WX BRIEF

Better knowledge of downbursts can save you from dangerous flying experiences

Strong storm-related winds can pummel unsuspecting aircraft.

By Karsten Shein
Comm-Inst, Climate Scientist


Schematic of an idealized encounter with a microburst between an aircraft and the runway. At first hint of an uncommanded increase of airspeed or altitude, pilots should apply full power and abort the approach.

Alittle after 6:30 pm on Jul 2, 1994, a US Air Douglas DC9 was lined up on approach to CLT (Charlotte NC) with both pilots remarking on the pop-up storms surrounding the airport.

Just before receiving clearance to the ILS for Runway 18R, the controller told the flight that there was rain south of the field. As they flew the approach, the pilots had a visual on the airport and the captain said to the first officer, "If we have to bail out ... it looks like we bail out to the right." He also indicated a "chance of shear."

Simultaneously, the crew of another aircraft elected to delay their departure after the controllers confirmed their transmission—"[It] looks like we've gotten a storm right on top of the field here."

Ahead of the DC9, however, a Fokker 100 reported a smooth ride to the runway, and the DC9 crew forged ahead, the first officer remarking, "Yep, lying right there this side of the airport, isn't it? ... the edge of the rain is, I'd say."

"Yeah," replied the captain, as he monitored the aircraft's radar screen. This "edge" was a thin veil of rain between the aircraft and the runway. A minute later, the tower controller transmitted to the aircraft, "Wind is showing one zero zero at one nine." Almost immediately after acknowledgement by the captain, this was followed with, "Wind now one one zero at two one," which was also acknowledged.

Differences between a dry and wet microburst. Despite these differences, both types of downburst can generate winds above 100 kts and can easily overcome the capabilities of even the stoutest aircraft.

Not 15 sec later, the controller trans­­mitted, "Windshear alert northeast boundary, wind one nine zero at one three." Within the minute the captain stated, "Here comes the wipers," and the first officer commented, "There's, oh, 10 kts right there." This was followed by the captain saying, "Okay, you're plus 20... Take it around—go to the right."
Within the next 30 sec or so, the captain called for max power, and the tower controller acknowledged the missed approach with a clearance to fly runway heading and climb to 3000 ft.

As the first officer moved the flaps to 15°, the captain shouted, "Down—push it down!" and the pilots transmitted that they were taking a right turn.

At that point, the ground proximity warning system began to whoop and the stick shaker activated. Only 1/10 sec later, the DC9 struck the ground, breaking up along a road adjacent to the runway, killing 37 passengers and destroying a house and a car.

Image of a dry microburst with wind patterns highlighted with red ribbons. Dry microbursts occur when the surface atmosphere is dry, and any rain is evaporated by the descending air, cooling it further.

Surviving passengers described the heavy rain and turbulence they encountered just before the crash, noting that they hit "an air pocket" and dropped as though "riding a roller coaster" just before they heard the engines spool up. Witnesses on the ground reported intense rain and noted that the wind was "blowing very hard."

The captain held an ATP and was a USAF reserve pilot with over 8000 hrs at the time of the crash. He had almost 2000 hrs in type. The first officer was also an ATP, with almost 13,000 hrs and over 3000 hrs in the DC9. The aircraft carried a Honeywell standard windshear detection system.

However, both pilots agreed that they had not received any aural or visual warnings from the system during the approach.

This was confirmed by the cockpit voice recorder, but Honeywell noted that, if the flaps had been in transition at the time, the system would likely have been de­sensitized to avoid false alerts, and so may not have activated.

Dry microburst as seen from the air. Virga often is present beneath the cloud, but the cold, dry air of the downdraft quickly evaporates it before the rain can reach the surface.

NTSB cited the lack of ATC weather and windshear reporting procedures, as well as the "inadequate soft­ware logic in the airplane's windshear warning system."

However, the NTSB report listed as probable causes of the crash of US Air Flight 1016 at CLT the flightcrew's "decision to continue an approach into severe convective activity that was conducive to a microburst," their "failure to recognize a windshear situation in a timely manner," their "failure to establish and maintain the proper airplane attitude and thrust setting necessary to escape the windshear," and "the lack of real-time adverse weather and windshear hazard information dissemination from air traffic control."

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