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VMCA testing

VMCA definition

Display of VMCA

VR and V2

VMC or VMCA?

What is Minimum Control speed VMC, or VMCA?

VMC, or more specifically VMCA, is the minimum speed for maintaining straight flight only 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.
CAUTION: The AFM-published VMCA (in older manuals VMC) does not apply at any other bank angle, such as required during turns.

On the Downloads page, papers can be downloaded that contain the content presented below at several levels of detail. The Accidents page presents reviews of several engine-failure related accidents to learn from.


Minimum Control Speed - VMCA

When an engine fails or is inoperative in-flight, the rudder is used to counteract the asymmetrical thrust yawing moment; roll effects are counteracted by the ailerons. The counteracting forces generated by these aerodynamic surfaces are proportional to the square of the airspeed (V2), to the area of the surfaces (S) and to the air density. So, 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. 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/CS 23.149). On the other side, a large tail results in a lower VMCA but in higher weight and production cost.
FAR/CS 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.
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 papers #2 and #3 on the Downloads page for details on tail design and the consequences for the use of VMCA.

A 42 min. video lecture, in which the real value of the minimum control speed airborne (VMCA) is explained as taught at all formal Test Pilot Schools and most aeronautical universities, including the review of two accidents using views from the cockpit, is available on YouTube. Click here

A pdf file with slides and script used in this video can be downloaded here.

Courses 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), click here. Links for downloading the complete flying qualities courses can be found on the Links page.

Graph Vmca & sideslip versus bank angle.

Bank angle versus sideslip and VMCA for a sample airplane when engine #1 is inoperative.
This graph shows:
When the Indicated Airspeed is near AFM published 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.

VMCA is a constant number, but only in the AFM. In flight, the actual VMCA varies with bank angle, engine thrust, rudder deflection, and other variables. This graph shows (actual) VMCA and sideslip angle, both versus bank angle of a sample airplane after failure of the left engine (#1) while the asymmetrical thrust and rudder are maximal.
The airspeed that results from the bank angle for which the sideslip is zero (minimum drag, max. ROC) is the VMCA that will be published in the AFM (85 kt). At bank angles larger than 6° away from the failed engine, for this sample airplane, the sideslip angle increases to 14°, being the fin stall angle of attack (with deflected rudder). The airspeed needs to be increased to prevent the fin from stalling at larger bank angles - the 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 AFM published VMCA.
This leads to the above announced condition that 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 5°), when the airspeed decreases to or is VMCA while the asymmetrical thrust is maximal. At higher airspeeds, the bank angle can be smaller; at VYSE mostly 3°.
This small bank angle does not result in a turn, but reduces both the drag and VMCA.

The vertical tail with rudder is not designed large enough for maintaining control during turns into or away from the inoperative engine at airspeed VMCA while maximum thrust is set, but only for maintaining straight flight. Therefore increase the airspeed by at least 30 kt before turning to either side. At any sign of loss of control, decrease thrust (a bit) and recover to straight flight. After establishing the favorable bank angle, asymmetrical thrust can be increased again.


How is VMCA measured in-flight

The airplane is brought in the VMCA test configuration, i.e. lowest weight possible and aft center of gravity, which result in the highest, worst case VMCA. Then in-flight, at a safe altitude of 5000 ft, an airspeed is attained well above the anticipated VMCA. Then the critical engine is shut down, or set at torque for minimum thrust, and the opposite engine at maximum thrust. The airspeed is slowly decreased until the increasing rudder and/or aileron cannot maintain the heading and/or wings level anymore. The airspeed at which this occurs is VMCA with the wings level; also mind the large unavoidable sideslip, i.e. drag. Then, while further decreasing the airspeed, a small bank angle is increased into the operating engine during straight flight to a maximum of 5°, or until the sideslip is zero. The airspeed at which this occurs is the VMCA of the airplane that will be published in the AFM, after extrapolation to sea level. Regulations do not require the much higher actual VMCA during turns to be determined.
Please refer to the formal FAA or EASA Flight Test Guides for a safe conduct of this test, via the
Links page.


VMCA Definition

For Manufacturers: For Pilots:

FAR 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:



VMC 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.

Once 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 of airplanes.

VMCA 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.

In addition, the manufacturer should specify the configuration for which VMCA and other published VMC's are valid.

For further details, refer to the papers for pilots presented on the Downloads page.


Display of VMCA


On 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 is valid only if a bank angle of 3 to 5 degrees (to be specified by the manufacturer) is maintained away from the inoperative engine. A larger bank angle, or a bank angle into the inoperative engine results in a much higher actual VMCA, a 14 or more degree sideslip, hence large drag, and to the loss of climb performance and possibly loss of control after which an accident cannot be avoided (when asymmetrical 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.

Dr. 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 airplane type and airspeed VYSE and VMCA resp.).
For 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. 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 for maintaining the glide path (and control will be lost because actual VMCA increases above the indicated airspeed). This happened many times.



NOTE

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 higher speed.
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 page.


ONE ENGINE INOPERATIVE
AIR MINIMUM CONTROL SPEED 80 KIAS

A similar placard is to be installed in full view of pilots of Part 23 commuter 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 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.

VMCA is 80 KIAS, provided straight flight is maintained while banking 5° toward operating engine

It is recommended to require a placard like this one in all Part 23 airplanes.

More options are possible.


VR and V2MIN

The standardized, 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 maximum takeoff).
Refer to the paper for Investigators and Flight Instructors for thorough explanation of takeoff speeds.


VMC or 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 its Airplane Flight Manual (AFM). Other published VMC's are Minimum Control speed on the Ground (VMCG) and, in some cases, also the Minimum Control speed during approach and Landing (VMCL).
VMC is often used in manuals rather than VMCA. Regulations, however, are changing VMC into VMCA, because "VMCA is more commonly used".


This figure, a safety improving suggestion of AvioConsult, 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.


Refer to the formal FAA and EASA Flight Test Guides via the Links page. In the reference list on the Downloads page, only the VMCA testing paragraphs can be downloaded.

For further details, refer to the paper for investigators and flight instructors presented on the Downloads page.


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