ME.2
Vmc — Aerodynamics and the Certification Conditions
Explain what minimum control speed means, the conditions under which the published red-line Vmc was determined, and how each real-world departure from those conditions moves the actual Vmc up or down.
References: 14 CFR part 23; FAA-H-8083-3 (AFH ch. 13); POH/AFM
Quick Review
Conversational Q&A — quiz yourself before the oral.
VMC is the calibrated airspeed at which, following the sudden critical loss of thrust, it is possible to maintain control of the airplane. It's marked with a red radial line on the airspeed indicator (14 CFR 23.2135(c); AFH ch. 13).
The previous definition, in 23.149, still applies to airplanes certificated under that rule: the calibrated airspeed at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane with that engine still inoperative, and thereafter maintain straight flight at the same speed with a bank angle of not more than 5 degrees.
No. There is no requirement under either determination that the airplane be capable of climbing at this airspeed — VMC only addresses directional control (AFH ch. 13).
This is the trap. Red line is a controllability floor, not a performance promise. Blue line (VYSE) is where performance lives.
No — VMC is a fixed airspeed only for the very specific set of circumstances under which it was determined during aircraft certification. This is the core of the subject: in reality VMC varies with a variety of factors, and the VMC you encounter in practice, in demonstration, or in an actual OEI situation could be less or even greater than the published value, depending on conditions and pilot technique (AFH ch. 13).
Historically, dynamic VMC was determined with (AFH ch. 13, citing 23.149):
- Maximum available takeoff power initially on each engine — 23.149(b)(1)
- Propeller controls in the recommended takeoff position — 23.149(b)(5)
- Most unfavorable weight and CG — 23.149(b)
- Landing gear retracted — 23.149(b)(4)
- Flaps in the takeoff position — 23.149(b)(3)
- Airplane trimmed for takeoff — 23.149(b)(2)
- Airborne, ground effect negligible — 23.149(b)
- Maximum 5 degrees of bank — 23.149(a)
Also: critical engine inoperative, and the value published as a sea level calibrated airspeed.
VMC increases as power is increased on the operating engine.
Normally aspirated: VMC is highest at takeoff power and sea level, and decreases with altitude as available power falls off.
Turbocharged: takeoff power, and therefore VMC, remains constant with altitude up to the engine's critical altitude (the altitude at which the engine can no longer maintain 100 percent power). Above critical altitude, VMC decreases as it would with a normally aspirated engine (AFH ch. 13).
VMC increases with increased drag on the inoperative engine — it's therefore highest when the critical engine's propeller is windmilling at the low pitch, high rpm blade angle.
VMC is normally determined with the critical engine propeller windmilling in the takeoff position — unless the engine is equipped with an autofeather system (AFH ch. 13, 23.149(b)(5)).
Practical consequence: feathering lowers actual VMC as well as reducing drag.
VMC increases as the CG moves aft, since the rudder's moment arm — and its effectiveness — is reduced; for a typical light twin the aft-most CG limit is the most unfavorable CG position. VMC also increases as weight is reduced (AFH ch. 13).
Historically part 23 called for VMC to be determined at the most unfavorable weight; for twins certificated under CAR 3 or early part 23, the weight at which VMC was determined was not specified (AFH ch. 13).
VMC increases when the landing gear is retracted, since extended gear aids directional stability, which tends to decrease VMC (AFH ch. 13, 23.149(b)(4)).
So the certification condition — gear up — is the conservative one.
Because bank angle moves VMC faster than any other variable — VMC increases as bank decreases. It can increase more than 3 knots per degree of bank lost between 5 degrees and wings level.
Since VMC was determined with up to 5 degrees of bank, loss of directional control may be experienced at speeds almost 20 knots above published VMC when the wings are held level (AFH ch. 13).
Wings level is not neutral. It is the worst case.
The horizontal component of lift generated by the bank balances the side force from the rudder, rather than using sideslip to do so. Sideslip requires more rudder deflection, which in turn increases VMC (AFH ch. 13, 23.149(a)).
The 5-degree allowance exists to prevent claims of an unrealistically low VMC — but the AFH warns that high bank angles may result in unsafe flight from both the large sideslip and the need to increase AOA to maintain the vertical component of lift.
No. The 5-degree bank maximum is a historical limit imposed upon manufacturers in aircraft certification. It does not inherently establish zero sideslip or best single-engine climb performance — zero sideslip, and therefore best single-engine climb, may occur at bank angles less than 5 degrees (AFH ch. 13).
Certification VMC is solely concerned with the minimum speed for directional control under a very specific set of circumstances, not the optimum attitude or configuration for climb.
With normally aspirated engines VMC decreases with altitude, but stalling speed (VS) remains the same.
- Except for a few models, published VMC is almost always higher than VS
- At sea level there is usually a margin of several knots
- The margin decreases with altitude, and at some altitude VMC and VS are the same (AFH ch. 13)
Should a stall occur under asymmetrical power, a spin entry is likely — and the airplane departs in the direction of the idle engine, not in the direction of applied rudder.
On the ground, engine fails below VMC: the takeoff must be rejected — directional control can only be maintained by promptly closing both throttles and using rudder and brakes as required. Airborne below VMC: directional control is not possible with the remaining engine producing takeoff power.
Therefore, on takeoff, the airplane should never be airborne before the airspeed exceeds VMC (AFH ch. 13). Use the manufacturer's VR or VLOF; if none is published, use a minimum of VMC plus 5 knots for VR.
Deep Dive
Dynamic vs. static determination
Two determinations exist, and knowing the difference explains why your VMC demo doesn't look like the flight test.
Dynamic: the critical engine is suddenly made inoperative and control is maintained thereafter. In certification, test pilots performed mixture cuts of the critical engine while gradually reducing speed with each attempt. VMC was the minimum speed at which directional control could be maintained within 20 degrees of the original entry heading (AFH ch. 13).
Static (steady-state): simply the ability to maintain straight flight at VMC with a bank angle of not more than 5 degrees.
If the two differ, the higher of the two is published as VMC (AFH ch. 13). The static determination more closely resembles the VMC demonstration task on the practical test.
Because the dynamic technique is used only by highly experienced test pilots during aircraft certification — attempting it outside those circumstances is unsafe (AFH ch. 13).
During those tests the climb angle with both engines operating was high, and pitch had to be lowered quickly after the cut to regain speed. The AFH's warning to transitioning pilots is explicit: attempting to demonstrate VMC with an engine cut from high power, or intentionally failing an engine at speeds less than VSSE, creates a high likelihood for loss of control and an accident.
Flying the VMC demonstration
The demonstration resembles the static determination. The description below assumes a twin with non-counter-rotating engines, so the left engine is critical.
- Select an altitude allowing the maneuver at least 3,000 ft AGL
- Landing gear retracted, flaps in the takeoff position
- Slow to approximately 10 knots above VSSE or VYSE, whichever is higher, and trim for takeoff — the trim setting then remains unaltered for the rest of the maneuver
- Select an entry heading; set high rpm on both propeller controls
- Throttle the left engine to idle as the right engine is advanced to takeoff power
- Counteract the left yawing and rolling moment primarily with right rudder; establish a bank of up to 5 degrees toward the operating engine as appropriate for the make and model
- Holding heading, slowly increase pitch to decelerate at 1 knot per second (no faster)
- Rudder pressure and aileron displacement both increase as control effectiveness decays
(AFH ch. 13)
The trigger is either of two things — the moment you first recognize uncontrollable yaw, or any symptom associated with a stall.
Recovery is simultaneous: retard the throttle of the operating engine to stop the yaw, and lower the pitch attitude to regain speed.
Recover to straight flight on the entry heading at VSSE or VYSE, then increase power on the operating engine and demonstrate controlled flight before restoring symmetrical power (AFH ch. 13).
No — maintaining altitude is not a criterion. This is a demonstration of controllability, not performance — many airplanes lose (or gain) altitude during it. Remaining at or above 3,000 ft AGL throughout is considered effective risk mitigation (AFH ch. 13).
Certification permitted 150 pounds of force under the historical rule (23.149(e)). Most twins will run out of rudder travel long before 150 pounds is required — but the pressure during any VMC demonstration may still seem considerable (AFH ch. 13).
An actual demonstration may not be possible at certain density altitudes, or in airplanes whose VMC is equal to or less than VS. Under those circumstances a demonstration may be safely conducted as a training technique by artificially limiting rudder travel to simulate maximum available rudder, at a speed well above VS — approximately 20 knots (AFH ch. 13).
This rudder-limiting technique avoids the hazard of spinning from a stall under high asymmetrical power while still demonstrating the loss of directional control.
A single-engine stall. A VMC demonstration that degrades into a single-engine stall with high asymmetrical thrust may result in an unrecoverable loss of control and a fatal accident (AFH ch. 13).
Terminate immediately on any stall warning light or horn, airframe or elevator buffet, or sudden loss of control effectiveness — reduce the AOA as the throttle is retarded. Be aware that flight deck noise may mask the sound of the stall warning horn (AFH ch. 13).
Also: avoid performing any VMC demonstration from a high pitch attitude with both engines operating and then reducing power on one.
The question behind the question
Any departure from the certification conditions in the unfavorable direction (AFH ch. 13):
- Less bank than 5 degrees toward the operating engine — the big one, more than 3 kt per degree
- Wings level — up to nearly 20 kt above published
- Aft CG — reduced rudder moment arm
- Lighter weight
- Windmilling (unfeathered) propeller on the failed engine
- High power on the operating engine
- Gear retracted
The same factors run in reverse lower it: feathering, gear down, more bank toward the good engine, and altitude with normally aspirated engines.
Turbocharging is the trap in that last one — it doesn't raise VMC, it takes away the decrease. Takeoff power, and therefore VMC, stays constant up to the engine's critical altitude, so at altitude your actual VMC is still the published sea-level value while Vs has climbed to meet it, narrowing the margin you were counting on (AFH ch. 13).
The same list also explains zero-sideslip technique: roughly 2 degrees of bank puts your actual VMC above the published figure — an accepted trade for climb performance, not an oversight (AFH ch. 13).