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Metal Fatigue Led to Fuselage Rupture

Fri, Oct 15, 2010 — David Evans

Articles

A hole blew open above the passengers sitting in the rear of the Southwest Airline’s B737 cabin, causing loss of pressurization and the pilots to make an emergency landing at Charleston, WV, rather than continue the flight from Nashville to Baltimore.

Buried in the National Transportation Safety Board (NTSB) 18 August 2010 report of the incident is this revealing tidbit:

“Boeing finite element modeling suggests stress levels are higher in the skin at the edges of chemically milled steps adjacent to non-chemically milled bays due to the difference in stiffness.”

The NTSB report does not say whether this Boeing metallurgical analysis took place before or after the 13 July 2009 rupture over WV. If the analysis took place after, Boeing was derelict in not doing it before, as part of the process of certificating the B737-300 for passenger-carrying service. If the analysis was conducted before this emergency landing, why weren’t its findings incorporated into the airplane’s recommended maintenance schedule?

All sorts of questions come to mind, virtually none of them answered by the NTSB investigation of this incident. (See Aviation Safety Journal, “Ruptured Southwest B737 Fuselage Being Investigated by Safety Board”)

The passengers' view of the hole in the ceiling.

The passengers' view of the hole in the ceiling.

Boeing issued a Service Bulletin (SB) on 3 September 2009 – a month and a half after the incident – calling for repetitive inspections in the area where the skin burst open. Not to be seen as “out of the loop” on the situation, the Federal Aviation Administration (FAA) issued an airworthiness directive (AD) on 12 January 2010 – six months after the metal failed and four months after Boeing issued its SB – making compliance with the SB mandatory.

The NTSB determined the probable cause of this event as:

“Fuselage skin failure due to preexisting fatigue at a chemically milled step.”

The NTSB was apparently satisfied with Boeing’s SB and did not issue any recommendations.

Pity; an opportunity was lost. NTSB recommendations following a fatal accident involving an Aloha Airlines B737 in 1988 spawned a host of overdue changes. In the Aloha accident, about a third of the upper fuselage departed the airplane when a lap joint failed. Flight attendant Clarabelle Lansing, standing in the aisle, was sucked out when the cabin depressurized. Her remains were never found. The passengers were shocked by the event and were buffeted by winds as they were kept in their seats by the lap belts. The pilots were able to land the airplane. The airplane was deemed beyond repair and was sold for scrap.

Before it was hauled to the junkyard, the airplane was carefully examined. Evidence of metal corrosion and fatigue set in motion the FAA-mandated supplementary structural inspection program (SSI) to inspect all older jetliners for evidence of metal failure, which could lead to a repeat of the Aloha Airlines accident.

In the Southwest incident, the B737-300 was 15 years old – younger than the Aloha jet by a good five years. If the Southwest B737 was not covered by the SSI, the gross failure of the metal should qualify jets of this age for the SSI program.

NTSB investigators found fatigue cracking of the fuselage skin near the leading edge of the tail. The cracking led to an 18 x 12-inch flap in the skin that rapidly depressurized the airplane as the airplane was cruising at35,000 feet. The yellow cups deployed (oxygen masks) and the pilots immediately diverted to Charleston and conducted an emergency landing.

In all the technical verbiage of the NTSB report, two sentences describe in layman’s terms the metal failure:

“The fuselage skin assembly near the leading edge of the vertical stabilizer was manufactured by bonding two full aluminum sheets together, then selectively chemically milling away pockets (bays) of the inner sheet. Continuous fatigue cracks initiated from multiple origins on the inner surface of the skin adjacent to the step formed at the edge of the chemically milled area and propagated outward.”

Piece of the Southwest B737 removed for analysis of metal fatigue.

Piece of the Southwest B737 removed for analysis of metal fatigue.

Note that the cracking occurred on the inner surface. This cracking would not normally be detected by the usual outside inspection. While a SB covers this unique inspection requirement, are other areas of the B737 structure subject to similar inner cracking? The NTSB doesn’t say, and the question may not even have occurred to NTSB investigators.

Moreover, if chemical milling leads to fatigue cracks, we have a problem. Normally, sufficient strength must be built in such that a part can miss two inspections without failing in service. But what if chemical milling is not considered a source of strength degradation? The area chemically milled may not ever be inspected.

Boeing evidently has determined that chemical milling can lead to higher stress levels – which mean a greater propensity for fatigue cracking. The lesson from this incident seems to be that every part of the structure where chemical milling was applied needs to be part of the airplane’s routine, repetitive inspection requirements.

Let the record reflect, the flap that ripped open was subject to a net force of about 1,500 pounds (the pressure differential between the inside of the cabin at 13,000 ft of altitude and the outside air at 34,000 ft). Fuselage skin about the thickness of a piece of cardboard is under terrific stress when the cabin is pressurized for the safety and comfort of the occupants. Given the need to contain enormous forces, routine inspection of known weaknesses ought to be required. And just maybe the use of chemical milling ought to be reconsidered.


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