Aging Aircraft and Structural Failures
Aloha Airlines Flight 243: Structural Failure of an Aging Aircraft Safety 335 aloha Airlines Flight 243: Structural Failure of an Aging Aircraft The age of the United States’ commercial aircraft fleet is a serious problem. The average age of commercial airline fleets is continuing to increase. As of year 2000, more than 2,500 commercial aircraft in the United States were flying beyond their original design lives. In 1988, a major incident in which the top peeled off an Aloha Airlines Boeing 737 in flight, sweeping a flight attendant to her death, was blamed on weak maintenance of the old aircraft’s structure.
The flight attendant was swept overboard at 24,000 feet after a spontaneous failure of one of the aircraft’s longitudinal joints. The aircraft involved, a Boeing 737, had been subjected to the severe operating environment particular to inter-island service during its 19-year lifep. The Aloha Airlines 737 was the second oldest aircraft still in service. The aircraft, which had been designed for 75,000 flight cycles, had actually accumulated 89,680 cycles with stage lengths of 20 to 40 minutes. This intensive use also inflicts the loads associated with repeated pressurization and de-pressurization of the aircraft’s cabin.
Fuselage fatigue damage is primarily caused by the application of the pressurization cycle that occurs on each flight. Typically, the inter-island carriers fly at 23,000 ft while the cabin is pressurized to 8,000 creating a 5 psi differential. The fuselage of this aircraft suffered from extensive Multiple Site Damage (MSD). MSD occurs when stress factors are fairly uniform, so that small cracks appear and grow at roughly the same rate. Each individual crack is difficult to see and by itself poses little problem; however, the small cracks can join together to form a large crack (Oster, Clinton, Strong, Zorn, 1992).
The Aloha 737’s MSD’s were cracks extending on both sides of rivet holes along the upper row of the lap joints along the fuselage. Two other major fuselage failures existed on the upper row of rivets on the S10L lap joint. Near the forward entry door, the MSD cracks had joined to form a single crack about 6-8 inches long. Two passengers noticed this crack as they boarded the aircraft in Hilo, HI. The crack was long enough and wide enough that the internal fiberglass insulation was being extruded from it. The passengers did not report the crack, feeling that if the aircraft was not safe, the airline would obviously not fly it (NTSB, 1988).
The focus of the National Transportation Safety Board’s (NTSB) hearings were the failure of the Boeing 737’s design to Safely Decompress. Contrary to the NTSB findings, the fuselage did tear open a Safe Decompression Flap as designed. If the Flight Attendant had not been standing directly underneath the Flap when it occurred, the plane would probably not have suffered an explosive decompression (Hinder, 2000). The forces exerted on the fuselage by leveling of the aircraft was the final blow that caused a link up of MSD cracks at BS500 (Approximately Row 5) which were arrested by the Safe Decompression design causing the Flap to open.
At the instant in time represented by Figure 1, the aircraft is in the process of rotating from climb to level flight, there is a tear in the S10L lap joint at approximately in front of row 1 and a Safe Decompression Flap at approximately Row 5. [pic] Figure 1 The cabin was pressurized. With the approximately 10″ x10″ opening, the internal cabin air began to escape at over 700 mph. The Flight Attendant who was reaching to pick up a cup from Passenger 5B was immediately sucked into but not through the Safe Decompression Flap. Only the Flight Attendant’s right arm and head were forced through the opening.
This effectively slammed the door shut on a 700 mph jet stream. The resultant reaction to corking a high velocity fluid flow is called a Fluid Hammer. The attempt to stop the high velocity airflow causes a pressure spike of high value (hundreds of pounds per square inch) and short duration (only tens of thousandths of a second). The fuselage integrity was severely degraded due to the MSD and its 0. 036″ (36 thousandths of an inch) pressure boundary wall thickness is only designed for about 8. 5 psi normal operating pressure differential. The fuselage could barely contain the normal operating pressure.
The Fluid Hammer caused the fuselage skin to crack (Hinder, 2000). Fluid flow always follows the path of least resistance. With the Flap at row 5 plugged and the fuselage skin between in front of row 1 and row 5 completely severed, the internal cabin pressure begins to push outward on the fuselage skin, sensing the weakest point as halfway between in front of row 1 and row 5. This is the location identified by the NTSB as the probable location of the initial failure. For the next 0. 6 seconds (6 tenths of a second) the aircraft is propelled nose down and to the right by the internal air escaping from the disintegrating fuselage.
The Flight Attendant begins to slide toward the rear of the aircraft as the lap joint separates. See Figure 2. [pic] Figure 2 For the next 1. 2 seconds the aircraft the moves up and to the left as sections of the fuselage continue to peel away. The section between row 1 and row 5 blows out and downward. The roof section blows up, tearing from the row 1 seam. At row 5, the roof crack angles diagonally back toward the top centerline of the aircraft. Aft of row 5, along the lap joint, above the joint, a diagonal piece folds back over on itself.
Below the joint, the window belt section tears in a backward direction. The Flight Attendant continues to slide rearward. See Figure 3. [pic] Figure 3 The window belt section aft of row 5 and below the lap-joint folds back over rearward. This pops out the window just forward of the row 6 seam and tears the fuselage from the window to the lap joint. This allows the Flight Attendant’s head and body to drop approximately 1 foot just as the section slams against the exterior fuselage. See Figure 4. [pic] Figure 4 The pilots told of a sudden whooshing sound at 24,000 ft. flying debris in the cockpit and a bouncing 25-mile descent with one engine out. The flight was diverted to Maui and a successful landing was accomplished with a significant portion of the fuselage missing. Sixty-nine of the 95 passengers sustained injuries from flailing wires, metal strips and wind burn (Hinder, 2000). According to the NTSB’s report on the investigation, contributing factors were improper inspection by company maintenance personnel, inadequate supervision of maintenance personnel, inadequate supervision by the FAA and inadequate aircraft equipment from the manufacturer.
Numerous other structural failure incidents of note in the same time period also brought to light significant problems to be addressed. In October 1988, a foot long crack was noted in a B-737 while stripping paint. In December 1988, a B-727 was noted with a 14″ crack in the fuselage. In February 1989, a B-747 cargo door failed, the fuselage was torn off and nine passengers were sucked to their deaths. In July 1989, a pre-flight inspection revealed a 20″ long fatigue crack in the wing of a B-727 (Oster, et al, 1992).
Though durability and damage tolerance were issues prior to this, the Aloha incident is generally considered to be the start of the Federal Aviation Authorities (FAA) Focused Aging Aircraft Program. The first response to the accident was an industry-wide review of the adequacy of aircraft design and efficacy of maintenance programs. In general, the aviation community found that with proper maintenance and structural modifications and with attention to service related damage such as fatigue and corrosion, the service lives of airplanes could be safely extended (Seher, Smith, 2001).
To identify and rectify issues related to operation of aircraft beyond their designed service objectives, the Air Worthiness Assurance Working Group (AAWG), the National Aging Aircraft Program, and the National Aging Aircraft Research Programs were established. The National Aeronautical and Space Administration (NASA) and the United States Air Force joined in and concentrated on research in fatigue and fracture issues associated with crack initiation, crack growth and residual strength of multi-site damaged fuselage skins (Seher, Smith, 2001).
Though progress has been made in the area of aging aircraft, the continued desire to maintain aircraft in revenue service beyond their design service objectives and the poor financial performance of carriers, there will almost certainly be new structural integrity problems. It is the mission of the FAA’s Aging Aircraft and Continued Airworthiness Programs to ensure that age-related problems are predicted and eliminated or mitigated prior to their having a major impact on safety.
References
- Hinder, Prof. , (2000, January 17).
