MCCONNELLSBURG, PA, USA
N515AU
Boeing 737-300
The Boeing 737 began an uncommanded roll during its climb, just prior to passing through 29,700 feet. The roll angle was most acute about 29,800 feet, when roll reversal began. The captain recovered the airplane, and diverted to a nearby airport without further incident. The weather, both prior to, and after the event, included clear conditions and a smooth ride. Radar analysis revealed that a DC-10, about 10 nautical miles ahead, had climbed through the same area, but 600 feet higher, approximately 1 minute and 11 seconds earlier.
HISTORY OF FLIGHT On August 7, 1998, at 1129 Eastern Daylight Time, a Boeing 737-300, N515AU, operating as USAirways flight 623, received minor damage when it experienced uncommanded rolls while climbing along Jet Route 110, near the Saint Thomas VOR (THS), McConnellsburg, Pennsylvania. Two cockpit flight crew members, 1 cockpit observer, 1 flight attendant, and 118 passengers were uninjured. Three flight attendants and seven passengers received minor injuries. Visual meteorological conditions prevailed at the time. The scheduled passenger flight was operating on an instrument flight rules flight plan, and originated from Philadelphia International Airport (PHL), Philadelphia, Pennsylvania, at 1043. The intended destination was McCarran International Airport (LAS), Las Vegas, Nevada, but the airplane diverted to Pittsburgh International Airport (PIT), Pittsburgh, Pennsylvania. The flight was being conducted under 14 CFR Part 121. In separate interviews, both the captain and the first officer stated that the flight had been in smooth air, and was in a climb from flight level 280 to flight level 310. The autopilot was engaged, and the captain was at the controls. The captain stated that the airplane started picking up "light chop" which, within seconds, was "bordering on moderate." The airplane began making "a very slow yaw," movement, and had a slight "wing waggling." He was not sure if the autopilot was causing it. "Then it was like someone wanted to roll the aircraft on its back," to the right. The captain grabbed the yoke and the throttles, and pressed the autopilot disconnect button. He turned the yoke to the left, to the 7 or 6 o'clock position to stop the right turn. The airplane then reversed into a left roll, and the captain corrected for it. The first officer said that prior to the event, in-flight weather was "severe clear with unlimited visibility, a tremendously smooth ride - not a ripple." Then, the smooth ride changed to moderate turbulence, and it "seemed like a wake event for 2 to 3 seconds." The airplane started a "roll rate to the right - slowly at first, like the autopilot was fighting it, to 15 to 20 degrees, then faster." He went on to say that after the event, the air was perfectly smooth again. The captain said that when the event occurred, he initially thought he had an "uncommanded yaw," but noted that the rudders were not displaced. He then thought wake turbulence may have caused the event. The first officer said he also had his feet on the rudder pedals, but did not feel them move. The crew did not disconnect the yaw dampener, and the controls exhibited "normal resistance." The captain stated that at the time of the event, there were no advisories for turbulence, "no reports - nothing." He also stated that another crew requested ride conditions at flight level 310, and the controller reported "no complaints." Approximately 30 seconds after the first officer reported the turbulence encounter to air traffic control, the controller said: "You're following a heavy...DC-10 out in front of you...about niner miles now, flight level 310." The captain also recalled he vaguely remembered that on the traffic alert and collision avoidance system (TCAS), there was an aircraft about 1,700 feet above and 10 miles in front of the flight. However, he couldn't remember exactly when he saw it. PERSONNEL INFORMATION The captain held an airline transport certificate with ratings for airplane single-engine and multi-engine land, and instrument airplane. He had over 14,800 total flight hours, with 5,000 hours in the Boeing 737, including about 2,000 hours of that as captain. He received Significant Event Training (SET) in March, 1998. SET training in a simulator involved recovery from two unusual attitudes. One recovery was from a nose high attitude, and the other involved recovery from a high bank angle. Training at that time also included a demonstration and recovery from an uncommanded yaw/roll. His most recent Federal Aviation Administration (FAA) First Class Medical Certificate was issued on June 26, 1998. The first officer held an airline transport certificate with ratings for airplane single-engine and multi-engine land, and instrument airplane. He had over 12,800 total flight hours, with about 5,500 hours as a first officer on the Boeing 737. During his last proficiency check in a simulator, he received unusual attitude training, including recovery from a nose-high attitude in a bank, and recovery from a nose-low attitude. In addition, he had between 400 and 600 hours of aerobatics. His most recent FAA Second Class Medical Certificate was issued on November 3, 1997. METEOROLOGICAL INFORMATION At the time of the event, there were no in-flight advisories in effect for the area, and experimental turbulence algorithms failed to indicate the presence of any clear air turbulence. A scientist from the National Oceanic and Atmospheric Administration (NOAA) Forecast Systems Laboratory (FSL) stated that wake vortices typically sink at approximately 400 feet per minute, and that vortices generated during a climb are generally stronger than those generated during level flight. FLIGHT RECORDERS A Solid State Flight Data Recorder (SSFDR) was removed from the aircraft and sent to the Safety Board Vehicle Recorders Laboratory for readout and evaluation. The SSFDR data indicated that the aircraft was climbing through 29,679 feet when it rolled to the right to a maximum of 37 degrees, and within 2 seconds, reached a maximum left roll of -27 degrees. The vertical acceleration reached a low of -.6 g's. Timelines were also determined by comparing air traffic control transmission times to the "VHF Keying" parameter of the SSFDR. The data revealed that the airplane was wings-level at 1129:09. The roll angle increased to the right, to about 2 degrees during the next 2 seconds, and to 4 degrees 2.25 seconds later. After another 1.5 seconds, the roll angle increased to 10 degrees, and approximately 1.75 seconds after that, at 1129:16.5, increased to about 37 degrees, when roll reversal began. The roll angle decreased to wings-level about 1 second later, and continued to a maximum left roll angle of about 27 degrees, approximately 1 second after that. The airplane rolled through a wings-level attitude once again about 1 second later, to a right roll angle of 4 degrees, followed by a left roll to 6 degrees, and then leveled. The total time for the event was less than 14 seconds. TESTS AND RESEARCH A review of National Track Analysis Program (NTAP) radar data was performed. The NTAP radar data provided secondary (transponder beacon) radar information, which included time, altitude, latitude and longitude for both the event airplane, and a DC-10 in the area, American Airlines flight 19. That information was recorded every 12 seconds, and provided x-y positions, as well as altitudes. Resolution was plus/minus 1/16 nautical mile, and plus/minus 50 feet, respectively. At the time of the incident, the tracks of the two airplanes were nearly co-linear in x-y space. The event occurred, per NTAP data, at 1129:15, about 29,800 feet, approximately 5 nautical miles northeast of the Saint Thomas VOR. At that time, the DC-10 was 10.17 nautical miles ahead of the Boeing. The approximate position of the event airplane at 1129:15 was 39 degrees, 58 minutes, 11 seconds north, and 077 degrees, 51 minutes, 43 seconds west. Approximately 1 minute, 11 seconds earlier, the DC-10 was in a climb at 39 degrees, 58 minutes, 10 seconds north, and 077 degrees, 52 minutes, 02 seconds west, proceeding in the same direction, but 600 feet higher. ADDITIONAL INFORMATION On August 8, 1998, the airplane was released to a representative from USAirways.
A high altitude encounter with wake vortices.
Source: NTSB Aviation Accident Database
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