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12/12/17    Home | Articles | Training | Instructor's Corner | Airplanes | Travelogues | PIREPS | For CFIs | ATPs | Pilot for Hire


Your multi-engine rating
By Ryan Ferguson

(cont'd from Page 3)

Decisions, decisions

Redundant systems are a blessing in the event of an emergency. Lose an alternator or generator? You've got another one that can handle most of the load and get you home. Lose a vacuum pump? Since you've got two, you won't lose your vacuum-powered gyros when you're flying in the clouds. Lose an engine on the takeoff roll? You have a Pilot-In-Command decision to make, and you have very little time to consider your options.

"If you experience an engine failure on the takeoff roll or climbout, you're going to have to make some decisions. First, are you still on the ground? If so, your course of action is actually simple close the throttles and stop. It doesn't matter if there's a freight train headed toward you down the runway you have no choice but to stop on whatever remaining runway you have and take your lumps with the obstacles. Hoping to climb out is a fool's fallacy."
It's not hard to visualize the asymmetric thrust condition caused by an inoperative engine producing zero thrust on one wing, and an operative engine providing 100% takeoff power on the other. This difference of thrust is going to contribute to a significant yawing/rolling moment. Below a certain airspeed in your twin, this moment will overcome the ability of the control surfaces (rudder and aileron, respectively) to counteract the yaw and roll. You get three guesses as to when you're going to be slow, generating full power, and close the ground and the first two don't count.

This airspeed is marked on the ASI with a red line. It is called "Vmc" minimum control speed. It is defined as the minimum speed at which the aircraft can maintain its heading with the critical engine failed and prop windmilling (not feathered). It is determined under a very specific set of test conditions and is not actually a number that always applies insofar as its literal definition is concerned. On any given day (what with temperature changes, aircraft gross weight, CG, etc.) the 'true' Vmc will change, but the placarded speed (also a red line on the ASI) is always what one should use as Vmc for practical purposes. View it as the 'worst case scenario' airspeed if you're above redline, you should able to maintain control. Note: I specifically said 'maintain CONTROL'. I said nothing about being able to continue to climb out at that speed. In fact, factors that adversely affect climb performance often enhance Vmc performance. The two are frequently at odds with each other.

So if you experience an engine failure on the takeoff roll or climbout, you're going to have to make some decisions. First, are you still on the ground? If so, your course of action is actually simple close the throttles and stop. It doesn't matter if there's a freight train headed toward you down the runway you have no choice but to stop on whatever remaining runway you have and take your lumps with the obstacles. Hoping to climb out is a fool's fallacy. As many multi-engine pilots have said, "It's better to be on the ground right side up than upside down." This is precisely what would happen if the pilot elected to continue the takeoff below minimum control speed the asymmetric thrust would overcome the control inputs of the frantic pilot, and the aircraft would probably roll inverted before impacting the ground.

Second, on the climbout, are you below the speed at which the yaw/roll moment will be greater than any control force you can input to counteract it? If you are again, you will have no choice other than to close the throttles and land. Ideally, you will never rotate under Vmc. If you do, you sacrifice your multi-engine safety factor. It's quite likely that your twin's POH will strongly discourage the practice.

Finally above the minimum control speed described above AND blue-line (Vyse) usually approximately 100 MPH or thereabouts for most light twins you have enough power to continue your climbout in the event of an engine failure. All of this happens in the first few seconds of your flight which means takeoffs are always by the numbers in a twin. Watch a multi-engine pilot and you'll see his or her eyes constantly flicking to and from the airspeed indicator during the takeoff. Vmc and blue line are critical speeds, and decisions will be made based on the ASI when engine failures occur.

These decisions will have to be made very quickly and under extreme stress. That's why you'll deal with so much single engine flying, and so many simulated engine failures, in your training. Let's dig in. This is the good stuff.

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