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"Vmc Upset"

Twin Training in the Extreme

Please say hello to "VELOCITY, minimum controllable"; Vmc, for short. Get to know what it means. If there's only one V-speed number you can remember when you fly a twin, make it Vmc. In a PA-30 Twin Comanche, always remember the number is 90 mph... and its the left engine that's critical. For a PA-39, or a PA-30 with counter rotating engines or a Robertson STOL kit, the number is 80. For Robertson PA-39 pilots, the number gets down to 75.


Why is this number so important? Because that's the lowest speed you can control the airplane if one of the engines loses power.  At best, Vmc is defined as "minimum control speed with one engine inoperative and maximum power on the other engine" At worst, get below that minimum speed and you'll have your world twisted upside down.


Being Introduced to Vmc

I first learned about Vmc before I started flying a Twin Comanche. It was in a Cessna 320.  I was an observer riding shotgun in the right rear seat while a very sharp FAA pilot demonstrated some techniques he'd learned at training headquarters in Oklahoma City. Note, I said sharp; not smart. This was back in the days when Vmc was demonstrated at low altitudes so the best simulation would be in dense air with the good engine producing maximum power. Unfortunately, some of the demonstrations went beyond simulation and resulted in an actual loss of control at an altitude too low for recovery. Since the Twin Comanche was a favorite for multi-engine training during this grim period, it had more than its share of accidents. It was a reputation that did not change until 1969 when a higher Vmc was mandated and low level training was done away with.


But, back to the Cessna 320. The FAA pilot had borrowed the airplane from a "willing" owner, and the demonstration started with a full bore climb right up to 20,000 feet... to show what those turbocharged Continental engines could do, without much consideration for their longevity.  After a few maneuvers on the way back down we landed at Aurora (OR) where our pilot-teacher would show us what the FAA had come up with as the latest and greatest method for giving Vmc instruction.


The 320, we were told, had a Vmc of 88 mph.  I remember the number vividly.  Field elevation was 200 feet and temperature was close to standard so we would get almost maximum power for takeoff.  Ideal for the "FAA-style" Vmc demonstration. We started our takeoff roll on runway 35, 5,000 feet long. We lifted off; the airspeed rose to 88 mph and the needle stayed there as the pilot held us at Vmc. Then, at about 400 feet, he cut power to the left engine.  I didn't even have time to utter, "Oh, s***" as we snapped to knife-edge flight with the left wing pointing straight down.  Now, this FAA pilot, as I said, was a very sharp pilot. Lucky for the three of us in the airplane, for he immediately chopped power on the right engine, let the wings roll back level as he lowered the nose and added power on both engines, skimming the overrun area as we recovered climb speed.  It was a startling, dramatic, indelibly impressive demonstration of Vmc and what can go terribly wrong in a twin.


That ended the Vmc demonstration.  We returned to base, and nothing more was said.  We all knew how close we'd come to making the evening news in a mess of crumpled aluminum off the departure end of runway 35.  And, we all now had a very graphic understanding of what Vmc was all about. Its very simple. At or below Vmc the airplane is not controllable with only one engine producing power. It will roll on its back and there's almost nothing you can do to stop it.


Vmc Reality

Vmc is a number not to be dealt with lightly.  Your actual Vmc speed is not necessarily what the book says.  It can be affected by a number of factors: gross weight; center of gravity; gear position, up or down; flap setting; propeller pitch; power setting; degree of bank; air temperature and density; and turbulent air. Changes in airframe configuration with items such as gap seals, stall strips, vortex generators, speed mods, tip tanks, etc. can also affect Vmc. In the 320, we must have been slightly below its actual Vmc speed when power on the left engine was chopped. Thankfully, the 320 with its tip tanks has a lumbering roll rate and we only rotated 90 degrees before quick action by the pilot stopped the roll and got us back upright. God bless his exceptional reaction time! But, no points for brains.


Second Meeting... Vmc in a Twin Comanche

About three years later, still with more than my share of respect for Vmc, I started multi-engine training... in a Twin Comanche. I'd had quite a bit of single engine Comanche time so my training was condensed, and concentrated mostly on multi-engine aspects, emergencies, and single engine procedures. By the second lesson, we were doing touch and goes, single engine maneuvering, balked landings, and emergency procedures.  The third lesson included stalls and single engine emergencies. Up to this time, we'd flirted with Vmc and now it was time to get serious.


You twin pilots know the drill: Go to 4,000 feet or higher above the terrain, in an acceptable maneuvering area and clear for traffic. Back off on left engine power to idle thrust and  feed full power to the right engine.  Slow to Vmc... very slowly. Start bringing in right rudder to hold the airplane straight as the speed bleeds off, and cock the right wing down about 5 degrees.  The ball will be about half a width off center toward the good engine if you're doing it right.  The idea is to recognize Vmc; not to experience it.  The procedure can be likened to stall training where you slow the airplane to just above stall speed and as the airplane begins to shudder, relax a little on the yolk; you've recognized the stall. Then, let the nose fall through as you pick up speed or add power to recover without ever actually stalling the airplane.


My first approach to Vmc in the Twin Comanche was very subtle. Airspeed bled off very slowly, the yoke was held back to slow the airplane.  Rudder pressure was increased till the rudder was hard over. At Vmc the airplane began to turn into the inoperative engine.  I was out of rudder and could not keep the airplane from turning. Very good. I had recognized Vmc.  My instructor was satisfied I'd had the experience.  I relaxed on the yoke; airspeed picked up and I could now control direction again. Just to make sure I had the idea, I decided to try it again. Only this time, I didn't have the patience to go slowly toward Vmc.  I knew what it took to get there so why not speed up the process a little?  Big mistake!


Vmc Snap Roll

I pulled back sharply on the wheel.  The power had not been touched. We slowed very quickly but the speed didn't stop at Vmc. We went through it suddenly.  Very suddenly!  The airplane snapped violently to the left and we were upside down in an instant. I reacted instinctively, pulling power, whipping ailerons, and stomping rudder all at the same time. I stopped the roll, but now we were on our back, nose down, headed for the ground, 4,000 feet below.


Don't ask me why, but the natural thing to do seemed to be to continue down in a split-S maneuver for recovery. I'd had enough rolling to last me awhile, but I suppose an alternative recovery would have been to roll on over and zoom out the bottom.  It just seemed important at the time to stop the roll to keep from spinning.  We lost about 2,000 feet, maybe more, in that half loop before we were level again.


My instructor was exceptionally cool.  He didn't have to lecture me about the lesson I'd learned. He knew I'd learned more about Vmc than I ever wanted to know... and that it was a lesson I'd never forget. He was right.  Flying with me in a twin since then has been almost boring as I recite the numbers from my check list and never get near Vmc until I'm over the runway threshold on landing, slowing for the flare, just before my tires touch the runway.


No Third Meeting... Please!

What have I learned from these Vmc encounters? First, I've learned not to mess around with Vmc. Second, if I want to see Vmc, I don't slow to book Vmc then cut an engine; I always reduce power first then slow the airplane. Third, actual Vmc may not be what the placard says; airframe configuration, accessories, air density, and loading will make a difference.  Fourth, unless training, I stay as far away from Vmc as I can.  At all times! Takeoffs and landings excepted.


If you should ever happen to stray below Vmc inadvertently, remember at all costs, you cannot control the airplane if there is power on only one engine.  Asymmetric thrust will roll the airplane upside down. If it starts to roll, or even hints at it, you must STOP THE ROLL!  You must CUT POWER TO THE GOOD ENGINE! 




Only then can you control the airplane.


Ancient Proverb

True happiness... and long life... means being in control.


Advice for Staying In Control

In your PA-30, always put the best parts in your left engine.


Glenn Plymate

Ex-ICS   2658


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