Initial reports of the Russian airliner crash over Sinai yesterday were quick to point out that the disaster occurred in an area where Egyptian security forces are fighting an Islamist insurgency. Several hours later a statement ostensibly issued by the Islamic State claimed to have shot the aircraft down. A video posted on the internet, and now widely discounted as a fake, purported to show the plane being hit by a missile and descending in a trail of smoke.

If the Airbus A-321 really had been destroyed by militants the most obvious conclusion would be to view it as a reprisal for Russia's military intervention in Syria. However, there is no reason at present to suppose the aircraft was shot down. Although some airlines had been warned not to fly over Sinai below 26,000 feet, Flight Radar shows the Russian plane was at 33,500 feet when the pilot lost control – far beyond the range of any weapons the Sinai militants are known to possess.

The only other way militants might conceivably have brought the plane down would be by planting a bomb on board or somehow sabotaging it before takeoff from Sharm el-Sheikh airport. That is one possibility the investigators will no doubt look at but, again, there is no evidence at present to suggest it is what actually happened.

Meanwhile, some of the technical discussion on aviation websites (PPRuNe and Aviation Herald) is focusing on the aircraft's tail section which, as can be seen from photographs of the crash site, became separated from other parts of the wreckage.

One fact which could be very relevant here is that 14 years ago the Airbus suffered a tailstrike which was then repaired. A tailstrike, as Boeing's website explains, occurs when an aircraft's tail makes contact with the runway during takeoff or landing (see graphic here).

Several comments on aviation websites point to a problem with the tail as a possible cause of yesterday's crash. For example, one posted at Aviation Herald says:

"It is thought-provoking that the entire tail-cone section housing the APU [auxiliary power unit] and the horizontal stabiliser has separated from the rest of the tail section rear of the aft doors and below the VS [vertical stabiliser] attachment. The start of the missing section corresponds to the position of the rear pressure bulkhead. This separation seems to have occurred while in-flight, as none of the missing wreckage is visible on any of the photos taken of the tail section from a number of angles."

A comment on PPRuNe notes that in photographs of the crashed tail section both sides of the horizontal stabiliser appear to be missing and goes on to suggest that investigators should focus on the tail section:

"I would be very interested in any recent maintenance/repairs of this area (rear bulkhead) as well as any historic damage.

"Jet suffered tailstrike damage in 2001 for which it received repairs. If I were investigating, I would be spending a fair old amount of time looking for any cyclic metal fatigue and crack propagation around the rear bulkhead area, any pre-crash impact damage with the screwjacks ... "

Another commenter writes:

"The tail section ... fell almost vertically and was crushed at the bottom, the impact also breaking the main spar of the VS [vertical stabiliser]. No trace of fire, and also no other wreckage is visible in the background on photos taken from different angles. The entire tail-cone section housing the APU and the entire HS structure is missing aft of the rear door and the pressure bulkhead.

"All this, together with reports of a large debris field, seems to confirm an in-flight breakup around the time radar contact ... was lost."

But how likely is it that a tailstrike in 2001 led to yesterday's crash? There have in fact been at least two previous crashes attributed to a tailstrike that occurred years earlier and had subsequently been repaired.

One was the crash of Japan Airlines Flight 123 in 1985. The Boeing 747 had been involved in a tailstrike seven years previously which damaged the rear pressure bulkhead. This was incorrectly repaired, reducing the bulkhead's resistance to metal fatigue. The Wikipedia entry says:

"During the investigation, Boeing calculated that this incorrect installation would fail after approximately 10,000 pressurisations; the aircraft accomplished 12,318 successful flights from the time that the faulty repair was made to when the crash happened.

"When the bulkhead gave way, the resulting explosive decompression ruptured the lines of all four hydraulic systems and ejected the vertical stabiliser. With the aircraft's flight controls disabled, the aircraft became uncontrollable."

Another example was China Airlines Flight 611 in 2002. The Boeing 747 had suffered a tailstrike 22 years earlier which was not repaired in accordance with Boeing's Structural Repair Manual. The Wikipedia entry says:

"Consequently, after repeated cycles of depressurisation and pressurisation during flight, the weakened hull gradually started to crack and finally broke open in mid-flight on 25 May 2002, coincidentally 22 years to the day after the faulty repair was made upon the damaged tail. An explosive decompression of the aircraft occurred once the crack opened up, causing the complete disintegration of the aircraft in mid-air."

    
Posted by Brian Whitaker
Sunday, 1 November 2015