Plane & Pilot
Tuesday, March 11, 2014

The Season Of Turbulence

The summer heat is around the corner, and so is the cobblestone sky

The flow characteristics of air and water are similar. Just as water in a stream flows over and around rocks, air manifests the same path, transitioning over rough terrain. Air can deflect horizontally and vertically, causing moderate to severe chop.

Pretty obviously, a mountain range is the worst possible manifestation of rugged terrain. In summer, when convection compounds the problem, the turbulence can be truly fierce. Consider yourself lucky if you only fly in the flatlands during summer.

For those who live in the mountains, there may be no way to avoid the rough stuff. You may be able to dodge the worst of it by staying as far as possible from ridgelines, however. If you must choose between the upwind (windward) or downwind (lee) side of a valley, opt for the upwind side. This is the side farthest from the direction of wind travel and should generate more updrafts than downdrafts.

In any area of moderate to severe up- or downdrafts, don't try to fight major displacements by maintaining altitude, even if you're filed IFR. Keep the wings level, avoid any significant control inputs and go with the flow. If the up- and downdrafts become severe, you may actually exacerbate damage to the aircraft by fighting to maintain altitude. If you're flying IFR, you'll need to advise ATC of your situation.

One tool that can be extremely valuable against turbulence is the radio. Don't be hesitant about jumping on the flight watch frequency (122.0) and asking for any and all pilot reports that pertain to your route of flight. Accept them with a grain of salt, however. Consider the time frame and the type of aircraft involved, and also remember that pilots tend to automatically exaggerate any turbulence they encounter. If there's no talk on 122.0, try 123.45. Many pilots use that as an air-to-air frequency, and you might learn something about what's ahead.

The FAA has laid out reasonable parameters for defining the various levels of turbulence, and if everyone subscribed to those definitions in reporting flight conditions, the job of analyzing the air ahead would be a lot easier. "Light" turbulence is a fairly gentle condition where roll disturbance rarely exceeds five degrees of bank and control corrections take effect immediately. "Moderate" can generate 20-30 degrees of bank, allows some lag between control application and response, and can move things around in the cockpit. "Severe" means the airplane often is unresponsive to control corrections, bank angles may exceed 50 degrees, and airspeed may shear by as much as 25 knots. With any luck at all, you'll never encounter severe.

As the name suggests, "extreme" is extremely rare. The airplane may be essentially out of control, you may experience roll moments past vertical and aileron, and elevator controls may be virtually ineffective. In 15,000 hours of flying all over the world, I've never seen anything worse than moderate. Sadly, it does happen, however. A few years back, we lost Scotty Crossfield, the famous North American X-15 test pilot, when his Cessna 210A apparently disintegrated in a thunderstorm over northern Georgia.


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