Handling In-flight Emergencies - Part 3

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My neighbor Ed was a serious smoker, at least until he accidentally placed a lit cigarette in his jacket pocket and set himself on fire. I heard the commotion next door and ran over to witness Ed jumping up and down on his smoldering jacket. I said "Nice jacket. Is it a blazer?" With minor burns to his arms, he was in no mood for humor.

Yes, it's hard to find anything funny to laugh about when talking about fire. It's a very dangerous thing. It's even more dangerous when it occurs after an airplane crash. Therefore, pilots need to know something about minimizing the chance of a postcrash fire when making an emergency landing.

Postcrash fire is a major concern when the possibility of a high-G impact exists. A fuel source (typically a ruptured fuel tank) and an ignition source are the two ingredients necessary for the start of a fire (the presence of oxygen is assumed). For all practical purposes, your only control over the source of fuel is shutting off fuel valves and deactivating electric fuel pumps prior to impact. Ruptured fuel tanks are a possibility, but their occurrence in these situations is often beyond your control. You do, however, have more control over reducing or eliminating the source of ignition.

There are two common sources of ignition in postcrash fires: electrical spark and a residual heat source. Electrical spark can virtually be eliminated by turning off the master switch (Figure 1) prior to impact. Remember, most fuel tanks are in the wing where the nav and strobe light circuitry is located. Eliminating this circuitry as an ignition source will surely help minimize the potential of a postcrash fire.

Unfortunately, our general aviation airplanes don't have inertially-deactivated master switches. The advantage of such a switch is that it would automatically shut off the electrical system during a high-G impact. We do have an ELT with a 5-G impact-activated switch. So why not a 5-G deactivated master switch to help minimize the chance of a postcrash fire? Hmmm.

Depending on the airplane, you may want to avoid shutting off the master switch until the last possible moment prior to impact. Remember, you may need to lower electrically operated gear or flaps or make radio calls and keep the transponder operative prior to landing.

The second source of ignition for a postcrash fire is residual heat. Hot exhaust stacks are a typical source of residual heat (Figure 2). Fuel from a ruptured tank pouring over such a heat source might easily ignite (depending on the source's temperature, of course). Hardly any information exists regarding residual heat sources and their ignition potential for general aviation airplanes. You might, however, find the following information useful.

During World War ll, the military did a study on the postcrash fire potential in P-47's and Corsairs (I will post this study at the bottom of this article via hypertext link when it's made available to me). Like most general aviation airplanes, the P-47 and Corsair have aircooled engines. The military discovered that the chances of a postcrash fire ignited by the failed engine's residual heat could be reduced by approximately 90% if the airplane had a chance to glide for at least 20 seconds before impact.

If I may take a little liberty to speculate, consider that the engines on a P-47 or a Corsair have more mass than the typical general aviation engine. Therefore, they probably retain their residual heat longer than do most smaller engines. It's reasonable to speculate that, in obtaining the same percentage reduction of postcrash fires caused by residual heat, a general aviation airplane may need less than 20 seconds of cooling before impact. This is only speculation on my part. Additionally, this suggests opening the cowl flaps (Figure 3) as part of the engine failure checklist (unless your POH states otherwise). After all, this does provide better engine cooling.

Consider the implications of this information if an engine fails on takeoff. Let's assume your engine quits on takeoff at 500 feet AGL. If, at the best glide speed, your airplane descends at 500 feet per minute, you have at most one minute before touchdown. This suggests adequate cooling time to prevent the ignition of fuel by a residual heat source.

But what if an engine fails on takeoff and you attempt a return to the airport at too low an altitude? No doubt that a spin on takeoff substantially increases your descent rate. It also increases the possibility of a ruptured fuel tank and the chance of a residual heat source remaining hot enough to ignite fuel. Need I say more? This is good enough reason to know the minimum altitude required for a turn-back before ever attempting such a thing.

The lesson provided here is for that rare occasion when it's necessary to set an airplane down in inhospitable terrain. Ensuring that the master switch is off prior to impact is the key to minimizing a postcrash fire. Make it a point to avoid flying over terrain that could result in a high-G impact during an emergency landing. Remember, you have a choice as to where you fly. Make sure you choose wisely.

In Part 4 of this series, we'll continue our investigation into preparing for the crash landing.

For more information on this subject, see "Aircraft Fires: Strategies For Survival" and "The Long Wait: What To Do Until Help Arrives."