Engineering and Construction
On July 6, 1996, the flight 1288 at Pensacola International Airport in Pensacola, Florida experienced an uncontained engine failure during a routine takeoff operation. The plane had 137 passengers and 5 aircraft personnel. The plane was flying from Pensacola International Airport to Atlanta International Airport, Georgia. At 14:24 central daylight time, the aircraft experienced the engine breakdown during takeoff as a result from which debris of the left engine fan hub passed through the left aft airframe (some pieces passing through the aircraft). After the engine failure, the plane was able to make a complete stop on the runway. The failure caused death of two passengers, two others were seriously injured, and 3 minor injuries occurred. It was an internal passenger flight conducted by Delta Airlines. The accident happened in the daylight visual meteorological conditions.
The probable cause of the engine failure was the fatigue cracks of the engine No.1 pressure fan hub. The National Transportation Safety Board determined that the engine breakdown happened due to parallel surface cracks (ladder cracks) that were missed by Delta Air Lines’ fluorescent penetrant examination process failure. This non destructive testing was performed in order to find a detectable fatigue cracks on the surface. Thus, manufacture process as well as the insufficient redundancy in the in-maintenance check program contributed the failure.
The failed hub was manufactured by Volvo for Pratt & Whitney who was building the engines for the MD-88. The fatigue cracks from manufacture were missed both during manufacture as well as inspections before installation.
On July 6, 1996, the McDonnell Douglas MD-88 flight 1288 was scheduled to take off from Pensacola International Airport, Florida to Atlanta International Airport, Georgia. The plane was carrying 137 passengers, 5 crew members, 2 pilots, and 3 flight attendants.
The preflight inspection began at 13:30 during the failure day which was performed by the first pilot as well as the captain of the plane. The officer noticed the oil coming from the nose bullet but there were only few drops of oil so the officer and captain made a conclusion that the amount of oil present did not appear to be significant. In addition, he noticed that two rivets were missing on the outboard section of the left wing. These factors were deemed insignificant and concluded the aircraft as being airworthy so the departure was to be performed without notifying maintenance. Both engines started normally first left then right without any extra sounds and or vibrations.
The first officer, who was the pilot of the aircraft, achieved the outbound clearance at 14:23. A minute later the start of takeoff, there appeared a “loud bang” followed by the loss of energy and instrumentation at a cockpit. The airplane stopped on the runway, and the first officer attempted to contact the tower and flight attendants in the cabin. The attempt failed, since the power was shut down. At 14:25 the flight crew turned on the emergency power and contacted the tower to report an emergency. Since there was no indication of fire, the captain did not initiate the evacuation. The flight attendant commanded the passengers remain seated via a portable megaphone.
The first officer went to inspect the cabin. What he saw was that the emergency left back door (L-2) was opened, and some passengers were already standing on the runway. The engines were operating, since there was no fire indication signal, and there was no need to shut them down; thus, the first officer went back to a cockpit and shut down engines. He also informed the tower that the aircraft was shut down on the runway. He also requested the medical supply at 14:27, since after the second cabin inspection; he noticed that some passengers were injured. In addition, he recommended the passengers to stay seated, since it was safer than evacuating via the over wing door.
The evacuation was initiated by flight attendants, who after unsuccessful attempts in contacting the cockpit began the evacuation based on serious airframe damage and passengers injuries. After recognizing the fire coming from the left engine, the L-2 door was closed, and passengers were redirected forward. The captain asked the tower to send the portable stairs in order to evacuate the passengers. However, the first stairway sent to an airplane was not suitable for this particular aircraft. Thus, about 25 minutes after the accident, the proper stairs were sent, and the passengers were evacuated from the board.
TECHNICAL SUMMARY OF EVENTS:
The back left airframe and interior of the aircraft were seriously damaged by debris from the engine. On the left fuselage exterior 16 holes were documented. Furthermore, seven punctures were documented on the right side of the plane, forward to the row 37. No penetrations to the floor or below floor level were found. There were 154 wires in the bundle to airframe, 146 of them were damaged by the entering debris. The interior of the cabin was seriously damaged near seat row 37, next to the left engine. The interior cabin walls were skewered by the debris from the fan hub and fan blades. Engine debris was found along the runway on the airplane’s path. The fan hub and blades were away from the engine completely. The case of the engine was torn, and there was a large amount of missing parts. The fan hub disassembled into three major pieces. One of the major pieces was found in the right side airframe interior above the window at the passenger seat row 37.
The probable cause of the accident is the fracture of left fan hub due to the fatigue cracks initiating. These fatigue cracks were a result of overuse of the end mill using a one pass drilling method approved to save time. As the chips would build if there were delays in the fluid flushing the chips out this would cause extra friction there for raising the temperature and wear to the end mill. As the high speed steel end mill would deteriorate the hub would consume (similar to a welding process) the end mill fragments changing the properties of the turbine hub by adding iron to the titanium hub. As this happened the hardness would increase while it would work harden and formed latter fatigue cracks in the deep half inch diameter holes. These latter cracks then would propagate upon duty cycle to a point as happened in flight 1288 ruptured the hub completely. The experiment had shown that the fatigue cracks started propagating almost immediately after the engine was put into service. Furthermore, the holes in the fan hub were recognized as non-detectable via existing inspection templates. The oil that was observed preflight by the first officer came from the No. 1 bearing housing and, therefore, was not related to the accident.
DECISION MAKING SUMMARY:
One of the most serious mistakes was made by the passenger sitting next to the over wing emergency door. He opened the emergency exit without total assurance that it is necessary when the plane was not at a complete stop. As a result, it led to the injury of passengers during the evacuation. On the other side, in the information sheet provided by Airlines, the circumstances in which the emergency doors are to be opened are not clearly identified. In addition, the passenger was under the pressure of other passengers on board, who were in panic. Thus, it must be said that it not a passengers fault for the individuals being injured.
Another issue is that the flight attendants began the evacuation despite the fact that there were no signs of fire and smoke. On the other side, the instruction for the emergencies requires the evacuation to be performed. However, this should be agreed with the board captain. Since the interphone connection was damaged, the flight attendants began the evacuation without the captain agreement according to the instruction. The situation when the interphone connection is broken was not once considered by various committees
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