To: Blacksheep
Airline metal bashers [or knockers] do the job to an approved repair scheme, work to approved drawings and do as good a job as any manufacturer. Airline development engineers holding regulatory authority design approvals have all the relevant degrees, specialise in the repair field and know how to liaise with the right people to get any extraordinary data.
In the case of the Japanese 747 the tail scrape caused a crack in the pressure bulkhead. A repair was developed by Boeing and implemented by Japanese technicians under the supervision of the Boeing repair reps. Somehow the repair deviated from the design fix in that the rivet lines were not redundant. The repair failed and the rip stop construction of the aft bulkhead was compromised and the bulkhead failed. The debris entering the aft compartment damaged the three hydraulic systems causing loss of the flight control systems. There is a vent in the aft compartment to allow equalization of pressure as the aircraft changed altitude and to accommodate minor leakage of the pressure bulkhead and it was sized because of the rip stop construction of the bulkhead. Boeing tests showed that the bulkhead would never fail catastrophically. There is also a hatch or cover which when removed allows access to the inside of the vertical fin. I understand that this hatch cover was not in place, which allowed the escaping air from the cabin to enter the vertical fin over pressuizing it and causing structural failure and subsequent loss of the vertical fin and the rudder.
After the accident Boeing constructed a test rig to test both the structural integrity of the aft bulkhead and the level of pressure that would cause it to fail. The bulkhead would not fail catastrophically until the structure was compromised in a similar manner to the defective repair. When it did fail it blew a TV camera over 500 feet.
The point is that the manufacture does not always get it right nor do the technicians from the airline.
The aircraft and its’ passengers and crew might still be alive if Boeing had incorporated hydraulic fuses in the three hydraulic systems.
