The Risk Of Low-Level Windshear: A Timeline Of Events
This is an abbreviated version of the article - Judging The Risk Of Low-Level Windshear.
On the stormy afternoon of Aug. 2, 1985, I drove out of the north tollbooth at Dallas/Fort Worth Airport (KDFW), headed for my home in Dallas. As I accelerated, I felt my car begin to rock from side to side with increasing force. In order to maintain control of the car, I slowed way down and crept along the divided highway at about 10 mph. About 30 min. later, I heard that a Delta Air Lines L-1011 had crashed into a water tank located only a few hundred yards to the east of where I had been driving when the winds struck me. The cause of that crash was low-level wind shear.
That experience was my most vivid memory of the effects on the ground of what we now call a microburst. The force of the downdraft from a low-ranging cumulus cloud can be astonishing as it strikes the ground and billows outward.
The investigation of a Piper Malibu accident near Harrisburg, Oregon, on April 7, 2017, provided another graphic example of the force a strong downdraft can have on the surface of the Earth. The airplane was destroyed when it impacted a field; the pilot and three passengers received fatal injuries. The NTSB investigator arrived at the crash site a few hours after the accident occurred. She found an area of disturbed, flattened, tall grass about 450 ft. southwest of the accident site. Her photos showed what clearly looks like the results of a microburst, and the flattened area appeared to lie along the flight path of the descending airplane.
The airplane was a PA-46-310P, N123SB, manufactured in 1984. It was powered by a 310-hp Continental TSIO-520BE2F engine, and had a pressurized cabin and retractable landing gear.
This is an interactive timeline so please give it a few moments to load
The crash site was 12 mi. north of Mahlon Sweet Field Airport (KEUG), Eugene, Oregon, at an elevation of 276 ft., and it was 450 ft. north of the area of disturbed, flattened tall grass indicative of a downburst or microburst. Both ailerons separated from the Piper Malibu at impact. The first point where the airplane impacted showed evidence of the landing gear being extended, and the left and right main landing gear were found near each other about 74 ft. beyond that spot. The main wreckage, including engine, fuselage, wings and empennage, was located 93 ft. beyond the point of impact.
An NTSB meteorologist examined and summarized all the available weather data for the Eugene area near the time of the accident, and obtained screen captures of the ForeFlight and Leidos text and graphics the pilot obtained before departure. The pilot received AIRMETs Sierra, Zulu and Tango; the appropriate METARs and TAFs; the Area Forecast; the Storm Prediction Center (SPC) Day 1 Convective Outlook; Winds Aloft Forecast; and a report of no urgent PIREPs. The screen shots of the pilot’s mobile application clearly show the deep low off the coast of Washington and the forecast rain and convective activity at the destination. The meteorologist could not find any evidence that the pilot checked any additional weather sources while in flight.
The NTSB found no evidence of airframe, engine or system abnormalities. Based on the photos of the flattened grass and National Weather Service (NWS) opinion, they found a downburst or microburst “could not be ruled out.” The pilot obtained weather information from his mobile application that indicated strong winds, heavy precipitation, turbulence and low-level windshear (LLWS) for the time of arrival, and he did in fact encounter an intense downdraft that resulted from a sudden change in wind velocity. The downburst probably exceeded the climb performance of his airplane. The Safety Board found the controller should have solicited from and provided to the pilot PIREP information.
Like with many general aviation accidents, we now know what happened but lack a complete understanding of why it happened. Without airborne data recorders and more in-depth knowledge about the pilot, we don’t have enough information to see why the pilot took the actions he did. However, from all the factual information the NTSB has provided, I think we can piece together some useful lessons.
I think the accident pilot had enough information to be able to avoid this accident, but the decision wasn’t entirely obvious.