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Does Maneuvering Speed Really Protect Your Plane?

Boldmethod

It's pretty much impossible to explain aerodynamics without heavily simplifying it. Aerodynamics is a field for engineers, based on differential equations that don't have much use in the cockpit.

So, when someone says ground effect is a "cushion of air," or airflow speeds up across the top of a wing because the "molecules flowing across the top and bottom have to meet up at the trailing edge" - they're really not hurting anyone, right?

How about this: When you're flying at or below maneuvering speed, you'll "stall before you break." Sound familiar?

gc232

That's exactly how I explained maneuvering speed as a flight instructor. It was an easy way to explain it - and everyone could remember it. But it's a dirty lie. Or, at least a whitish-gray lie.

So - what does design maneuvering speed really do? To understand it, you need to consider how an airplane is certified. And, no maneuvering speed discussion is complete without covering why it changes with weight.

And, for the rest of this article, we'll call maneuvering speed "Va."

Va - Most of The Protection You Assumed, But Not All Of It

Prior to 2001, everyone assumed you could manhandle flight controls in any direction while flying at or below Va, and the aircraft would stall before the structure broke. Not that you wanted to fly that way, but - if you did - you'd be fine.

Chad Horwedel

It turns out that at or below Va, you'll stall before you break, but only if you:

Move a single flight control, in one direction only, in smooth air.

If it's turbulent, or if you roll and pitch and kick the rudder, all bets are off.

Too Many Stresses, All At The Same Time

You're great at handling stress, right? You can manage radio calls, a full pattern, and a gusty crosswind all without breaking a sweat. But, add in the fact that you're behind on your bills, your dog left you and your boss hates you, and things start to break down.

The same things happen to your plane while maneuvering. In a normal category airplane, your wings and horizontal stabilizer are tested to handle 3.8 Gs pulling up, and 1.52 Gs pulling down. But, what happens if you kick the rudder in full to add some horizontal loading?

The answer is - who knows? That's not tested. The combination of horizontal and vertical forces can cause extra stress that your structure wasn't designed to handle. And, in this case, it could fail before you hit your positive or negative G limits.

The same goes for rolling Gs. Lets say you pull back on the yoke abruptly until you hit 3.8 Gs, then shove forward to hit -1.52 Gs, and do that over and over again. If your stomach doesn't stop you, your airframe might. Those rolling Gs can cause structural failure, even though you never exceeded the G limits.

What Can You Do Below Va

Your airplane will stall before it breaks when you're flying at or below Va and you:

Move one control (elevator, rudder or aileron), in one direction, in smooth air.

To verify this, the manufacturer performs several tests at Va:

  • Suddenly moving the elevator back fully to the stop
  • Suddenly moving the elevator forward fully to the stop
  • Suddenly moving the ailerons fully to the right
  • Suddenly moving the ailerons fully to the left
  • Suddenly moving the rudder fully to the right
  • Suddenly moving the rudder fully to the left

(Actually, they're deflected to the full stop or the limit of a pilot's physical ability.)

I broke each test out in a separate point for a specific reason. They're not tested as "checked maneuvers," where the controls move forward and back in quick succession. Each of those movements is an independent test.

Manufacturers do certify one checked maneuver, and they perform the test at speeds above Va. The test pilot suddenly moves the elevator aft, and then moves it forward. But, the movements are carefully executed so that they don't exceed the aircraft's G limits. The test pilot also performs the maneuver using a limited amount of angular acceleration.

Manufacturers normally don't test checked maneuvers using the ailerons or rudder. (Those tests will be done if the aircraft's approved for "flick" maneuvers, like snap rolls.)

Finally, the manufacturer executes a full aileron movement while pulling 2/3 of the load limit. So, if you're flying a normal category aircraft with a positive limit of 3.8 Gs, they test that maneuver at 2.5 Gs. If you try that maneuver at the full 3.8 Gs, you just became a test pilot.

Va Changes With Weight

When the manufacturer certifies Va, they do it at maximum gross weight. But, as your weight decreases, Va also decreases. This confuses nearly everyone.

As you increase G loading, your angle of attack also increases. If you're flying at Va and you pull the yoke back to the stop, your angle of attack will increase and you'll reach the critical angle of attack right as you hit the G limit. In other words - you'll stall right before you break.

If you're flying at your certified Va speed, but you're below max gross weight, you'll fly at a lower angle of attack. So now, as you pull back and increase Gs, you'll hit your G limit before you reach the critical angle of attack. So, as you lose weight, your Va slows down - putting your 1 G angle of attack back in the safe range.

The Va Formula

Most aircraft flight manuals and operating handbooks have a chart to compute Va at various weights. If your handbook doesn't have a chart, you can use the following formula:

Aren't you glad you carry a smartphone? Try calculating that in your head.

Written In Blood

They say every FAR is written in blood. That's the case for Va's new stipulations, as well. The NTSB discovered the oversimplification when investigating the American Airlines Flight 587 accident.

In 2001, Flight 587 encountered wake turbulence during climb-out from JFK. The pilot moved the rudder pedals nearly full right, then full left, then full right, then full left, then full right in 6.5 seconds. The resulting stress sheared off the vertical fin, and the aircraft lost control. The entire event happened below design maneuvering speed.

Wikipedia

You can read the NTSB's findings here. And, you can read the resulting Special Airworthiness Bulletin (CE-11-17) here.

The Takeaway

Design maneuvering speed provides lots of protection. But even if you're flying below it - there's no excuse for manhandling the controls. Flying with smoothness and accuracy is always your best bet.

Aleks Udris

Aleks is a Boldmethod co-founder and technical director. He's worked in safety and operations in the airline industry, and was a flight instructor and course manager for the University of North Dakota. You can reach him at aleks@boldmethod.com.

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