Did We Just See A VMC Rollover?


I’m up early every morning and my normal sweep of the overnight aviation news turned up that stunning video of the ATR crash in Taipei. Even in my pre-caffeinated state, the video practically lifted me out of my chair. My brain couldn’t quite parse what my eyes were seeing.

And now, what I think all of us were seeing was a classic VMC rollover of the sort that used to be the scourge of multi-engine training. On first viewing, I thought the airplane just got into a ordinary stall and one wing dropped-the left-provoking what might have been an incipient spin. But later in the day, a report surfaced that the pilot reported a flameout and if you scrub the video carefully, you can almost detect less motion in the left propeller than the right. Given the likelihood of rolling shutter from the camera, I wouldn’t take that observation to the bank; the FDR will shed light on it. But it sure looks plausible.

In case you haven’t done any multi-engine training, a VMC rollover occurs when one engine fails and the airplane is slowed first to minimum controllable airspeed and then a little below. Air flow over the wing from the working engine produces more lift and there’s a natural yawing tendency toward the bad engine. This results in a potentially vigorous roll toward the dead engine. In some twins, it’s leisurely, but in others, it’s scary. VMC characteristics change with CG, so the phenomenon is hardly predictable. From what I can tell, the left engine is critical in the ATR 72 and if that’s the one that failed, yaw forces from the right engine would be that much greater because the clockwise turning propeller exerts more yawing moment on its downward rotation thanks to having a longer lever arm from the center of thrust.We’ll see what the investigation reveals.

Back in the heyday of multi-engine training, VMC rollovers were more common in training because instructors tended to shut down engines with the mixture, rather than setting them to zero thrust as they learned to do in the 1970s. VMC rolls were never a huge percentage of the multi-engine accident picture, but they were particularly grating because in addition to being uniformly fatal, they were also uniformly avoidable, at least in training.

I never saw one in the multi-engine training I did–other than the required demo–and I sure as hell never expected to see a large turboprop airliner do one, if indeed that’s what the footage revealed. But given the fact that the world is now populated by so many always-running video cameras, maybe it was just a matter of time.

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