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The air safety reports from FlyDubai aircrew leaked to the Guardian contain some alarming stories about crew rostering, tiredness and duty times, but another one caught my eye, a description of an encounter with severe clear air turbulence on a flight from Moscow to Dubai (search for 511 VKO-DXB):

Aircraft was uncontrollable at time for 10-15 seconds every now and then. Total duration probably around one and half minute.

What does the pilot mean by uncontrollable? That the aircraft wasn't responding to commands and the crew were helpless passengers, at the mercy of the weather, and that everyone survived simply because it didn't go on long enough to tumble the plane out of the sky? Or that it was just being a bit difficult?

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    $\begingroup$ I means exactly what is written, the crew was not able to control the aircraft attitude, altitude and speed due to the turbulence. The aircraft was close to the maximum Mach number for operation (MMO). At cruise altitude (low air density), the difficulty is to remain in the small range of speed allowed (between MMO and stall) materialized in the speed indicator between the red and amber bands. $\endgroup$ – mins Aug 7 '16 at 22:03
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    $\begingroup$ If someone asks "What is meant by 'controllable' in this context?", "It means exactly what it says" is not really a good answer, unless you want to imply that the person who asked it was too stupid or too lazy to work it out for themselves. I'm not lazy or stupid, and it's not completely clear to me what a pilot might mean by those words in that kind of report. $\endgroup$ – Daniele Procida Aug 8 '16 at 11:58
  • $\begingroup$ You might be interested in the AIM section 7-1-22, which gives guidelines for reporting turbulence. The ones for severe turbulence say "Occupants are forced violently against seat belts or shoulder straps. Unsecured objects are tossed about. Food Service and walking are impossible." That might give you some idea about the actual experience, if that's what you're looking for. $\endgroup$ – Pondlife Aug 8 '16 at 15:29
  • $\begingroup$ @DanieleProcida: "[this] is not really a good answer". This is not an answer, but a constructive comment. I don't have any reason to suggest what you imagined, your question is legitimate to me. $\endgroup$ – mins Aug 8 '16 at 17:06
  • $\begingroup$ @Pondlife That is indeed interesting ("Eyeballs bounce so hard in their sockets that it causes bruising") but it's not an account of the experience that interests me here. I want to know what it means for the pilot to have lost control. If I lose control of my bicycle for 10-15 seconds, I'll be upside-down in a ditch. Or in my car, I'd expect to be giving my post-incident report from a hospital ward, or in front of a judge. In either case, loss of control implies loss of safety. $\endgroup$ – Daniele Procida Aug 8 '16 at 17:47
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"Uncontrollable" is a technical term (actually, in literature it's more often called "loss of control"). It means the aircraft is not responding to at least one normal command. Be it elevator or ailerons or rudder etc.

Really, really good AND lucky pilots can sometimes still control aircraft that's uncontrollable. Either by using another control surface to influence the control desired (eg. by using rudder to get out of a bank) or by using non-primary surfaces like trim tabs or flaps.

When no control input will influence what the aircraft is doing then it's called total loss of control (or totally uncontrollable).

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  • $\begingroup$ Note that even total loss of control is not in and of itself catastrophic. An airplane is normally balanced so that it'll fly straight and level in the absence of control inputs, so you may have several hours for troubleshooting before things become critical. $\endgroup$ – Mark Feb 12 at 4:11

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