreline, lengthening its yawing arm.
Losing the left engine therefore leaves the right engine operating with the longer yawing arm, producing the worse asymmetric yaw of the two possible failures. That worse case is what the term critical engine describes, and it is why the left engine, on this common direction of propeller rotation, is the one certification and training treat as critical.
What raises and lowers VMCA
VMCA is not one fixed number; it moves with anything that changes the yawing moment to be balanced or the rudder's ability to balance it.
- Higher density altitude: the operating engine produces less thrust, shrinking the yawing moment to be balanced, so VMCA decreases.
- Aft centre of gravity: the moment arm between the centre of gravity and the fin and rudder shortens, so more speed is needed for the rudder to produce the same corrective moment, and VMCA increases.
- Bank toward the live engine, up to the certified 5 degrees: a helpful sideforce component reduces the speed needed, so VMCA decreases.
Worked example
Worked example: which change raises VMCA
Not quite. The correct answer is B.
Correct answer: B
- A. This reverses the true direction: higher density altitude reduces the thrust the operating engine can produce, shrinking the yawing moment and lowering VMCA, not raising it.
- B. Correct: an aft centre of gravity shortens the moment arm between the centre of gravity and the rudder, so more speed is needed for the same corrective moment, which raises VMCA.
- C. This lowers VMCA, since the certified bank allowance exists precisely to help the rudder with a sideforce component, not to hinder it.
- D. Less thrust on the operating engine means less asymmetric yawing moment to counter in the first place, which lowers VMCA rather than raising it.
Step by step
- VMCA is the minimum speed at which the rudder, assisted by up to 5 degrees of bank toward the live engine, can still balance the yawing moment produced after the critical engine fails at take off thrust.
- Anything that increases the yawing moment to be balanced, or reduces the rudder's ability to balance it at a given speed, raises the speed needed, and so raises VMCA.
- Moving the centre of gravity aft shortens the moment arm between the centre of gravity, the point the aeroplane actually yaws about, and the fin and rudder, so the rudder needs more dynamic pressure, meaning more speed, to produce the same corrective moment.
- By contrast, climbing to a higher density altitude, banking toward the live engine, and reducing operating engine thrust all shrink the yawing moment or assist the rudder, and all lower VMCA.
- Only the aft centre of gravity case makes the rudder's job harder at a given speed, so it is the only option that raises VMCA.
Common mistakes
Assuming altitude always makes engine failure handling worse
Higher density altitude actually lowers VMCA, because the available thrust, and so the yawing moment to be balanced, falls with altitude. Treating altitude as universally more critical reverses this specific relationship.
Forgetting the 5 degree bank is part of the VMCA definition, not an optional extra
VMCA is certified assuming up to 5 degrees of bank toward the live engine. Reading a question as if wings level flight were the baseline, or forgetting that the bank helps rather than hinders control, misapplies the whole definition.
Confusing centre of gravity's effect on VMCA with its effect on longitudinal stability
Aft centre of gravity shortens the rudder's moment arm and raises VMCA, a directional control effect, which is a separate mechanism from aft centre of gravity reducing longitudinal static stability, and treating them as the same fact loses the distinction the exam is testing.
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Last reviewed July 2026