The company is developing a new fuse pin to replace the part thought to be the cause of the Amsterdam disaster last October which killed about 50 people, mostly on the ground, and a crash in Taiwan in December 1991 which a crew of five died. Both aircraft were freighters.
Boeing is considering supplying the fuse pins free to the airlines. Four pins, each about the size and shape of a large baked bean tin, are used to connect each of the four engines to the wings. They are designed to break in a crash landing or if there is such heavy vibration from the engine that the wing is at risk of breaking.
Boeing has a long history of problems with the fuse pin on the 747s. The original design was found to be subject to corrosion and a second generation was introduced from 1980. The Taiwan aircraft had the newer pins while the Amsterdam jumbo was one of the few still with the original type, which have subsequently been phased out. Now a new type of pin should be ready by the summer. A Boeing spokesman, Christopher Villiers, told the Independent: 'We hope airlines will replace all existing pins. We expect the Federal Aviation Administration (FAA) in the US to set up a timetable to make it mandatory to put in the new pins.' He expected other aviation authorities to follow suit.
Boeing has carried out extensive research since the two 747 crashes, including a test on the fuse pins using a super-fast computer which revealed previously unknown microscopic regions where stress levels were eight to ten times higher than expected.
On 31 March this year, a third 747 freighter lost an engine - at Anchorage in Alaska - and in the past year two 707 freighters have had engines fall off during flights. All three managed to land safely.
The 707 and 747 have many similarities. Michael Ramsden, editor of Aerospace, the magazine of the Royal Aeronatical Society, and himself a pilot, said: 'The 747's engine attachments are designed to the same structural and certification principles as the 707's. My personal view is that there is an airworthiness crisis because these parts should fail safe and they are not. There should not be a hard failure more than once in the lifetime of the fleet. And we've had five incidents in less than 18 months.'
Investigators from the US National Transportation Safety Board have found that the fuse pins were intact on the Anchorage 747 but discovered metal fatigue in part of the fittings which might, along with the very heavy turbulence which caused the aircraft to roll 50 degrees, have been responsible for the loss of the engine.
The fact that all five incidents involve freighters, a small proportion of the world's fleet, has led investigators to seek a link. All initially involved inner engines, known to suffer more stress than outer ones.
The two incidents involving 707s were found to have been caused by metal fatigue in the fittings which hold engines on to the wing. The frequency of inspections has been stepped up as a result. One possible explanation of why freighters have been affected has been revealed by investigators into the Amsterdam air disaster. They found that the extra stress placed on the suspect fuse pin was probably caused by one of the fans in the engine being slightly out of alignment and rubbing against its housing.
Thomas McSweeny, FAA director, explained: 'This would have caused vibration placing extra stress on the fuse pin over a period of time. In a passenger aircraft, people on the plane would notice the extra vibration and complain.'
Another theory is that cargo places extra stresses because it is concentrated in part of the fuselage, whereas passengers and baggage are more spread out. However, Mr McSweeny said: 'We have considered this but can't explain that it would have a different effect on the stresses.'
Overloading is another possible cause. A pilot who regularly flies freighters said: 'Freighters are frequently full to the gunwales. You rely on third parties to ensure that the weights given are correct. If you have five parts each listed as eight tonnes and they weigh nine, you have a significant problem.'Reuse content