Air Pockets or Worse !

May 14, 2014 by Bruce Landsberg

wavesToo many airlines think we can’t handle the truth, and so the flight crew refers to “bumps” rather than turbulence. You’ll even hear media types occasionally mention “air pockets,” a long outdated terminology. For light GA, turbulence is often uncomfortable and occasionally dangerous.

The Air Safety Institute, working with the FAA and the NTSB, is highlighting turbulence this month as part of our 2014 weather education campaign. Turbulence comes in a variety of forms and can be merely aggravating or really bad news.

Convective turbulence from thunderstorms is probably the worst and disassembles aircraft about six times a year. It often takes the form of the pilot first losing control, and then the hardware starts to come apart after all speed limits are exceeded.

But sometimes the bounces are in the form of waves and rotors. If you’ve done much flying in any sort of mountainous or hilly terrain you’ve likely experienced the wave—sometimes visible with lenticular clouds but not always, especially when the air is dry.

My first exposures to the wave were perplexing at best. In cruise flight with the autopilot in “altitude hold,” the airspeed was going up and the A/P was trimming nose down. Pretty cool—free lunch? You know the deal on lunches, because a few miles down the road just the opposite happened as the nose went up and the airspeed just went to pot. Thinking a navigational error had placed me into the Bermuda Triangle I called ATC to ask for a block altitude. It was, of course, the smooth part of the down wave.

However, altitudes below the smoothness can be a dentist’s delight—rough enough to jar loose fillings. Airspeed control is the key, and on descent in those types of conditions one should be well acquainted with VAmaneuvering speed. VA is one of those counter-intuitive concepts. As the gross weight declines, the slower the maneuvering speed. The published number in the handbook or on the placard is appropriate at max gross. If you’re lighter than that—slow down.

Airspeed bumps around a good bit in turbulence, and if everything is stable “right at” VA, a gust can increase the speed by 10 knots in the blink of an eye. When flying an older aircraft there is no comfort knowing that if the bozo that flew it before you exceeded VA, and got into heavy turbulence, some of the margins built into the structure when new have been used up. How much? Good question. Slow down.

Pay attention to pireps and Center Weather Advisories—the loss of a modified Cessna P210 is instructional. Even moderate winds over big mountains deserve to be treated with great respect. Maybe there’s something to this air pocket thing after all.

Bruce Landsberg
Senior Safety Advisor, Air Safety Institute

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  • Mike Brown

    > VA is one of those counter-intuitive concepts. As the gross weight declines,
    > the slower the maneuvering speed. The published number in the handbook or
    > on the placard is appropriate at max gross. If you’re lighter than that
    > —slow down.

    Better yet, if your fillings are being shaken loose by turbulence, forget Va – slow down regardless!

  • Lawrence Stalla

    Bruce, your advice to slow to VA in turbulence is good but — for those of us who fly airplanes designed for cruising and not maneuverability — it may not be enough. There is another airspeed, called variously the “design speed for maximum gust intensity” and the “maximum turbulence penetration speed”, denoted VB and not usually published by manufacturers, that may be a better choice.

    Both VA and VB are defined in FAR part 23.335 as the product of the clean stall speed at current weight and the square-root of an induced load factor. For VA, this load factor is assumed to be induced by deflecting a flight control (usually the pitch control) to its limit (part 23.337). For VB, this load factor is assumed to be caused by turbulence-induced gusts (part 23.341; see also part 23.333(c) for the FAA’s definition of what a “gust” is). An airplane manufacturer can increase VA simply by making the control surfaces less effective (“turning an aerobat into a cruiser”). VB, however, remains a fundamental property of the airfoil, not affected by control surface size or deflection.

    On the evening of 1/30/2003, I dealt with continuous severe turbulence in my Mooney M20K on the VFR airway west of Denver at “about” 7,500′ by slowing past VA to VY, and countering large gust-induced pitch and bank angle deviations with only gentle control inputs. I made it home to KFLY, shaken and with the contents of the cabin strewn about, but alive and with no airframe damage.

  • Bruce La Fountain

    Nortriptyline is some pretty nasty stuff.

    Even in small doses, as used for pain, it will cause dizziness and mild hallucinations.
    Regular use for depression and flying an airplane is a certain death wish.

    Many pilots are known to hide prescription drug usage in their medical history due to the consequence of loosing their flying privileges.

    Add in some significant turbulence and the recipe is fatal.

  • Bruce Landsberg

    Lawrence…. I was aware of Vb but haven’t found much reference to it for CAR3 or other light GA . There is documentation in larger aircraft and it makes good sense to use there.

    Bruce… I think there is certainly some evidence that pilots are hiding drugs from the FAA. Our current system is counterproductive in the Catch 22 aspect of if you disclose, you’re grounded. It is also important for pilots to understand that there are some meds that are absolutely inappropriate to use when flying.

    We need to have an open discussion on that and the 3rd class medical reform project is working on that.

    Many thanks to both of you for commenting!