Turbine Pilot

Red lights and two shots

Though an engine fire is dangerous in any airplane, turbine-powered airplanes are most likely to have the luxury — and safety — of fire detection and extinguishing systems. These systems are one more significant advantage of flying turbine-powered aircraft.

Fire detection systems are usually part of a turbine aircraft's standard equipment list. These systems monitor the engines and, in those airplanes equipped with them, the auxiliary power unit (APU). APUs are smaller turbine engines that can be used for ground power and air conditioning, and engine start power. Airliners take fire detection a step further and include smoke detectors in cargo and equipment bays and, of course, lavatories.

Conventional engine fire detection setups use thermal detector units strategically placed near the locations most likely to experience a fire — near an engine's combustion chamber ("hot section") and close by an accessory case attached to an engine. The sensors are connected by a wiring harness and calibrated to complete an electrical circuit when the temperature near them reaches a predetermined value — say, 450 degrees Fahrenheit. Some fire detection systems use sensing elements that are continuous loops and surround the entire engine. When the sensors or elements are triggered, a signal is sent to prominently located cockpit warning indicators.

A fire warning annunciation is sure to get your attention. First of all, you see a large flashing red master warning light, then a fire warning annunciator light near eye level, either on the glareshield or receiving top billing on a display-mounted annunciator. In case this display eludes your attention, a fire bell sounds. The fire bell — intentionally loud, annoying, and conforming to international warning-sound conventions — does a great job of scaring the life out of you and provoking a fast response.

For this reason, many checklists recommend that your first response to a fire warning be to silence the bell. The theory is that in the confusion of lights and bells you may react too quickly, and unintentionally do something to worsen the situation — such as shutting down the wrong engine. The relative calm after bell-silencing lets you positively identify the fire's location, then shut down the affected engine and activate the appropriate fire shutoff valves. After completing any remaining fire-shutdown checklist items, it's time to hope the fire has burned itself out — in which case the fire warning light should go out. If the fire dies down and then flares up again, the fire warning system will reset during any cool-down and come on again as soon as the detection sensors are tripped. Now you've got a huge problem; the only solution for a second burn is to get on the ground as soon as possible. Diving the airplane on the way down may help put out a fire, but just pray that the heat hasn't melted or weakened any structural components such as engine mounts or wing spars.

Fire extinguishing systems are standard equipment on larger business jets and all airliners, but costly options on the specification sheets of most light turboprop singles and twins. But they pay their way when the fire bell sounds. Then the usual drill is to shut down the offending engine and hit the extinguisher switch.

These systems commonly use one or two fire-extinguishing supply cylinders per engine compartment, each one filled with Halon or some other fire-killing agent. Some systems give you two centrally located, interconnected cylinders designed to fire into either engine. The cylinders are pressurized with dry nitrogen and activated by guarded switches or fire handles. To discharge a fire bottle, lift the extinguisher switch's cover (you may have to break a safety wire to do this) and press on the switch or pull on a fire handle. Switches are guarded to discourage misactivation. To empty a fire bottle on a perfectly good engine would be disastrous: You'd have to deal with one engine still burning and the other one flamed out (the extinguishing agent is designed to put out all fires, including the one in your combustion chamber). Pilots of single-engine turboprops and helicopters, of course, are spared the stress of identifying and extinguishing the proper engine.

Some extinguishing systems give you two shots at putting out a fire. If a fire returns after discharging one fire bottle, you can hit the extinguisher switch once more. After that, you're on your own. Even so, there's little downside to having fire detection and extinguishing capability. The more you think about it, the more you wonder why all aircraft don't have it.

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