CONTROL COLUMN Turbulence Penetration Speed

Structural failure is every pilot’s boogeyman. We know the possibility of a problem is exceedingly low, but the fear of having an emergency completely out of one’s control lends itself to some irrational fears.

Except for structural problems, which often are the result of a pilot’s actions. He or she may not have yanked hard on the yoke or overstressed the wings coming out of a dive, but considering that weather is a major factor in many accidents where structural failure occurred, it seems that pilots can play a key role.

Some suggest training is to blame. From early in private pilot training we are taught three important airspeed limits. Never exceed speed, or redline, is never to be crossed; max structural cruising speed, or the top of the green arc, is only to be exceeded in smooth air; and maneuvering speed is the turbulence limit. It’s impossible for a structural failure to occur so long as we are operating below maneuvering speed, or so the texts tell us.

Yet, that’s not necessarily the case. For severe turbulence scenarios, there’s evidence that using V_B, or turbulence penetration speed, is a better option. Douglas Boyd, writing a few years ago for IFR Magazine, uses the excellent example of the American Airlines Airbus A300 accident in New York in 2001, when the first officer’s excessive rudder input caused a structural failure, despite the fact that the aircraft was well below maneuvering speed.

The problem with using maneuvering speed as a limit is that by its nature, turbulence isn’t constant. While you or the autopilot may be able to largely maintain maneuvering speed, the gusts associated with the turbulence could quickly and easily exceed the limit, erasing any protective benefit.

Turbulence penetration speed takes these gusts into account and offers a lower target airspeed. The speed has long been known to operators of turbojet aircraft, as it’s required to be established for Part 25 aircraft. Being a Part 23 aircraft, the Twin Commander doesn’t have a published turbulence penetration speed. But Boyd and others offer some guidance on how to develop your own. Writing for Air Facts, O.C. Hope references the FAA’s design regulations to say that turbulence penetration speed must be low enough that the associated gusts don’t make the aircraft exceed maneuvering speed. It’s clearly impossible to know exactly what the gusts will be in a given scenario, but research has shown that even moderate turbulence can result in momentary gusts of 25 knots.

Hope suggests splitting the difference and flying a speed approximately halfway between stall speed and maneuvering speed. That would give the same margin above stall and below structural failure, which seems reasonable. By using the autopilot to maintain attitude, airspeed will be allowed to vary up and down.

While no one could know or study how many aircraft could have been saved in cases of structural failure by flying turbulence penetration speed, it’s a well-known paradox that maneuvering speed only works in smooth air, but is generally only used in moderate or higher turbulence. For those times, establish a turbulence penetration speed and give yourself a better margin for gusts.