To Check or Not to Check?

flight controllability check
By Ben Filippini
Whenever I teach Upset Prevention and Recovery Training, I make it a point to ask if the students have ever heard of a controllability check. While you would hopefully never have to use it, this may be one of the most important procedures to know. So what is a controllability check, and when would one ever use it?
Let us imagine you are on initial descent into your scheduled destination field. Passing through around 8,000 feet, a large flock of birds comes into view in the windscreen. Unfortunately, despite your best efforts to avoid the birds, you hear the dreadful thud of several hitting portions of the aircraft. Immediately, you level off to try to assess the situation. The plane is controllable and both engines seem to be functioning appropriately.
You inform ATC that you have just experienced a series of birdstrikes and are declaring an emergency. ATC asks for your intentions. At this point you start to weigh your options. The aircraft seems to be handling well, so you don’t have to land immediately. You have plenty of gas and will surely make the airport with fuel to spare. However, just because the airplane is flying well at the moment does not necessarily mean it will continue to do so as you go through the landing process. What if by decreasing speed the aircraft goes below its controllable airspeed (Vmc)?
It is into this type of a situation where you should consider performing a controllability check.
A flight controllability check is, in essence, a process by which you take an aircraft that has sustained suspected structural damage and configure it for landing slowly and methodically. It starts by initially flying to a safe altitude where recovery is likely should the aircraft depart controlled flight. Generally, a good altitude is 10,000 feet. Also, make sure you inform ATC of your actions, perhaps even requesting a block of protected airspace.
Once established at a safe altitude, you will slowly reduce speed and begin configuring for landing step by step. With each change in configuration you must remain poised to revert back if the plane behaves poorly. Likewise, you should fly for a few seconds after each change to ensure continued controllability. When comfortable, you will proceed to further configure the aircraft, vigilant should the plane begin to depart.
In the end, you should continue like this until you are at an appropriate landing configuration and speed. If you are able to remain in control at this point, you can generally assume that the aircraft will remain stable during actual landing.
However, what happens if during your methodical changes to landing configuration the airplane begins to behave poorly or even departs controlled flight? Well, first you must recover the aircraft as expeditiously as possible. This is why we do controllability checks at safe altitudes. Also, whether it was extending gear or increasing flaps, you must reverse the last thing you did that caused the aircraft to depart flight. Once you have reversed the last step of configuring and are under control again, you must not attempt that configuration again.
Upon reaching your most-configured-but-under-control state, the procedure requires that you remain in said configuration throughout the remainder of the approach and landing. While it may be costlier on gas, you have suffered a structural issue. Even though you were able to configure for landing, can you be so sure you will be able to do so again? Just fly the entire approach and landing in this configuration. Let ATC know, though, as it will certainly require them to modify sequencing due to your slower speed.
Hopefully this is a skill that you will never have to use in your career, but it is better to understand what to do if you are ever faced with the prospect of a structurally damaged aircraft.