Figure 13-12. Forces created during single-engine operation.
Many twins are designed with a counter-rotating right engine. 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 V MC demonstration may be performed with either
engine windmilling.
The following bullets describe the way several factors affect V MC speed for those multiengine airplanes often used during training,
which were certified in accordance with historical 14 CFR part 23, section 23.149. They also describe the conditions used to
determine the manufacturer's published speed. Historically, in aircraft certification, dynamic VMC has been determined under the
following conditions outlined in historical 14 CFR part 23, section 23.149:
⦁ Maximum available takeoff power initially on each engine (section 23.149(b)(1)). VMC increases as
power is increased on the operating engine. 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 with increases in altitude up to the engine's critical altitude (the altitude where the
engine can no longer maintain 100 percent power). Above the critical altitude, VMC decreases just as it
would with a normally aspirated engine whose critical altitude is sea level. In order to avoid accidents, test
pilots conduct VMC tests at a variety of altitudes, and the results of those tests are then extrapolated to a
single, sea level value.
⦁ All propeller controls in the recommended takeoff position throughout VMC determination
(section 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. VMC is
normally determined with the critical engine propeller windmilling in the takeoff position, unless the
engine is equipped with an autofeather system.
⦁ Most unfavorable weight and center-of-gravity position (section 23.149(b)). VMC increases as the
center-of-gravity (CG) is moved aft. 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 CG limit is the most unfavorable
CG position. Historically, 14 CFR part 23 calls for VMC to be determined at the most unfavorable weight.
For twins certificated under CAR 3 or early 14 CFR part 23, the weight at which VMC was determined was
not specified. VMC increases as weight is reduced. [Figure 13-13]
Figure 13-13. Effect of CG location on yaw.
⦁ Landing gear retracted (section 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 (section 23.149(b)(3)). This normally includes wing flaps and cowl
flaps. For most twins, this will be 0° of flaps.
⦁ Airplane trimmed for takeoff (section 23.149(b)(2)).
⦁ Airplane airborne and the ground effect negligible (section 23.149(b)).
⦁ Maximum of 5° angle of bank (section 23.149(a)). VMC is highly sensitive to bank angle. To prevent
claims of an unrealistically low V MC speed in aircraft certification, the manufacturer is permitted to use a
maximum of a 5° bank angle toward the operative engine. 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. The bank angle works in the manufacturer's favor in
lowering VMC since using high bank angles reduces required rudder deflection. However, this method may
result in unsafe flight from both the large sideslip and the need to increase the angle of attack in order to
maintain the vertical component of lift.
VMC increases as bank angle decreases. In fact, V MC may increase more than 3 knots for each degree of bank reduction between 5°
and wings-level. Since VMC was determined with up to 5° of bank, loss of directional control may be experienced at speeds almost 20
knots above published VMC when the wings are held level.
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°. The determination of V MC in certification is solely concerned with the
minimum speed for directional control under a very specific set of circumstances, and not the optimum airplane attitude or
configuration for climb performance.
During dynamic VMC determination in aircraft certification, cuts of the critical engine using the mixture control are performed by
flight test pilots while gradually reducing the speed with each attempt. V MC is the minimum speed at which directional control could
be maintained within 20° of the original entry heading when a cut of the critical engine was made. During such tests, the climb angle
with both engines operating was high, and the pitch attitude following the engine cut had to be quickly lowered to regain the initial
speed. Transitioning pilots should understand that attempting to demonstrate V MC 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.
VMC Demo
The actual demonstration of V MC and recovery in flight training more closely resembles static VMC determination in aircraft
certification. For a demonstration that avoids the hazard of unintended contact with the ground, the pilot selects an altitude that will
allow performance of the maneuver at least 3,000 feet AGL. The following description assumes a twin with non-counter-rotating
engines, where the left engine is critical.
With the landing gear retracted and the flaps set to the takeoff position, the pilot slows the airplane to approximately 10 knots above
VSSE or V YSE (whichever is higher) and trims for takeoff. For the remainder of the maneuver, the trim setting remains unaltered.
The pilot selects an entry heading and sets high rpm on both propeller controls. Power on the left engine is throttled back to idle as
the right engine power is advanced to the takeoff setting. The landing gear warning horn will sound as long as a throttle is retarded,
however the pilot listens carefully for the stall warning horn or watches for the stall warning light. The left yawing and rolling
moment of the asymmetrical thrust is counteracted primarily with right rudder. A bank angle of up to 5° (a right bank in this case)
may be established as appropriate for the airplane make and model.
While maintaining entry heading, the pitch attitude is slowly increased to decelerate at a rate of 1 knot per second (no faster). As the
airplane slows and control effectivity decays, the pilot counteracts the increasing yawing tendency with additional rudder pressure.
Aileron displacement will also increase in order to maintain the established bank. An airspeed is soon reached where full right rudder
travel and up to a 5° right bank can no longer counteract the asymmetrical thrust, and the airplane will begin to yaw uncontrollably to
the left.
The moment the pilot first recognizes the uncontrollable yaw, or experiences any symptom associated with a stall, the pilot
simultaneously retards the throttle for the operating engine to stop the yaw and lowers the pitch attitude to regain speed. Recovery is
made to straight flight on the entry heading at V SSE or VYSE. The pilot increases power to the operating engine, and demonstrates
controlled flight before restoring symmetrical power.
To keep the foregoing description simple, there were several important background details that were not covered. The rudder pressure
during the demonstration can be quite high. During certification under historical 14 CFR part 23, section 23.149(e), 150 pounds of
force was permitted. Most twins will run out of rudder travel long before 150 pounds of pressure is required. Still, the rudder
pressure used during any VMC demonstration may seem considerable.
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. Remaining at or above a minimum of
3,000 feet AGL throughout the maneuver is considered to be effective risk mitigation of certain hazards.
VMC Demo Stall Avoidance
As discussed earlier, with normally aspirated engines, V MC decreases with altitude. Stalling speed (V S), however, remains the same.
Except for a few models, published V MC is almost always higher than V S. At sea level there is usually a margin of several knots
between VMC and VS, but the margin decreases with altitude, and at some altitude, VMC and VS are the same. [Figure 13-14]
Should a stall occur while the airplane is under asymmetrical power, a spin entry is likely. The yawing moment induced
from asymmetrical thrust is little different from that induced by full rudder in an intentional spin in the appropriate model of single-
engine airplane. In this case, however, the airplane will depart controlled flight in the direction of the idle engine, not in the
direction of applied rudder. Twins are not required to demonstrate recoveries from spins, and their spin recovery characteristics are
generally very poor.
Where V S is encountered before V MC, the departure from controlled flight might be quite sudden, with strong yawing and rolling
tendencies to the inverted orientation and a spin entry. Therefore, during a V MC demonstration, if there are any symptoms of an
impending stall such as a stall warning light or horn, airframe or elevator buffet, or sudden loss of control effectiveness; the
pilot should terminate the maneuver immediately by reducing the angle of attack as the throttle is retarded and return the airplane
to the entry airspeed. Note that noise within the flight deck may mask the sound of the stall warning horn.
While the V MC demonstration shows the earliest onset of a loss of directional control when performed in accordance with the
foregoing procedures, avoid a stalled condition. Avoid stalls with asymmetrical thrust, such that the V MC demonstration does not
degrade into a single-engine stall. A V MC demonstration that is allowed to degrade into a single-engine stall with high asymmetrical
thrust may result in an unrecoverable loss of control and a fatal accident.
Figure 13-14. Graph depicting relationship of VMC to VS.
An actual demonstration of V MC may not be possible under certain conditions of density altitude, or with airplanes whose V MC is
equal to or less than V S. Under those circumstances, as a training technique, a demonstration of V MC may safely be conducted by
artificially limiting rudder travel to simulate maximum available rudder. A speed well above V S (approximately 20 knots) is
recommended when limiting rudder travel.
The rudder limiting techniqu e avoids the hazards of spinning as a result of stalling with high asymmetrical power, yet is effectiv e in
demonstrating the loss of directional control.
To reduce the risk of a loss of control, avoid performing any V MC demonstration from a high pitch attitude with both engines
operating and then reducing power on one engine.
OEI Climb Performance
Best OEI climb performance is obtained at V YSE with maximum available power and minimum drag. After the flaps and landing
gear have been retracted and the propeller of the failed engine feathered, a key element in best climb performance is minimizing
sideslip.
For any airplane, sideslip can be confirmed through the use of a yaw string. A yaw string is a piece of string or yarn approximately 18
to 36 inches in length taped to the base of the windshield or to the nose near the windshield along the airplane centerline. In two-
engine coordinated flight, the relative wind causes the string to align itself with the longitudinal axis of the airplane, and it positions
itself straight up the center of the windshield. This is zero sideslip. Experimentation with slips and skids vividly displays the location
of the relative wind. A particular combination of aileron and rudder also establishes zero sideslip during OEI flight. Adequate
altitude, flying speed, and caution should be maintained if attempting these maneuvers.
With a single-engine airplane or a multiengine airplane with both engines operative, sideslip is eliminated when the ball of the turn
and bank instrument is centered. This is a condition of zero sideslip, and the airplane is presenting its smallest possible profile to the
relative wind. As a result, drag is at its minimum. Pilots know this as coordinated flight.
In a multiengine airplane with an inoperative engine, the centered ball is no longer the indicator of zero sideslip due to asymmetric
thrust. In fact, there is no flight deck instrument that directly indicates conditions for zero sideslip. In the absence of a yaw string, the
pilot needs to place the airplane at a predetermined bank angle and ball position. Since the AFM/POH performance charts for one
engine inoperative flight were determined at zero sideslip, this technique should be used to obtain the charted OEI performance.
There are two different control inputs that can be used to counteract the asymmetric thrust of a failed engine:
1. Yaw from the rudder
2. The horizontal component of lift that results from bank with the ailerons
Used individually, neither is correct. Used together in the proper combination, zero sideslip and best climb performance are achieved.
Three different scenarios of airplane control inputs are presented below. The first two are not correct and can increase the risk of a
loss of control. They are presented to illustrate the reasons for the zero sideslip approach to best climb performance.
1. Engine inoperative flight with wings level and ball centered requires large rudder input toward the operative
engine. [Figure 13-15] The result is a moderate sideslip toward the inoperative engine. Climb performance is
reduced by the moderate sideslip. With wings level, V MC is significantly higher than published as there is no
horizontal component of lift available to help the rudder combat asymmetrical thrust.
Figure 13-15. Wings level engine-out flight.
2. Engine inoperative flight using ailerons alone requires an 8–10° bank angle toward the operative engine.
[Figure 13-16] This assumes no rudder input, the ball is displaced well toward the operative engine, and climb
performance is greatly reduced by the large sideslip toward the operative engine. Due to the increased risk of
loss of control, instructors should not normally demonstrate this.
Figure 13-16. Excessive bank engine-out flight.
3. Rudder and ailerons used together in the proper combination result in a bank of approximately 2° toward the
operative engine. The ball is displaced approximately one-third to one-half toward the operative engine. The
result is zero sideslip and maximum climb performance. [Figure 13-17] Any attitude other than zero sideslip
increases drag, decreasing performance. VMC under these circumstances is higher than published, as less than
the 5° bank certification limit is employed.
Figure 13-17. Zero sideslip engine-out flight.
When bank angle is plotted against climb performance for a hypothetical twin, zero sideslip results in the best (however marginal)
climb performance or the least rate of descent. Whether the airplane can climb depends on the weight of the airplane, density altitude,
and pilot technique. If the pilot uses zero bank (all rudder to counteract yaw), climb performance degrades as a result of m oderate
sideslip. Using bank angle alone (no rudder) severely degrades climb performance as a result of a large sideslip.
The precise condition of zero sideslip (bank angle and ball position) varies slightly from model to model and with available power
and airspeed. If the airplane is not equipped with counter-rotating propellers, it also varies slightly with the engine failed due to P-
factor. The foregoing zero sideslip recommendations apply to reciprocating engine multiengine airplanes flown at V YSE with the
inoperative engine feathered. The zero sideslip ball position for straight flight is also the zero sideslip position for turning flight.
The actual bank angle for zero sideslip varies among airplanes from one and one-half to two and one-half degrees. The position of the
ball varies from one-third to one-half of a ball width from instrument center toward the operative engine.
During certain flight training scenarios, pilots and instructors simulate propeller feathering. Zero thrust means the pilot sets power on
one engine such that drag from its rotating propeller equals that of a stopped feathered propeller. With an engine set to zero thrust (or
feathered) and the airplane slowed to V YSE, a climb with maximum power on the remaining engine reveals the precise bank angle
and ball deflection required for zero sideslip and best climb performance. Again, if a yaw string were present, it aligns itself vertically
on the windshield as an indication of zero sideslip. There are very minor changes from this attitude depending upon the engine failed
(with non-counter-rotating propellers), power available, airspeed, and weight; but without more sensitive testing equipment, these
changes are difficult to detect. The only significant difference would be the pitch attitude required to maintain V YSE under different
density altitude, power available, and weight conditions.
Low Altitude Engine Failure Scenarios
In OEI flight at low altitudes and airspeeds such as the initial climb after takeoff, pilots should operate the airplane so as to guard
against the three major accident factors: (1) loss of directional control, (2) loss of performance, and (3) loss of flying s peed. All have
equal potential to be lethal. Loss of flying speed is not a factor, however, when the airplane is operated with due regard for directional
control and performance.
A takeoff or go-around is the most critical time to suffer an engine failure. The airplane will be slow, close to the ground, and may
even have landing gear and flaps extended. Altitude and time is minimal. Until feathered, the propeller of the failed engine is
windmilling, producing a great deal of drag and yawing tendency. Airplane climb performance is marginal or even non-existent, and
obstructions may lie ahead. An emergency contingency plan and safety brief should be clearly understood well before the takeoff roll
commences. An engine failure before a predetermined airspeed or point results in an aborted takeoff. An engine failure after a certain
airspeed and point, with the gear up, and climb performance assured result in a continued takeoff. With loss of an engine, it is
paramount to maintain airplane control and comply with the manufacturer’s recommended emergency procedures. Complete failure
of one engine shortly after takeoff can be broadly categorized into one of three following scenarios.
Landing Gear Down
If the engine failure occurs prior to selecting the landing gear to the UP position [Figure 13- 18]: Keep the nose as straight as
possible, close both throttles, adjust pitch attitude to maintain adequate airspeed, and descend to the runway. Concentrate on a normal
landing and do not force the aircraft on the ground. Land on the remaining runway or overrun. Depending upon how quickly the pilot
reacts to the sudden yaw, the airplane may run off the side of the runway by the time action is taken. There are really no other
practical options. As discussed earlier, the chances of maintaining directional control while retracting the flaps (if extended), landing
gear, feathering the propeller, and accelerating are minimal. On some airplanes with a single-engine-driven hydraulic pump, failure of
that engine means the only way to raise the landing gear is to allow the engine to windmill or to use a hand pump. This is not a viable
alternative during takeoff.
Figure 13-18. Engine failure on takeoff, landing gear down.
Landing Gear Control Selected Up, Single-Engine Climb Performance Inadequate
When operating near or above the single-engine ceiling and an engine failure is experienced shortly after lift-off, a landing needs to
be accomplished on whatever essentially lies ahead. [Figure 13-19] There is also the option of continuing ahead, in a descent at
VYSE with the remaining engine producing power, as long as the pilot is not tempted to remain airborne beyond the airplane’s
performance capability. Remaining airborne and bleeding off airspeed in a futile attempt to maintain altitude is almost invariably
fatal. Landing under control is paramount. The greatest hazard in a single-engine takeoff is attempting to fly when it is not within the
performance capability of the airplane to do so. An accident is inevitable.
Figure 13-19. Engine failure on takeoff, inadequate climb performance.