Introduction
A hazard unique to operating a multi-engine airplane is losing directional control following an engine failure or precautionary shutdown. However, a loss of control is preventable if the pilot can recognize the onset of a loss of directional control and apply the corrective actions. When equipped with the knowledge and skill in this lesson, a pilot may see that the benefits of having a second engine outweigh the risks.
Note: Although “demonstration” is included in the title, learners must practice this procedure to proficiency before a practical test.
Objectives
After this lesson, the learner will be able to:
- Define VMC and describe how it is determined during the aircraft certification process.
- Describe situations that may result in a loss of directional control with one engine inoperative (OEI).
- Differentiate between the operating techniques for best climb performance (zero sideslip) and maximum directional control.
- Recognize the onset of a loss of directional control and recover properly.
Lesson Briefing
- What is a VMC Demonstration?
- Definition of VMC
- How the Manufacturer Determines VMC
- Control and Performance After an Engine Failure
- Four Factors that Make the Left Engine Critical
- Critical Density Altitude
- How to Perform a VMC Demonstration
- Sideslip During the VMC Demonstration
- Safety Considerations for VMC Demonstrations
- Common Errors for VMC Demonstrations
- Airman Certification Standards for VMC Demonstrations
Aircraft Specific Training
- A review of airspeeds including VMC (red line), VYSE (blue line), and VSSE (safe single-engine speed)
- Proper recovery procedures specified by the manufacturer
Risk Management
- Improper airplane configuration
- Maneuvering with one engine inoperative
- Distractions, loss of situational awareness, or improper task management
Scenario
You are flying a multi-engine airplane on a visual approach. On a one-mile final, you add power because you are slightly low and slow. The aircraft suddenly yaws and rolls to the right, and the nose pitches down.
What would you do? If you are still thinking, you are too late.
Resources
- Airplane Flying Handbook (FAA-H-8083-3):
- Chapter 13, Transition to Multi-Engine Airplanes
Schedule
- Lesson Briefing (0:30)
- Demonstrations and Practice (0:20)
- Lesson Debriefing (0:10)
Equipment
- Whiteboard, markers, and erasers
- Airplane models
- Airplane checklists
- Headsets and flight gear
Lesson Debriefing
This lesson concludes with a collaborative assessment and review of the main points and risk management items.
Additionally, the instructor ensures:
- All of the learner’s questions are resolved.
- The learner is made aware of his or her performance and progress.
Completion Standards
This lesson is complete when the lesson objectives are met and the learner’s knowledge, risk management, and skills are determined to be adequate for the stage of training. Ultimately, the learner must meet or exceed the Airman Certification Standards.
Lesson Content
What is a VMC Demonstration?
VMC demonstrations closely resemble how manufacturers determine VMC during the airplane’s certification process:
- Power on the critical engine is reduced to idle.
- The airplane is configured according to the VMC certification criteria.
- The pitch attitude is increased to reduce airspeed gradually until a loss of control occurs.
- Proper recovery procedures are performed to reestablish controllable, single-engine flight.
Definition of VMC
Reference: 14 CFR 23.2135
VMC is the calibrated airspeed at which, following the sudden critical loss of thrust, it is possible to maintain control of the airplane. For multi-engine airplanes, VMC must be determined, if applicable, for the most critical configurations used in takeoff and landing operations.
Directional control has been lost when full rudder deflection is applied towards the operating engine, and the aircraft begins to yaw toward the inoperative engine.
How the Manufacturer Determines VMC
References: 14 CFR 23.2100, 14 CFR 23.2105, 14 CFR 23.2135, Wang (2015) Legal Interpretation
Note: Changes to 14 CFR Part 23 do not impact how an airplane was certificated. Reference the historical regulations when applicable.
14 CFR Part 23 requires airplane manufacturers to determine VMC, if applicable, for the most critical configurations used in takeoff and landing operations. Specific configurations are not addressed; however, the following are typically the most critical.
Standard Day Conditions at Sea Level
An airplane must meet performance requirements for certification in still air and standard atmospheric conditions at sea level. This is necessary to establish a standard of measurement.
Most Unfavorable Weight (Light)
A heavier airplane has a lower VMC due to the horizontal component of lift and inertia.
For a given bank angle, an increase in weight will require more lift to maintain altitude. The increase in the horizontal component of lift will require less rudder pressure to keep the aircraft from yawing.
A heavily loaded airplane has more inertia than a lightly loaded one; thus, a heavier airplane will have a higher resistance to yawing.
Most Unfavorable CG Location (Aft)
The aft-most CG limit is the most unfavorable CG position. As the CG moves aft, the rudder’s moment arm is shortened, resulting in less leverage. The rudder will have less authority in overcoming yawing forces, causing VMC to increase. At the same time, the moment arm of the propeller blade is increased, aggravating asymmetrical thrust.
Critical Engine Windmilling (Propeller Controls in the Takeoff Position)
VMC increases as drag increases on the inoperative engine. A windmilling propeller creates more drag than a stationary propeller. When the propeller is stationary, an unfeathered position creates more drag than a feathered propeller. Therefore, VMC is highest when the critical engine’s propeller is windmilling at a low pitch, high RPM blade angle.
Flaps, Gear, and Trim in the Takeoff Position with the Airplane out of Ground Effect
- Extending the flaps increases the drag behind the operative engine. This can have a stabilizing effect that may reduce VMC.
- VMC increases when the landing gear is retracted. Extended landing gear creates a keel effect that aids directional stability and decreases VMC.
- Since the airplane is usually trimmed to a neutral position on takeoff, having the airplane pre-trimmed as an aid to an engine failure would be “cheating.”
- Ground effect decreases drag and increases VMC.
Up to 5° (Not More) of Bank Toward the Operating Engine
VMC is highly dependent on bank angle. In a bank, the horizontal component of lift helps the rudder counteract the operative engine’s thrust. Historically, 14 CFR Part 23 prevented airplane manufacturers from using more than 5° of bank toward the operative engine when determining VMC.
When banking toward the operating engine, VMC decreases by approximately 3 knots for every degree of bank less than 5°. Banking away from the operating engine increases VMC.
Note: A 5° bank does not inherently establish zero-sideslip or give the best single-engine climb performance. Zero-sideslip occurs at bank angles less than 5°. The determination of VMC in certification is solely concerned with the minimum speed for directional control under a specific set of circumstances.
Maximum Available Takeoff Power Initially on Each Engine (Engine Failure Should Happen Suddenly)
A higher power imbalance results in more asymmetrical thrust. VMC increases as power is increased on the operating engine.
Control and Performance After an Engine Failure
Directional Control
When a multi-engine aircraft experiences an engine failure and the engines are not mounted on the longitudinal axis, there are unbalanced forces and turning moments about the CG.
Pitch Down: The loss of induced airflow over the horizontal stabilizer results in less negative lift produced by the tail. Additional back pressure is required to maintain level flight.
Roll Toward the Inoperative Engine: The loss of the accelerated slipstream air over the wing of the inoperative engine results in a reduction of lift on that wing. Therefore, the aircraft tends to roll toward the inoperative engine due to asymmetrical lift. This requires additional aileron pressure into the operative engine.
Yaw: Asymmetrical thrust requires rudder pressure toward the operating engine.
Climb Performance
The loss of one engine results in a 50% loss of power but an approximate 80% loss of climb performance. This can verified by comparing the climb performance charts for one engine operating and two engines operating under the same atmospheric conditions.
Four Factors that Make the Left Engine Critical
Note: The following factors apply to a conventional twin-engine airplane with both propellers turning clockwise when viewed from the cockpit.
Asymmetrical Thrust (Yaw)
Each engine’s descending propeller blade will produce greater thrust than the ascending blade when the airplane is operated under power and at positive angles of attack.
Even though both propellers produce the same thrust, the descending blade on the right engine has a longer moment arm (greater leverage) than the left engine’s descending blade. As a result, the left engine’s failure will result in the most asymmetrical thrust.
Accelerated Slipstream (Roll and Pitch)
Asymmetrical thrust (P-Factor) results in a longer moment arm to the center of thrust of the right engine than the left engine. Therefore, the center of lift is farther out on the right wing, resulting in a greater rolling tendency with a loss of the left engine. The left engine’s failure also results in a more significant downward pitching moment due to the greater loss of negative lift produced by the tail.
Notes:
- P-Factor is greatest when flying at a high AOA.
- With both engines operating, 18% to 30% of total lift is generated from the accelerated slipstream.
Spiraling Slipstream (Yaw)
A spinning propeller produces a spiraling wind pattern that rotates in the same direction as the propeller’s rotation. The spiraling slipstream of air from the left engine strikes the vertical stabilizer from the left. This helps counteract the yaw caused by a failure of the right engine.
The right engine’s slipstream does not reach the vertical stabilizer. If the left engine fails, the right engine’s slipstream will not counteract the yaw toward the inoperative engine.
Note: This phenomenon is similar to the corkscrew effect, one of the four turning tendencies described during ASEL training. The spiral elongates and becomes less effective as the airplane’s forward speed increases.
Torque (Roll)
For every action, there is an equal and opposite reaction. A propeller that rotates clockwise (right) will tend to roll the aircraft counter-clockwise (left). The airplane will tend to roll toward the inoperative engine regardless of which engine has failed.
On a multi-engine airplane with clockwise rotating propellers, the effect of torque during OEI flight will:
- Increase the tendency of the airplane to roll toward the inoperative engine (left engine failed); or
- Counteract the tendency of the airplane to roll toward the inoperative engine (right engine failed).
Critical Density Altitude
With normally aspirated engines, an increase in altitude or temperature results in reduced engine performance and propeller efficiency. This results in a lower minimum control speed (VMC) as altitude is increased. However, the calibrated stall speed (VS) does not decrease with altitude.
There exists an altitude where each of the following exists:
- VMC is less than VS (stall occurs first)
- VMC is the same as VS (stall and yaw coincide)
- VMC is greater than VS (yaw occurs first)
The density altitude where VMC and VS are equal is called the critical density altitude. Here, the airplane will “stall and yaw” simultaneously under a condition of asymmetrical thrust. The airplane could experience an abrupt change in attitude or enter into a spin.
How to Perform a VMC Demonstration
Note: 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, a VMC demonstration may 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.
Pre-Maneuver Checks
- Clear the area
- Heading established and noted
- Altitude established:
- No lower than 3,000′ AGL or or the manufacturer’s recommended altitude (whichever is higher)
- Position near a suitable emergency landing area
- Set power and aircraft configuration:
- Adjust the mixture controls as required
- Set the propeller controls full forward
- Retract the landing gear
- Set the flaps to the normal takeoff setting
- Set the trim for takeoff and do not readjust it during the maneuver
- Set the cowl flaps for takeoff
- Establish a normal cruise speed
Execution
- Simulate a failed engine by retarding the throttle lever of the critical engine to idle.
- Slow the airplane to 10 knots above VSSE or VYSE, as appropriate, or the manufacturer’s recommended speed.
- Apply full power on the operative engine while maintaining heading.
- Bank the aircraft up to 5° towards the side of the operative engine and sustain 1/2 ball deflection toward the operative engine.
- Increase the pitch attitude slowly to decrease airspeed by one knot per second. A fast reduction of airspeed can be hazardous and does not provide an adequate demonstration.
- Aileron control deflection should be increased as necessary to keep the bank angle constant as the ailerons become less effective as airspeed decreases.
- Maintain heading as long as possible by increasing rudder input.
- Recognize, announce, and recover at the first sign of a loss of directional control, stall warning, or buffet, whichever occurs first.
Recovery
- Promptly and simultaneously reduce power sufficiently on the operating engine and decrease the AOA as necessary to regain airspeed and directional control. To maintain directional control as the throttle is reduced, rudder pressure needs to be reduced.
- Once above VMC and control is regained, smoothly advance the throttle on the operative engine to full power. Simultaneously reapply rudder pressure as needed.
- Accelerate to VXSE or VYSE as appropriate. Do not use the failed engine (simulated).
- Recover to the original heading.
Exit
- Return to cruise speed with both engines, trimming as necessary.
- Complete the cruise checklist.
Sideslip During the VMC Demonstration
During the VMC demonstration, the pilot applies rudder pressure to maintain directional control. The loss of control occurs under conditions of sideslip. VMC is not determined under conditions of zero-sideslip during aircraft certification; therefore, it is not part of a VMC demonstration.
A zero-sideslip may be established after the initial VMC recovery procedure is completed. Pilot certification standards require the pilot to accelerate to VXSE or VYSE as appropriate during the recovery, which is normally maintained in a zero-sideslip condition for best climb performance.
Safety Considerations for VMC Demonstrations
- Airplanes with normally aspirated engines will lose power as altitude increases because of the reduced density of the air entering the engine’s induction system. This loss of power will result in a VMC lower than the stall speed at higher altitudes. Therefore, recovery should be made at the first indication of loss of directional control, stall warning, or buffet.
- 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.
- Do not perform this maneuver by increasing the pitch attitude to a high angle with both engines operating and then reducing power on the critical engine. This technique is hazardous and may result in loss of airplane control.
- Instructors should be alert for any sign of an impending stall. The learner may be focused on the maneuver’s directional control aspect to the extent that impending stall indications go unnoticed.
- Follow the airplane manufacturer’s guidance regarding the CG location during slow flight maneuvers. Generally, a forward CG aids in stall recovery, spin avoidance, and spin recovery.
Common Errors for VMC Demonstrations
- Inadequate knowledge of the causes of loss of directional control at airspeeds less than VMC, factors affecting VMC, and safe recovery procedures
- Improper entry procedures, including pitch attitude, bank attitude, and airspeed
- Failure to recognize an imminent loss of directional control
- Initiating recovery steps too early
- Failure to use proper recovery procedures
Airman Certification Standards for VMC Demonstrations
References: FAA-S-ACS-6, FAA-S-ACS-7
Entry Airspeed | Recovery Airspeed | Bank | Heading |
---|---|---|---|
Decrease at 1 knot per second | VXSE/VYSE +10/-5 KIAS (PVT) ±5 KIAS (COM) | Not to exceed 5° toward the operating engine | ±20° |