The minimum control speed (V
MC) of a multi-engine
aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines ...
(specifically an
airplane
An airplane or aeroplane (informally plane) is a fixed-wing aircraft that is propelled forward by thrust from a jet engine, propeller, or rocket engine. Airplanes come in a variety of sizes, shapes, and wing configurations. The broad spe ...
) is a
V-speed
In aviation
Aviation includes the activities surrounding mechanical flight and the aircraft industry. ''Aircraft'' includes fixed-wing and rotary-wing types, morphable wings, wing-less lifting bodies, as well as lighter-than-air craft ...
that specifies the
calibrated airspeed
Calibrated airspeed (CAS) is indicated airspeed corrected for instrument and position error.
When flying at sea level under International Standard Atmosphere conditions (15 °C, 1013 hPa, 0% humidity) calibrated airspeed is the same as equiva ...
below which
directional or
lateral
Lateral is a geometric term of location which may refer to:
Healthcare
*Lateral (anatomy), an anatomical direction
*Lateral cricoarytenoid muscle
*Lateral release (surgery), a surgical procedure on the side of a kneecap
Phonetics
*Lateral cons ...
control of the aircraft can no longer be maintained, after the failure of one or more engines. The V
MC only applies if at least one engine is still operative, and will depend on the stage of flight. Indeed, multiple V
MCs have to be calculated for landing, air travel, and ground travel, and there are more still for aircraft with four or more engines. These are all included in the
aircraft flight manual
An aircraft flight manual (AFM) is a paper book or electronic information set containing information required to operate an aircraft of certain type or particular aircraft of that type (each AFM is tailored for a specific aircraft, though aircraft ...
of all
multi-engine aircraft. When design engineers are sizing an airplane's
vertical tail
A vertical stabilizer or tail fin is the static part of the vertical tail of an aircraft. The term is commonly applied to the assembly of both this fixed surface and one or more movable rudders hinged to it. Their role is to provide control, sta ...
and
flight control surfaces
Aircraft flight control surfaces are aerodynamic devices allowing a pilot to adjust and control the aircraft's flight attitude.
Development of an effective set of flight control surfaces was a critical advance in the development of aircraft. Ea ...
, they have to take into account the effect this will have on the airplane's minimum control speeds.
Minimum control speeds are typically established by
flight test
Flight testing is a branch of aeronautical engineering that develops specialist equipment required for testing aircraft behaviour and systems. Instrumentation systems are developed using proprietary transducers and data acquisition systems. D ...
s
as part of an aircraft certification process.
They provide a guide to the pilot in the safe operation of the aircraft.
Physical description
![Vmc Article Fig 3 - Forces and moments during wings-level equilibrium](https://upload.wikimedia.org/wikipedia/commons/f/f8/Vmc_Article_Fig_3_-_Forces_and_moments_during_wings-level_equilibrium.png)
When an
engine
An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy.
Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power gen ...
on a multi-engine aircraft fails, the thrust distribution on the aircraft becomes
asymmetrical
Asymmetry is the absence of, or a violation of, symmetry (the property of an object being invariant to a transformation, such as reflection). Symmetry is an important property of both physical and abstract systems and it may be displayed in pre ...
, resulting in a
yawing moment
Moment or Moments may refer to:
* Present time
Music
* The Moments, American R&B vocal group Albums
* ''Moment'' (Dark Tranquillity album), 2020
* ''Moment'' (Speed album), 1998
* ''Moments'' (Darude album)
* ''Moments'' (Christine Guldbrand ...
in the direction of the failed engine. A
sideslip
A slip is an aerodynamic state where an aircraft is moving ''somewhat'' sideways as well as forward relative to the oncoming airflow or relative wind. In other words, for a conventional aircraft, the nose will be pointing in the opposite directio ...
develops, causing the total drag of the aircraft to increase considerably, resulting in a drop in the aircraft's
rate of climb
In aeronautics, the rate of climb (RoC) is an aircraft's vertical speed, that is the positive or negative rate of altitude change with respect to time. In most ICAO member countries, even in otherwise metric countries, this is usually expressed ...
. The
rudder
A rudder is a primary control surface used to steer a ship, boat, submarine, hovercraft, aircraft, or other vehicle that moves through a fluid medium (generally aircraft, air or watercraft, water). On an aircraft the rudder is used primarily to ...
, and to a certain extent the
aileron
An aileron (French for "little wing" or "fin") is a hinged flight control surface usually forming part of the trailing edge of each wing of a fixed-wing aircraft. Ailerons are used in pairs to control the aircraft in roll (or movement around ...
s via the use of bank angle, are the only aerodynamic controls available to the pilot to counteract the asymmetrical thrust yawing moment.
The higher the speed of the aircraft, the easier it is to counteract the yawing moment using the aircraft's controls. The minimum control speed is the airspeed below which the force the rudder or ailerons can apply to the aircraft is not large enough to counteract the asymmetrical thrust at a maximum power setting. Above this speed it should be possible to
maintain control of the aircraft and maintain straight flight with asymmetrical thrust.
Loss of engine power of wing-mounted-propeller aircraft and
blown lift aircraft affects the lift distribution over the wing, causing a roll toward the inoperative engine.
In some aircraft roll authority is more limiting than rudder authority in determining V
MCs.
Certification and variants
![Vmc Article Fig 1 - Overview of all Vmc's](https://upload.wikimedia.org/wikipedia/commons/f/fc/Vmc_Article_Fig_1_-_Overview_of_all_Vmc%27s.png)
Aviation regulations (such as
FAR and
EASA
The European Union Aviation Safety Agency (EASA) is an agency of the European Union (EU) with responsibility for civil aviation safety. It carries out certification, regulation and standardisation and also performs investigation and monitori ...
)
define several different V
MCs and require design engineers to size the vertical tail and the aerodynamic
flight controls of the aircraft to comply with these regulations. The minimum control speed in the air (V
MCA) is the most important minimum control speed of a multi-engine aircraft, which is why V
MCA is simply listed as V
MC in many aviation regulations and
aircraft flight manual
An aircraft flight manual (AFM) is a paper book or electronic information set containing information required to operate an aircraft of certain type or particular aircraft of that type (each AFM is tailored for a specific aircraft, though aircraft ...
s.
On the
airspeed indicator
The airspeed indicator (ASI) or airspeed gauge is a flight instrument indicating the airspeed of an aircraft in kilometers per hour (km/h), knots (kn), miles per hour (MPH) and/or meters per second (m/s). The recommendation by ICAO is to use km/h, ...
of a twin-engine aircraft of less than 6000 lbs (2722 kg), the V
MCA is indicated by a red radial line, as standardised by
FAR 23
The Federal Aviation Regulations (FARs) are rules prescribed by the Federal Aviation Administration (FAA) governing all aviation activities in the United States. The FARs comprise Title 14 of the Code of Federal Regulations (CFR). A wide variety ...
.
Most test pilot schools use multiple, more specific minimum control speeds, as V
MC will change depending on the stage of flight. Other defined V
MCs include minimum control speed on the ground (V
MCG) and minimum control speed during approach and landing (V
MCL). In addition, with aircraft with four or more engines, V
MCs exist for cases with either one or two engines inoperative on the same wing. Figure 1 illustrates the V
MCs that are defined in the relevant civil aviation regulations
and in military specifications.
Minimum control speed when airborne
![Vmc Article Fig 5 - Graph Vmc and sideslip](https://upload.wikimedia.org/wikipedia/commons/a/a8/Vmc_Article_Fig_5_-_Graph_Vmc_and_sideslip.png)
The
vertical tail
A vertical stabilizer or tail fin is the static part of the vertical tail of an aircraft. The term is commonly applied to the assembly of both this fixed surface and one or more movable rudders hinged to it. Their role is to provide control, sta ...
or
vertical stabilizer
A vertical stabilizer or tail fin is the static part of the vertical tail of an aircraft. The term is commonly applied to the assembly of both this fixed surface and one or more movable rudders hinged to it. Their role is to provide control, sta ...
of a multi-engine aircraft plays a crucial role in maintaining directional control while an engine fails or is inoperative. The larger the tail, the more capable it will be of providing the required force to counteract the asymmetrical thrust yawing moment. This means that the smaller the tail is, the higher the V
MCA will be. However, a larger tail is more costly and harder to accommodate, and comes with other aerodynamic issues such as increased prevalence of
slipstream
A slipstream is a region behind a moving object in which a wake of fluid (typically air or mustard) is moving at velocities comparable to that of the moving fluid, relative to the ambient fluid through which the object is churning. The term slip ...
s. Engineers designing the vertical tail must make a decision based on, amongst other factors, their budget, the weight of the aircraft, and the maximum
bank angle
A banked turn (or banking turn) is a turn or change of direction in which the vehicle banks or inclines, usually towards the inside of the turn. For a road or railroad this is usually due to the roadbed having a transverse down-slope towards the ...
of 5° (away from the inoperative engine), as stated by
FAR.
V
MCA is also used to calculate the minimum
takeoff safety speed.
A high V
MCA therefore results in higher takeoff speeds, and so longer runways are required, which is undesirable for airport operators.
Factors influencing minimum control speed
Any factor that has influence on the balance of forces and on the yawing and rolling moments after engine failure might also affect V
MCs. When the vertical tail is designed and the V
MCA is measured, the worst-case scenario for all factors is taken into account. This ensures that the V
MCs published in the
AFMs guaranteed to be safe.
Heavier aircraft are more stable and more resistant to yawing moments, and therefore have lower V
MCAs.
The
longitudinal
Longitudinal is a geometric term of location which may refer to:
* Longitude
** Line of longitude, also called a meridian
* Longitudinal engine, an internal combustion engine in which the crankshaft is oriented along the long axis of the vehicle, ...
centre of gravity
In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force may ...
affects the V
MCA as well: the further from the tail it is, the lower the minimum control speed, because the rudder will be able to provide a larger yawing moment, and so it is easier to counteract the imbalance in thrust.
The
lateral
Lateral is a geometric term of location which may refer to:
Healthcare
*Lateral (anatomy), an anatomical direction
*Lateral cricoarytenoid muscle
*Lateral release (surgery), a surgical procedure on the side of a kneecap
Phonetics
*Lateral cons ...
centre of gravity also has an effect: the nearer the inoperative engine it is, the larger the moment of the working engine, and so the more force the rudder has to apply. This means that if the lateral centre of gravity shifts towards the inoperative engine, the aircraft's V
MCA will increase.
The thrust of most engines depends on altitude and temperature; increasing altitude and higher temperatures decrease thrust. This means that if the air temperature is higher and the aircraft has a higher altitude, the force of the operative engine will be lower, the rudder will have to provide less counteractive force, and so the V
MCA will be lower.
The bank angle also influences the minimum control speed. A small bank angle away from the inoperative engine is required for smallest possible sideslip and therefore lower V
MCA. Finally, if the
P-factor
P-factor, also known as asymmetric blade effect and asymmetric disc effect, is an aerodynamic phenomenon experienced by a moving propeller,) where the propeller's center of thrust moves off-center when the aircraft is at a high angle of attack. ...
of the working engine increases, then its yawing moment increases, and the aircraft's V
MCA increases as a result.
Other minimal control speeds
Aircraft with more engines
Aircraft with four or more engines have not only a V
MCA (often called V
MCA1 under these circumstances), where the critical engine alone is inoperative, but also a V
MCA2 that applies when the engine inboard of the critical engine, on the same wing, is also inoperative.
Civil aviation regulations (FAR, CS and equivalent) no longer require a V
MCA2 to be determined,
although it is still required for military aircraft with four or more engines.
On turbojet and turbofan aircraft, the outboard engines are usually equally critical. Three-engine aircraft such as the
MD-11
The McDonnell Douglas MD-11 is an American tri-jet wide-body airliner manufactured by American McDonnell Douglas (MDC) and later by Boeing.
Following DC-10 development studies, the MD-11 program was launched on December 30, 1986.
Assembly of t ...
and
BN-2 Trislander do not have a V
MCA2; a failed centerline engine has no effect on V
MC.
When two opposing engines of aircraft with four or more engines are inoperative, there is no thrust asymmetry, hence there is no rudder requirement for maintaining steady straight flight; V
MCAs play no role. There may be less power available to maintain flight overall, but the minimum safe control speeds remain the same as they would be for an aircraft being flown at 50% thrust on all four engines.
Failure of a single inboard engine, from a set of four, has a much smaller effect on controllability. This is because an inboard engine is closer to the aircraft's centre of gravity, so the lack of yawing moment is decreased. In this situation, if speed is maintained at or above the published V
MCA, as determined for the critical engine, safe control can be maintained.
Ground
If an engine fails during
taxiing
Taxiing (rarely spelled taxying) is the movement of an aircraft on the ground, under its own power, in contrast to towing or pushback where the aircraft is moved by a tug. The aircraft usually moves on wheels, but the term also includes aircra ...
or
takeoff
Takeoff is the phase of flight in which an aerospace vehicle leaves the ground and becomes airborne. For aircraft traveling vertically, this is known as liftoff.
For aircraft that take off horizontally, this usually involves starting with a t ...
, the thrust yawing moment will force the aircraft to one side on the runway. If the airspeed is not high enough and hence, the rudder-generated side force is not powerful enough, the aircraft will deviate from the runway centerline and may even veer off the runway.
The airspeed at which the aircraft, after engine failure, deviates 9.1 m from the runway centerline, despite using maximum rudder but without the use of nose wheel steering, is the minimum control speed on the ground (V
MCG).
Approach and landing
The minimum control speed during approach and
landing
Landing is the last part of a flight, where a flying animal, aircraft, or spacecraft returns to the ground. When the flying object returns to water, the process is called alighting, although it is commonly called "landing", "touchdown" or ...
(V
MCL) is similar to V
MCA, but the aircraft configuration is the landing configuration. V
MCL is defined for both part 23
and part 25 aircraft in civil aviation regulations. However, when maximum thrust is selected for a go-around
In aviation, a go-around is an aborted landing of an aircraft that is on final approach or has already touched down. A go-around can either be initiated by the pilot flying or requested by air traffic control for various reasons, such as an unstab ...
, the flaps will be selected up from the landing position, and VMCL no longer applies, but VMCA does.
Safe single-engine speed
Due to the inherent risks of operating at or close to VMCA with asymmetric thrust, and the desire to simulate and practise these manoeuvres in pilot training and certification VSSE may be defined. VSSE safe single-engine speed is the minimum speed to intentionally render the critical engine inoperative, established and designated by the manufacturer as the safe, intentional, one engine inoperative speed. This speed is selected to reduce the accident potential from loss of control due to simulated engine failures at inordinately slow airspeed.
References
{{reflist, 35em
Airspeed
Aerodynamics
Aviation safety