NTSB/AAR-84-09 - AirDisaster.Com

can pass into tine open space beneath the amenities section from the area beneath the
sink. Thus, if the fit between the trash chute and waste disposal door or between the
chute and trash receptacle had not been secure, it is possible that a lighted cigarette
placed in the chute partition could have fallen from the chute and thereafter rolled or
maSe its way through the hole into the open area below the amenities section. The fact
Lhat debris can enter this area was illustrated by the discovery of the vial and
m8.intenance tag below Flight 797's vanity, w,d the waste materials found in this area on
another air carrier's DC-9. The Safety Board concludes that the possihility thet this
occurred cannot be p:iled out.
To investigate the possibility that the fire was ignited as a result Of a?
overheated flush motor, the Safety Board conducted two tests. The highest temperature
achieved during the tests was 803OF, which was not high enough to ignite isvatory
components in the vicinity of the flush motor. k addition, the izotors used in the tests
were damaged internally by heat. The flush motor involved in the accident did not show
any evidence of internal heat damage or internal failure.
In order for the flush motor to overheat in service, three ma!functions must
take place: the motor must seize; the flush button must be held in or fail in the
depressed, or power on, position; ard finally, the timer must either be defective or fail.
(A properly functioning timer will limit the flash motor to a LO-second cycle even if the
flush button is held in the depressed position. Once the timer cycles the motor off, the
flush button must be released and then depressed again to restart tne motor.) The
recovered components of the timer circuitry were tested functionally after the accident
and were found to be operationel.
The last known person to use the lavatory did so about 35 to 40 minutes before
the fire was detected. She stated that the flush motor worked properly at that time. She
also stated that another passenger was waiting to enter the lavatory when she left;
therefore, the Safety Board conciudes that the flush motor was operating normally within
35 to 40 minutes before the fire was Ciscovered. Given the facts that (1) the test
demonstrated that an overheated flush motor would not produce temperstures high enocgh
to ignite edjacen: lavatory materials; (2) evidence showed that the flush motor had not
failed internally and was not damaged internally by heat: (3) there was no evidence to
indicate that tke flush motor timer had failed; and (4) the flush motor was most probably
operating normally before the fire was discovered, the Safety Board cor,c!udes it uniiely
that the flush motor was the source of ignition of the fire.
Another possible source of ignition Fear the area where the fire was
discovered was the flush motor wiring harness. The tripping of the three circuit breakers
accompanied by the arcing sounds recorded by the CVR occurred at 1851:14. The three
circuit breakers tripped almost simultaneously indicating that the circuitry of all three
phases shorted at the same time. The only evidence of wiring damage was found where
the flush motor wiring harness passed through the lightening hole in the partition between
the amenities section and the toilet section of the lavatory. The damage noted in the
wiring harnes et this location could only have been the result of fire and heat, and the
Safety Board concludes :hat the damage ?o the wiring which caused the three &ash motor
circllit breakers to Pip was caused by heat md fire.
numerous arcing sounl;s were recorded on the CVR. The.:e sounds were accompanied by
Beginning 3 minutes 2 seconds before the 'hree circuit breakers tripped,
voltage fluctuetions and the electrical components whic!. showed these fluctuations, as
well as the flush motor, were ell powered by the right LC. bus. Because of the extensive