NTSB Factual Report

On March 6, 2002, at 0715 central standard time, a Fokker F-28 MK-100 turbojet airplane, N1425A, operating as American Airlines Flight 334 (AAL 334) experienced an uncontained rupture of the auxiliary power unit's (APU) turbine wheel while operating during de-icing procedures at the Dallas/Fort Worth International Airport (DFW), Dallas, Texas. AAL 334 was being operated by American Airlines, Inc., of Fort Worth, Texas under 14 Code of Federal Regulations Part 121, as a scheduled domestic passenger flight from DFW, to the Nashville International Airport (BNA), near Nashville, Tennessee. The two airline-transport pilots, two flight attendants, and thirty passengers were not injured. Instrument meteorological conditions prevailed, and an instrument flight rules (IFR) flight plan was filed. The flight was in preparation for departure to BNA at the time of the accident.

AAL 334 departed gate Alpha 14 at 0633, and taxied to ramp spot 22 for de-icing. The aircraft was parked at a northeast direction. The ground de-icing crew notified the flight crew to prepare for de-icing. Approximately ten minutes after de-icing began, the Captain reported that he heard and felt a "thud-like" noise from outside the aircraft. At the same time, the APU shutdown. One of the de-icing crewmembers on the ground stated he saw a flame come from the APU exhaust and discharged the ground APU fire bottle. Approximately the same time, the #2 flight attendant who was seated in the jumpseat at the rear of the cabin, phoned the Captain via the intercom and informed him that the emergency first aid kit sustained damage in the form of a three-inch hole in the back of the kit. The flight attendant stated that an apparent impact on the first aid kit had propelled it from the stowed position into the center aisle of the aircraft. The rear cabin jumpseat was located directly in front of the aft pressure bulkhead. The emergency first aid kit was stowed directly onto the aft wall of the cabin underneath the rear cabin jumpseat.

Two spray trucks were used to de-ice the airplane with Type 1 de-icing fluid. The five-person crew consisted of a checker, a driver for each truck, and a spray bucket operator for each truck. The members of the de-icing crew stated that at the time they were de-icing the airplane, the wind was blowing at 20 knots and gusting. Due to the windy conditions, the drivers of the de-icing trucks had coordinated prior to de-icing the airplane to start at opposite ends of the airplane so the bucket operators would not be spraying each other as they de-iced the airplane. The bucket operators both stated that at times, the wind was whipping the stream of de-icing fluid such that they could not even see the airplane, let alone where the de-icing fluid was being sprayed. Bucket operator #1stated that he had completed de-icing the nose, right wing, and right horizontal stabilizer of the airplane, and had just started to de-ice the right side of the vertical fin, when he heard a noise and saw that the APU was on fire. Bucket operator #2 stated that he had de-iced the left horizontal stabilizer, the left side of the vertical fin, and aft fuselage, and after the truck was repositioned, he had started to de-ice the left wing when the checker discovered the APU fire. The bucket operators stated that they knew where the F-100's APU air inlet was located, and were aware that de-icing fluid should not be sprayed into the APU inlet.

The Automated Surface Observing Station for DFW at 0700 reported winds from 320 degrees at 20 knots gusting to 27 knots, visibility 2.5 statute miles in freezing rain and ice pellets, overcast skies at 2,500 feet, temperature minus 4 degrees Celsius, dew point minus six degrees Celsius, and an altimeter setting of 30.01 inches of Mercury. The Investigator-In-Charge calculated the density altitude to be approximately minus 1,586 feet.

The AlliedSignal (Honeywell) GTCP 36-150RR APU is transversely mounted in the tail compartment directly behind the aft pressure bulkhead. According to American Airlines, the APU had a total time of 19,637 hours and 15,674 cycles since new. The APU had operated 1,764 hours and 1,167 cycles since its last overhaul. Air for the APU is supplied through an intake duct. The APU intake duct inlet is located on the upper right side of the fuselage adjacent to the vertical stabilizer, just aft of where the leading edge of the vertical stabilizer attaches to the crown of the fuselage. The APU's air inlet door, which is hinged at the front, opens inward to an angle of 15 degrees when the airplane is on the ground and the APU is operating. The F-100 does not have a flow diverter strip over the APU air inlet, similar to what is installed over the cabin doors, to divert the flow of any fluids away from the doors.

Examination of the airplane revealed APU damage, which was located inside the tailcone. The turbine wheel was found fractured into five pieces. The fracture surfaces were coarse and grainy, with no evidence of fatigue. The impeller was found intact and the impeller vane leading edges had nicks and gouges and were bent opposite the direction of rotation. Ridges on the rear face of the impeller were in line with the vanes on the front face. A fragment of the turbine wheel penetrated the aft pressure bulkhead, and was found embedded in the first aid kit that was stored directly underneath the flight attendant's aft jump seat located at the rear of the cabin. Examination of the APU was conducted at American Airlines' maintenance facility in Tulsa, Oklahoma. The examination revealed two large holes in the APU's compressor case in line with the turbine wheel's plane of rotation. The turbine wheel was broken into several pieces. The broken pieces of the turbine wheel were submitted to Honeywell's Materials Laboratory for a metallurgical examination that confirmed the APU had experienced an overspeed. Further examination did not reveal evidence of fatigue on any of the fracture surfaces. Additionally, the containment ring was broken into three pieces.

The APU has an electronic control unit (ECU) that meters fuel to the APU while it is in operation. The ECU monitors the rotor speed and will shut off fuel to the APU if it senses an overspeed, which is 107 percent. The ECU was removed from the engine and shipped to Honeywell's Anniston, Alabama facility to interrogate the ECU's non-volatile memory (NVM) and perform a diagnostic test. The NVM indicated that the APU had experienced an overspeed, which is a rotor speed of 107 percent or greater, and had commanded a shutdown. The diagnostic test showed that there were no discrepancies with the ECU.

A review of American Airlines' de-icing training program showed that each of the 5 had completed annual refresher training in de-icing procedures for the F-100. During de-icing training, personnel are advised not to spray the F-100 APU inlet area, and crews are furnished diagrams of each type of airplane that higlight prohibited spray areas. According to company records the following is a summary of de-icing experience for the crew:

Checker 14 years

Driver #1 14 years

Driver #2 17 years

Bucket Operator #1 13 years

Bucket Operator #2 15 years

In February 2001, American Airlines issued a 'Winterization Bulletin' for the F-100 advising that de-ice spray should not be sprayed into the APU inlet. Additionally, the F-100 maintenance manual section 12-31-00, page 301 specifically advises, "Do not let de-icing and/or anti-icing fluid/water mixture go into the APU inlet. Injury to persons and/or damage to equipment can occur." American Airlines' policy had been to operate the APU during de-icing. As a result of this event, pilots were instructed to not operate the APU during de-icing operations, and to refer to the operating manual and follow procedures for de-icing with an inoperative APU. De-icing crews were also informed of the change in de-icing procedures, regarding the operation of the APU.

As a result of this event, on March 29, 2002, the National Transportation Safety Board issued Safety Recommendation A-02-05 that recommended the Federal Aviation Administration (FAA) immediately issue an airworthiness directive (AD) for the Fokker F-28 MK-100 and F-28 MK-4000 airplanes that prohibits APU operation during deicing operations. The FAA issued AD 2001-07-03 that prohibits the operation of the APU during deicing operations for all series of the F-28 airplane.

NTSB Probable Cause

The ground crew inadvertent application of deicing fluid in the auxiliary power unit resulting in an overspeed and turbine wheel burst. A contributing factor was the gusty wind.