It’s been more than nine years since the vertical fin of an Airbus A300 operated by American Airlines broke, causing the wide body jet to crash shortly after takeoff from JFK Airport. All aboard were killed.
The NTSB determined that rapid and repeated movement of the rudder pedals in both directions by the pilot overstressed the vertical fin, causing it to snap off. What alarmed most pilots is that the Airbus was flying slower than maneuvering speed (Va) when the failure occurred.
As far back as I can remember, we were told that our control inputs could not overload and break the airplane if it were flying slower than Va. The exact words out of the FAA-approved limitations sections of the pilot’s operating handbook for my Baron says, “Do not make full or abrupt control movements above this speed.”
The book doesn’t exactly say that “full and abrupt” control movements at slower than Va airspeed are harmless, but it implies that they are. In fact, generations of flight instructors have told pilots that flying slower than Va protects the airplane from overload either by the pilot or from the gust loads of turbulence. And dozens, even hundreds of magazine articles – yes, some written by me – have repeated the same.
And, in a clinical way, there is an element of truth in the belief that you can’t break the airframe when flying slower than Va. That kernel of fact applies only to the wing and probably the horizontal tail. The reason you probably can’t break the airplane by applying full elevator travel at an airspeed slower than Va is that the wing will pitch to the stalling angle of attack before structural design loads are exceeded. The stall would then unload the wing, protecting it from failure.
I say Va “probably” protects the wing structure from an abrupt elevator input because in a real-life situation, turbulence or unintended bank angle change could add unexpected loads.
The fact that flying slower than Va could protect an airframe from failure was never completely true, whether the loading came from the pilot moving the controls or from severe turbulence. But the concept made us as pilots feel good. If things got really rough we could slow down to Va and take comfort in the legend that turbulence couldn’t break the airplane because the wing would stall before failing. Or, if we wanted to throw the airplane around and show off a little, doing so at speeds slower than Va would make it okay even if the airplane were only certified in the normal category.
What the Airbus crash taught us—or at least should have taught us—is that Va certification standards, and certification flight test results, protect the airplane from only a single control input in only one direction at a time. Any combination of control inputs that rotate the airplane around more than a single axis creates loads for which Va does not necessarily consider or test.
Flying slower than Va also only protects the airframe from moving a flight control – elevator, ailerons, or rudder – to its full travel in a single direction, not from stop to stop. So certification calculations and flight testing show that moving the ailerons fully and abruptly full left at a speed slower than Va, for example, will not break the airplane. But if the ailerons are suddenly moved fully back to the right without the airplane stabilizing in a steady attitude, Va offers no guarantee.
These airframe structural loadings have been understood by airplane designers and certification authorities for many decades. That’s why an airplane can be certified in the aerobatic category but be restricted to certain maneuvers in which the loads are understood and have been tested. Only in unlimited aerobatics have all possible loads been considered and sufficient structural strength designed into the airframe to handle them.
Building an airframe with enough structural strength for normal flying – with an additional 50-percent strength margin between limit load and ultimate load – is a useful tradeoff between a low enough empty weight to provide a high useful load, and enough strength to handle powerful turbulence and normal maneuvering. Modern unlimited aerobatic airplanes, and the almost unbelievable combinations of maneuvers pilots put them through, are proof that it is possible to build an unbreakable airplane. But that works only if you want to carry one or, at most, two people, and not much fuel or baggage. Taking your family to Disney World in your Extra isn’t going to work.
The military did a good job of teaching its pilots to avoid combinations of rolling and pitching maneuvers. But the general aviation pilot was told Va is his insurance against breaking the airplane.
Finally, the FAA is convinced that many, even most, pilots do not understand the true significance of Va and its limits on control inputs and has just issued one of its Special Airworthiness Information Bulletins (SAIB) to try to explain what Va really means.
The SAIB is written pretty much in FAA speak, so allow me to translate: Even when flying at or slower than the correct Va airspeed adjusted for the actual weight of the airplane, there is no guarantee that the airframe won’t break if you make full or abrupt control movements in combination, or if you move a control abruptly and fully from stop to stop.
There aren’t many guarantees in flying, but the protection of flying slower than Va seemed to be one. It’s not.