Sunday, 11 September 2016

The AAIB technical investigation

[What follows is excerpted from a long and detailed article by K P R Smart, AAIB Chief Inspector of Air Accidents, entitled The Lockerbie Investigation: Understanding of the Effects of the Detonation of `Improvised Explosive Devices’ on Aircraft Pressure Cabins that was published on this date in 1997:]

At 19:03 hrs UTC on 21 December 1988 Pan American World Airways Flight PA 103 from London, Heathrow to Kennedy Airport, New York was receiving its oceanic clearance from Shanwick Oceanic Control. Seconds later the secondary radar return disappeared from the controller’s screen and multiple primary radar returns were seen to fan out in an easterly direction for a considerable distance.

An improvised explosive device (IED) had detonated in the forward baggage compartment of the Boeing 747 at station 700. The structural damage to the aircraft forward fuselage caused the forward section of the aircraft to detach and pivot to the right around the window belt on the right side. The nose section of the aircraft struck the No 3 engine intake causing the engine to detach from its pylon. This element of the structural break-up was complete within three seconds of the detonation of the device. The aircraft then entered a steepening descent path with the forward fuselage structure detaching until it reached a vertical descent at some 19,000 feet over the town of Lockerbie. At about this time the tail surfaces of the aircraft started to disintegrate, probably by a flutter mode, and as a consequence the rear fuselage started the break-up. A large section of cabin floor and baggage hold from the rear fuselage together with three landing gear units fell onto a residential area in Lockerbie. The main wing structure struck the ground a short distance away, destroying a bungalow and creating a huge crater in the ground. There was a very strong westerly wind at the time of the accident (115 knots at the aircraft’s cruising altitude of 31,000 feet). These winds produced a wreckage trail that stretched from Lockerbie in the south west of Scotland to the east coast of northern England, some 80 miles away. The recorded primary radar returns showed debris falling over the east coast of northern England more than one hour after the initiating event. All 259 passengers and crew on board the aircraft were killed and 11 residents of Lockerbie lost their lives as the wreckage fell onto the town.

At the time of the disaster it was dark and the initial emergency service response was concentrated in and around the town of Lockerbie. The area to the east of Lockerbie is sparsely populated and includes one of the largest manmade forests in Europe, the Kielder Forest. The police had initially identified some seven major wreckage sites in or near the town and the rescue teams set about the task of recovering bodies whilst at the same time preserving essential evidence for the criminal and technical investigations. Increasingly it became clear that wreckage was being discovered at greater and greater distances from Lockerbie and the eventual wreckage and evidential trail was established to have covered an area of 840 square miles.

From the start of the investigation into the causes of the Lockerbie disaster, the police and the AAIB were considering two possible scenarios. The first involved sabotage, which would obviously have resulted in the police conducting a criminal investigation. The second, that the aircraft had been destroyed by defects in the aircraft structure, which would have resulted in the AAIB taking the lead in an investigation under the Civil Aviation (Investigation of Air Accidents) Regulations. On 26 December a small section of baggage container was recovered from the open countryside to the east of Lockerbie. This piece of wreckage showed evidence of being in the vicinity of detonating high explosive. Forensic examinations conducted on 26/27 December confirmed the initial  findings and the world was notified of these facts in a press release on 28 December. At that time the AAIB decided that the technical investigation, conducted under the Civil Aviation (Investigation of Air Accidents) Regulations, required clear boundaries to ensure that no conflict arose with the criminal investigation. It was decided that the AAIB investigation would determine the position of the device within the aircraft, the sequence of structural failures that led to the break-up of the Boeing 747, and consider what safety action could be recommended to provide the aircraft with enhanced protection against explosive devices. The technical investigation was therefore able to concentrate on the aviation safety aspects arising out of this disaster whilst at the same time assisting and supporting the criminal investigation being conducted by the police.

The initial AAIB team of ten accident investigators arrived in Lockerbie at 01.30 hrs, some six hours after the accident occurred. Over that first night they started to assess the task ahead and co-ordinate their activities with the police. In the days that followed the disaster the numbers of agencies and personnel increased to peak at around 2000 personnel working on the accident site. On the day after the disaster the AAIB arranged for the Royal Air Force to fly a series of photographic reconnaissance missions in an attempt to establish the boundaries of the wreckage trail. It quickly became clear that there were in fact two wreckage trails. One, the `northern trail’ , was bounded at its western end by the town of Lockerbie where a number of large sections of aircraft structure and three of its engines fell. This trail extended to the east by some 15 km. The second trail, `the southern trail’ , was far longer and stretched from the site of the initial explosion, south of Lockerbie, to the east coast of northern England some 80 miles away. These very long wreckage trails were a result of the upper level winds on the evening of 21 December which were from the west at 115 knots at flight level 310 (31000 feet), the cruising altitude of the aircraft prior to the explosion taking place. The first priority for the technical investigation was to identify and record the position of all the items of wreckage over the very long wreckage trail. This was done with the aid of military photographic interpreters and large teams on the ground who examined, identifed and recorded each piece of wreckage.

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