What Brought Down the Russian Jet in Egypt?

Last week’s tragic crash of an Airbus A321 in the Sinai Peninsula took the lives of all 224 people on board and, like all tragedies that occur in the Middle East, it has become a political football.

Between claims of responsibility from ISIS, reports from US intelligence sources about flashes on satellite imagery and statements from sources as high-level as British prime minister David Cameron, everyone has a theory about the cause of the crash.

Few, however, are thinking about the engineering angle.

MetroJet Flight 9268 was operated by a Russian airline, so it’s understandable that politicos assume a connection between the tragedy and terrorism given Vladimir Putin’s involvement in the conflict in Syria. Somewhere, lost in the ludicrous media speculation about the true cause of the crash, a simple fact has emerged. In 2001, the same Airbus A321 aircraft had a tail scrape incident that required structural repair.

It’s not uncommon for a pilot to over-rotate occasionally on takeoff and rub the tail against the runway, but the consequences can be more serious than they look. Modern jetliners are built with monocoque construction. This means that the external skin is an important and stressed structural part of the fuselage.

Jetliners are also very large, lightweight pressure vessels. The critical rear bulkhead is particularly vulnerable if damaged and part of the repair procedure for tail strike incidents is an inspection of that bulkhead. Approved repair procedures range from riveted doubler plates to replacement of the entire bulkhead, but as mechanical engineers know, stopping crack propagation is more complex than just drilling holes.

For an airliner fuselage, the pressure loads are cyclical, occurring with every takeoff and landing. The MetroJet A321 had over 20,000 cycles, which is not unusual for a midsized airliner on European routes entering the latter half of its service life.

Could an inadequate repair of a damaged rear pressure bulkhead lead to catastrophic depressurization and loss of the aircraft? Without a doubt.

It’s happened before, most tragically in August 1985 when a Japan Airlines 747 suffered an explosive decompression due to failure of the rear pressure bulkhead. The failure released cabin pressure into the normally unpressurized tail section of the fuselage, blowing off the vertical stabilizer. 32 minutes later, the aircraft crashed – killing 505 passengers and crew.

That aircraft had some 18,000 cycles in its logbook and had been repaired after a tail strike incident seven years earlier, which had damaged the rear pressure bulkhead. In that case, Boeing repair technicians used two riveted splice plates instead of the approved procedure which called for a single continuous doubler. This resulted in a repair with 70 percent lower resistance to metal fatigue, according to the Federal Aviation Administration.

Boeing later calculated that the incorrect repair would fail after approximately 10,000 cycles. The aircraft in fact underwent 12,318 cycles from the time of repair until the crash.

So it’s happened before. The inadequate repair of a tail strike incident that damages the rear pressure bulkhead can rear its head years later and result in the breakup of the aircraft at altitude.

Could a bomb have brought the aircraft down? Sure, but the engineering reality is that this aircraft had endured damage to a critical part of the airframe which can produce an in-flight breakup of the aircraft if it fails at altitude.