In
1990, after a Boeing 737-300 exploded and burned on the ground at
the
Manila airport, the National Transportation Safety Board made several
recommendations to the Federal Aviation Administration.
With yesterday's urgent airworthiness directive calling for inspections
of
almost all 737s to detect a possible electrical problem inside center
fuel
tanks, the FAA has now heeded most of the NTSB's 9-year-old
advice. It took
years of well-documented problems among the worldwide fleet
of jetliners for those
recommendations to become rules. They were deemed by the FAA as unnecessary
in 1990, when the NTSB first theorized there were hazards.
Such has been the history of aviation-safety changes: A problem documented
by
multiple incidents or discoveries is far more likely to sway government
or
industry engineers than theories or evidence deemed inconclusive from
a single
accident. That threshold for change has been weakened somewhat by
public and political pressure after a cluster of major air crashes
in the mid-1990s.
The FAA has showed signs of being more aggressive, especially since
the 1996
midair explosion of TWA Flight 800 near Long Island, which killed
230 people
aboard a 747-100. A fuel-tank explosion of internal, though undetermined,
electrical origin was blamed for that accident. TWA Flight 800 inspired
intensive
scrutiny of airplane wiring and fuel systems. The unsolved Manila
explosion,
in which eight of 119 Philippine Air Lines passengers were killed,
drew attention again during the TWA investigation as the only other
similar explosion in recent
commercial-jetliner history.
But it's still problems found on airplanes in service, not single
catastrophic
events, that prompt most of the hundreds of airworthiness directives
issued by
the FAA, and government and industry officials say that's simply the
system
working as it should.
Such was the case with yesterday's airworthiness directive covering
737 fuel-
tank wiring. The safety order focused on a device called a float switch,
which
prevents overflow during fueling. Chafed float-switch wires inside
center
wing tanks recently were found in eight 737-200s operated by seven
airlines.
The condition could enable electrical arcing, which could ignite volatile
fumes. That
discovery apparently was the result of fuel-tank inspections stemming
from the
Flight 800 investigation.
The FAA ordered that operators of all but the newest 737s, the world's
most-
popular airliner, inspect the wires for chafing, which is caused by
vibration,
and for signs of arcing. The inspections must be completed before
a plane
reaches 30,000 flying hours or before April 17, whichever is later.
Boeing
already had issued a service bulletin recommending the inspections.
Back in 1990, the NTSB urged the FAA to do nearly the same thing:
require
inspections to detect faulty float switches and damaged wires that
could
enable a spark of sufficient energy to ignite fumes, according to
government
documents.The NTSB then alerted the FAA that the Philippine Air Lines
investigation had revealed "potential defects involving the center-tank
float switch and the wiring for the float switch, both of which could
have been the source of the
ignition." The NTSB is often invited by foreign countries to
assist in disaster inquiries involving U.S.-made aircraft.
But the FAA decided not to take action, saying that certain defects
investigators found were unique to that airplane's float switch and
that tests
showed that an electrical fault in the system would generate a spark
too small
to cause an explosion. Nine years later, eight examples of chafing
in aging
airplanes prompted yesterday's order. The planes with the wire chafing
each had between 32,000 and 85,000 flight hours. The inspections,
which are not costly, are
required for 1,181 U.S. 737-100s, -200s, -300s, -400s and -500s. There
are
2,984 such 737s worldwide. Other aviation authorities are certain
to issue
their own directives. Newer Boeing 737-600s, -700s and -800s
are not subject
to yesterday's airworthiness directive.
Boeing spokesman John Dern said the float-switch issues then and now
are
different. "The issue we're dealing with today wouldn't have
even been
manifested in the PAL airplane," he said of the Philippine Air
Lines 737. In
the 1990 case, there had been modifications to the float switch and
insufficient evidence, in the opinion of the FAA, of an inherent design
hazard. Moreover, yesterday's safety order is based on discoveries
of problems
that developed in high-hours airplanes, and the Philippine Air Lines
737 was
relatively new, Dern said.
Still, the float-switch issue is just one of three potential in-tank
hazards
to emerge from scrutiny of jet fuel systems since the TWA explosion.
In the
TWA investigation, safety experts discovered that fuel pumps are vulnerable
to
wear - that parts could dislodge and collide, possibly causing a spark.
A
number of documented cases in recent years of worn fuel-pump parts
prompted
airworthiness directives calling for inspections. In 1990, the NTSB
had recommended exactly that after the Manila explosion. The FAA at
the time said that, in tests, it was unable to cause a spark in such
a manner, and it declined to take action.
Also after TWA Flight 800, the FAA and the aviation industry began
a massive
effort to determine ways to detect wiring defects and scrutinize other
systems
in aging aircraft. In 1990, the NTSB urged the FAA to require inspections
of
certain wire bundles for chafing and other wire-insulation damage
that could
cause a short circuit, but the FAA then concluded that the established
maintenance regimen for wiring was sufficient to discover such problems.
Chuck Taylor's phone-message number is 206-464-2465.
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