Training and Safety Tip: Every twin has a dark side
The airspeed indicator in most twin-engine airplanes has two redlines: one at the top of the yellow arc, and one near the bottom of the green arc.
The top redline, as it is in single-engine airplanes, is the VNE—never exceed airspeed—the airframe’s speed limit. The bottom redline is there because, in the words of my favorite crusty old flight instructor: “If an airplane has two engines, it’s because it doesn’t fly very well with one.”
Right. So much for the safety of two engines.
Don’t get me wrong. Night over the Rockies? Dawn over a fogbank? Anytime over piranha-infested water? Give me the spare engine, please. But just know that there’s a dark side to that twin-engine redundancy that you need to master as the pilot in command of a multiengine airplane.
The lower redline marks what is called VMC—the minimum controllable airspeed in an engine-out emergency. You’ll learn the full emergency recovery details during your multiengine training but keeping it simple, in a conventionally designed twin, if one engine fails, the aircraft rolls and yaws toward the failed engine. Now, you will need to counter the roll with aileron and counter the yaw with rudder. As you’ve got a ton of drag on one side and a ton of thrust on the other side of the aircraft, you’ll need to compensate by using rudder. A lot of rudder. But if you get too slow, you…well…run out of rudder. There simply isn’t enough to prevent what is called a VMC rollover—a loss of control that is invariably fatal.
I know what you are thinking: If pilots train for it, and if it’s marked on the airspeed indicator, how can anyone get in trouble with this? I think there are three reasons.
The first is that, although painted in red, VMC is not carved in stone. Minimum controllable airspeed varies with engine power, altitude, temperature, propeller position, center of gravity, bank angle, and more. In some scenarios, VMC can be almost 20 knots higher than the lower redline. Secondly, the loss of control happens wickedly fast compared to how it is practiced in airborne training; so, get some good simulator time. And thirdly, just as with stall training, pilots who have practiced VMC demos with their instructor before the checkride may lack proficiency when an engine-out emergency happens after they get their multiengine rating.
So, for safety’s sake, ask your flight instructor to periodically work with you on practicing stalls and how to control engine-out scenarios in your twin-engine airplane.
