Dealing With In-Flight Electrical Fires, Part 3
The NTSB found the in-flight fire most likely originating from electrical arcing from the coat closet lamp assembly, which subsequently created a hole in an oxygen line.
Further energy-dispersive x-ray spectra (EDS) from within the pits at the perforation detected aluminum in local areas in addition to the previously detected elements on the line surface.
The electrical wires connected to the curtain strip and the light fixture showed evidence of high temperature exposure, and the insulation was missing from all wires.
The outer housing of the light fixture and the aluminum fuselage attachment bracket had areas of severe localized heating and melting."
Examination of maintenance records from a year or so earlier revealed an entry that stated, "Closet light circuit breaker trips whenever closet light switch is turned on." The corrective action described in the entry stated that the closet light fixture was removed and inspected and a fixture mount screw was found to be contacting an oxygen line and allowing the circuit breaker to pop. The oxygen line was inspected, and no damage was observed. The screw tip was then protected with Teflon tape to prevent it from shorting any further, and the light fixture was reinstalled.
A second entry dated about a year later, three months before the incident, stated, "Forward closet light circuit breaker popped." Again, the corrective action noted was that the closet light assembly was arcing on an oxygen line. Teflon tape was again installed between the light assembly and the oxygen line, and no further damage was observed.
The says report states the fixture mount screw was used to bond the fixture to the airframe, and it served as an electrical ground.
Once again, there was no light to indicate what was wrong. There is no light for “coat closet electrical connection arc welding a hole in oxygen supply line with sparks.” There was no early warning because the smoke detector was basically cremated within a second of the oxygen line rupture. I personally cannot think of a more dangerous situation than an oxygen fire in the pressure vessel.
There’s an old saying: "If something goes wrong in the cockpit, undo the last thing you did.”
I remember once coming up the old “Control” channels from San Juan to New York at night. We turned the cockpit instrument floodlights on to try to see something a little clearer. The cockpit filled with smoke. We immediately turned the floods off, and the smoke dissipated.
In the above case, it would be obvious that if you turned on the closet light and smoke came pouring out, turn the light off. Or if the circuit breaker pops when you turn the light switch on, turn it off. But in the case above, the screw had finally managed to get to ground, and the subsequent current from the fixture to the oxygen line opened the line. There was probably nothing that could be done to prevent that grounding from sparking the oxygen. The electrical problem turned into an oxygen fire problem. Thankfully, the airplane was in a visual flight rules (VFR) pattern and back on the ground within 4 min. The crew, by cutting all electrical power in the area, removed the ignition-sparking source and then put out the fire, but this was an oxygen-fed fire within the pressure vessel. It flared up again—and, as stated above, the ground emergency vehicles were not able to put it out until the oxygen system was depleted.
Remember, the first rule in emergencies is: “Maintain aircraft control.” In an electrical situation, even where there is not a fire, the pilot’s attention is drawn into a labyrinth of possible reasons why such-and-such electrical component is not working. If it is not a critical item, we carry on. If it is critical and there are other factors such as weather and location—for example, mid-ocean—the potential for distraction goes up rapidly.
The pilot that fought the fire and returned to the cabin mentioned one other electrical fire and smoke problem that I did not know. The type of smoke that is created when a “fire-retardant” or “fire-resistant” item does burn—the smoke is heavy, dense and builds from the ground up. When it envelops you, you cannot see and you cannot hear. He describes it as yelling at the person next to you as if you were shouting into a pillow and that pillow is absorbing all the sound. It is so thick that when he felt the main gear touch down, he was almost unconscious and only able to open the emergency hatch because he had already placed his hand on the mechanism.
What do I recommend? I recommend whatever the manufacturers of our airplanes tell us is the best way to handle the potential of an electrical fire in the airplane. But I will say that the one diagram I always studied the hardest was the electrical layout of any plane I was flying. There are common areas in all of them. Generators or alternators feed the buses. The buses are usually divided into essential and main buses. There are sometimes buses that are absolutely non-essential and can be completely shed with the flip of a switch. There are ways the batteries can feed all or some of the buses, depending on how long you need them to last. There are procedures for using external or emergency (RAT) power.
In all the years I flew, electrical problems or fires were my biggest concern.