Beware The Downburst

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.

That's what happened to Delta Airlines flight 191, a Lockheed L-1011, approaching DFW airport in August, 1985. The captain was anything but unwary. A 14,000-hour professional, he was well respected as one of Delta's best, and his actions during the flight suggest he knew exactly what was happening. That accident was associated with thunderstorms in the area, but it presented a classic case of the dangers of a microburst, a phenomenon not always associated strictly with thunderstorms.

The Delta flight was following a Learjet from four miles out on the ILS to runway 17 when the copilot, flying the approach, called the captain's attention to lightning coming out of a thunderstorm slightly above and straight ahead. The Learjet landed safely without any reported problems, so the Delta crew continued the approach. Curiously, as the copilot attempted to keep the needles caged on the ILS, the airspeed began to increase, suggesting updrafts.

As the aircraft passed 800 feet AGL with speed still increasing, the captain recognized what was about to happen and ordered the copilot to add full power. "You're going to lose it all of a sudden," he said. "There it is. Push it up, way up, way up, all the way up."

The brief period of updrafts was followed by a series of violent downdrafts. The L-1011 lost 40 knots in about eight seconds, and the flight crew couldn't recover from the descending air, at one point speeding downhill at 5,000 fpm. The captain's final word was, "TOGA," ("Takeoff/Go Around Power"). Full power from all three engines did arrest some of the aircraft's descent, but the Delta flight finally crashed to the ground short of the runway, impacting a highway with a final descent rate of 600 fpm and an airspeed over 200 knots. After touchdown, the big Lockheed slammed into two water tanks and disintegrated, killing 136 passengers and crew.

Granted, most general aviation pilots don't fly anywhere near thunderstorms, but airline flight crews sometimes do. This exposes large passenger jets to more risk from thunderstorms, but all pilots should learn to recognize the symptoms of a downburst. In its simplest form, a thunderstorm-generated downburst is a flow of air that can hide inside a rain shaft, though relatively dry downbursts, like the one I flew through in Albuquerque, are possible. Dry downbursts sometimes originate from clouds at high altitude that seem to pose no threat to aircraft flying at lower levels.

Rain may fall as virga and affect the atmosphere below. When rain or moisture drops below the cloud base or mixes with dry air, it begins to evaporate and cool. Since warm air rises and cool air descends, the colder air falls progressively faster as it approaches the ground.

Downbursts go through three distinct stages; the actual descent or downburst toward the ground, the outburst during which the air is deflected by terrain and the cushion effect associated with rising air at the edges of the phenomenon. When a downburst actually arrives at the surface, its advancing edge spreads abruptly in all directions, the signature of a downburst. The air also effectively "splashes" back up toward the outside of the rain shaft. This accounts for the updraft phenomenon that may precede violent downdrafts as the air currents swirl and shift in the sky.

A microburst is simply a small downburst, usually no more than 2.5 miles in diameter, and it may generate temporary surface winds as high as 100 mph in all directions, usually highly localized and lasting no more than 10 minutes. Thunderstorms generally travel fairly quickly across the ground, so conditions can change in a few minutes. The winds may be erratic and almost unmanageable, shifting from head- to tailwinds in an instant, making it virtually impossible for a pilot to fly a consistent, stabilized approach.

Delta 191 wasn't the first wind shear accident. An Eastern Airlines jet approaching JFK Airport in New York had crashed 10 years before, killing all aboard, and a Pan Am 727 had crashed on takeoff from New Orleans in 1982, again fatal to all aboard. Both accidents had been attributed to wind shear.

As the pilots of flight 191 taught us, the only solution is avoidance, because once you're caught in wind shear, you may become nothing more than a passenger. The forces generated are often too strong for even many military fighters to overcome. To that end, several manufacturers have developed Doppler radar capable of predicting major shifts of wind direction and velocity well ahead of an appropriately equipped aircraft.

Trouble is, you and I will probably never fly behind such equipment. The radar systems are so expensive that it's unlikely they'll ever trickle down to general aviation. We're left with warnings generated by other sources. So far, 45 major airports, principally those that support airline traffic, are equipped with ground-based Doppler wind shear warning systems. These allow controllers to detect possible wind shear and warn all aircraft of potentially dangerous wind situations. NEXRAD uplink also can provide information on wind shear.

Of course, the best advice when flying around thunderstorms is the age-old adage, "Don't." This doesn't address situations when you're already en route and thunderstorms spring up unexpectedly. The good news/bad news about wind shear is that it normally occurs close to the ground. Even if you did encounter the phenomenon at cruise altitude, you'd probably have time to recover.

If you encounter what seems a strange updraft on final approach in convective weather that tempts you to reduce power and follow the glideslope to avoid increasing speed, be aware that you may be about to fly through the mother of all downdrafts in a few seconds. It's probably safer to climb above the glideslope just in case you're about to be forced downhill by a microburst.

Perhaps the only good thing about a downburst is that it usually provides some warning of what's about to happen. Considering what's at stake in a downburst and the fact that the forces unleashed may countermand any amount of skill, you need to understand the warning signs. Unlike me, you may not be allowed a second chance.

Pilot precautions for handling wind shear are fairly simple. If thunderstorms are in the area and you're flying adjacent to significant convective activity, be aware that you may be required to take evasive action to avoid being seduced by a downburst. In Albuquerque, I had the luxury of landing long. That's a good option if you're operating into an airline airport with runway to spare.

Don't blindly follow the ILS in VFR conditions, and don't allow a controller to coerce you into landing short with an order to expedite your approach for traffic behind you. That's a rare occurrence these days, anyway, as controllers have been educated to the dangers of wind shear and will do everything in their power to avoid a hazardous situation.