Tuesday, February 24, 2009

Internal Combustion


Few things in aviation are more frightening to contemplate than an in-flight cabin fire, which is what the pilots of this Cessna 310 were apparently attempting to deal with. Most GA pilots don't give much consideration to what parts of the aircraft might provide a source of ignition nor how they would handle an in-flight cabin fire. As with all things in life, forewarned is forearmed and there are some important things to consider regarding in-flight fires. Contemplating the ingredients required for an in-flight fire is a good place to start, followed by knowing if your aircraft is equipped with a fire extinguisher and what type it is, inspecting the fire extinguisher during your preflight, knowing how to remove and use the fire extinguisher, and an awareness of how much time the average GA pilot might have to successfully deal an in-flight fire. To be fair, other factors in the above mentioned accident appear to involve the improper handling of inoperative equipment and circuit breakers, which I'll cover in a future installment.

This just in: To have a fire you need a source of combustible material, an ignition source that will get that material to its kindling temperature, and oxygen. A lot of things inside an aircraft will burn with the most obvious being fuel, but insulation on wiring, circuit boards, and other electrical components will combust when given the right sort of push. Heat from an engine, open flames, bleed air, and electrical sparks and resistance are all possible sources of ignition. Oxygen is widely available at no charge. Once a material reaches its kindling temperature, the source of ignition will probably no longer be required to keep the party going. Even so, the first step in fighting an in-flight fire is to remove the source of ignition. Always follow the checklist procedures in your FAA-Approved Aircraft Flight Manual, but a safe first step is to de-energize the electrical system - Turn off the battery master, alternator and/or generator.

The next step is to extinguish the fire and many, but not all GA aircraft are equipped with at least one hand-held fire extinguisher. If you own the aircraft, you probably know whether or not there's an extinguisher on board. If you are renting an unfamiliar aircraft, take the time to determine if a fire extinguisher is installed. Fire extinguishers are often located between the two front seats, but in some aircraft they are squirreled away in a weird location. Even though AC 20-42C recommends that fire extinguishers be mounted where they are readily accessible, I've seen extinguishers mounted underneath the pilot's seat and in locations that are not only inconvenient, they could be downright dangerous in an emergency when every second counts. And if you aren't following a preflight checklist that includes the fire extinguisher and you can't actually see the fire extinguisher, you're likely to forget about it. Out of sight, out of mind.

Once you've found the extinguisher, make sure it is securely held in its mounting bracket. If you are unsure how to remove the extinguisher, the preflight inspection is a good time to get familiar the process rather than during an in-flight fire. If the extinguisher is mounted in a hard-to-reach location, you may decide to remove the extinguisher to complete the next steps.

Check to see what type of extinguisher you have. Halon (halogenated hydrocarbon, a liquified gas) is the most common type for use in aircraft because it can be used on most types of fires, except when the combustible agent is a metal, such as magnesium or titanium. Halon comes in two basic varieties: 1211 and 1301. If you're interested, the four digits represent (in order from left to right) the number of carbon, fluorine, chlorine, and bromine molecules contained in the product. 1211 is a liquid streaming agent that can be projected over a distance and directed at the source of the fire while 1301 is a flooding agent that is designed to smother hard to reach fires. Some extinguishers contain a mixture of these two agents. The main advantage of Halon is that it can reach fires that you might not be able to see, like behind an instrument panel. Halon will also minimize residue left on electronic components, but fire extinguisher damage to a radio stack may be the least of your worries when the cockpit fills with smoke.

If you're worried about the in-flight effects of Halon on you, AC 120-80 advises that Halon discharged in confined spaces may result in dizziness, impaired coordination, and reduced mental sharpness. Since a fire requires a source of ignition, a source of combustible fuel, and oxygen, the goal is to remove the source of ignition, extinguish the fire and then ventilate the cabin. The effects of Halon, if any, should be short-lived if the fire is extinguished quickly and besides, you don't really have many other options.

One issue with Halon is that halogenated hydrocarbons harm the earth's ozone layer and the production or importing of Halon in the US has been prohibited since the Clean Air act went into effect in 1994 (as part of the Montreal Protocol). Though production of Halon is banned, possessing or discharging Halon is not prohibited nor is it against the law to sell a new unit or recharge an existing Halon fire extinguisher. In case you're wondering, discharging a Halon extinguisher just to test it is a really dumb idea: It's bad for the environment and it's expensive, too. In spite of the manufacturing ban, Halon continues to be available because it is recycled from used extinguisher systems. It's been estimated that about 40% of the remaining supply of Halon exists in the US, though the limited supply has certainly driven up the cost. Replacements for Halon will have to be identified eventually.

Virtually all hand-held extinguishers have a safety pin to prevent accidental discharge. Locate the pin and verify that it is in place. The safety pins will usually be held in place by a thin plastic tie that must be broken before the pin can be removed.

If the extinguisher has a pressure gauge, it should be reading in the green. Not all extinguishers have a pressure gauge in spite of the fact that this is the primary way to know if the extinguisher is operable, short of actually pulling the pin and discharging it. If the extinguisher has a service tag, check when it was last serviced and when it is next due for service. If you have removed the fire extinguisher from its bracket, you can also try this test: Note the position of the pressure indication, then turn the extinguisher upside down and right side up five times, then verify that the needle is in the same position. If the needle has moved, have the fire extinguisher serviced.

The standard method for using a fire extinguisher is often summed up with the acronym PASS: Pull the safety pin, Aim the fire extinguisher nozzle or hose at the base of the fire, Squeeze the handle, and Sweep the extinguisher's stream back and forth. This sequence may sound simplistic, but if you start squeezing the handle as you're trying to remove the safety pin, the pin may not come out freely if at all. An in-flight fire behind the instrument panel will make it impossible to accurately aim the extinguisher's stream. Some extinguishers don't have a hose, they just have a nozzle. Many fire extinguishers are marked with instructions saying that you should hold the fire extinguisher upright while discharging or you may reduce the effectiveness.

A Halon extinguisher is well-suited for situations where you can't see the source of the fire. When I used to fly freight, we had a single, large canister Halon extinguisher, equipped with a hose, and mounted between the pilot seats. Since there was no air-tight barrier between the cockpit and the freight area, the procedure for a fire in the freight hold was to 1) don the oxygen mask 2) pull the safety pin on the extinguisher 3) aim the hose back toward the freight area 4) discharge the entire contents of the extinguisher 5) declare an emergency and land as soon as possible. As always, if in doubt, follow the instruction in your aircraft's FAA-approved Flight Manual.

As for how long you have to control an in-flight fire and get on the ground in a GA aircraft, my research only uncovered survivability data for transport category aircraft. In these aircraft there was an average of 17 minutes from the first indication of a fire until the results became fatal. For smaller aircraft, one would assume that even less time would be available.

In the next installment I'll discuss recommendations for handling tripped circuit breakers and the importance of properly dealing with inoperative equipment.

1 comment:

John said...

Great post....very interesting