June 1, 2003
By Bruce Landsberg
It's been said that history has a way of repeating itself for those who do not heed past lessons. In April 1977 a Southern Airways McDonnell Douglas DC-9 tangled with a thunderstorm that smashed the windshield and flamed out both engines. That resulted in a dead-stick landing with heavy loss of life (see " Safety Pilot Landmark Accidents: Deadly Surprise," August 1998 Pilot). The crew misread the weather radar and flew into a cell with massive rainfall that literally snuffed out the engines. Twenty-one years later, on May 7, 1998, about 7:20 p.m. Eastern Daylight Time, another DC-9, operated as AirTran Airlines Flight 426, nearly replicated the earlier accident and, again, confirmed that thunderstorms should be given a wide berth.
Flight 426 was scheduled to fly from The William B. Hartsfield Atlanta International Airport to Chicago Midway International Airport. The captain, looking at a weather display in the AirTran operations area, observed a "line of weather" along the Georgia and Tennessee borders. He stated that he was "not sure if the weather depiction was from a satellite or radar, but the line was significant with red displays in north Georgia and was more like a blob instead of a single cell." The first officer also viewed the radar display that depicted a line of weather with red-, yellow-, and green-colored cells along the Georgia and Tennessee borders.
The captain briefed a flight attendant that there was no hazardous weather around Atlanta, that Chicago weather was improving, but that there was some adverse weather en route. After a brief ground delay, the flight departed Atlanta at 7:07 p.m. on an assigned heading of 350 degrees and a clearance to climb to 14,000 feet. The weather radar was on and selected to the 80-mile range in the "normal" mode with the gain set to "auto." The captain stated that the weather radar showed a solid line with dark returns in northwest Georgia. He also visually observed the line by looking out of the cockpit windows.
The captain delayed the 10,000-foot departure announcement to the flight attendants so they would remain seated rather than begin food service. The forward flight attendant started to get up from her seat, but was told to sit down.
The flight was cleared up to 23,000 feet and the crew requested a 330-degree heading to remain clear of weather. The pilots stated that they were in visual meteorological conditions and that their radar showed an area of light precipitation west of the line of weather. Their radar also showed an approximate 10-mile gap between two storm cells, and through this gap they saw no adverse weather on the other side.
As the DC-9 proceeded through the gap an egg-size hailstone shattered the outer pane of the center windshield. At the same time, the aircraft began experiencing turbulence that lasted about 10 seconds. Approximately two seconds after the first hail encounter, the airplane entered significant hail, which lasted about three to five seconds and shattered the outer panes of the captain's and first officer's windshields. The nose radome cover was torn off. The airspeed indication went to zero and the altimeter indications became erratic.
The flight data recorder (FDR) showed that the aircraft had encountered some moderate turbulence that had dissipated before the hail encounter. At 7:18 p.m., while climbing through 20,500 feet, many of the FDR parameters became erratic again. The aircraft rolled as far as 40 degrees, and oscillated between 10 and 20 degrees. Pitch angle increased to about 10 degrees, then decreased to negative 6 degrees, and remained erratic throughout the incident. Vertical acceleration varied between 2.9 Gs and negative 0.9 Gs. Indicated airspeed fluctuated but generally decreased from 350 knots to 280 kt. At 7:19 p.m. the crew reduced power as engine pressure ratios decreased from approximately 1.9 to 1.5. During the incident, pressure altitude increased from approximately 19,500 feet to approximately 23,000 feet. The almost 4-G vertical movement and significant airspeed reduction coincided with a power reduction as the crew slowed the aircraft in response to the turbulence.
The crew advised ATC what had happened, requested vectors direct to Chattanooga, declared an emergency, and asked to have emergency equipment standing by when they landed. They also asked for a surveillance approach with verbal groundspeed readouts every 10 seconds. The crew reported seeing the runway through small unshattered areas of the windshield about 10 miles out, and the landing was successful. The cockpit voice recording that frequently gives us insight into the pilots' thoughts was not available. By the time the DC-9 reached the gate at Chattanooga more than 30 minutes had elapsed from the time of the incident and the tape had recorded over the transcript.
In airliners, many injuries occur to passengers and cabin crew during unexpected turbulence encounters. In this case, the flight crew was well aware of the weather and turbulence potential. One of the forward flight attendants stood because she thought the captain had forgotten to make the "departure check" announcement that would have been the signal to begin meal service. The captain immediately told her to sit down. The aft flight attendant saw the forward flight attendant get up from her jump seat and thought she had missed the departure-check announcement. As she stood up the forward flight attendant called via interphone to tell her to sit down. As she was refastening her seat belt, the aircraft dropped suddenly and she was seriously injured when her head struck the ceiling.
A female passenger was also seriously injured when bags fell out of an overhead compartment during the turbulence encounter. The fuselage either flexed significantly or the latch on that compartment was defective or improperly secured.
Portions of the DC-9's radome were ingested into the right engine. All three outer panes of the front windshield were shattered. The wing leading-edge devices, horizontal stabilizer leading edge, vertical stabilizer leading edge, and both left and right engine inlet cowls were dented. Both engines sustained foreign-object damage.
Nexrad ground weather radar from the National Weather Service (NWS) showed Flight 426 to be operating in an area of Level 6 echoes (extreme). The GOES 8 infrared satellite image recorded at 7:15 p.m. indicated that Flight 426 passed about three nautical miles west of an estimated cloud top of 44,000 feet.
Two local communities within 25 miles of Flight 426's encounter reported hail between 1 and nearly 2 inches in diameter. The National Weather Service had forecast the system accurately. A convective sigmet was current for the area with severe thunderstorms moving from 230 degrees at 30 kt, with tops above FL450, tornadoes, 2-inch hail, and wind gusts to 70 kt. There was a severe weather forecast alert for tornadoes and finally a center weather advisory issued prior to the accident indicating a broken line of VIP (video integrator and processor) Level 5 to isolated Level 6 thunderstorms, 15 miles wide. Tops stretched to FL400. The line was moving east at 25 kt, with cells moving northeast at 40 kt. Hail to 1 inch and wind gusts to 55 kt were possible. Most of this was included in the flight release provided to the crew before departure, although three pertinent convective sigmets and a tornado watch report were not included because of a programming flaw in the airline's dispatch computer system.
The captain, age 57, had accumulated about 7,200 hours in the DC-9 and about 4,800 hours as a DC-9 pilot in command. Before his employment with ValuJet, AirTran's predecessor, the captain served 21 years as a pilot for Eastern Air Lines and six years as a pilot in the U.S. Air Force. In early 1994, the captain lost control of a DC-9 during a takeoff roll in adverse weather conditions. The takeoff was aborted and the airplane departed the runway onto a grassy area. Again in July 1994, the captain was in command of a DC-9 that encountered moderate convective turbulence during climbout, causing a flight attendant to suffer multiple leg fractures. After the May 1998 accident, the captain was reassigned as a first officer.
The first officer, age 30, had accumulated a total of 5,500 flight hours, including 1,500 hours in the DC-9 with prior experience in regional airline aircraft.
The following is excerpted from FAA Advisory Circular (AC) 00-24B, titled Thunderstorms: "Hail competes with turbulence as the greatest thunderstorm hazard to aircraft. Hail may be encountered in clear air several miles from dark thunderstorm clouds.... Hailstones larger than one-half inch in diameter can significantly damage an airplane in a few seconds. National Weather Service radar...is able to objectively determine storm intensity levels with VIP equipment. These radar echo intensity levels are on a scale of one to six.... VIP Level 6 is 'extreme' with severe turbulence, lightning, large hail, extensive surface wind gusts, and turbulence. Thunderstorms build and dissipate rapidly. Therefore, do not attempt to plan a course between echoes.... You must avoid individual storms...either by visual sighting or by airborne radar. It is better to avoid the whole thunderstorm area than to detour around individual storms unless they are scattered.... Do avoid by at least 20 miles any thunderstorm identified as severe or giving an intense radar echo.... Do remember that vivid and frequent lightning indicates the probability of a severe thunderstorm. Do regard as extremely hazardous any thunderstorm with tops 35,000 feet or higher whether the top is visually sighted or determined by radar."
The Aeronautical Information Manual (AIM) states in part, "Severe turbulence can be expected up to 20 miles from severe thunderstorms. This distance decreases to about 10 miles in less severe storms.... No flight path through an area of strong or very strong echoes separated by 20 to 30 miles or less may be considered free of severe turbulence."
Other air carrier flights were diverting east and west to avoid the area where Flight 426 penetrated. There is much to be said for going with the herd in these situations because the decision-making power of many pilots is then brought to bear. The rewards for being a trailblazer are frequently not worth the risk. Another 10 minutes of flying time might well have solved the problem completely.
Although this captain had two weather encounters before his demotion, it's tough to second-guess a PIC, but here are some subjective observations. One certainty about thunderstorms is that they are unpredictable. Anyone who has spent much time avoiding them knows the temptation of sliding through a narrow hole in a line. If the tops are low, arbitrarily below 25,000 feet or described as a VIP Level 2, a 20-mile gap might work (that's 10 miles from each cloud) with nothing more than moderate turbulence, but there are no guarantees. When storms are described as severe, it's best not to be in the same county.
A critical point regarding VIP levels is that a growing storm can change in a matter of minutes from Level 2 to Level 5 or 6. What you, the controller, or another aircraft ahead is viewing may be altogether unflyable by the time your flight is next to the storm. Cheating on storm-avoidance distance is sometimes a great short-term strategy, but no one can predict when it will fail catastrophically. The tough part to reconcile is that successful tactics that worked before may not work if the weather situation is different. In this case, it seems inconceivable that there was a question about the severity of the weather.
A light GA aircraft in this situation might well have been lost. Almost any turbocharged or pressurized single is capable of operating at 20,000 feet, and there is good reason in convective weather to be high enough to be able to see the "saddles" in the clouds. However, those mid-altitudes are notoriously unfriendly.
The Gs reported by the FDR in this accident approach Normal category aircraft certification limits. The aircraft will generally withstand turbulence encounters from moderate thunderstorms, but not always. The controllability of lightly wing-loaded aircraft is not comparable to an airliner. What is flyable in a heavy jet may be all but impossible in our airplanes.
Finally, it is probable that a typical light-aircraft windshield would have broken completely during the hail encounter. This would greatly complicate recovery because of a rapid depressurization and a compromised cockpit environment. If you ever needed incentive to give the big ones a wide berth, this accident should provide it.
Bruce Landsberg is executive director of the AOPA Air Safety Foundation.
Shortly after this accident, AirTran required all pilots to view a training video on weather and weather avoidance.
On August 27, 1998, AirTran issued a revision to the flight operations manual that contained guidance for flight in areas with thunderstorms in addition to radar operating techniques for weather avoidance. The revision also contained guidance for operations around turbulence, hail, lightning, icing conditions, and low-level wind shear. AirTran also issued a flight operations bulletin to all DC-9 pilots and dispatchers that addressed flight in hazardous meteorological conditions. — BL
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