WX BRIEF

Better knowledge of downbursts can save you from dangerous flying experiences

Strong storm-related winds can pummel unsuspecting aircraft.


On Aug 1, 1983, a microburst was recorded at ADW (Andrews AFB, Camp Springs MD) minutes after Boeing VC25 (747-200) Air Force One landed. Typical of most microbursts, this one lasted just a few minutes, but winds went from about 20 mph to over 120 mph within a few seconds. Often simply going around for a 2nd approach will allow the microburst time to dissipate.

Now the pilot is faced with a sudden downward force push­ing the aircraft toward the ground. Even when full power is applied, most aircraft take a few seconds to respond to firewalled thrust levers. In that time, the aircraft may continue beyond the downdraft core into the outflow moving in the same direction as the aircraft. This flow robs the aircraft of airspeed, possibly to a point below stall speed. At such low altitudes, the resulting drop in altitude can be disastrous.

In reality, many downburst encounters do not include the first part. The vortex tends to be relatively close to the ground, and an approaching aircraft may fly right over the vortex, directly into the core.

In that situation, there may not be time to identify the downburst before it slams down on the aircraft stuck in a low and slow landing configuration only a few hundred feet above the surface. If the aircraft just clips the downburst core, the resulting force could cause one wing to dip or stall. Similarly, a roll may occur if the aircraft flies tangentially into the ringing vortex rather than meeting it head on.

Downbursts are equally dangerous on takeoff. Although the aircraft is already producing maximum thrust on its roll, it is vulnerable just after rotation, when its airspeed is just enough for it to leave the ground and begin to climb.

If it should encounter a strong low-level shear that reduces that airspeed—such as if the downburst occurs behind the aircraft just as it is rotating—it could lack sufficient lift to continue to climb away from the danger, and may have already traveled beyond the end of the runway.

The key to avoiding downburst is simple, though. Do not take off or land within about a mile of a storm, and never, ever fly beneath a storm cell. If there are thunderstorms in the vicinity and you must fly through rain on your approach, you are flying beneath a convective cell and should consider aborting the approach until it can be made in clear air.

Most pop-up storms live for only about a half hour, and if they are already producing rain, their end will come much sooner. But if they are producing rain, they will have a downdraft, and can easily produce a downburst with little or no warning.

Stronger storms that are tilted by upper-level windshear can last much longer, but most of them will move along at the speed of the mid-level winds (ie, those at around 10,000-18,000 ft), so they often won't linger near your flightpath or the airport for too long.
Many larger airports have parallel runways that can be a mile or more apart.

In those cases, ask for the use of the runway furthest from any storm. At any airport, however, where there are thunderstorms in the vicinity, pay close attention to the wind reports or even the windsock. If the winds are strong, any rapid shifting of direction by more than 10–15 degrees is an indicator of low-level windshear. Similarly, any significant gustiness or sudden increases in speed from relatively calm air can indicate outflow from a nearby storm.

If your aircraft is equipped with onboard radar, tilt it upward on approach so that you can see the cells that may be ahead and above your path. Most importantly, if conditions around the airport lead you to think that a downburst is a possibility, carry extra power and altitude on your approach, and be ready to miss the approach at the first hint that the atmosphere may be altering your plans.

If you do find your aircraft ballooning unexpectedly or losing altitude rapidly, don't hesitate to apply full power and—unless it would place the aircraft in greater danger—execute a straight-ahead missed approach.

Climbing or turning immediately are actions best avoided, except if needed to clear terrain or obstacles, as these actions will increase your angle of attack and bring you closer to a low-level stall. Also, experts advise not to waste time changing flap or landing gear configurations while trying to recover—these can be dealt with later.

Maintaining positive control, building speed and arresting any further descent are paramount to recovering from a downburst. A level recovery along the runway centerline should keep you clear of obstacles and give you time to build speed for a climb.

When practical, let ATC know about the missed approach, and that you encountered a downburst—they will want to know so as to keep other aircraft from harm's way. Of course, the best course of action whenever you even suspect the possibility of a downburst is to go around—and do so before you even get near that rain shaft between you and the runway or fly beneath that area of virga.

Downbursts are short-lived phenomena and, by the time you're lined up for your 2nd approach, it's likely that the danger will have passed.

Karsten Shein is a climatologist with the National Climatic Data Center in Asheville NC. He formerly served as an assistant professor at Shippensburg University. Shein holds a commercial license with instrument rating.


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