Metal fatigue cited
in helicopter crash
By David R. Baker
Daily News Staff Writer
In their first report on the deadly crash of a Los Angeles Fire
Department helicopter, federal investigators said Wednesday that a key part of the tail
rotor assembly cracked from metal fatigue.
Experts found a fatigue fracture in the yoke holding the twin tail rotors, which
are needed to control the aircraft, according to the report from the National
Transportation and Safety Board.
The rear rotors have been under suspicion since they were found, along with
their gearbox, about a mile away from the rest of the air ambulance that crashed March 23
in Griffith Park, killing four people.
We are pursuing additional avenues, but the primary focus is investigating
the separations of the tail rotors, said lead investigator Wayne Pollack.
Its an in-flight structure failure.
Pollack cautioned that Wednesdays report is just the first review of evidence
uncovered during what could be a months-long investigation. Determining exactly what
happened to the Bell 205A-1 helicopter might never be known for certain.
Metal fatigue--the weakening of material because of prolonged stress --can build up
over many years of light use or during shorter periods of heavier use. Rough flying
conditions can add to it as well.
Although safety board investigators said they do not know yet how the fracture
started, aviation experts said Wednesday that metal fatigue is a persistent problem for
all types of aircraft.
Most important components on a helicopter must be replaced after enduring a certain
amount of wear and tear. But even that is no guarantee against a crash, experts said.
You meet all the regulations and still have a failure, said Peter
Lissaman, who teaches aerospace engineering at the University of Southern California.
The Federal Aviation Administration has issued several air worthiness
directives to repair or inspect parts in the Bell 205A-1s tail rotor assembly,
but none specifically target the yoke. An FAA spokesman said Monday he did not know
weather the yoke was covered by any of the directives.
The Fire Department helicopters was built in 1976 but city officials
said they did not know weather the yoke had ever been replaced. All records that would
show a replacement or upgrade have been taken by the safety board, according to Les Iden,
assistant general manager for the city department that maintains the helicopters.
Iden said the helicopter that crashed had received new main rotors. But Iden and a
Bell Helicopters spokesman said they did not know the life expectancy of the tail rotor
Iden said that NTSBs report doesnt cause him to questions the safety of
the Fire Departments remaining helicopters, which were inspected after the crash.
The entire tail rotor assembly of those helicopters was inspected in fine
detail, he said.
Bob Wood, a professor of aeronautics at the Naval Postgraduate School in Monterey,
said he had seen metal fatigue problems during the mid-1960s and early 1970s in the tail
rotor yokes of some military helicopters manufactured by Bell.
Although the problems seemed similar to what happened to the Fire Department
helicopter, with both rotors and the gearbox dropping from the helicopters, Wood said Bell
had long ago redesigned the parts that were failing.
He noted that the loss of the tail rotors tends to suddenly pitch the helicopter
forward. A pilot has little time to react.
This all happens really fast, he said. If youre a pilot,
youve got to move.
Wednesdays report from the safety board provided few other clues about the
About six minutes after picking up a young traffic collision victim in Shadow
Hills, the helicopters pilot experienced a partial loss of control in the craft and
radioed that he was experiencing an emergency, the report states.
About 30 seconds later, pilot Steven Robinson survived the crash, along with