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I was reading about the F-35 flight tests and it mentions a "departure" at a couple of places.

What does that word mean in this context? I understand spins and stalls but what's a "departure"?

AF-2 was the first F-35 to be flown to 9g+ and -3g, and to roll at design-load factor. Departure/spin resistance was also proven during high angle-of-attack (AOA) testing which eventually went as high as 110 deg. AOA

Pilots really like maneuverability, and the fact that the aircraft recovers so well from a departure allows us to say [to the designers of the flight control system laws], ‘you don’t have to clamp down so tight,

(Wikipedia Lockheed Martin F-35 Lightning II article)

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Departure means that the aircraft has departed from controlled flight. From FAA news:

A Loss of Control (LOC) accident involves an unintended departure of an aircraft from controlled flight

In this video about flight testing F-16s, the test pilot says 'intentionally departed from controlled flight' and later 'the aircraft is prone to departure if mishandled' and 'the aircraft departed in yaw'.

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    $\begingroup$ How exactly is "controlled flight" defined? e.g. Not all spins are loss of control, right? $\endgroup$
    – curious_cat
    Dec 15, 2015 at 15:51
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Expanding on the other answer...

A loss of controlled flight (departure) is what happens when the aircraft is moving in an uncontrolled manner (on any one or all axis). For example in the video that the other answer linked, the departure from controlled flight happens when the aircraft exceeds its operating envelope and begins uncommanded movement in any axis, in this case when the pilot yanks back on the stick and the aircraft yaws uncommanded (like a "boot full of rudder" as the person in the video comments).

This is why an aircraft can depart controlled flight in individual axis while retaining control in others. You can intentionally make an aircraft depart controlled flight (spin for example) and if the control surfaces still have enough authority, it can be recovered (altitude withstanding). You aren't really "in control" but you are coaxing the aircraft back into the flight envelope.

If you don't have enough control authority to get back into the flight envelope, or altitude, this is otherwise known as a crash... Some aircraft cannot be recovered from maneuvers like spins (Cirrus SR-20/22 for example, not certified to at least).

So, controlled flight is defined as providing control inputs without uncommanded movement on any axis (significant, for example just using ailerons may result in yaw). Any departure from controlled flight is uncontrolled flight, even if you regain control later.

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  • $\begingroup$ Very interesting, thanks. So if an acrobatic aircraft or military craft executes a spin is that still uncontrolled flight?i.e. Are there planes which can be in a spin & still be in controlled flight? $\endgroup$
    – curious_cat
    Dec 16, 2015 at 2:15
  • $\begingroup$ Isn't it mighty hard to deduce from a crash whether there was control authority or not enough to return to the envelope because that will also be a strong function of what sort of inputs, in what sequence & of what intensity / duration were provided to the control surfaces by the test pilot? At a time which I'm pretty sure is very stressful even for a test pilot? $\endgroup$
    – curious_cat
    Dec 16, 2015 at 2:17
  • $\begingroup$ I think you are extrapolating any crash to mean a departure, when typically I'd say a majority of crashes are "controlled". Either CFIT (controlled flight into terrain), fuel outages, flying into IMC, etc. But yes, its very hard to determine that, but typically test aircraft have a LOT of telemetry on them and the inputs can be recreated against mathematical models to see exactly where the aircraft was in the flight envelope, even during crashes. Usually test pilots are chosen because they have a high ability of doing the right thing in stressful situations. $\endgroup$
    – Ron Beyer
    Dec 16, 2015 at 2:28
  • $\begingroup$ And to answer your first question, yes I'd consider any aircraft, military or civilian that does a spin to be a departure from controlled flight, even if control is regained. Acrobatic aircraft depart flight into a spin in such a manner that its "cool to watch", but I don't think it degrades from it being a departure in the basic sense. Since a spin is where one wing is stalled more than the other, I'm not sure you can say that any aircraft is both in controlled flight and a spin. See this link for good reading on spins $\endgroup$
    – Ron Beyer
    Dec 16, 2015 at 2:32
  • $\begingroup$ Is there a possibility, of having the Control System Logic itself provide the right inputs to return a plane into the Flight Envelope after a departure is detected or indicated by the controlling pilot? i.e. Is it possible to algorithmize the stick inputs that are needed to return a flight into its envelope assuming control authority exists? Can a flight computer can apply these inputs faster & more reliably than a human pilot? $\endgroup$
    – curious_cat
    Dec 16, 2015 at 3:57

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