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The Vmc Demo

Vmc Demo

During your transition to twin training, you’ll be asked to perform a “Vmc Demo“.  This is a training maneuver that is very similar to a power on stall demonstration where you will set the aircraft up in a particular configuration in order to demonstrate your ability to recognize the point at which a loss of control is imminent.  Vmc is defined the minimum airspeed at which a twin engine aircraft is controllable with only one engine running — think V SPEED with MINIMAL CONTROL.  This is an important V speed in flying twins because it it represents the slowest you can fly the aircraft while still maintaining directional control which is important when you are trying to land with only one engine operative.   Vmc, like stall speed, is not constant, it changes with changes in the location of the CG and altitude so recognized and dealing appropriately with what happens just before and after the aircraft slows through this speed is very important.

A loss of airspeed to a point below Vmc sounds like a pilot’s worse nightmare — running on one engine and unable to maintain control, which results in a roll-over close to the ground — and so it is the subject of dedicated training and evaluation when stepping up to a multi-engine rating.  In a recent review of causes of crashes of light twin aircraft, 7 of 545 sequential crashes were due to Vmc rollovers, which turns out to be less than 1%.  This review noted that this rate was “better than imagined, suggesting that Vmc-roll avoidance training may work”.*

The Vmc Demo

Vmc is established during aircraft certification testing in the least favorable circumstances and the way this V speed is determined is covered by Federal Aviation Regulation 23.149, which reads as follows:

Part 23 AIRWORTHINESS STANDARDS: NORMAL, UTILITY, ACROBATIC, AND COMMUTER CATEGORY AIRPLANES
Subpart B–Flight
Controllability and Maneuverability
Sec. 23.149

Minimum control speed.

(a) VMC is the calibrated airspeed, at which, when the critical engine is suddenly made inoperative, it is possible to recover control of the airplane with that engine still inoperative, and maintain straight flight either with zero yaw or, at the option of the applicant, with an angle of bank of not more than five degrees. The method used to simulate critical engine failure must represent the most critical mode of powerplant failure with respect to controllability expected in service.
(b) For reciprocating engine-powered airplanes, VMC may not exceed 1.2  Vs1 (where Vs1 is determined at the maximum takeoff weight) with–
(1) Takeoff or maximum available power on the engines;
(2) The most unfavorable center of gravity;
(3) The airplane trimmed for takeoff;
(4) The maximum sea level takeoff weight (or any lesser weight necessary to show VMC);
(5) Flaps in the takeoff position;
(6) Landing gear retracted;
(7) Cowl flaps in the normal takeoff position;
(8) The propeller of the inoperative engine–
(i) Windmilling;
(ii) In the most probable position for the specific design of the propeller control; or
(iii) Feathered, if the airplane has an automatic feathering device: and
(9) The airplane airborne and the ground effect negligible.
(d) At VMC, the rudder pedal force required to maintain control may not exceed 150 pounds, and it may not be necessary to reduce power or thrust of the operative engines. During recovery, the airplane may not assume any dangerous attitude and it must be possible to prevent a heading change of more than 20 degrees.

In most of the study guides for a twin engine rating they suggest that most designated pilot examiners will ask for a list of the 10 factors that are used to determine Vmc.  In plain English, here are the 10 factors:

  1. the critical LEFT engine is not producing power and the propeller is windmilling
  2. the RIGHT engine is producing maximum thrust at sea level
  3. the airplane is at max gross weight
  4. the CG is at the rear limit
  5. the flaps are in the take-off configuration (normally up on a BE-95)
  6. cowl flaps are in the take-off configuration (open on a BE-95)
  7. the aircraft is trimmed for takeoff
  8. we’ve rotated, are airborne and out of ground effect
  9. the gear is up
  10. less than 5 degrees of bank into the good engine is being used to counteract the asymmetrical thrust

Why all the specifics?  Vmc is all about testing the ability of the airplane to stay stable with the most asymmetrical thrust that it is capable of producing, and this occurs in a take-off configuration when the airplane has slow airspeed and the engines are set for maximum power.  This test is designed to simulate the least opportune time for an engine failure and the situation is worse when the aircraft is slow and heavy with minimal rudder authority and maximal adverse yaw and roll created by operating right engine.

That makes sense, but why the gear up?  While having the gear down creates a lot of drag, the gear acts like a vertical stabilizer.  The presence of the gear in the slipstream has a keel like effect and increases the stability of the airplane about the yaw axis.

According to Sheble’s website, the procedure for performing a Vmc demonstration is as follows:

Full Power
Critical Engine to Idle
Slow aircraft and pitch up until nose yaws
Power to Idle on GOOD engine
Pitch for blue line
Add power on GOOD engine to stop descent
Pitch for blue line

Alright….I memorized that procedure and the list of the 10 factors used to establish Vmc,. so we’ll see what happens during training.

References

* http://www.avweb.com/news/safety/185267-1.html

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