Tuesday, December 7, 2010
Beware The Downburst
An insidious risk that can undercut airspeed and drive you into the ground
Contrary to the advice that aviation usually allows you to make most mistakes only once, I’ve been fortunate in 50 years of flying to make virtually all the bad mistakes, in some cases more than once. That’s supposed to be a no-no in aviation, but apparently, the gods of the sky haven’t chosen to strike me down yet.
One lesson I hadn’t learned by example until a few years ago was taught to me on approach to the Albuquerque Sunport in New Mexico. I was delivering a Piper Cheyenne II from Brussels, Belgium, to Tucson, Ariz., for Belgian Formula One driver Thierry Boutsen. A member of the Williams racing team, Boutsen had recorded a successful year and was trading his Cheyenne for a Learjet. He was flying into Tucson by airline to train in and pick up the new Lear, and I had been hired to deliver his Cheyenne trade-in across the North Atlantic to Arizona.
Albuquerque was my last fuel stop. The weather was more than 10 miles’ visibility with high, scattered cumulus clouds, and I was cleared number two for a straight-in visual approach to runway 26 behind a Jet Commander. As I flew through Tijeras Canyon to the east, I spotted the Commander directly ahead of me on the approach. I casually watched him descend toward the runway from my position two miles in trail.
Suddenly, I saw the airplane begin to drop toward the sagebrush, descending away from the security of the glideslope. As the jet approached the threshold of the Sunport’s huge 13,800-foot runway, it looked, from my vantage point, as if he was certain to land short.
He did exactly that. The Commander slammed into the ground 50 yards short of the asphalt, spraying a huge cloud of dust behind him, then took a giant bounce up onto the runway. From where I sat, it looked as if the jet rolled out normally and turned left off the active toward Cutter Aviation.
Forewarned, I added power to adjust my glide to a higher angle and planned touchdown several thousand feet down the runway. At about the same time, the tower announced, “Attention, all aircraft. Wind shear alert, threshold of runway 26.” My Cheyenne experienced the same tendency to climb, followed a minute or so later by a vicious downdraft, but I was able to catch the descent with power and land without incident.
Later, I saw the pilot out checking his aircraft and asked about the experience. He said a reported 12-knot headwind had suddenly turned to a strong tailwind, and he lost about 30 knots in five seconds, forcing him to pitch the nose down to avoid a stall. Even full power couldn’t fully arrest the Commander’s fall. What the Jet Commander pilot had flown through was wind shear, probably precipitated by a downburst, and he was lucky to have walked away without injury or damage to his aircraft. Not everyone is so fortunate.
Many pilots have been caught in downdrafts of varying intensity, and the phenomenon is definitely not fun. Downbursts are vertical air currents that are even more dangerous as they operate on a much larger scale. They flow directly toward the earth and, because they’re often violent and unpredictable, they can catch an unwary pilot by surprise.
Page 1 of 3