VMC Demonstration (AMEL, AMES)

• Current, 14 CFR part 23 section 23.2135(c): the calibrated airspeed at which, following the sudden critical loss of thrust, it is possible to maintain control of the airplane.
• Historical, section 23.149: the calibrated airspeed at which, when the critical engine is suddenly made inoperative, it is possible to maintain control with that engine still inoperative and thereafter maintain straight flight at the same speed with a bank angle of not more than 5°. This definition still applies to airplanes certificated under that rule — which is most of the training fleet (AFH 13-2).

Neither definition requires any ability to climb. VMC addresses directional control only.

Dynamic VMC is determined by test pilots making mixture cuts of the critical engine at progressively lower speeds — the minimum speed at which control could be maintained within 20° of the original entry heading. That technique "is unsafe to be attempted outside of these circumstances." Static VMC is simply the ability to maintain straight flight with a bank angle of not more than 5°. If the two differ, the higher of the two is published (AFH 13-23).

The ACS demonstration closely resembles the static determination (AFH 13-23, 13-26).

Because published VMC is "a fixed airspeed only for the very specific set of circumstances under which it was determined during aircraft certification. In reality, VMC varies with a variety of factors" — and the value seen in practice, demonstration, or an actual failure "could be less or even greater than the published value, depending on conditions and pilot technique" (AFH 13-23). Treat the red line as a certification data point, not a guarantee.

Enormously — it is the single most sensitive factor. "VMC may increase more than 3 knots for each degree of bank reduction between 5° and wings-level," so with the wings held level, "loss of directional control may be experienced at speeds almost 20 knots above published VMC" (AFH 13-25). The bank works by using the horizontal component of lift to balance the rudder's side force instead of paying for it with sideslip, which would demand more rudder deflection (AFH 13-25).

No — and confusing the two is a common oral failure. "The 5° bank angle maximum is a historical limit imposed upon manufacturers in aircraft certification. The 5° bank does not inherently establish zero sideslip or best single-engine climb performance. Zero sideslip, and therefore best single-engine climb performance, may occur at bank angles less than 5°" (AFH 13-25). VMC determination "is solely concerned with the minimum speed for directional control… not the optimum airplane attitude or configuration for climb." Zero sideslip lives near 2° (AFH 13-29).

Altitude requirement: at least 3,000 feet AGL (AFH 13-26) — because with normally aspirated engines VMC decreases with altitude while stall speed (VS) stays the same. Published VMC is almost always higher than VS at sea level, but "the margin decreases with altitude, and at some altitude, VMC and VS are the same" (AFH 13-26).

Stall relationship: where VS arrives first, "the departure from controlled flight might be quite sudden, with strong yawing and rolling tendencies to the inverted orientation and a spin entry." Twins are not required to demonstrate spin recovery "and their spin recovery characteristics are generally very poor" (AFH 13-26).

Recover at the first indication of loss of directional control, stall warning, or buffet (CA.X.B.S5). Then, simultaneously:

• Reduce power on the operating engine sufficiently to stop the yaw
• Decrease the angle of attack as necessary to regain airspeed and directional control
• Never add power on the simulated failed engine (CA.X.B.S6)

Recover within 20° of entry heading (CA.X.B.S7), then advance power smoothly on the operating engine and accelerate to VSSE/VYSE as appropriate, ±5 knots (CA.X.B.S8).

No. "Maintaining altitude is not a criterion in accomplishing this maneuver. This is a demonstration of controllability, not performance. Many airplanes will lose (or gain) altitude during the demonstration" (AFH 13-26). What you owe the examiner is the entry configuration, the deceleration rate, the recognition, the recovery, and staying at or above 3,000 feet AGL throughout.

"VMC increases as power is increased on the operating engine" (23.149(b)(1)). With normally aspirated engines VMC is highest at takeoff power and sea level and decreases with altitude. With turbocharged engines takeoff power — and therefore VMC — remains constant up to the engine's critical altitude, the altitude at which the engine can no longer maintain 100 percent power; above that it decreases like a normally aspirated engine "whose critical altitude is sea level" (AFH 13-24). That is why a turbocharged twin can present a VMC/VS convergence problem far higher up.

All propeller controls in the recommended takeoff position throughout the determination (23.149(b)(5)). "VMC increases with increased drag on the inoperative engine. VMC is highest, therefore, when the critical engine propeller is windmilling at the low pitch, high rpm blade angle" — which is exactly the condition you are in during the seconds before you feather. VMC is normally determined windmilling unless the engine has an autofeather system (AFH 13-24).

Certification uses the most unfavorable weight and center-of-gravity position (23.149(b)).

• Aft CG raises VMC — "the moment arm of the rudder is reduced, and therefore its effectivity is reduced, as the CG is moved aft." For a typical light twin the aft-most limit is the most unfavorable.
• VMC increases as weight is reduced (AFH 13-24) — counterintuitive, and a favorite oral question. A heavier airplane's greater bank-induced side force helps oppose the yaw.

For twins certificated under CAR 3 or early part 23, the weight used was not specified (AFH 13-24).

• Landing gear retracted (23.149(b)(4)) — "VMC increases when the landing gear is retracted. Extended landing gear aids directional stability, which tends to decrease VMC."
• Flaps in the takeoff position (23.149(b)(3)), normally including cowl flaps; for most twins this is 0° of flaps.
• Airplane trimmed for takeoff (23.149(b)(2)).
• Airborne with ground effect negligible (23.149(b)).
• Maximum of 5° angle of bank toward the operating engine (23.149(a)) (AFH 13-25).

The left engine's failure leaves the remaining thrust acting through the longer moment arm — the most asymmetrical thrust and the worst yaw (AFH 13-23). That's because multiengine airplanes are subject to P-factor like any airplane: at positive angles of attack under power, each engine's descending blade produces greater thrust than its ascending blade. On a twin with both propellers rotating clockwise as viewed from the pilot's seat, the descending blade of the right engine is farther from the center of gravity and therefore has a longer moment arm — so it's the left engine's loss that leaves that longer arm unopposed. The critical engine is simply "the engine whose failure had the most adverse effect on directional control."

"With this design, the degree of asymmetrical thrust is the same with either engine inoperative. No engine is more critical than the other, and a VMC demonstration may be performed with either engine windmilling" (AFH 13-23). Know which type you are flying the checkride in — the examiner will ask which engine you are about to retard and why it is a legitimate choice.

• VS is aerodynamic: the wing exceeds its critical angle of attack. It is essentially constant in indicated airspeed regardless of altitude.
• VMC is a control limit: the rudder plus up to 5° of bank can no longer balance asymmetric thrust. It scales with the power the good engine is making, so it falls with density altitude on a normally aspirated twin (AFH 13-26).

The lethal combination is meeting them together. "Should a stall occur while the airplane is under asymmetrical power, a spin entry is likely… the airplane will depart controlled flight in the direction of the idle engine, not in the direction of applied rudder" (AFH 13-26).

Per the manufacturer, or in its absence (CA.X.B.S1, AFH 13-26):

1. Landing gear retracted; flaps set for takeoff; cowl flaps set for takeoff; trim set for takeoff — and the trim setting then remains unaltered for the rest of the maneuver
2. Propellers set for high rpm
3. Slow to approximately 10 knots above VSSE (AFH: VSSE or VYSE, whichever is higher), select an entry heading
4. Critical engine throttled to idle, propeller windmilling; operating engine to takeoff or maximum available power
5. Establish a single-engine climb attitude with a bank of not more than 5° toward the operating engine
6. Raise pitch slowly to decelerate at approximately 1 knot per second — no faster — feeding in rudder to hold heading (CA.X.B.S1–S4)

• Rudder pressure climbing steadily — certification under 23.149(e) permitted up to 150 pounds of force, though "most twins will run out of rudder travel long before 150 pounds of pressure is required" (AFH 13-26)
• Aileron displacement also increasing to hold the bank
• The landing gear warning horn sounds continuously as long as a throttle is retarded, so listen carefully for the stall warning horn and watch for the stall warning light
• Noise masking: the AFH cautions that "noise within the flight deck may mask the sound of the stall warning horn" — so feel for airframe or elevator buffet as well as listening (AFH 13-26)

"An actual demonstration of VMC may not be possible under certain conditions of density altitude, or with airplanes whose VMC is equal to or less than VS." The accepted training technique is to artificially limit rudder travel to simulate maximum available rudder, with the loss of control staged at a speed well above VS — approximately 20 knots above (AFH 13-27). This "avoids the hazards of spinning as a result of stalling with high asymmetrical power, yet is effective in demonstrating the loss of directional control."

• Cutting an engine from high power or intentionally failing an engine below VSSE — "creates a high likelihood for loss of control and an accident" (AFH 13-25).
• Entering from a high pitch attitude with both engines operating and then reducing power on one — explicitly to be avoided (AFH 13-26).
• Pressing past the first stall symptom because you are fixated on directional control — terminate immediately by reducing angle of attack as the throttle is retarded (AFH 13-26).
• Letting the demo degrade into a single-engine stall — "may result in an unrecoverable loss of control and a fatal accident" (AFH 13-26).

This maneuver deliberately loads you up: rising rudder force, a deceleration to hold at 1 knot per second, a bank to keep at 5°, a heading to track, and three separate recovery cues to watch for. The predictable failures:

• Fixating on heading and flying past the first stall symptom instead of recovering at the first indication (CA.X.B.S5)
• Losing the altitude picture — controllability is what is graded, but the 3,000 feet AGL floor is not optional (AFH 13-26)
• Ambiguity about who has what. The AFH is blunt: "any ambiguity as to who is operating what systems or controls increases the likelihood of an unintended outcome" (AFH 13-36)

The mitigation is the preflight briefing — the AFH asks for "a clear understanding… as to how simulated emergencies will be introduced, and what action the learner is expected to take" before the flight begins (AFH 13-35). Brief the entry, the recovery triggers, and the knock-it-off call on the ground, and the maneuver stops being a surprise.