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The exact cause of the ignition of the hydraulic fluid could not be determined. However, from all the physical evidence in this instance the Board believes that the cargo in "G" compartment was ignited by contact with an unguarded light bulb. The fire smoldered and progressed until it burned through the forward aluminum wall. Any break in the compartment wall would be accompanied by an increase in airflow concentrated at that point. The intensity of the fire increased with the increase in airflow (which is normally forward) and was carried into the area of the rear spar where numerous hydraulic units and lines are located. Fire damage to one of these components resulted in a leak which fed the flames to uncontrollable proportions.
In determining the cause of this fire the Board considered, among other things: a. previous baggage compartment fires caused by cargo contacting unguarded light bulbs which have been experienced by other airlines; b. the unguarded light bulbs as installed in N 7840B; c. the probability that one or more of the mail sacks containing clothing pressed against an unguarded and lighted bulb; d. the probability that the compartment sealing ta was deteriorated permitting an airflow exceeding a rate of 1,500 cu. ft./hr. A flow greater than this rate would supply sufficient oxygen to support combustion within the compartments; e. in compartment "G", the normal airflow in this area which would carry fire forward to the rear spar.
A second possibility considered by the Board was that of a hydraulic leak in the area of the rear spar. This was considered unlikely as a second element would have had to have been present simultaneously, i. e., an electrical fault of a nature sufficient to generate enough heat to ignite the fluid. Also, had the fire originated in this manner, there would be no accounting for its rearward progress against the normal air flow through the rear cargo compartment.
The bright fire the ATC operator observed falling from the aircraft was a flare. It is believed that this flare was released either by fire damage in the area or by fire damage to the flare release circuit and adjacent circuitry which resulted in voltage being applied to the flare circuit.
Conclusions
As a result of this investigation the Board has made a number of recommendations for corrective action to the FAA and to the company, several of which have already been accomplished. One of these recommendations was that the conpany require cargo lights to be "off" during flight and that attention be directed to the requirement that guards be installed over these lights; in addition, electrical relays and electrical terminals in the hydraulic compartment area should be protected. Further, it has been recommended that the tape which is used to seal class "D" cargo compartments be subjected to periodic inspections to insure that the proper control of airflow is maintained.
The company has also instituted a program to design and install fire-detection and fire-extinguishing equipment in these lower cargo compartments.
