Plane & Pilot
Tuesday, June 3, 2014

Treat Vmc with Respect


Nearly all twin accidents are related to an abuse of Vmc


The hard reality of twin-engine accidents is that the vast majority are a function of loss of control during single-engine operations. In fact, though it may sound like an oversimplification, response to practically any emergency in a twin is almost identical to the same problem in a single unless the twin has lost power on one engine and is suffering from some aspect of asymmetrical thrust.

As every multi-engine pilot knows, Vmc is "Velocity minimum controllable," the slowest speed at which the airplane will accept full power on one side and still remain controllable with the other engine inoperative. On a Cessna 310R, for example, Vmc is 80 knots and stall speed is 79 knots. If you've ever had to demonstrate minimum controlled flight in a 310 with one engine zero thrusted and the other running at full power, you'll see this phenomenon at work, and you'll feel it, too.

The airplane is obviously angry, control pressures are heavy, performance is barely positive (even if you're doing everything right) and the pilot won't be having fun. Perhaps worst, you'll be subceeding the Vyse speed of 106 knots by a wide margin, and that's never a comfortable feeling.

As you slow the airplane, you'll need progressively more rudder on the power side to hold up the "dead" wing. Eventually, you'll get to a point where you're totally out of top or "power" rudder, and the airplane will want to stall and roll uncontrollably toward the dead engine.

There are only two ways you can recover in a real emergency. One is to reduce power on the good engine. The other is to lower the nose and allow the airplane to accelerate out of the stall, obviously sacrificing altitude in the process.

Yes, I appreciate that a Vmc roll is somewhat more complicated than the explanation above, but for purposes of definition, this is a summary of the forces involved.

The two most typical dangerous reactions of pilots in emergency situations are to freeze, sit there unbelievingly, and watch the problem expand and become unmanageable, or to go into panic mode and try to correct the situation by doing the wrong thing. In a twin, pilots may react to a power loss by misidentifying the sick engine and feathering the good one, or try to solve an engine fire on the left engine by shutting off fuel to the right.

A primary reason some pilots lose control in a twin during an emergency is that they're reluctant to practice handling anything other than normal, balanced, multi-engine flight. The cost may be prohibitive, or it may be impractical or impossible to practice emergencies in their own model.



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