Airplane Flying Handbook (FAA-H-8083-3C)
Chapter 13: Transition to Multiengine Airplanes
Introduction
This chapter is devoted to the factors associated with the operation of small multiengine airplanes. 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 pounds or less. This discussion assumes a conventional design with two engines —one mounted on each wing. Reciprocating
engines are assumed unless otherwise noted. The term “light-twin,” although not formally defined in the regulations, is used herein as
a small multiengine airplane with a maximum certificated takeoff weight of 6,000 pounds or less.
There are several unique characteristics of multiengine airplanes that make them worthy of a separate class rating. The one engine
inoperative (OEI) flight information presented in this chapter emphasizes the significant difference between flying a multiengine and
a single-engine airplane. However, all pilots need appropriate knowledge, risk management strategies, and skills to fly safely in any
airplane they fly, and mastery of OEI flight is only one aspect of safe multiengine flying. The modern, well-equipped multiengine
airplane can be remarkably capable under many circumstances, but, the performance and system redundancy of a multiengine airplane
only increase safety if the pilot is trained and proficient.
The airplane manufacturer is the final authority on the operation of a particular make and model airplane. Flight instructors and
learners should use the Federal Aviation Administration’s Approved Flight Manual (AFM) and/or the Pilot’s Operating Handbook
(POH). The airplane manufacturer’s guidance and procedures take precedence over any general recommendations made in this
handbook.
General
Multiengine and single-engine airplanes operate differently during an engine failure. In a multiengine airplane, loss of thrust from one
engine affects both performance and control . The most obvious problem is the loss of 50 percent of power, which reduces climb
performance 80 to 90 percent. In some cases after an engine failure, the ability to climb or maintain altitude in a light-twin may not
exist. After an engine failure, asymmetrical thrust also creates control issues for the pilot. Attention to both these factors is crucial to
safe OEI flight.
Terms and Definitions
Pilots of single-engine airplanes are already familiar with many performance “V” speeds and their definitions. Twin-engine airplanes
have several additional V- speeds unique to OEI operation. These speeds are differentiated by the notation “SE” for single engine. A
review of some key V-speeds and several new V-speeds unique to twin-engine airplanes are listed below.
⦁ VR —rotation speed—speed at which back pressure is applied to rotate the airplane to a takeoff
attitude.
⦁ VLOF —lift-off speed—speed at which the airplane leaves the surface. (Note: Some manufacturers
reference takeoff performance data to VR, others to VLOF.)
⦁ VX —best angle of climb speed—speed at which the airplane gains the greatest altitude for a given distance
of forward travel.
⦁ VXSE —best angle-of-climb speed with OEI.
⦁ VY —best rate of climb speed—speed at which the airplane gains the most altitude for a given unit of time.
⦁ VYSE —best rate of climb speed with OEI. Marked with a blue radial line on most airspeed indicators.
Above the single-engine absolute ceiling, VYSE yields the minimum rate of sink.
⦁ VSSE —safe, intentional OEI speed—originally known as safe single-engine speed. It is the minimum speed
to intentionally render the critical engine inoperative.
⦁ VREF —reference landing speed—an airspeed used for final approach, which is normally 1.3 times
VSO, the stall speed in the landing configuration. The pilot may adjust the approach speed for winds
and gusty conditions by using VREF plus an additional number of units (e.g.,VREF+5).
⦁ VMC —currently defined in 14 CFR part 23, section 23.2135(c) as the calibrated airspeed at which, following
the sudden critical loss of thrust, it is possible to maintain control of the airplane. VMC is typically marked
with a red radial line on most airspeed indicators [Figure 13-1]. VMC was previously defined in 14 CFR
part 23, section 23.149 as 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. This
definition still applies to airplanes certified under that regulation. There is no requirement under either
determination that the airplane be capable of climbing at this airspeed. VMC only addresses directional
control. Further discussion of VMC as determined during airplane certification and demonstrated in pilot
training follows later in this chapter.
Figure 13-1. Airspeed indicator markings for a multiengine airplane
Unless otherwise noted, when V-speeds are given in the AFM/POH, they apply to sea level, standard day conditions at maximum
takeoff weight. Performance speeds vary with aircraft weight, configuration, and atmospheric conditions. The speeds may be stated in
statute miles per hour (mph) or knots (kt), and they may be given as calibrated airspeeds (CAS) or indicated airspeeds (IAS). As a
general rule, the newer AFM/POHs show V-speeds in knots indicated airspeed (KIAS). Some V-speeds are also stated in knots
calibrated airspeed (KCAS) to meet certain regulatory requirements. Whenever available, pilots should operate the airplane from
published indicated airspeeds.
Rate of climb is the altitude gain per unit of time, while climb gradient is the actual measure of altitude gained per 100 feet of
horizontal travel, expressed as a percentage. An altitude gain of 1.5 feet per 100 feet of travel (or 15 feet per 1,000 or 150 feet per
10,000) is a climb gradient of 1.5 percent.
There is a dramatic performance loss associated with the loss of an engine, particularly just after takeoff. Any airplane’s
climb performance is a function of thrust horsepower, which is in excess of that required for level flight. In a hypothetical twin
with each engine producing 200 thrust horsepower, assume that the total level flight thrust horsepower required is 175. In this
situation, the airplane would ordinarily have a reserve of 225 thrust horsepower available for climb. Loss of one engine would leave
only 25 (200 minus 175) thrust horsepower available for climb, a drastic reduction.
The performance characteristics of an airplane depend upon the rules in effect during type certification and do not depend on the
production year after certification. The current amendment to 14 CFR part 23, 81 FR 96689, went into effect on December 30, 2016.
This includes certification of normal category airplanes with passenger seating configuration of 19 or less and a maximum certificated
takeoff weight of 19,000 pounds or less (section 23.2005(a)). Current 14 CFR part 23 certification rules (section 23.2005(b)) classify
airplanes into certification levels 1 through 4 based on maximum passenger seating configuration. For example, a level 2 airplane has
a passenger seating configuration between two and six passengers. The rule further divides airplanes into two different performance
levels based on speed (section 23.2005(c)). After a critical loss of thrust, a level 2 low speed airplane (V NO or VMO less than or equal
to 250 knots calibrated airspeed and M MO less than or equal to 0.6) that does not meet single-engine crashworthiness requirements
requires a climb gradient of at least 1.5 percent at a pressure altitude of 5,000 feet in the cruise configuration for certification (section
23.2120(b)(1)).
While, the various subsets of airplanes receiving certification under the current part 23 meet specific single-engine climb performance
criteria as listed in 14 CFR part 23, section 23.2120(b), the historical 14 CFR part 23 single-engine climb performance requirements
for reciprocating engine-powered multiengine airplanes are broken down as follows:
⦁ More than 6,000 pounds maximum weight and/or VSO more than 61 knots: the single-engine rate of climb
in feet per minute (fpm) at 5,000 feet mean sea level (MSL) must be equal to at least 0.027 VSO 2. For airplanes type
certificated February 4, 1991, or thereafter, the climb requirement is expressed in terms of a climb gradient, 1.5 percent.
The climb gradient is not a direct equivalent of the .027 VSO 2 formula. Do not confuse the date of type certification
with the airplane’s model year. The type certification basis of many multiengine airplanes dates back to the Civil
Aviation Regulations (CAR) 3.
⦁ 6,000 pounds or less maximum weight and VSO 61 knots or less: the single-engine rate of climb at 5,000
feet MSL must simply be determined. The rate of climb could be a negative number. There is no
requirement for a single-engine positive rate of climb at 5,000 feet or any other altitude. For light-twins
type certificated February 4, 1991, or thereafter, the single-engine climb gradient (positive or negative) is
simply determined.
Operation of Systems
This section deals with systems and equipment that are generally installed in multiengine airplanes. Multiengine airplanes share many
features with complex single-engine airplanes. However, there are certain features that are found more often in airplanes with two or
more engines.
Feathering Propellers
Although the propellers of a multiengine airplane may appear identical to a constant-speed propeller used in many single-engine
airplanes, this is usually not the case. The pilot of a typical multiengine airplane can feather the propeller of an inoperative engine.
Since it stops engine rotation with the propeller blade streamlined with the airplane’s relative wind, feathering the propell er of an
inoperative engine minimizes propeller drag. [Figure 13-2] Depending upon single-engine performance, this feature often permits
continued flight to a suitable airport following an engine failure.
Feathering is important because of the change in parasite drag with propeller blade angle. [Figure 13-3] When the propeller blade
angle is in the feathered position, parasite drag from the propeller is at a minimum. In a typical multiengine airplane, the parasite drag
from a single, feathered propeller is a small part the airplane's total drag.
At the smaller blade angles near the flat pitch position, the drag added by the propeller is large. At these small blade angles, the
propeller windmilling at high revolutions per minute (rpm) can create enough drag to make the airplane difficult or impossible to
control. A propeller windmilling at high speed in the low range of blade angles can produce parasite drag as great as the parasite drag
of the entire airframe.
Figure 13-2. Feathered propeller.
Figure 13-3. Propeller drag contribution.
As a review, the constant-speed propellers on almost all single-engine airplanes are of the non-feathering, oil-pressure- to-increase-
pitch design. In this design, increased oil pressure from the propeller governor drives the blade angle towards high pitch, low rpm.
In contrast, the constant-speed propellers installed on most multiengine airplanes are full feathering, counterweighted, oil-pressure-to-
decrease-pitch designs. In this design, increased oil pressure from the propeller governor drives the blade angle toward low pitch,
igh rpm—away from the feather blade angle. In effect, the only thing that keeps these propellers from feathering is a constant supply
of high-pressure engine oil. This is a necessity to enable propeller feathering in the event of a loss of oil pressure or a propeller
governor failure.
Aerodynamic forces acting upon a windmilling propeller tend to drive the blades to low pitch, high rpm. Counterweights attached to
the shank of each blade tend to force the blades to high pitch, low rpm. Inertia, or the apparent force (called centrifugal force) acting
through the counterweights, is generally slightly greater than the aerodynamic forces. Therefore, centrifugal force would drive the
blades to high pitch and low rpm were it not for an additional force acting through the propeller governor. A controlling for ce
generated from high pressure oil from the propeller governor pushes the propeller blade angles toward low pitch and high rpm. Thus,
a reduction in oil pressure allows the counterweights to drive the blades to a higher pitch and decreases engine rpm. [Figure 13-4]
To feather the propeller, the propeller control is brought fully aft. All oil pressure is dumped from the governor, and the
counterweights drive the propeller blades toward feather. As centrifugal force acting on the counterweights decays from decreasing
rpm, additional forces are needed to completely feather the blades. This additional force comes from either a spring or high-pressure
air stored in the propeller dome, which forces the blades into the feathered position. The entire process may take up to 10 seconds.
Figure 13-4. Pitch change forces.
Feathering a propeller only alters blade angle and stops engine rotation. To completely secure the engine, the pilot turns off the fuel
(mixture, electric boost pump, and fuel selector), ignition, alternator/generator, and closes the cowl flaps. If the airplane is
pressurized, there may also be an air bleed to close for the failed engine. Some airplanes are equipped with firewall shutoff valves that
secure several of these systems with a single switch.
Completely securing a failed engine may not be necessary or even desirable depending upon the failure mode, altitude, and tim e
available. The position of the fuel controls, ignition, and alternator/generator switches of the failed engine has no effect on aircraft
performance, and the pilot might manipulate the incorrect switch under conditions of haste or pressure.
To unfeather a propeller, the engine should be rotated so that oil pressure can be generated to move the propeller blades from the
feathered position. The ignition is turned on prior to engine rotation with the throttle at low idle and the mixture rich. With the
propeller control in a high rpm position, the starter is engaged. The engine begins to windmill, start, and run as oil pressure moves the
blades out of feather. As the engine starts, the propeller rpm should be immediately reduced until the engine has had several minutes
to warm up; the pilot should monitor cylinder head and oil temperatures.
An unfeathering accumulator is a device that permits starting a feathered engine in-flight without the use of the electric starter. An
accumulator is any device that stores a reserve of high pressure. On multiengine airplanes, the unfeathering accumulator stores a small
reserve of engine oil under pressure from compressed air or nitrogen. To start a feathered engine in-flight, the pilot moves the
propeller control out of the feather position to release the accumulator pressure. The oil flows under pressure to the propeller hub and
drives the blades toward the high rpm, low pitch position, whereupon the propeller usually begins to windmill. If fuel and ignition are
present, the engine starts and runs. High oil pressure from the propeller governor recharges the accumulator just moments after engine
rotation begins making it available for another unfeathering cycle, if needed. For airplanes used in training, an unfeatherin g
accumulator may prolong the life of the electric starter and battery. If the accumulator fails to bring the propeller out of feather, the
electric starter may be engaged.
In any event, the AFM/POH procedures should be followed for the exact unfeathering procedure. Both feathering and starting a
feathered reciprocating engine on the ground are strongly discouraged by manufacturers due to the excessive stress and vibrations
generated.
As just described, a loss of oil pressure from the propeller governor allows the counterweights, spring, and/or dome charge to drive
the blades to feather. Logically then, the propeller blades should feather every time an engine is shut down as oil pressure falls to
zero. However, below approximately 800 rpm, a reduction in centrifugal force allows small anti-feathering lock pins in the pitch
changing mechanism of the propeller hub to move into place and block feathering. Therefore, if a propeller is to be feathered, it needs
to be done before engine rpm decays below approximately 800. On one popular model of turboprop engine, the propeller blades do,
in fact, feather with each shutdown. This propeller is not equipped with such centrifugally-operated pins due to a unique engine
design.
Propeller Synchronization
Many multiengine airplanes have a propeller synchronizer (prop sync) installed to eliminate the annoying “drumming” or “beat” of
propellers whose rpm are close, but not precisely the same. To use prop sync, the propeller rpms are coarsely matched by the pilot
and the system is engaged. The prop sync adjusts the rpm of the “slave” engine to precisely match the rpm of the “master” engine and
then maintains that relationship.
The prop sync should be disengaged when the pilot selects a new propeller rpm and then re-engaged after the new rpm is set. The
prop sync should always be off for takeoff, landing, and single-engine operation. The AFM/POH should be consulted for system
description and limitations.
A variation on the propeller synchronizer is the propeller synchrophaser. A propeller synchrophaser acts much like a synchronizer to
precisely match rpm, but the synchrophaser goes one step further. It not only matches rpm but actually compares and adjusts the
positions of the individual blades of the propellers in their arcs. There can be significant propeller noise and vibration reductions with
a propeller synchrophaser. From the pilot’s perspective, operation of a propeller synchronizer and a propeller synchrophaser are very
similar. A synchrophaser is also commonly referred to as prop sync, although that is not entirely correct nomenclature from a
technical standpoint.
As a pilot aid to manually synchronizing the propellers, some twins have a small gauge mounted in or by the tachometer(s) with a
propeller symbol on a disk that spins. The pilot manually fine tunes the engine rpm so as to stop disk rotation, thereby synchronizing
the propellers. This is a useful backup to synchronizing engine rpm using the audible propeller beat. This gauge is also found installed
with most propeller synchronizer and synchrophase systems. Some synchrophase systems use a knob for the pilot to control the phase
angle.
Fuel Crossfeed
Fuel crossfeed systems are also unique to multiengine airplanes. Using crossfeed, an engine can draw fuel from a fuel tank located in
the opposite wing.
On most multiengine airplanes, operation in the crossfeed mode is an emergency procedure used to extend airplane range and
endurance in OEI flight. There are a few models that permit crossfeed as a normal, fuel balancing technique in normal operation, but
these are not common. The AFM/POH describes crossfeed limitations and procedures that vary significantly among multiengine
airplanes.
Checking crossfeed operation on the ground with a quick repositioning of the fuel selectors does nothing more than ensure freedom of
motion of the handle. To actually check crossfeed operation, a complete, functional crossfeed system check should be accomplished.
To do this, each engine should be operated from its crossfeed position during the run-up. The engines should be checked individually
and allowed to run at moderate power (1,500 rpm minimum) for at least 1 minute to ensure that fuel flow can be established from the
crossfeed source. Upon completion of the check, each engine should be operated for at least 1 minute at moderate power from the
main (takeoff) fuel tanks to reconfirm fuel flow prior to takeoff.
This suggested check is not required prior to every flight. Crossfeed lines are ideal places for water and debris to accumulate unless
they are used from time to time and drained using their external drains during preflight. Crossfeed is ordinarily not used fo r
completing a flight with one engine inoperative when an alternate airport is nearby. Pilots should never use crossfeed during takeoff
or for normal landing operations with both engines operating. A landing with one engine inoperative using crossfeed may be
necessary if setting normal fuel flow would cause the operative engine to fail.
Combustion Heater
Combustion heaters are another common item on multiengine airplanes not found on single-engine airplanes. A combustion heater is
best described as a small furnace that burns gasoline to produce heated air for occupant comfort and windshield defogging. M ost are
thermostatically operated and have a separate hour meter to record time in service for maintenance purposes. Automatic over-
temperature protection is provided by a thermal switch mounted on the unit that cannot be accessed in flight. This requires the pilot or
mechanic to visually inspect the unit for possible heat damage in order to reset the switch.
Manufacturers often suggest a cool-down period when shutting down a combustion heater. Most heater instructions recommend that
outside air be permitted to circulate through the unit for at least 15 seconds in flight or that the ventilation fan can be operated for at
least 2 minutes on the ground. Failure to provide an adequate cool down usually trips the thermal switch and renders the heater
inoperative until the switch is reset.
Flight Director/Autopilot
Multiengine airplanes are often equipped with flight director/autopilot (FD/AP) systems. The system integrates pitch, roll,
heading, altitude, and radio navigation signals in a computer. The outputs, called computed commands, are displayed on a flight
command indicator (FCI). The FCI replaces the conventional attitude indicator on the instrument panel. The FCI is occasionally
referred to as a flight director indicator (FDI) or as an attitude director indicator (ADI).
The entire flight director/autopilot system is called an integrated flight control system (IFCS) by some manufacturers. Others may
use the term automatic flight control system (AFCS).
The FD/AP system may be employed at the following different levels:
⦁ Off (raw data)
⦁ Flight director (computed commands)
⦁ Autopilot
With the system off, the FCI operates as an ordinary attitude indicator. On most FCIs, the command bars are biased out of view when
the FD is off. The pilot maneuvers the airplane as though the system were not installed.
To maneuver the airplane using the FD, the pilot enters the desired modes of operation (heading, altitude, navigation (NAV)
intercept, and tracking) on the FD/AP mode controller. The computed flight commands are then displayed to the pilot through either a
single-cue or dual-cue system in the FCI. On a single-cue system, the commands are indicated by “V” bars. On a dual-cue system, the
commands are displayed on two separate command bars, one for pitch and one for roll. To maneuver the airplane using computed
commands, the pilot “flies” the symbolic airplane of the FCI to match the steering cues presented.
On most systems, the FD needs to be operating to engage the autopilot. At any time thereafter, the pilot may engage the autopilot
through the mode controller. The autopilot then maneuvers the airplane to satisfy the computed commands of the FD.
Like any computer, the FD/AP system only does what it is told. The pilot should ensure that it has been programmed properly for the
particular phase of flight desired. The armed and/or engaged modes are usually displayed on the mode controller or separate
annunciator lights. When the airplane is being hand-flown, if the FD is not being used at any particular moment, it should be off so
that the command bars are pulled from view.
Prior to system engagement, all FD/AP computer and trim checks should be accomplished. Many newer systems cannot be engaged
without the completion of a self-test. The pilot should also be familiar with various methods of disengagement, both normal and
emergency. System details, including approvals and limitations, can be found in the supplements section of the AFM/POH.
Additionally, many avionics manufacturers can provide informative pilot operating guides upon request.
Yaw Damper
The yaw damper is a servo that moves the rudder in response to inputs from a gyroscope or accelerometer that detects yaw rate or
lateral Gs, respectively. The yaw damper reduces motion about the vertical axis caused by turbulence. (Yaw dampers on swept wing
airplanes provide another, more vital function of damping Dutch roll characteristics.) Occupants feel a smoother ride, particularly if
seated in the rear of the airplane, when the yaw damper is engaged. The yaw damper should be off for takeoff and landing. There may
be additional restrictions against its use with one engine inoperative. Most yaw dampers can be engaged independently of the
autopilot.