The Risks of Maneuvering Speed Myths

Sure, we know what maneuvering speed is, we learned it in private pilot ground school. You know, Va-Design Maneuvering Speed. "This is the maximum speed at which the limit load can be imposed (either by gusts or full deflection of the control surfaces) without causing structural damage."

That's the definition straight out of the old Flight Training Handbook. Older versions of the Pilot's Handbook of Aeronautical Knowledge, another heavily-relied upon source of aviation information and a flight instructor favorite, defined Va as "The rough air speed and the maximum speed for abrupt maneuvers." Legions of pilots learned these definitions during their primary training.

We were comforted by the knowledge that if we just slowed down toVa, in rough air or during maneuvers, the wings would unload and stall before we bent or broke the airplane.

It turns out that we were rather badly mislead.

The issue at hand is not so much that Va has changed, but rather that some of the definitions we learned during primary ground school were (and are) purposely simplified in order to ease the learning process. Usually, these simplifications cause no harm. That is not so with maneuvering speed.

It took a tragic event to bring a significant training deficiency to the surface.

The in-flight breakup of American Airlines flight 587, in late 2001, was the event that taught a lot of pilots that what they had been told about maneuvering speed was terribly incomplete.

An Airbus 300 had just departed JFK when it encountered wake turbulence from a previously departing Boeing 747. Counteracting the wake turbulence, the Airbus First Officer used alternating full rudder inputs to control the airplane. These inputs caused the aerodynamic loads on the tail to be twice as high as the structure's load limit.

What was stunning to many pilots was that the tail broke off the Airbus when it was flying well below its Design Maneuvering Speed, a speed which at or below, according to many aircraft flight and training manuals, it is safe to use full and abrupt control travel without exceeding the aircraft's structural limits.

Surprisingly, to nearly every pilot, Va is not a speed which at or below the pilot is allowed full unrestricted control surface movement without the danger of structural damage or failure. Another big surprise is that it should never be considered a gust penetration speed. The old Flight Training Handbook only hinted about this little bit of very important information: "Regardless of speed held, there may be gusts that can produce loads which exceed the load limits."

The actual truth is significant and two-fold: Only when Va equals Vs times the square root of the load factor will the aircraft stall in a nose up pitching maneuver at or near its load limit factor. Moreover, any time the value of Va is greater than the value of VS times the square root of the load factor, as is often the case, the loads imposed by the maneuver or gust need to remain along a single rotational axis, otherwise the aircraft's load limits will be exceeded.

The reader may be saying to herself, "What the expletive? That's not what I learned." Unfortunately the cause of most misunderstandings pilots have concerning Va are the over simplifications taught during primary flight training.

A devil does exist in the details. Part 23 regulations do require an aircraft to have adequate strength for a full control deflection below VA, but the regulations do not require the aircraft design to withstand full control deflection in one direction followed by another full control deflection in the opposite direction, even when operating below Va.

Further, regulations don't require the aircraft to be designed to withstand the forces caused when two or more control surfaces are simultaneously moved to their stops. These types of control movements can place incredible asymmetric loads on the airframe known as rolling Gs.

Perhaps this new and better definition of Va, which can be found in a recent Special Airworthiness Information Bulletin, number CE-11- 17, can summarize the above. "The Design Maneuvering Speed (Va) is the speed below which you can move a single flight control, one time, to its full deflection, for one axis of airplane rotation only (pitch, roll or yaw), in smooth air, without risk of damage to the airplane." This definition is not even close to what so many pilots learned long ago.

First and foremost, it is a speed which an aircraft designer chooses and uses to calculate aircraft structural strength and to comply with regulatory requirements.

It is also a speed which many light aircraft manufacturer's list as a proxy for Turbulence Penetration Speed, Vb. However, this may not be the best advice, since we now know Va should not be considered a gust (nee turbulence) penetration speed.

Perhaps a speed 5 to 10 knots below Va or 1.7 times Vs would be a better substitute for Vb, should your aircraft operating manual not list a specific Turbulence Penetration Speed. This estimated speed should give a pilot enough of a margin over stall speed to be comfortable, and if a gust does cause a sudden stall, the wing, at these speeds, should quickly self recover.

Finally, we need to understand that Va is a variable speed which decreases as aircraft weight decreases. Most manuals list only one speed forVa, but buried in the fine print there will be a note stating the listed speed is only valid when the aircraft is at gross weight.

Calculating a new Design Maneuvering Speed, based on aircraft weight, is a matter of simple math: Va-New = Va √ (WNew/WMax-Gross). A prudent pilot should take a moment before flight to calculate his aircraft's Va speed at take-off weight and as fuel is burned off.

In IMC we don't purposely perform chandelles and lazy eights, two maneuvers if done incorrectly can place serious loads on an airframe.

However, we fly IFR where an inadvertent unusual attitude caused by an upset in turbulence, vertigo, disorientation or diverted attention, coupled with poor recovery technique can load up the Gs in a big hurry.

It is a safe wager to assert that most in-flight break ups are not caused by a serious gust, but rather by the pilot's improper control inputs during an attempt to recover from an upset. The key, which admittedly is difficult to accomplish when you are under pressure while experiencing a sudden upset in the clouds (or out), is to make smooth and deliberate single axis control surface changes until the recovery is complete.

The rolling pull out has long been demonstrated to be an effective way to overstress an airframe. Fortunately, most of us received good primary training for such an upset: Roll the wings level first, then center the ailerons and raise the nose while keeping the rudder centered. Rolling and pulling at the same time can be your last act as a pilot.

In summary, here are some important points which are easy to remember: Do not apply a full control surface deflection followed by an immediate full control deflection in the opposite direction, even when below VA.

Do not apply simultaneous multiple control surface inputs to their stops. If you are going to fly at the edge of the aircraft's velocity and load envelope, on purpose or not, it behooves you to dig a little deeper and go beyond the basic information you learned long ago.

Armand Vilches is a commercial pilot, instructor and FAASTeam member who lives in Brentwood, TN. He brings an extensive background in risk management and insurance to aviation and flight instruction.

This article originally appeared in the August 2012 issue of IFR Refresher magazine.