What is Minimum Control speed VMC, i.e. VMCA?
VMCA, the Minimum Control speed -
in the Air (or Airborne), is one of the Minimum Control speeds (VMC's) of a
multi-engine airplane that is published as operational limitation in Airplane Flight Manuals
multi-engine airplanes. Other published VMC's are Minimum
Control speed on the Ground (VMCG) and Minimum Control speed
during approach and Landing (VMCL).
is often used in manuals rather than VMCA. Regulations
are changing VMC into VMCA, because "VMCA
is more commonly used" (Flight Test Guide AC
When an engine fails or is inoperative
rudder is used to counteract the asymmetrical thrust yawing moment.
Roll effects are counteracted by the ailerons. The counteracting
forces generated by the aerodynamic control surfaces are proportional to the square of the airspeed (V2),
to the area of the control surfaces (S) and to the air density. For a given size of the vertical tail with rudder, there is a
speed below which the generated rudder side force is not large enough to counteract
the asymmetrical thrust, or below which the ailerons are not effective
anymore: the heading and/or bank angle cannot be maintained below this speed.
is called Minimum Control speed VMC or better: Minimum Control speed
in the Air VMCA.
VMCA is already determined
(i.e. assumed) by the design engineer for sizing the vertical tail (fin). A vertical
tail may not be that small that VMCA increases above 1.2 VS
(FAR 23.149). On the other side, a large tail results in a smaller VMCA
but in higher weight and production cost.
FAR 23 allows the design engineer
to use a small bank angle of maximum 5º (away from the failed engine) which can be used to reduce
the size of the vertical tail and also reduce both
the sideslip (drag) and
VMCA while an engine is
inoperative. A larger bank angle increases the sideslip and might
result in a fin stall.
However, the saved hardware weight of a smaller tail needs to be replaced by a quite 'heavy'
software condition (on paper in the AFM) for pilots when an engine is
inoperative. This condition is presented below.
Refer to the paper Control and Performance during Asymmetrical Powered
Flight on the downloads page (#2) for details on tail design and the use
The course on asymmetric
powered flight that test pilots and flight test engineers receive at formal
Test Pilot Schools can be downloaded from the References list (No. 10 and 11),
Bank angle versus sideslip and VMCA
for a sample airplane when engine #1 is inoperative. This graph shows:
When Indicated Airspeed is near VMCA, then bank 4°
away from the inoperative engine to avoid loss of control and for minimum drag,
i.e. for max. Rate of Climb.
graph shows (actual) VMCA and sideslip angle versus bank angle of a sample
airplane after failure of the left engine (#1).
The airspeed that results from the bank angle for which the sideslip is zero
is the VMCA that will be published in the AFM (85 kt). At bank angles
6° away from the failed engine, for this sample airplane, the sideslip angle
increases to 14°, the maximum possible angle of attack of the fin.
airspeed needs to be increased to avoid the fin to stall - actual VMCA
increases (blue line).
Notice that the actual VMCA for wings level (100 kt) of this
sample airplane is 15 kt higher than the
Therefore the pilot should not maintain a bank angle of maximum 5°
(either side) as is presented in VMC definitions in most AFM's, but
the exact bank angle that was used to design the vertical tail and at which
the drag is minimal (in this example 4°, usually
and EASA/CS 23.149 and equivalent present the definition of VMCA for the
design and certification of multi-engine airplanes
that is also inappropriately copied into most AFM's:
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 airplane is designed and build, the selected tail size imposes a
limitation on, i.e. a constraint to, pilots. The VMCA
definition for use by pilots is therefore different than the VMCA
definition out of FAR/CS 23.149 that is for manufacturers, for designing and certification
is the minimum speed for maintaining straight flight when an engine
fails or is inoperative and the corresponding opposite engine is set to
provide maximum thrust, provided a bank angle is being maintained of 3
– 5 degrees (exact number to be provided by the manufacturer) away
from the inoperative engine.
addition, the manufacturer
should specify the configuration for which this, or other published VMCA's
is/ are valid.
For further details,
refer to the papers for pilots presented on the Downloads
the airspeed indicator of Part 23 twin-engine airplanes, the
standardized AFM-published VMCA is indicated by a red
radial line, in this example at 80 kt. However, neither a placard on the
instrument panel nor
a note or warning in the AFM tells the pilot that the redlined VMCA
if a bank angle of 3 to 5 degrees (to be specified by
the manufacturer) is maintained away from the inoperative engine.
Any other bank angle results in a much higher actual
VMCA and to the loss of control after which an accident cannot
be avoided (when asym. thrust is not reduced).
The airspeed for maximum single-engine rate of climb VYSE
is indicated by a blue radial line, here at 105 kt.
In the legend of some Performance Data Tables of Graphs,
a note tells the pilot that
the presented performance data, including the performance at VYSE, are valid only
if a small bank angle is being maintained of 2 - 3 degrees away from the
inoperative engine. For other bank angles, the maximum climb performance
or the performance to maintain altitude (i.e. to prevent drifting down)
is not guaranteed.
Jan Roskam (KU): "The VMCA value ultimately used
ties take-off performance to engine-out controllability."
If the pointer is at or near the red line and the thrust on the
remaining engine(s) is or is increased to maximum, only straight flight should be maintained while
maintaining a bank angle of 3 to 5 degrees away from the inoperative
engine, depending on the airspeed (VYSE and VMCA
turning safely while the asymmetrical thrust is high, gain altitude
first during straight flight to allow for some altitude loss during
reduced thrust turns, because of the
increased sideslip (drag) during turns.
The vertical tail is simply not large enough for turns at max.
It is safer
to reduce the thrust a little during the turns to keep the actual VMCA
low. Also consider a long straight-in approach rather than a
tight final turn during which the thrust might have to be increased to
maximum (and control will be lost because actual VMCA
increases above the indicated airspeed).
2° to 3° BANK TOWARD OPERATING ENGINE
This note is included in the legend of the Climb Performance Chart - One
Engine Operating in the Piper PA-44 Pilot's Information Manual. It
is included, because not maintaining this bank angle renders the
presented performance data invalid; the airplane might not even be able
to maintain altitude. The bank angle is smaller than 5 degrees, because the
presented performance data requires VYSE, the blue line speed, which is
higher than VMCA. The vertical tail is more effective at
Keeping the wings level or turning means loss of
performance; altitude cannot be maintained on most
multi-engine airplanes if this NOTE is neglected. The reason why
this NOTE is included is explained in the papers presented on the Downloads
AIR MINIMUM CONTROL SPEED 80 KIAS
similar placard is to be installed in full view of pilots of commuter Part 23 airplanes to comply
with Aviation Regulations (23.1563). The
required small bank angle for the listed VMCA to be valid
is regrettably not included on the placard, because this is not
required by the Aviation Regulations, but is essential for flight
safety and performance.
Not maintaining the small bank angle (i.e. straight
flight) at airspeeds as low as VMCA,
while the power setting of the remaining engine is high, is the real
cause of most engine failure related accidents.
is 80 KIAS, PROVIDED straight flight is maintained while
BANKing 5° TOWARD OPERATING ENGINE
is recommended to require a placard like this one in all Part 23
options are possible.
AFM-published VMCA is one of the factors for calculating the
rotation speed VR of all multi-engine airplanes, and for
calculating the minimum takeoff safety speed V2MIN
of big Part 25 airplanes. Since this VMCA is
valid only while maintaining a bank angle of 3 to 5 degrees, as to be
specified by the manufacturer, away from the inoperative engine, both
the calculated VR and V2MIN are also valid only
when maintaining the same bank angle (when the thrust setting is
Refer to the paper
for Investigators and Flight Instructors for thorough explanation of
figure, a safety improving suggestion
shows that the actual VMCA in this example has become higher than VR
because the wings are kept level. Bank angle and rudder advisories
are presented to decrease the actual VMCA to a safe level to
prevent the loss of airplane control. The bank angle advisory
widens up as the airspeed increases.
Also refer to the formal FAA and EASA Flight Test Guides in the reference list on
the Downloads page.
refer to the paper for investigators and flight instructors presented on the
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