Following last week’s Southwest airlines fuselage in-flight decompression the FAA is announcing it will issue new Airworthiness Directive that would mandate new inspection procedures for earlier model Boeing 737-300/-400/-500. The measure is taking place after the roof of a Southwest Airlines Boeing 737-3H4 (modified with fuel efficient blended winglets) registered N632SW ruptured during flight 812 from Phoenix to Sacramento on Friday April 1st 2011. Passengers were grateful after the plane landed safely in Yuma, Arizona following the emergency diversion. The National Transportation Safety Board who is leading the investigation is already analyzing data contained on both of the 15 years old plane ‘black boxes’ Flight Data Recorder and Cockpit Voice Recorder. In the meantime the carrier has announced Monday that after thorough inspection of its 67 Boeing 737-300 (out of 79 total in the fleet) it was ready to return 64 of them to service while three aircraft which had shown subsurface cracks would obviously have to be repaired.

We have conducted a cursory review of FAA Airworthiness Directives on the older 737 which identify newly mandated procedures changes as new safety requirements appear for the industry. We found a minimum of 17 Airworthiness Directives affecting Boeing 737-300/-400/-500 which generally expounded where and how cracks caused by fatigue and load were likely to develop but also the method to spot signs of cracks. Age of an aircraft was the most powerful factor. Aircraft with very high cycles were encountered and required strenuous attention from their operator. Generally 4,500 cycles would separate inspections on aircraft with a certain ages. By the 10,000 cycle bar an aircraft deserved full attention. FAA-developed model fuselage structure of a 737 test and fatigue analysis showed fuselage skin cracking between 21,000 and 42,000 cycles. However one carrier operated one 737 with 52,000 cycles at some point during the last decade. The favored inspection method was the repeated, careful visual inspections except when the time had come for a thorough High or Low Frequency Eddy Current Test. Frequency for conducting such tests was a pre-determined finite number of cycles.

( A federal investigator carrying the flight 812 Cockpit Voice Recorder and Flight Data Recorder to NTSB headquarter in Washington DC on Saturday April 2nd 2011)

The second factor was the places at which cracks primarily developed; at places where fuselage elements came into contact with one another and attach; the combination fasteners/lap joints that ties lower or upper skin sheet metal to various components was the most likely place for cracks to evolve, with the hidden lower skin more vulnerable than the upper. Surfaces where attachments like fasteners heads, stringers tie clips and dorsal fin resided were also to receive thorough inspection. At places where repairs had been previously conducted, fasteners, tear straps (hot bonding technology), chemically milled surfaces showed likelihood of cracks. Moreover bear straps that had been applied at certain lap joints in order to slow down the propagation of cracks was a hindrance to both visual and eddy current inspection. Stringers were the most numerously mentioned core structural elements to show cracks at many points along the fuselage. Frames and stub beams had also shown vulnerabilities. All structures and elements around the forward, main and aft cargo doors were prone to fatigue; skin, doubler, bear strap, stiffeners, frames surrounding the main forward and aft cargo. The overall fuselage skin aft and forward section was vulnerable on both sides of the airplane along longitudinal edges of the bonded skin doubler.

The Boeing 737 first rolled out of the assembly line in 1967 and benefits from an iconic stature with operators and enthusiasts. Today 737 still show their fuselage kinship tracing back to the Boeing 707 and 727 aircraft. At 44 years of age, which is very unlikely for a passenger aircraft program, the manufacturer has been able to continuously improve the design into the current Next Generation -600/-700/-800/-900 series. The industry-wide true commitment for the type took place after the tragic in-flight depressurization of Aloha Air flight 243 in 1988. That event marked the single mots decisive moment when the industry, carriers, manufacturer, regulatory agencies began implementing crucial but effective measures that have allowed the aircraft to remain viable and safe to operate.

Aloha Air Flight 243 on April 24th 1988. The 737-200 structural integrity had been compromised by advanced corrosion after maintenance program failed. Years of operating from Hawaiian airport in direct proximity to the salt-rich maritime environment may have played a factor.

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