Airbus Avoided Use Of Ill-Fated Fuse Pins -- Engines Designed To Stay Attached To Jets' Wings During Emergencies
Airbus Industrie, the European consortium, does not use breakaway safety bolts, called fuse pins, to mount jet engines to the wings of its jetliners.
Instead, when Airbus engineers began designing jetliners for the first time in the early 1970s, they ruled out fuse pins in favor of permanently attaching the engine and its supporting structure, called a strut, to the wing.
"We have no fuse pins," said a high-ranking Airbus engineer. "In other words, we have designed it so that (the engine and strut) will stay attached to the wings in all circumstances."
That difference points up a potential hazard posed by fuse pins used on jetliners built by The Boeing Co.
Designed to snap and release an engine in certain emergencies, fuse pins used on more than 930 Boeing 747 jumbo jets have been found to be susceptible to corrosion and cracking, which can make them dangerously weak. And fuse pins used on more than 480 757 twinjets can develop "fatigue cracks."
Moreover, evidence emerging from two recent fatal 747 crashes suggest that powerful gyroscopic and aerodynamic forces can cause an engine that breaks loose in flight to veer disastrously into other parts of the aircraft.
On Airbus jets, the engine mounts are designed to hold the engines in place even if the airplane belly lands or the engine starts breaking up internally during flight.
"The rules require the fuel tanks (inside the wing) and the wing to remain intact to avoid the risk of fire," said the Airbus engineer. "The Boeing and (McDonnell) Douglas approach is that, in the event of a survivable belly landing, the engine would come off the wing, hopefully cleanly.
"Our approach is that the engine would stay on the wing and slide along, again leaving the fuel tanks intact and providing a degree of protection for the fuselage."
Boeing spokesman Chris Villiers said the company would not comment on fuse pins.
Dave Duff, FAA spokesman, said the divergent Boeing-Airbus philosophies both meet existing safety rules.
"There are different approaches to design," Duff said. "As long as our rules are met and the safety margins we require are there, we will approve it."
Boeing is developing an upgraded fuse pin to replace the corrosion-susceptible pins now used on 747s and has asked airlines to periodically check 757s for cracked fuse pins and replace them with fresh ones.
But safety experts are concerned that fuse pins will continue to pose a hazard as airlines increasingly fly heavier loads using ever larger, more powerful jet engines.
When they were developed in the late 1950s and early 1960s, fuse pins were aimed primarily at planes forced to make belly landings, then a high risk, more survivable. Now that belly landings are rare, the bigger risk may be a weak fuse pin breaking in flight.
In the late 1970s, Boeing considered and then ruled out using the Airbus design and eliminating fuse pins on engine mounts for its then-developing 767 and 757 models, said aviation consultant Dick Sears, a retired Boeing flight engineer.
"There's nothing wrong with the Airbus philosophy; it will, indeed, prevent the engine and strut from coming off in flight," Sears said.
However, Sears said that while the Airbus mounts probably would hold up in a belly landing on a hard airport runway, the mounts could break apart catastrophically if the airplane skidded off the runway onto uneven ground.
"There are tradeoffs," Sears said. "You design for different situations, and you can never cover everything."
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