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Southport, Manitoba, Canada
Steve Pomroy is a professional flight instructor and aviation writer. He has been teaching since 1995 and holds an Airline Transport Pilot License, Class 1 Instructor and Aerobatic Instructor Ratings, military QFI, and an undergraduate degree in Mechanical Engineering. He's written and published three flight training books through his company, SkyWriters Publishing, and has several other books under development. Steve currently teaches RCAF pilot candidates on their Primary Flight Training course.

Thursday, September 23, 2010

Va: Not a Bad Speed, Just Misunderstood (Part 1)

Maneuvering speed, denoted as ‘Va’ in V-speed notation, is probably the most poorly understood speed in all of aviation. As the speed is associated with structural limitations, one might anticipate that not understanding it could lead to trouble. Sure enough, there have been fatal accidents in which the pilot’s incomplete (or incorrect) understanding (or application) of Va has been identified as a contributing factor.

One notable example, which has received plenty of media attention: On November 12th, 2001, American Airlines Flight 587 crashed after takeoff from JFK Airport in New York. The accident was caused by an encounter with wake turbulence from a preceding aircraft while flying at 250 knots. In attempting to recover from the wake turbulence upset, the pilot-flying used multiple large rudder control inputs below Va, which was 270 knots. The vertical stabilizer was structurally overloaded and separated from the aircraft. The resulting flight characteristics led to a complete loss of control followed by ground impact, killing all 260 people on board and 5 people on the ground.

This crash resulted in lots of finger pointing: pilots, manufacturers, airlines, training companies, regulatory officials, all blaming each other. But the truth is that flight crews are not properly taught about maneuvering speed, and the resulting misdirection has not only been allowed, but promoted by regulatory authorities. This situation has improved slightly since AA587, but there is plenty more work to be done.

Common wisdom is that at speeds lower than Va, it is impossible to overstress the airframe with any control input. Less common, but still out there, is the belief that at speeds below Va, it is impossible to overstress the airframe with turbulence or a combination of turbulence and control inputs. Both of these beliefs are false.

So, what is Maneuvering Speed? Tracing the certification standards around can be tedious and time consuming, so let’s boil it all down to a single sentence:
"Maneuvering speed is the maximum speed at which a single full deflection of a single control will not result in structural failure of the control or it’s supporting structure."
That’s quite a mouthful, so take a minute to re-read it. The highlighted parts of the definition are where people get into trouble. Lets look at them in turn.

"A single full deflection" means that we aren’t oscillating the control back and forth. To illustrate how this might be a problem, let’s look at the rudder. Full deflection of the rudder will result in some known side load on the vertical tail. This load is known by the aircraft designer(s) and the structure is strong enough to withstand the load at speeds at or below Va. However, if we start with our rudder fully deflected to the left, the aircraft will be in a sideslip. The sideslip means that the vertical tail is at an angle of attack, and therefore, if we reverse the rudder deflection, it will produce more “lift” (sideload) than in coordinated flight—thus creating more stress on the rudder and tail structure. This effect is amplified if we oscillate the rudder, because the momentum of the rotating (yawing) aircraft will carry it momentarily to a higher slip angle than it can maintain. At this higher slip angle, if we suddenly reverse the rudder deflection, structural failure can result even though we are below Va. A similar effect applies to both the ailerons and elevator.

"A single control" means that the interactions of multiple control actions can result in structural problems even below Va. For example, on the Grob 120A,the utility maneuvering speed is 145 knots and the aerobatic maneuvering speed is 165 knots, however, simultaneous full deflection of the elevator and rudder is not permitted at speeds above 110 knots. This is because the maneuvering speed is based on single application of a single control. The combined lateral and vertical load of rudder and elevator together can lead to a structural failure somewhere in the tail. Similarly, applying elevator to increase the angle of attack of the wings while also applying aileron may result in overstressing the ailerons at large deflections—even below Va.

(TO BE CONTINUED ... )

1 comment:

Pat Flannigan said...

Va is a remarkably misunderstood speed, and you're right to highlight American Airlines Flight 587 as the poster child for this one.

I actually dug into this subject pretty deep and discovered a number of surprising facts, notably:
1. Maneuvering speed as we learn in Aerodynamics for Naval Aviators is not necessarily the same thing as Va.
2. Va - Design Manuevering Speed is legally defined as a very arbitrary number and is not required to offer any protection per Part 23 and Part 25 aircraft certification.

I wrote about it in depth on my site here: http://www.aviationchatter.com/2012/06/the-myth-of-maneuvering-speed/

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