Microburst: A Micro-Hazard With Maxi Impact

Sometimes in the field of aviation safety, there are revelations: As a result of an accident investigation, a hitherto unexplored hazard emerges and everyone becomes cognizant of it and vows that never again shall it be permitted to take a life. That's just what happened more than 33 years ago on August 8, 1985, when Delta Air Lines Flight 191, a Lockheed L-1011-385-1, was brought down while flying through a microburst during an approach to Dallas/Fort Worth International Airport (KDFW). A microburst is a relatively small, violent downburst creating damaging winds and positive and negative wind shear spreading out in all directions. Although the term is nothing new, many pilots had never even heard of a microburst until the 135 deaths in the flight Flight 191 accident. It suddenly became a hot topic and remained so long after the Safety Board's accident report came out on August 15, 1986. In fact, every pilot now does, or at least should, hear something about microbursts during training. Even so, some pilots may have a perception that microbursts are things that go after airliners and don't bother with little general aviation planes. That perception is very wrong, as demonstrated just over a year ago when a Piper PA-46-310P Malibu crashed near Harrisburg, Oregon, killing the pilot and his three passengers.

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The Malibu was flying from Van Nuys Airport (KVNY) at Van Nuys, California, to Mahlon Sweet Field Airport (KEUG) at Eugene, Oregon. Instrument meteorological conditions existed and the airplane was on an IFR flight plan. The airplane had taken off at 7:27 a.m., and the accident took place at 10:48 a.m. Pacific time. The airplane was a registered to a corporation in Park City, Utah.

The single-engine aircraft had been built in 1986 and was powered by a Continental TSIO-520BE2F engine rated at 310 horsepower. The six-seat pressurized aircraft used a two-blade constant speed prop. Maximum cruising range with reserves was 1,550 nautical miles. Maximum takeoff weight was 4,100 pounds. Maximum structural cruising speed (Vno) was 173 knots indicated (KIAS) and never exceed speed (Vne) was 203 KIAS. A chart in a 1986 FAA-approved "Airplane Flight Manual" for a PA-42-310P showed wings level stall speed in landing configuration with the gear and flaps down was about 58 kts. In a 45-degree bank, it rose to about 75 kts., and at a 60-degree bank, it was about 86 kts.

Those numbers are quite different from what you'd find for the L-1011 that crashed at KDFW. Those three-engine widebodies typically were configured to carry up to 256 passengers, had a maximum takeoff weight of 430,000 lbs., a cruise speed of 520 KIAS at FL300, a range of 2,680 nautical miles, and a wings level stall speed of 108 kts. in landing configuration.

Flight 191 was en route from Fort Lauderdale to Los Angeles with a stop at Dallas/Fort Worth. There were 152 passengers and 11 crew. The possibility of rain showers and thunderstorms had been forecast for the evening, and by about 5:30 p.m., the captain had decided to take a longer, more northerly route into KDFW to avoid developing weather. When they were still about 20 minutes out, the captain advised Fort Worth Center about a cell ahead, saying, "I'd rather not go through it, I'd rather go around it one way or another." After being given a turn, the captain told the first officer, "I'm glad we didn't have to go through that mess. I thought for sure he was going to send us through it."

Although they had been doing visual approaches at KDFW, conditions were, indeed, changing, and they were starting to make ILS approaches to Runway 17L. A thunderstorm had moved close to the airport, and, as it was dissipating, a secondary cell it had generated was intensifying. As the L-1011 was handed off to an arrival controller, the first officer commented, "We're gonna get our airplane washed." A few minutes later, after the flight had been handed off to the tower and had been cleared to land, the first officer pointed out to the captain that a cloud ahead had lightning coming out of it. The captain asked, "Where?" and the first officer replied, "Right ahead of us." They were about to fly into a VIP Level 4 thunderstorm, which is characterized as "very strong" with severe turbulence likely. Level 4 storms typically have cloud tops to 40,000 feet.

This thunderstorm produced an outflow containing a microburst that was centered just under two nautical miles north of the approach end of Runway 17L and about 1,000 feet west of the ground track being flown by Flight 191. The horizontal wind shear across the microburst was at least 73 knots, and the speed of the downdraft was at least 29 knots. When the airplane first entered the microburst, the first officer shoved the throttles forward to almost takeoff power and pulled the nose up to greater than 15 degrees. Then, about 17 seconds before impact, the stall warning stickshaker activated as the airplane encountered rapid reversals in wind direction inside the microburst. The first officer pushed the yoke forward to avoid a stall, then pulled back again to avoid ground contact. But the L-1011 touched down and stuck a car, killing the driver, hit a light pole and then struck a water tank. The airplane burst into flames.

This accident resulted in research, especially by NASA, on microburst and wind shear recovery techniques, such as increasing power and climbing at more than 13 degrees nose up or powering up and keeping the nose low if altitude permits to fly through the disturbance as quickly as possible.

Could the Malibu pilot have recognized what was going on when his plane got into trouble and recalled learning something related to Flight 191? While we'll never know for sure, data recovered from onboard equipment by investigators could have indicated at least some intentional maneuvering by the pilot while the airplane was being tossed around. They focused on data recorded by an Appareo Stratus 2S receiver. It is intended to feed data to an iPad and integrate with the ForeFlight Mobile application to create graphics, show weather information, display the projected ground track and more. The self-contained battery-powered unit has an attitude heading reference system, a GPS/WAAS receiver and an ADS-B receiver. It has the capability to store more than 13 hours of flight data and recorded approximately five data records per second.

The device showed that it was powered up at about 7:07 a.m., and by just after 7:12 the airplane was taxiing. At 7:21:37, the airplane began its takeoff roll on Runway 16R at KVNY. By 7:41, it had reached 14,200 feet GPS altitude.

The airplane held about 14,000 feet in cruise on a generally direct route from KVNY to KEUG. At 10:38 a.m., the pilot was cleared by Cascade Approach to descend to 4,000 feet MSL and told to expect the ILS approach to Runway 16L at Eugene. KEUG is at 373 feet MSL.

At about 10:40, the controller advised the pilot that there was heavy precipitation at the 11 o'clock to 2 o'clock position. He asked the pilot whether he had received the latest weather at the airport, and the pilot said he had. The pilot then radioed that he was in heavy precipitation and asked for vectors to the localizer and for clearance to descend to 2,000 feet. At 10:45, the Cascade Approach controller told the pilot there was heavy to extreme precipitation in the area.

What the controller did not offer to the pilot were recent pilot reports. The Safety Board could not determine whether the pilot had updated himself on the weather during the flight other than getting the KEUG observation. If he had done so, he might have received the PIREPs. The Safety Board pointed out that FAA guidance called for the controller to have offered pilots being handled existing pilot reports while soliciting new reports from them because of the weather conditions in the area.

At 9:19, the crew of a Boeing 737 over Portland, Oregon, about 104 miles from Eugene, had put in a PIREP of moderate turbulence below 8,000 feet. At 9:43, the pilot of a Cessna 525 Citation reported moderate to light turbulence at 5,000 feet when 18 miles from Eugene. At 10:45, a Bombardier CRJ200 regional jet reported light to moderate turbulence and light mixed icing during descent from 13,000 feet to 9,000 feet and low-level wind shear at 300 feet at Redmond, Oregon, about 103 miles from Eugene.

Recovered data showed that at about 10:47, the Malibu was at 1,870 feet and its ground speed had changed from 144 knots to 75 knots. At 10:47:41, its altitude was 1,923 feet, and the bank angle had increased to 42 degrees left wing down and the attitude was 13 degrees nose up. Just 7 seconds later, the left bank angle had reached 95 degrees, and the nose was pointing down by 35 degrees. The airplane sank to 1,125 feet. It continued to descend as it rolled level and then continued rolling to the right.

Cascade Approach issued a low altitude alert to the pilot and instructed him to climb. There was no response. The controller again issued a low altitude alert and again there was no response. The controller tried two more times, but there was only silence on the frequency.

At 10:48:12, the airplane's ground speed had dropped to zero, consistent with ground impact. It had slammed into the ground about 12 miles north of KEUG. The impact site was at an elevation of 276 feet. The first point of contact covered an area that resembled the silhouette of an airplane. The NTSB said the impression in the ground was consistent with the Malibu hitting in a level attitude. The engine was still attached, with the propeller in place. One of the prop's blades was bent back at a 90-degree angle, consistent with it having impacted the ground vertically and some forward motion.

A witness who was about one-half mile from the accident site said she heard a loud engine noise and looked up to see the airplane flying over her house. She said the engine then went completely silent, and the airplane descended out of sight behind a tree line. Another witness who was just over a mile from the accident site said he saw the airplane flying just above treetop level and then entering a near-vertical dive before going out of sight behind trees. That seemingly contradicted what the NTSB said about the ground impression made by the airplane. The wreckage examination failed to find evidence of engine failure that would have caused it to go silent as the first witness suggested.

About six hours before beginning the flight, the pilot had self-briefed on the weather. Forecasts called for strong winds, heavy rain, turbulence and low-level wind shear in the KEUG area at his anticipated time of arrival. Included in the weather information the pilot received were AIRMETs Sierra and Tango, which included the accident area at the time of the accident. They warned of mountain obscuration in clouds and precipitation, low-level wind shear and moderate turbulence below 16,000 feet.

At 10:54, the automated observation at KEUG included wind from 200 degrees at 18 knots with gusts to 26 knots, visibility 7 miles in light rain, a few clouds at 2,400 feet and an overcast ceiling at 3,500 feet. The temperature was 8 degrees C., the dew point was 7 degrees C., and the altimeter was 29.51. Subsequent data showed wind gusts to 35 knots.

When an NTSB investigator was at the scene, she saw that an area of tall grass about 450 feet from the wreckage had been flattened. The area was about 140 feet by 150 feet. When National Weather Service personnel looked at photos she provided, they concluded that it was possible for a microburst to have produced the flattening.

The NTSB said the probable cause of the accident was an encounter with a downburst during an instrument approach, which resulted in a loss of control at low altitude. Contributing to the accident was the air traffic controller's failure to solicit and disseminate pilot reports from arriving and departing aircraft in order to provide pilots with current and useful weather information near the airport.

The 67-year-old commercial pilot had single-engine, multi-engine and instrument airplane ratings. His second-class FAA medical certificate was current and required him to wear corrective lenses. While investigators could not determine his total instrument flight experience, his last FAA medical application, which was filed about two months before the accident, reported 5,060 total flight hours, with 163 in the Piper Malibu. He reported 25 hours of instrument time in the six months before the medical application. The NTSB did not report on the pilot's training background, and there's no indication in the report that they looked further than what the FAA had on file. So, there's nothing to go on regarding any training the pilot might have received regarding downburst and microburst recognition and recovery. But, with 5,060 hours and 67 years, it would be a reasonable bet that he had heard of the Flight 191 accident and had learned at least something about severe and unusual weather hazards like downbursts in general and microbursts in particular. Perhaps this accident demonstrates we all should learn even more about the subjects.

Peter Katz is editor and publisher of NTSB Reporter, an independent monthly update on aircraft accident investigations and other news concerning the National Transportation Safety Board. To subscribe, visit www.ntsbreporter.us or write to: NTSB Reporter, Subscription Dept., P.O. Box 831, White Plains, NY 10602-0831.