MANEUVERING SPEED....WHAT IS IT REALLY ? This weeks lets rethink this lesson.....maneuvering speed, commonly known as.....dont break the plane speed or....sometimes as turbulent air penetration speed . This is something of a point with me because so many were not correctly taught what this number is, how to find it and its importance. I was one of those. As I transitioned to jet aircraft, I was educated a little better, but the information applies to just about any aircraft.Try the formula included with some real world numbers and you may find that it is different from what you thought you knew. If you want to see an example, skip to the longer paragraph towards the end of this post. 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. Thats the definition straight out of the old Flight Training Handbook. Older versions of the Pilots 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 to Va, 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 structures 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 aircrafts 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 Vs 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 aircrafts load limits will be exceeded. Unfortunately the cause of most misunderstandings pilots have concerning Va are the over simplifications taught during primary flight training. Part 23 Regulations - the details ? 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 dont 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. Ok then....what is Va good for ? 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 manufacturers 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 for Va, 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. The formula for finding this new Va is : New Va = Old Va * Square Root ( current weight/ max gross weight ). Lets look a a typical Cherokee 140 on a training flight with fuel to the tabs. plugging in the numbers our formula should look something like this. X = 129 * square root (1830/2150). The result for X gives us 119 mph, a full 10 mph less than the speed at max gross weight. In other words, if you make an abrupt maneuver at 125 mph, you may be below the max gross weight maneuvering speed (129), but above the current gross weight maneuvering speed (119). This could result in damage to the aircraft. Since there is no published number for turbulent air penetration speed for a Cherokee 140, we can use 5-10 knots below our New Va number if we fly into some really rough air. Just a hint. slow down at the first sign of pending turbulence, if it doesnt get rough, you can always speed up. A speed between 114 to 109 would be a number to shoot for in this event. Remember, you are slowing down because you encountered rough air. The airspeed indicator will bounce around so dont worry about hitting exactly 111.5 ! As the airspeed indicator bounces around between 109 and 114 you can rest assured that you will not break anything and you are at a speed that a loss of control stall is not likely. Of further note, maneuvers like steep turns in the 140 show an entry speed of 129 ( the old maneuvering speed ). Perhaps 110-112 is a better speed ? Interestingly enough, a power setting of 60 % is listed as instructional cruise power setting for the 140 and yields a resulting true airspeed of 110 mph. Hmmmmm. Im sure that number was picked for fuel savings as well, but we can see that tooling along at 60 % and suddenly hitting a huge bump or making a quick maneuver to avoid that huge California Condor should not result in any damage. Maybe when they wrote the manual, they just didnt want to explain why they picked that power setting ! For those of you with electronic flight instruments, trying to hit exact numbers when it is bumpy can drive you crazy and if nothing else will lead to fixating on the airspeed. Dont for get that scan. A prudent pilot should take a moment before flight to calculate his aircrafts Va speed at take-off weight and as fuel is burned off. Fly safe !
Posted on: Tue, 02 Sep 2014 15:40:24 +0000
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