Investigators have uncovered more evidence implicating problem fuse pins in the case of a jet engine ripping off the wing of a 747-100 freighter shortly after the jet took off from Anchorage last Wednesday.
The National Transportation Safety Board released a description of damage to the fallen engine that indicates - according to industry sources - that the engine was operating properly when the accident occurred.
"There doesn't appear to be any indication of major engine distress," said Mark Bobbi, jet-engine expert at Forecast International, an aerospace consulting company.
Moreover, one of the four fuse pins that secured the engine to the wing was found sheared in two, while the other three remained intact and fastened to remnants of the airplane's wing.
No one was injured when the Evergreen International Airlines jumbo jet lost its left inboard engine over a populated neighborhood, minutes after taking off from Anchorage International Airport.
Safety board spokesman Brent Bahler said no official conclusions have been reached.
"Good intensive detective work will continue," Bahler said.
Weakened fuse pins - safety bolts that secure jet engines to the wings of Boeing jetliners - already have been implicated in two fatal 747 crashes in the past 15 months. Designed to snap in certain emergencies, fuse pins have proven to be susceptible to corrosion and cracking that can make them dangerously weak.
After an El Al Israel Airlines 747-200 freighter missing its two right engines crashed into a crowded 10-story apartment building in Amsterdam last October, authorities ordered ongoing inspections of the world's fleet of 930 747s - and replacement of weak pins.
The Federal Aviation Administration inspection order required inspection of all four pins, but mandatory replacement of only the two "mid-spar" pins implicated in the El Al crash and a virtually identical crash in Taiwan in December 1991.
The Evergreen jet's two mid-spar pins, in fact, were replaced in mid-January, at which time the forward and aft pins were inspected, but not replaced, Bahler said.
Now investigators are analyzing why the aft pin on the Evergreen jet sheared, and yet the mid-spar and forward pins remained intact.
Industry sources noted that the pins are designed as the weak points in the 747's engine mount system. When all four pins are at full strength, only a major event, such as massive engine failure or a belly landing, should be capable of snapping them. The four pins are designed to snap more or less simultaneously in emergencies, sources said.
One explanation for the evidence found by the safety board is that the Evergreen jet's rear pin was weak and snapped as the heavily laden plane began to turn left away from the airport under maximum thrust, said a veteran 747 mechanic at a major U.S. airline who is familiar with engine mounts.
With the rear pin no longer keeping the mounting system in equilibrium, the engine could have twisted violently, ripping loose the other mounting points, the mechanic said.
In theory, the other fuse pins should have snapped permitting the engine to break away cleanly. Instead, the center and forward pins remained intact. The result: the loose engine tore out a large chuck of the leading edge of the wing.
"Once this unusual load of the (rear) brace cutting loose took effect, the other attach points gave way," the mechanic theorized. "It looks like the attachment lugs (on the wings) were weaker than the fuse pins."
Unusually turbulent winds in the area also may have been a factor. A weather specialist has been called in to join the investigation team, Bahler said.
Bahler said investigators had not yet begun focusing on a cause for the crash, but were reviewing flight recorders from the 747 and another jet that took off just prior to the Evergreen flight. The voice cockpit recorder from the stricken jet also was being analyzed in Washington, D.C.