Never Again Online: Gravity waves: A pilot's perspective

April 1, 2004

As pilots, we know quite a bit about many things. For instance, while we are not mechanics, we understand our aircraft's systems. We need not be air traffic controllers to navigate along complex routes. Similarly, we interpret weather without being meteorologists. Relying on official weather observers and forecasters, we add our experience to make important flight decisions.

I noticed no significant wind during my preflight inspection on that April afternoon. My weather briefing for a scheduled 7 p.m. departure from Burlington International Airport, in Vermont, to Clinton County Airport in Plattsburgh, New York, in our Piper Seneca yielded no significant weather. There was no frontal activity, storm cells, or precipitation along my route, and thus no concern about this night's flight. Once I was aboard, though, the new ATIS report included winds from 140 degrees at 20 knots gusting to 35 knots with an altimeter setting of 29.74. The remarks stated that the pressure was falling rapidly.

I copied my IFR clearance to Clinton County and was cleared to taxi to Runway 15. As I taxied, the controller informed me that the new altimeter setting was 29.67, remarking that he had never seen such a rapid pressure drop. The wind was unchanged and there were no other signs of weather unrest, so I continued as planned. The controller broke in again and said that the National Weather Service observer called this radical drop in pressure a gravity wave. I requested a definition, but he was not familiar with the term either.

We have all heard about thunderstorms, microbursts, and tornadoes. While most of us have not encountered one in flight, we have been trained to recognize the conditions favorable to their formation. We have learned through education to avoid flight in their vicinity. But the gravity wave phenomenon was not in my vocabulary.

I was cleared for takeoff: "Turn right heading to 190, climb to 2,600, altimeter 29.60!" On my takeoff roll, the controller interjected again: "Altimeter 29.65!" It was coming back up again rapidly. I became airborne rather quickly, retracted the gear, and set climb power and speed. The turbulence was as rough as I had ever encountered. My hand flailed wildly as I made the frequency change to Departure, and my eyeballs were rattling around as I tried to focus on my instruments.

On my first call to Departure, I told the controller I was climbing to 2,600 feet. I couldn't believe my bouncy eyes as my altimeter wound up through 3,600 feet. The vertical speed indicator was pegged up. Powering back to idle had no effect. ATC directed me to turn right direct to Plattsburgh. The landing-gear warning horn was blaring as I tugged the throttles back as far as possible, still climbing uncontrollably.

It was apparent to me I probably should not have launched in the first place. I requested vectors to return to Runway 15 instead of continuing to Clinton County. Once established on my new northwest heading, I was able to descend. The clouds parted, offering me a view of the airport 3,000 feet below. Planning for a high downwind leg for Runway 15 and cleared for the visual approach, I lowered the gear.

The wind was now 270 at 25 knots, and now I was in a healthy descent. "Wind is 290 at 25, suggest Runway 33," the tower controller said. Entering a left downwind for 33, my rate of descent became alarming. I was no longer at idle, but at full throttle as I turned base leg to attempt to bring the vertical speed indicator needle off the bottom peg. I relied on my GUMPS prelanding check; reading my instruments was hard enough, let alone reading a checklist! Thankfully my landing was uneventful.

In retrospect, I had all the signs I needed to delay my flight: a rapid pressure drop with a sharp wind increase. I was even informed of the weather phenomenon; I simply did not know what it was. I know now! As any wave contains vertical movement, so does a gravity wave. Unlike a typical mountain wave, which remains mostly stationary, a gravity wave can migrate. Remember that I encountered tremendous lift, then tremendous sink just a few moments later at the same location. The phenomenon also passed very quickly; a commuter carrier took off right after me with no complaints.

Gravity waves tend to propagate between layers of cold, stable air topped by a layer of warm air. If there are different wind speeds associated with each layer, turbulence and wind shear are likely. Now I know, a delay of only a few minutes would have prevented the encounter. Pausing to allow the yo-yo pressure and wind to stabilize would have meant an uneventful departure.

Gregory Bean, AOPA 834253, is a certified flight instructor with instrument, airplane, glider, single-engine, and multiengine ratings.

Additional information on gravity waves can be found at the following links:

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Posted Thursday, April 08, 2004 10:49:25 AM