FAA Airplane Flying Handbook: Multiengine Transition Notes

Introduction to Small Multiengine Airplanes

• Small Multiengine Airplane Definition: For the purpose of this handbook, a small multiengine airplane is a reciprocating or turbopropeller-powered airplane with a maximum certificated takeoff weight of $12,500\,lbs$ or less.

• Design Assumption: The discussion assumes a conventional design with two engines, one mounted on each wing, with reciprocating engines assumed unless otherwise noted.

• Light-Twin Definition: While not formally defined in regulations, it is used herein to describe a small multiengine airplane with a maximum certificated takeoff weight of $6,000\,lbs$ or less.

• Unique Characteristics: The necessity of a separate class rating is due to unique multiengine characteristics, specifically One Engine Inoperative (OEI) flight.

• Importance of Proficiency: Performance and system redundancy only increase safety if the pilot is properly trained and proficient. The airplane manufacturer remains the final authority; the Federal Aviation Administration's (FAA) Approved Flight Manual (AFM) and the Pilot’s Operating Handbook (POH) take precedence over general recommendations.

Key Performance V-Speeds and Multiengine Definitions

• Multiengine vs. Single-Engine Failure: In multiengine planes, losing an engine affects both performance and control. A $50\%$ loss of power typically results in an $80$ to $90\%$ reduction in climb performance. Asymmetrical thrust also creates significant control issues.

• Standard V-Speeds:   - $V_R$ (Rotation Speed): The speed at which back pressure is applied to rotate the airplane to a takeoff attitude.   - $V_{LOF}$ (Lift-off Speed): The speed at which the airplane leaves the surface.   - $V_X$ (Best Angle of Climb Speed): Gains the greatest altitude for a given distance of forward travel.   - $V_{XSE}$: Best angle-of-climb speed with OEI.   - $V_Y$ (Best Rate of Climb Speed): Gains the most altitude for a given unit of time.   - $V_{YSE}$ : Best rate of climb speed with OEI, marked with a blue radial line on the airspeed indicator. Above the single-engine absolute ceiling, $V_{YSE}$ yields the minimum rate of sink.   - $V_{SSE}$ (Safe, Intentional OEI Speed): The minimum speed to intentionally render the critical engine inoperative.   - $V_{REF}$ (Reference Landing Speed): Speed used for final approach; normally $1.3 \times V_{SO}$ (stall speed in landing configuration). Pilots adjust for winds (e.g., $V_{REF}+5$).

• $V_{MC}$ (Minimum Control Speed):   - Marked with a red radial line on the airspeed indicator.   - Current Definition (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.   - Previous Definition (14 CFR part 23, section 23.149): The calibrated airspeed at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane and maintain straight flight at the same speed with a bank angle not exceeding $5^\circ$.   - Limitation: $V_{MC}$ only addresses directional control; there is no requirement that the airplane be capable of climbing at this speed.

• Standard Conditions: Unless noted, V-speeds in the AFM/POH apply to sea level, standard day conditions, and maximum takeoff weight. These may be given as Knots Indicated Airspeed (KIAS) or Knots Calibrated Airspeed (KCAS).

Multiengine Performance and Certification Standards

• Performance Loss Mechanics: Climb performance is a function of thrust horsepower in excess of that required for level flight.   - Example: A twin-engine plane where each engine produces $200$ thrust horsepower ($400\,total$). If level flight requires $175\,hp$, the reserve for climb is $225\,hp$.   - Loss of one engine leaves $200\,hp$. If level flight still requires $175\,hp$, the reserve drops to only $25\,hp$ ($200 - 175 = 25$).

• Certification Levels (14 CFR Part 23 effective Dec 30, 2016):   - Applies to normal category planes with $\le 19$ seats and $\le 19,000\,lbs$ takeoff weight.   - Levels 1 to 4 based on seating; Level 2 is $2$ to $6$ passengers.   - Speed Levels: Low speed is defined as $V_{NO}$ or $V_{MO} \le 250\,KCAS$ and $M_{MO} \le 0.6$.   - Level 2 low speed planes (not meeting crashworthiness requirements) require a climb gradient of at least $1.5\%$ at $5,000\,ft$ pressure altitude in cruise configuration.

• Historical Single-Engine Performance Requirements:   - Max weight > 6,000\,lbs and/or V_{SO} > 61\,kts: Single-engine rate of climb at $5,000\,ft$ MSL must be equal to at least $0.027 V_{SO}^2$. Airplanes type certificated Feb 4, 1991, or later use a $1.5\%$ climb gradient.   - Max weight $\le 6,000\,lbs$ and $V_{SO} \le 61\,kts$: The single-engine rate of climb at $5,000\,ft$ MSL must simply be determined; it can be negative.

Feathering Propeller Systems

• Purpose: Feathering minimizes propeller drag by stopping engine rotation with the blade streamlined with the relative wind. This allows continued flight after an engine failure.

• Drag Considerations: A windmilling propeller at high RPM/low blade angle can produce as much parasite drag as the entire airframe. Drag is minimized when blades are in the feathered position.

• Governing Mechanics:   - Single-engine constant-speed props: Non-feathering, oil-pressure-to-increase-pitch; increased pressure drives blades to high pitch (low RPM).   - Multiengine constant-speed props: Full feathering, counterweighted, oil-pressure-to-decrease-pitch; increased pressure drives blades to low pitch (high RPM).

• Forces involved in multiengine feathering:   - Centrifugal force on counterweights: Drives blades toward high pitch/feather.   - High-pressure oil: Opposes the counterweights to push blades toward low pitch/high RPM.   - Spring/High-pressure air (in the dome): Provides final force to feather the blades as RPM/centrifugal force decays.

• Feathering Procedure: Propeller control is moved fully aft. Oil pressure is dumped, counterweights and springs drive the blades. The process takes up to $10\,seconds$.

• Anti-feathering Lock Pins: Small pins move into place below approximately $800\,RPM$ to prevent the prop from feathering every time the engine is shut down on the ground. To feather in flight, it must be done before engine RPM drops below $800$.

• Unfeathering Accumulator: Stores engine oil under pressure (via nitrogen or air). Releasing the pressure (moving prop control out of feather) flows oil to the hub to rotate blades out of feather, allowing the engine to windmill and start without the electric starter.

Accessory Multiengine Systems

• Propeller Synchronization (Prop Sync):   - Eliminates the ‘beat’ or ‘drumming’ from slightly mismatched RPMs.   - A "master" engine is matched by a "slave" engine.   - Prop Synchrophaser: Further adjusts the phase angle of the blades in their arcs to reduce noise and vibration.   - Must be OFF for takeoff, landing, and OEI operation.

• Fuel Crossfeed:   - Uses fuel from the opposite wing tank to power an engine. Primarily an emergency procedure to extend range during OEI flight.   - Ground checks: Engines should run at $1,500\,RPM$ minimum for $1\,minute$ on crossfeed, then $1\,minute$ on main tanks prior to takeoff.   - Water and debris often accumulate in crossfeed lines if not used and drained periodically.

• Combustion Heater:   - Small gasoline-burning furnace; thermostat-controlled with a separate hour meter.   - Over-temperature safety: A thermal switch trips if the unit overheats, which requires visual inspection to reset.   - Cool-down requirement: Circulate outside air through the unit for $15\,seconds$ in flight or run the ventilation fan for $2\,minutes$ on the ground post-operation.

• Yaw Damper: A gyro-driven servo that reduces vertical axis oscillation (Dutch roll) and provides a smoother ride. Should be OFF for takeoff and landing.

• Flight Director/Autopilot (FD/AP):   - Includes Flight Command Indicator (FCI), also known as an ADI (Attitude Director Indicator).   - Operating Levels: Off (raw data), Flight Director (computed commands), and Autopilot.   - Single-cue systems use ‘V’ bars; dual-cue systems use separate pitch and roll bars.

• Nose Baggage Compartment: Important to secure locks, as open doors can distract pilots even if they don't impact flight stability. Items like tow bars and oil containers must be secured against shifting.

Anti-icing and Deicing Systems

• Anti-icing: Prevents ice formation. Typical systems include heated pitot tubes, static ports, fuel vents, electrothermal prop boots, and alcohol windsheild spray.

• Deicing: Removes ice already formed. Typical systems are pneumatic wing/tail boots that inflate to break ice and deflate with vacuum.

• Operating Constraints: The presence of equipment does not automatically mean approval for known icing conditions. Autopilot should not be used in icing as it masks control feedback; sudden disconnects can lead to loss of control.

• Performance Impact: Ice adds weight and drag, reduces thrust and lift. Approaches should use minimum flaps and higher speeds. Thin airfoil sections (like horizontal stabilizers) may ice faster than wings.

• Alternate Induction Air: Used if impact ice blocks main air sources. Detected by a loss of manifold pressure or RPM.

• Alternate Static Source: Usually vented to the cabin (non-pressurized) or baggage area (pressurized). May cause errors in the altimeter and airspeed indicator.

Takeoff and Climb Performance Concepts

• Accelerate-Stop Distance: Runway required to accelerate to $V_R$ or $V_{LOF}$, experience engine failure, and stop.

• Accelerate-Go Distance: Horizontal distance to continue takeoff and climb to $50\,ft$ after engine failure at $V_R$ or $V_{LOF}$.

• Climb Gradient: Altitude gain per $100\,ft$ of travel. A $1.5\%$ gradient is $1.5\,ft$ gain per $100\,ft$ forward. Winds affect gradient (headwind improves it).

• Service Ceilings:   - All-engine service ceiling: Altitude where a $100\,fpm$ climb is maintained.   - Single-engine service ceiling: Altitude where a $50\,fpm$ climb is maintained OEI.

• The Go/No-Go Decision: If an engine fails before the decision point (usually gear retraction), the takeoff should be rejected. If it fails after, and performance is available, the climb may continue.

Weight and Balance Standardization

• GAMA Standards (Post-1975):   - Standard Empty Weight: Standard plane + full hydraulic fluid + unusable fuel + full oil.   - Basic Empty Weight: Standard empty weight + optional equipment (includes full oil).

• Legacy Standards (Pre-1975):   - Licensed Empty Weight: Includes standard plane + options + hydraulic fluid + unusable fuel + undrainable oil (must add full oil manually).

• Zero Fuel Weight (ZFW): Maximum allowable weight of the plane and payload assuming no usable fuel is on board. This protects wing spar structural integrity from heavy fuselage loads.   - Calculation Example:     - $Basic\,Empty\,Weight = 3,200\,lbs$     - $Zero\,Fuel\,Weight = 4,400\,lbs$     - $Max\,Takeoff\,Weight = 5,200\,lbs$     - $Payload = ZFW - Basic\,Empty\,Weight = 1,200\,lbs$     - $Fuel\,at\,Max\,Payload = Max\,Takeoff - ZFW = 800\,lbs$ (approximately $133.3\,gal$).

Takeoff and Ground Procedures

• Ground Handling: Use differential power to assist turns and minimize brake wear. Conduct operations with cowl flaps fully open. Avoid pivoting on a stationary inboard wheel.

• Normal Takeoff: Set full throttle (normally aspirated) or smooth, deliberate throttle for turbocharged engines. Hold brakes until boost is established if runway is short.

• Scan and Monitoring: Match fuel flows, fuel pressures, EGTs, and oil pressures before rotation speed. Do not fixate on the airspeed indicator.

• Airspeeds: Never be airborne before exceeding $V_{MC}$. Use $V_R$ or a minimum of $V_{MC} + 5\,kts$.

• Gear Retraction: Retract when there is insufficient runway for landing and a positive rate of climb is established. General recommendation is to retract no later than reaching $V_{YSE}$.

• Short-Field Takeoff: Accelerate to $V_X$ initially, transition to $V_Y$ once obstacles are cleared. Partial flaps may cause premature lift-off; the pilot should be prepared to abort if failure occurs below $V_X$.

Maneuvering: Slow Flight, Stalls, and Spins

• Slow Flight: Conduct no lower than $3,000\,ft$ AGL. Monitor engine temps. Do not simulate engine failures during slow flight.

• Stall Training: Practice at $3,000\,ft$ AGL minimum. The primary recovery step is reducing Angle of Attack (AOA). Avoid asymmetrical power in stalls as it induces spins.

• Power-Off Stall (Landing Simulation): Establish a $500\,fpm$ descent. At buffet, reduce AOA, roll wings level, and apply power. Accelerate to $V_X$ or $V_Y$.

• Power-On Stall (Takeoff Simulation): Configure for takeoff. Use $65\%$ power (or recommended). At buffet, reduce AOA, level wings, and recover to $V_X$ or $V_Y$.

• Accelerated Stall: Perform bank at $45^\circ$ at a speed no greater than $V_{A}$ or $V_{O}$. Recover at first indicator.

• Spin Awareness: No multiengine plane is approved for spins. Recovery requires simultaneously retarding both throttles to idle, full opposite rudder, and full forward elevator. Spin recovery consumes considerable altitude.

Landing Procedures

• Descent Planning: Use rule of thumb of $500\,fpm$. Avoid low manifold pressure/high RPM combinations to prevent excessive cooling.

• Normal Approach: Maintain $V_{YSE}$ until short final. Confirm gear down abeam the touchdown point. Avoid full stall landings; allow main wheels to touch first.

• Crosswind Landing: Two methods (Crab and Wing-low). Sideslip (Wing-low) should be limited to $30\,seconds$ on some models to avoid fuel starvation. Differential power is an alternative but responds slower than flight controls.

• Short-Field Landing: Use full flaps and a stabilized constant descent angle. Maintain $V_{REF}$ or normal approach speed. Retracting flaps after touchdown adds weight to wheels for braking.

• Go-Around: Advance throttles to takeoff power, adjust pitch, retract flaps to intermediate, then landing gear once positive rate of climb is confirmed. Retract flaps before gear because they usually produce more drag.

One Engine Inoperative (OEI) Principles and $V_{MC}$

• Factors Affecting $V_{MC}$. It is determined under very specific certification conditions:   - Maximum takeoff power on operating engine ($V_{MC}$ increases as power increases).   - Propeller windmilling (low pitch) on critical engine ($V_{MC}$ increases with drag).   - Most unfavorable (aft) CG ($V_{MC}$ increases as CG moves aft).   - Most unfavorable (light) weight ($V_{MC}$ increases as weight decreases).   - Landing gear and flaps retracted.   - Max $5^\circ$ bank toward operative engine ($V_{MC}$ increases as bank decreases).

• Critical Engine: The engine whose failure most adversely affects directional control. Due to P-factor (descending blade having a longer moment arm), the LEFT engine is typically critical in conventional clockwise-rotating twins.

• Counter-rotating Props: These have no critical engine as asymmetrical thrust is equal on both sides.

• V-Speed Relationship: At sea level, $V_{MC}$ is typically higher than stall speed ($V_S$). As altitude increases, $V_{MC}$ decreases due to less power, while $V_S$ stays the same. At some altitude, they meet, creating a high-risk area for stalling under asymmetrical power.

OEI Control and Best Climb Performance

• Zero Sideslip Rule: Best OEI climb performance is achieved by eliminating sideslip. Centering the ball is NOT zero sideslip in OEI flight.

• Sideslip Options:   - Wings level, ball centered: Results in moderate sideslip toward inoperative engine; reduces performance.   - Bank only ($8$-$10^\circ$), no rudder: Results in large sideslip toward operative engine; severely reduces performance.   - Zero Sideslip (Correct Method): Approximately $2^\circ$ bank toward the operative engine and the ball displaced $1/3$ to $1/2$ toward the operative engine.

• Memory Aid for Control: "Raise the Dead" — Lift the wing of the inoperative (dead) engine about $2^\circ$ to achieve zero sideslip and maximum performance.

• Identification: Use the "Dead Foot — Dead Engine" rule. The foot that is not working the rudder corresponds to the failed engine side.

Emergency Management: Engine Failure Scenarios

• Scenario 1: Gear Down: Close both throttles and land on the runway or overrun ahead.

• Scenario 2: Gear Up, Inadequate Performance: Land under control on the most suitable surface ahead. Attempting to maintain altitude by bleeding airspeed is usually fatal.

• Scenario 3: Gear Up, Adequate Performance:   - 1. Maintain control (stop yaw with rudder, then a slight bank).   - 2. Configure (identify, verify with throttle, feather prop, retract gear/flaps).   - 3. Climb (maintain $V_{YSE}$ and zero sideslip).   - 4. Checklist (review securing failed engine checklist items).

• Identification vs. Verification: Identify the failed engine by control response (rudder pressure). Verify by retarding the throttle of the suspected engine. If no change occurs, verification is complete.

Training Considerations and Risk Mitigation

• Simulated Failures: FAA recommends in-flight simulations below $3,000\,ft$ AGL be done with a smooth throttle reduction (zero thrust), not the mixture.

• Simulated failures on the ground roll should happen at no more than $50\%$ of $V_{MC}$.

• Zero Thrust Setting: Mimics the drag of a feathered propeller. The instructor sets specific manifold pressure and RPM so the student can experience OEI performance without actually stopping the engine.

• Safety Limits: Simulated OEI at low altitude should only start at a minimum of $400\,ft$ AGL. Actual feathering/unfeathering exercises should be planned to finish no lower than $3,000\,ft$ AGL.

• Checklists: Mandatory for safe operation. If a discrepancy exists, the AFM/POH takes precedence over aftermarket checklists. Immediate action items (OEI takeoff) should be memorized.

• Touch-and-Go Precautions: Not recommended during initial training. If performed, the instructor should handle resetting flaps and trim while the student maintains directional control.