• • • VMCA – Minimum control speed with the

critical engine

inoperative while airborne.

– The

critical engine

is the engine which if failed would most adversely affect the performance or handling qualities of an aircraft.

VMCA is marked with a red radial line on most airspeed indicators (see figure).

VMCA is the minimum speed at which directional control can be maintained under a very specific set of circumstances outlined in 14 CFR part 23.

Remember this:

VMCA only guarantees directional control, it does not guarantee the ability to climb with one-engine inoperative.

The

critical engine

is the engine whose failure would most adversely affect the performance or handling qualities of an aircraft.

On a conventionally designed twin engine aircraft both propellers rotate the same direction, usually clockwise (as viewed from the pilot’s point of view). Conventionally designed aircraft have a critical engine, usually the left engine.

Some twin engine aircraft are designed so the propellers rotate in opposite directions, these designs are called counter-rotating (left engine propeller rotates clockwise, right engine propeller rotates counter-clockwise). On these designs neither engine is critical, the loss of one engine is no more adverse to the performance and handling of the aircraft than the other.

The following slides will discuss the four factors that are responsible for making the left engine critical in conventionally designed, non-counter rotating propeller twin engine aircraft.

The four factors that make the left engine critical on a conventional twin can be abbreviated using the acronym, PAST: P – Factor (asymmetric thrust) A ccelerated slipstream S piraling slipstream T orque P-Factor During high angles of attack, the descending blade (right blade) produces more thrust than the ascending blade (left blade). The descending (right blade) on the right engine has a longer arm from the center of gravity (CG) than the descending (right) blade of the left engine, creating a yaw force to the left.

Accelerated Slipstream As a result of p-factor, stronger induced lift is produced by the propeller wash on the right side of the right engine than on the left side of the left engine.

Spiraling Slipstream The spiraling slipstream from the left engine hits the tail from the left. In case of a right engine failure on a conventional twin, this tail force will counteract yaw towards the left, dead, engine; but in case of a left engine failure, the slipstream does not hit the tail to counteract the yaw, so the loss of directional control is greater.

Torque For every action there is an equal and opposite reaction. When the propeller spins clockwise, torque will cause the airplane to roll counter-clockwise.

P-Factor During high angles of attack, the descending blade (right blade) produces more thrust than the ascending blade (left blade). The descending (right blade) on the right engine has a longer arm from the center of gravity (CG) than the descending (right) blade of the left engine, creating a yaw force to the left.

P- Factor causes a conventional twin to yaw to the left. Failure of the left engine will cause more loss of directional than loss of right engine because of the longer arm of the right engine's thrust from the CG.

Counter-rotating twin Conventional Twin

P- Factor counter-rotating engines, no yaw produced. Failure of either left or right engine will cause the same amount of directional control loss.

Accelerated Slipstream As a result of p-factor, stronger induced lift is produced by the propeller wash on the right side of the right engine than on the left side of the left engine.

Accelerated slipstream. Conventional twin. In case of a left engine failure, there would be a strong moment rolling the plane to the left. Also on a failure of the left engine, less negative lift will be produced by the tail, resulting in a pitch down.

Conventional Twin Counter-rotating twin

Accelerated slipstream. Counter-rotating engines. Failure of either engines will result in the same loss of control. The arms from the CG are much closer than they are in case of a left engine failure on a conventional twin.

Spiraling Slipstream The spiraling slipstream from the left engine hits the tail from the left. In case of a right engine failure on a conventional twin, this tail force will counteract yaw towards the left, dead, engine; but in case of a left engine failure, the slipstream does not hit the tail to counteract the yaw, so the loss of directional control is greater.

Spiraling slipstream - Conventional twin

Conventional Twin

Spiraling slipstream - Counter rotating engines

Counter-rotating twin

Torque For every action there is an equal and opposite reaction. When the propeller spins clockwise, torque will cause the airplane to roll counter-clockwise.

Torque - Conventional twin. As a result of the propellers turning clockwise on a conventional twin, there is a left rolling tendency of the airplane. If the right engine fails, this left roll tendency will help us maintain control and resist the right roll towards the right, dead engine, caused by asymmetric thrust; but if the left engine fails, the left roll tendency by torque will add to the left turning force caused by asymmetric thrust, making it much more difficult to maintain directional control. This makes the left engine critical.

Conventional Twin

Torque - Counter rotating engines. On a counter-rotating twin, No matter which engine fails, torque will oppose the roll created by asymmetric thrust.

Counter-rotating twin

Conventional Twin Left engine failure Tendency

Yaw towards the inoperative engine Yaw towards the inoperative engine Counter clockwise roll adding to force created by asymmetric thrust

Factor

P-Factor

Roll towards the inoperative engine & pitch down

Accelerated Slipstream Spiraling Slipstream Torque Tendency

Yawing moment due to loss is the same

Counter-Rotating Twin– Either engine failed

Loss of directional control is the same per engine Yaw counters the effect of drag Torque opposes the roll created by asymmetric thrust

a) VMC is 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 an angle of bank of not more than 5 degrees. The method used to simulate critical engine failure must represent the most critical mode of power plant failure expected in service with respect to controllability .

b) 1) 2) 3) 4) 5) VMC for takeoff must not exceed 1.2 VS1, where VS1 is determined at the maximum takeoff weight. VMC must be determined with the most unfavorable weight and center of gravity position with the airplane airborne and the ground effect negligible, for the takeoff configuration(s) with – Maximum available takeoff power initially on each engine; The airplane trimmed for takeoff; Flaps in the takeoff position(s); Landing gear retracted; and All propeller controls in the recommended takeoff position throughout.

C O M B A T S

Critical engine failed, propeller windmilling Operating engine(s) at max takeoff power Most unfavorable weight (light = higher VMC) Bank up to 5 degrees into operating engine Adverse CG (Aft = bad) Takeoff configuration (gear retracted, cowl flaps in takeoff position, flaps in takeoff position, propeller controls in takeoff position, out of ground effect) Standard Day (15 degrees C, 29.92, Sea Level)

VMCA Determination Maximum available takeoff power:

VMC increases as power is increased on the operating engine. More power will result in an increased yawing force due to P-factor, an increase in the accelerated slipstream and an increase in torque.

Windmilling propeller:

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 (see figure 12-3).

Most unfavorable weight and center-of-gravity position:

VMC increases as the center of gravity is moved aft. The moment arm of the rudder is reduced, and therefore its effectiveness is reduced, as the center of gravity is moved aft. At the same time, the moment arm of the propeller blade is increased, aggravating asymmetrical thrust. Invariably, the aft-most CG limit is the most unfavorable CG position (see figure 12-20). VMC increases as weight is reduced.

Landing gear retracted:

VMC increases when the landing gear is retracted. Extended landing gear aids directional stability, which decreases VMC.

Wing flaps in the takeoff position :

For most twins, this will be 0 degrees of flaps.

Cowl flaps in the takeoff position.

Airplane trimmed for takeoff .

Airplane airborne and ground effect negligible.

Maximum of 5 degrees angle of bank:

The horizontal component of lift generated by the bank assists the rudder in counteracting the asymmetric thrust of the operating engine. Manufacturers are limited to 5 degrees of bank angle during certification.

Factor

Increase in Density Altitude Increase in weight Windmilling prop (vs. feathered) Aft CG Flaps extended Gear Retracted Up to 5 degrees bank towards good engine

VMCA

Decreases (Good) Decreases (Good) Increases (Bad) Increases (Bad) Decreases (Good) Increases (Bad) Decreases (Good)

Performance

Decreases (Bad) Decreases (Bad) Decreases (Bad) Increases (Good) Decreases (Bad) Increases (Good) Increases (Good)

Critical Density Altitude (Stall vs. Yaw altitude regions)

As the aircraft increases in altitude, V

MCA

decreases, but calibrated stall speed does not change (indicated stall speed will not change on any given day if we assume no compressibility) for a specific weight, configuration, and altitude. So, there exists an altitude where each of the following exists:

V MCA V MCA V MCA

is less than V S is the same as V S is greater than V S The density altitude where V

V MCA

and V S

MCA

and V S are equal is called Critical Density Altitude. At this altitude, the aircraft slows to at the same time. Loss of directional control and a stall will happen at the same time, which may result in a spin.

Above the critical density altitude the aircraft will reach the calibrated stall speed before V MCA .

Below the critical density altitude the aircraft will reach V

MCA

before the calibrated stall speed.

Normally aspirated engines lose efficiency as altitude increases and are unable to develop 100% rated sea level power.

This power loss causes VMCA to decrease; however, the stalling speed remains the same.

Part 23 of 14 CFR divides the requirements for single engine climb performance for reciprocating engine-powered multiengine airplanes into two categories: 1) 2) Aircraft weighing more than 6,000 pounds maximum weight and/or VSO more than 61 knots.

Aircraft weighing 6,000 pounds or less maximum weight and VSO 61 knots or less.

The Piper Seminole has a maximum weight of less than 6,000 pounds and the VSO is 55 knots, meaning Piper was only required to determine a single-engine steady gradient of climb or descent at a pressure altitude of 5,000 feet MSL. This means the Seminole is not required to be able to climb with one-engine inoperative.

You might think that since an aircraft has two engines it would be able to climb quite rapidly. This can be true when both engines are operating; however, when one engine is inoperative climb performance can be reduced by 80 to 90 percent.

Any airplane’s climb performance is a function of thrust horsepower which is in excess of that required for level flight.

In a hypothetic twin with each engine producing 200 thrust horsepower, (400 thrust horsepower total) assume that the total level-flight thrust horsepower required is 175. In this situation, the airplane has an additional 225 thrust horsepower available for climb.

Losing one engine means the loss of 200 thrust horsepower, leaving a total of 200 thrust horsepower available. If the total-level flight thrust horsepower required is 175, this means the aircraft only has 25 thrust horsepower in excess of level flight requirements, thus drastically reducing the available power for climb.

Hypothetical Twin

Engine 1 Thrust Horsepower = 200 Engine 2 Thrust Horsepower = 200 Level-flight thrust horsepower requirement = 175

Both Engines Operating

Engine 1 HP: 200 Engine 2 HP: +200 Total HP available: 400 Total HP available: 400 Level-flight HP Req.

-175 HP available for climb: 225

One-Engine Inoperative

Engine 1 HP: Engine 2 HP: +200 Total HP available: 200 200 Total HP available: 200 Level-flight HP req.

-175 HP available for climb: 25

Single engine climb performance was determined by the manufacturer while flying the aircraft at VYSE and in a condition of zero sideslip. To approximate the single engine climb performance during single engine operations the pilot must know how to fly in a condition of zero sideslip when one engine is inoperative.

In a single engine or 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 a minimum. Pilots know this as coordinated flight.

Centering the ball of the turn and bank instrument will not yield zero sideslip for multi-engine aircraft with one engine inoperative due to asymmetric thrust. There is no instrument that directly tells the pilot when the aircraft is in a zero sideslip configuration with one engine inoperative. Therefore, a combination of yaw from the rudder and the horizontal component of lift must be used to place the aircraft into an approximate bank angle and ball position.

The following slides will discuss the effects of the rudder and horizontal component of lift individually and why using both at the same time is required to achieve a zero sideslip configuration during one engine inoperative flight.

Keeping the wings level and using the rudder to position the ball of the turn and bank instrument in the center will expose a greater aircraft surface area to the relative wind causing a moderate sideslip. Climb performance will be reduced and VMC will be significantly higher than published.

Using ailerons alone requires an 8 to 10 degrees of bank angle towards the operative engine, assuming no rudder input. The ball will be displaced well towards the operative engine. The result is a large sideslip towards the operative engine. Climb performance will be greatly reduced by the large sideslip.

Rudder and ailerons used together in the proper combination will result in a bank of approximately 2 degrees towards the operative engine. The ball will be displaced approximately one-third to one-half towards the operative engine. The result is zero sideslip and maximum performance. Any attitude other than zero sideslip increases drag, decreasing performance. VMC under these circumstances will be higher than published, as less than the 5 degree bank certification limit is employed.

Incorrect Incorrect Correct

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. 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. Zero bank (all rudder to counteract yaw) degrades climb performance as a result of moderate sideslip. Using bank angle alone (no rudder) severely degrades climb performance as a result of a large sideslip.