There are two main considerations for OEI operations—performance and control. Multiengine pilots learn to operate the airplane for maximum rate of climb performance at the blue radial indicated airspeed by training to fly without sideslip. Pilots also learn to recognize and recover from loss of directional control associated with the red radial indicated airspeed by performing a VMC demonstration. Since the object of a VMC demonstration is not performance, sideslip occurs during the maneuver. Detailed discussion on both the loss of directional control and maximum OEI climb performance follows.
Derivation of VMC
VMC is a speed established by the manufacturer, published in the AFM/POH, and marked on most airspeed indicators with a red radial line. A knowledgeable and competent multiengine pilot understands that VMC is not a fixed airspeed under all conditions. 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 as outlined below. The VMC noted in practice and demonstration, or in actual OEI operation, could be less or even greater than the published value, depending on conditions and pilot technique.
Historically, in aircraft certification, VMC is the sea level 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 then maintain straight flight at the same speed with an angle of bank not more than 5°.
The foregoing refers to the determination of VMC under dynamic conditions. This technique is only used by highly experienced test pilots during aircraft certification. It is unsafe to be attempted outside of these circumstances.
In aircraft certification, there is also a determination of VMC under static, or steady-state conditions. If there is a difference between the dynamic and static speeds, the higher of the two is published as VMC. The static determination is simply the ability to maintain straight flight at VMC with a bank angle of not more than 5°. This more closely resembles the VMC demonstration task in the practical test for a multiengine rating.
The AFM/POH-published VMC is determined with the critical engine inoperative. The critical engine is the engine whose failure had the most adverse effect on directional control. On twins with each engine rotating in conventional, clockwise rotation as viewed from the pilot’s seat, the critical engine will be the left engine.
Multiengine airplanes are subject to P-factor just as single-engine airplanes are. The descending propeller blade of each engine will produce greater thrust than the ascending blade when the airplane is operated under power and at positive angles of attack. The descending propeller blade of the right engine is also a greater distance from the center of gravity, and therefore has a longer moment arm than the descending propeller blade of the left engine. As a result, failure of the left engine will result in the most asymmetrical thrust (adverse yaw) as the right engine will be providing the remaining thrust. [Figure 1]
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| Figure 1. 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 VMC demonstration may be performed with either engine windmilling.
The following bullets describe the way several factors affect VMC 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. VMCC 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 2]
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| Figure 2. 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 VMC 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, VMC 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 VMC 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. VMC 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 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.
VMC Demo
The actual demonstration of VMC 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 VYSE (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 VSSE 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, VMC decreases with altitude. Stalling speed (VS), however, remains the same. Except for a few models, published VMCC is almost always higher than VS. 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 3]
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| Figure 3. Graph depicting relationship of VMC to VS |
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 VS is encountered before VMC, 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 VMC 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 VMC 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 VMC demonstration does not degrade into a single-engine stall. A VMC 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.
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. Under those circumstances, as a training technique, a demonstration of VMC may safely be conducted by artificially limiting rudder travel to simulate maximum available rudder. A speed well above VS (approximately 20 knots) is recommended when limiting rudder travel.
The rudder limiting technique avoids the hazards of spinning as a result of stalling with high asymmetrical power, yet is effective in demonstrating the loss of directional control.
To reduce the risk of a loss of control, avoid performing any VMC 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 VYSE 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:
- Yaw from the rudder
- 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 4] The result is a moderate sideslip toward the inoperative engine. Climb performance is reduced by the moderate sideslip. With wings level, VMC is significantly higher than published as there is no horizontal component of lift available to help the rudder combat asymmetrical thrust.
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| Figure 4. Wings level engine-out flight |
2. Engine inoperative flight using ailerons alone requires an 8–10° bank angle toward the operative engine. [Figure 5] 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.
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| Figure 5. 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 6] 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.
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| Figure 6. 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 moderate 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 VYSE 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 VYSE, 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 VYSE under different density altitude, power available, and weight conditions.