Denver, CO, USA
N570UA
BOEING 757-222
A United Airlines Boeing 757-222, N570UA, experienced a tailpipe fire on the No. 2, right, engine, a Pratt & Whitney PW2037(M) during engine start at the Denver International Airport. The airplane's right wing in back of the engine and adjacent fuselage suffered minor damage. Interrogation of the cockpit engine display showed several faults related to the electronic engine control (EEC) burner pressure (Pb) sensors. The interrogation of the cockpit engine display also showed the Pb indications were abnormally high for an engine start. Fuel is scheduled to the an engine as a function of Pb. So a high Pb indication would consequently result in a high fuel flow to the engine. Because the engine was just starting, the limited amount of airflow through the engine was insufficient for the excessive amount of fuel and resulted in a tailpipe fire. The EEC was removed from the engine and sent to the manufacturer for testing. The testing could not identify a cause for the malfunction and subsequent tailpipe fire.
History of Flight On June 10, 2009, about 1230 mountain standard time, a Boeing 757-222 airplane, N570UA, operated by United Airlines as flight 146, experienced a tailpipe fire in the No. 2 (right) engine, a Pratt & Whitney (P&W) PW2037(M) during engine start at the Denver International Airport (DEN), Denver, Colorado. Airport fire department personnel and equipment were dispatched to the airplane, but the fire had gone out by the time they arrived. The airplane was operating as a regularly scheduled passenger flight in accordance with the provisions of 14 Code of Federal Regulations Part 121 from DEN to Sacramento International Airport, Sacramento, California. The pilots reported that after receiving clearance from the push back crew, they started the left engine normally and without incident while the airplane was being pushed back from the gate. The pilots further reported that after the airplane had been stopped on the ramp, the push back crew cleared them to start the right engine as the push crew was removing the tow bar. The pilots also reported while starting the right engine, and the cockpit data confirmed, they noted that the right engine’s starting fuel flow and exhaust gas temperature (EGT) were significantly higher than normal. The captain declared an abnormal engine start and aborted the start by putting the right engine’s fuel control switch to cutoff as he continued to motor the engine and the first officer accomplished the abnormal start checklist. Concurrently, the ground personnel who just pushed the airplane back from the gate observed smoke and flames from the rear of the engine and advised the pilots via the intercom system. At the same time as the ground crew was advising the pilots via the intercom of the smoke and flames from the tailpipe, the pilots also heard someone on the ramp control frequency talking about an engine on fire on a 757. The pilots stated that they did not receive an engine fire warning in the cockpit. The report from the ground crew and the comment they overheard on the ramp control frequency was their first indication that they had an engine tailpipe fire and they then accomplished the engine tailpipe fire checklist. The flight attendants reported that during the playing of the safety video as the airplane was being pushed back, there were multiple passenger call lights on the right side of the cabin between rows 19 and 22, which are just aft of the wing. The passengers reported the right engine was on fire and at least one flight attendant reported seeing flames in back of the right wing. The flight attendants reported that they also called the cockpit to report that the right engine was on fire. After United Airlines maintenance and airport fire department personnel had determined the fire was out and the airplane was safe to move, the airplane was pulled back to the gate where the passengers and crew deplaned normally. Injuries to Persons There were no injuries to the 2 pilots, 4 flight attendants, and 182 passengers on board. Damage to Airplane The fire was confined to the right engine’s tailpipe and to the wing and adjacent structure aft of the engine. There was no damage to the engine, however, the wing flaps aft of the engine, flap fairings near the engine, and the right side fuselage wing fairing were scorched and blistered. Other Damage There was no other reported property damage. Airplane Information The airplane was a Boeing 757-222, registered as N570UA. Engine Information The engine was a P&W PW2037(M) turbofan engine serial number 726585. According to United Airlines’ maintenance records, the engine had accumulated 54,336 hours and 19,345 cycles since new at the time of the incident. The engine had accumulated 4,745 hours and 1,763 cycles since it had been installed on the airplane on December 15, 2007, at San Francisco, California. Fire There was a fuel-fed fire that was confined to the right engine’s tailpipe. The examination of the right engine showed that there was no evidence of an undercowl fire. The pilots reported and the FDR data confirmed that there was no engine fire warning. Tests and Research Following the incident, investigators interrogated the cockpit EICAS engine performance and auxiliary power unit (APU) page. For the right engine, the engine pressure ratio (EPR) was 1.002, N1 speed was zero, N2 speed was 46.3 percent, fuel flow was 3,927 pounds per hour (pph), the exhaust gas temperature (EGT) was 492 degrees C, and burner pressure (Pb) was 132 pounds per square inch (psi).In comparison, the left engine’s indications showed the EPR was 1.010, N1 speed was 23.3 percent, N2 speed was 64.2 percent, fuel flow was 904 pph, EGT was 386 degrees C, and Pb was 41 psi. In addition, the EICAS displayed a number of fault codes that according to the 757 PW2000 Engine Series Fault Isolation Manual, referred to Pb sensor faults. When the Pb line between the diffuser case and the electronic engine control was disconnected, the B-nuts on either end of the line were tight. The Pb line was blown out with dry nitrogen and only a minute amount of water came out. The Pb trap did not have any water or debris in it. The right engine’s two ignition system were tested and both were found to function normally. The APU duct pressure was 28 psi, which was slightly low in comparison to the 757 Airplane Maintenance Manual minimum pressure of 30 psi. However, even though the APU duct pressure was only 28 psi, investigators were able to start the left engine successfully a number of times. The EEC was removed from the engine and sent to Hamilton Sundstrand, Windsor Locks, Connecticut. At Hamilton Sundstrand, the eight faults stored in the EEC were downloaded and all were related to Pb sensor failures. The EEC underwent a continuity test and passed. The EEC then underwent an initial acceptance and failed because the Channel A Pb sensor 2.0 psi test (range: 1.6 to 2.4 psi) measured 2.66 psi. The Channel A and B Pb sensors were removed and underwent thermal and pressure tests and both sensors passed all of the tests. The EEC with replacement Pb sensors installed underwent another acceptance test and it passed. The replacement Pb sensors were removed from the EEC and the original Pb sensors were reinstalled and the EEC passed a third acceptance test. The EEC underwent several vibration tests and passed each time. The Pb sensors were removed from the EEC and disassembled. The disassembly revealed several pieces of metallic and nonmetallic debris. Hamilton Sundstrand stated that the material that was found was foreign to the Pb sensor, but none of the material that was found should have resulted in the high Pb indication that was observed on the EICAS display at Denver. Fuel is scheduled to any jet engine as a ratio of fuel flow over Pb. The ratio is fixed for the jet engine's operating conditions such as starting, acceleration, steady-state, and deceleration. If the Pb is unnaturally high, the EEC maintaining the specified ratio for the operating condition will deliver a correspondingly high amount of fuel to the engine. The incident engine, after it was removed from the airplane, was shipped to United Airlines maintenance to be tested in an engine test facility to confirm it could be started. The engine was successfully started eight times; three times on ignition system 1, three times on ignition system 2, and twice with both systems.
There was a malfunction of the electronic engine control (EEC) burner pressure (Pb) sensor that had an abnormally high indication that resulted in an excessive amount of fuel being pumped into the engine that was still at a sub-idle condition. Because the engine was still accelerating to idle, the limited airflow through the engine engine was insufficient for the amount of fuel and resulted in a tailpipe fire. The cause of the EEC's Pb sensor fault could not be determined.
Source: NTSB Aviation Accident Database
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