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I recently watched an Air Crash Investigation episode where Qantas flight 32 lost an engine shortly after takeoff. It was revealed that a stud pipe in a Rolls-Royce engine was made incorrectly, causing the pipe to snap and burst through the engine and the aircraft's wing, severing major hydraulic systems, and causing the ECAM to display pages and pages of errors.

On the final approach, the captain announced he wished to perform a ‘control check’. He said (and this is approximate):

When the aircraft is damaged, you have to certify it is safe to land before you try to land.

I assume this is so that the captain can make the necessary preparations if he feels his aircraft is not safe to land. On flight 32 this was done by moving the stick left and right to simulate lining up with the runway.


Perhaps a better question is back to the post's title.

What are the purposes of in-flight Control Checks?

In my example of Qantas Flight 32, the stick was moved left and right on final approach to simulate lining up with the runway. This was only small left and right control deflections. The flight deck had only minutes until landing. What was the captain expecting to see, or not to see? Can the captain after that brief manoeuvre be aware of his aircraft enough to make a decision?

Does this mean then that a Control Check only comes, or should only be done, if you are worried that the aircraft cannot land? Can it be done at any other time? In other words: is an in-flight control check only done for landing?


SOURCES:

Wikipedia Flight 32: https://en.m.wikipedia.org/wiki/Qantas_Flight_32

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    $\begingroup$ We do a flight control check at least twice before every takeoff. $\endgroup$ – J Walters Dec 17 '17 at 19:02
  • $\begingroup$ @mins in my example. The joystick was moved left and right to simulate lining up with the runway. The captain wouldn’t have been able to tell all of your examples in such a short space of time would he? $\endgroup$ – cmp Dec 17 '17 at 19:20
  • $\begingroup$ @cmp You can tell if the airplane is moving pretty easily as you can see what changes out of the cockpit, plus there are plenty of instruments that you can look at to see which way the airplane is moving. $\endgroup$ – SMS von der Tann Dec 17 '17 at 19:23
  • $\begingroup$ When you know the hydraulic system is compromised being able to land the plane safely or not isn't the first thing you should worry about. The pilot was probably worrying if he/she can still control the plane at all. In those very sad situations, when the pilot knows for a fact the plane could not land safely, evaluating the capability of controls is still useful because the pilot could use the remaining controls to "dispose" the plane in a way that minimizes collateral damage. $\endgroup$ – user3528438 Dec 17 '17 at 19:39
  • $\begingroup$ Fundamentally the job of the pilot is to control the plane, so for obvious reasons he needs to have constant awareness over the status of his control authority. $\endgroup$ – user3528438 Dec 17 '17 at 19:46
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Technically speaking, a control check is part of any pre-flight.

In the case of Qantas 32, one engine had exploded, punching holes in the wing and setting off a host of warnings, so there was a reasonable expectation that some of the controls might also have been damaged.

What the pilot did was put the plane through some basic maneuvers to see how the controls responded in their now potentially damaged form. And found that roll rate had been affected, something he factored in on the subsequent landing. He also found that pitch control had not been affected, which meant he could make a reasonably conventional approach to the runway.

Better to find out how much the controls had been affected when there is time and altitude to recover from an unexpected response, than to find out on final approach, where the margin for recovery is very slim to none at all.

During flight, this is not normal. It is prudent if an inflight incident may have damaged some of the controls, so that the pilot knows the level of controllability they have to work with.

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Why controllability checks matter

The procedure you're referring to from the QF32 mishap is called a "controllability check". While a standard control check is done as part of a ground preflight to make sure that the flight controls work well enough to fly, a controllability check is an in-flight operation, performed either as part of a functional check flight or, more importantly, after the aircraft has sustained damage.

The controllability check is different from an ordinary control check in that while a control check is looking only for flight control motion, range of motion, binding, or free-play, a controllability check evaluates the airframe's response to control inputs in-flight at various speeds, typically from a stable clean speed at 10,000' down to a normal landing Vref if all is functional. Flap and gear extension will also be tested if possible as part of this exercise due to their impact on aerodynamics. Of course, if the controls start getting too close to full deflection during what basically is a mock landing approach, the pilots will knock it off and accelerate back to a safer speed, cleaning up the airplane if possible.

As a result of this, the pilots could determine what impact the damage had on airplane handling. For instance, damage to a wing or wing control surfaces could have an impact in roll, while damaged or missing engines can be problematic at low speeds in roll and/or yaw. Tail damage will often manifest itself as pitch control issues, but sometimes can be a yaw problem instead. A load shift or other C.G. issue will also show up as pitch control trouble at low speeds.

A good example of a flight where one should have been performed was El Al 1862. Had they performed a controllability check, they'd have noticed that the departure of engines #3 and #4 from the aircraft, as well as the loss of the right outboard aileron and right leading edge devices, had effectively jacked their Vmca up sky-high. With this knowledge in hand, the crew could have recalibrated their expectations, shooting a partial flap approach at a much higher Vref speed to a choice of runway that gave them more favorable winds as they could still maintain flight at this higher speed and would need the headwind to help slow down after landing.

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Aircraft control their flight using movable portions of the wing and tail called control surfaces; a control check is a check performed to make sure that they're capable of moving through the range of motion that they're expected to be capable of moving through. In the case of small airplanes, they're connected to the control column directly through the use of small cables, but in large airplanes, they use electrical sensors in the avionics to create electrical signals that then control hydraulics in the wings and tail that move the control surfaces.

In the case of the Qantas plane, the damaged engine might have damaged the control surfaces or the fluid lines that power the hydraulics, so they carried out the check to figure out how much functionality they had so that they could get an idea of how damaged the plane was, and how it would control as they came in for an emergency landing.

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