Chain of events

During last week’s rigging workshop in Oregon I was discussing chains of events which lead to system failures. Oftentimes the actions of the person involved in the accident are merely the final straw in a long enchainment of causalities which end in catastrophic failure.

In this respect you can sometimes almost say that the person is set up to fail.

One of my favourite examples to illustrate this effect is the story of British Airways flight 5390.

Flight 5390 was a scheduled passenger flight between Birmingham and Malagà in Spain. On 10 June 1990, Tim Lancaster was due to pilot the BAC 1-11, his co-pilot was Alastair Aitchison, plus a crew of three flight attendants. After an uneventful take-off, the cabin crew was just busy preparing the in-flight meal, when at a height of 5’300m, passing over Oxfordshire, the left cockpit window explosively blew out of its frame without any warning. Lancaster, who had unfastened his harness, was sucked out of the window due to the sudden decompression, his feet snagging on the flight controls. This resulted in his torso being pressed down by the wind against the fuselage outside the aircraft, with only his legs in the cockpit. Flight attendant Nigel Ogden grabbed Lancaster’s legs to prevent him being sucked all the way out. The decompression had caused the door to the flight deck to blow out, which was now jammed against the throttle, causing the aircraft to continue accelerating whilst descending.

Lancaster, after the event, went on record as saying that when he saw the tail fin of the plane, he knew that something was really wrong. Well yes, you can’t fault that reasoning.

The poor chap was being battered by 550km/h winds at temperatures of -17° Celsius, rapidly loosing consciousness in the thin air.

Atchinson initiated an emergency descent. Due to the noise of the wind rushing past the cockpit, he was struggling to establish two-way communication with air traffic control, in order to inform them of the emergency.

Meanwhile Ogden, still holding onto Lancaster, was also developing frostbite and was exhausted. He was relieved by the two remaining members of the flight crew. Lancaster had at this point been pulled a further fifteen to twenty centimetres out of the window. The crew assumed that Lancaster was dead, now mainly holding onto him to prevent him from being sucked into the engine, potentially causing an engine failure.

After a twenty minute ordeal, Aitchison finally managed to land the plane safely in Southhampton, the passengers were evacuated and the emergency crews rescued Lancaster – who, amazingly, apart from some frostbite, bruising and shock, as well as fractures to the arm, thumb and wrist, escaped otherwise unscathed from this extraordinary incident.

The post-incident investigation revealed that the window which failed had been replaced 27 hours before the incident. However, 84 of the 90 windscreen retention bolts were 0.66 mm too small in diameter, while the remaining six were 2.5 mm too short. This was due to an earlier botched repair on the window, where wrong bolts had been used to fix it in place. During this repair, like had been replaced for like, so the faulty bolts went undetected. Furthermore, the inquest revealed a design quirk in the 1-11, whereby the window is bolted onto the airframe from the outside rather than from inside, which exerts further pressure on the bolts when the external air pressure is low.

The inquiry panel identified a number of action to be taken as consequence of this incident, such as the need for British Airways to review their quality assurance protocols, that the Civil Aviation Agency should consider the need for periodic training and testing of engineers or that, if prescribed, aircraft engineers should use corrective glasses when undertaking engineering tasks.

This case illustrates how a couple of millimetres can literally make all the difference – it also drives home the importance of being attentive to details and diligent when it comes to safety critical systems and to not let mistakes slip by unnoticed or to become self-perpetuating, but rather to identify and deal with them.