9.5 Detecting and Extinguishing Fires
Engines require systems that minimize the risk of engine fires (Ref. 9.5-6). There are several options that can be used in combination.
These systems include: “reactive” systems that extinguish a burning fire before it causes serious damage (extinguishing systems), “preventive” systems such as cooling or ventilating air flow in the nacelles (Fig. "Engine behaviour during fire"), and “protective” systems, such as firewalls, that are intended to limit fires (Fig. "Firewall safety").
Fire extinguishing systems are made up of several components:
- A detection system identifies the fire with the aid of sensors (Fig. "Principle of fire warning system in engine nacelle").
- A warning system informs the pilot of the fire, i.e. sets off the fire alarm (Fig. "Principle of fire warning system in engine nacelle")
- The actual extinguishing system (Fig. "Principle of engine fire extinguishing system")All of this equipment must take up as little space and be as light as possible.
There are also systems that indicate overheating, even if there is no immediate danger of a fire. These early warning systems are designed to allow the pilot to take appropriate measures, such as shutting down the engine, before a fire starts. These systems use sensors similar to those of fire warning systems. These sensors are placed in the cooling air flow around the engine and at the nacelle vent openings in order to detect unusually high temperatures in the cooling air.
Fire warning systems are prone to false alarms (examples 9.5-1 to 9.5-5). The causes of malfunctions are varied. For this reason, service and maintenance of these systems is especially important.
Example "False fire warning" (Ref. 9.5-7):
Excerpt: “After leveling at fl 330, the No. 2 engine warning light & bell activated. The crew followed the published procedures, shut down the engine and used a fire bottle…They then diverted…made a precautionary landing… Subsequently, the aircraft was stopped and evacuated using the escape slides… Investigation revealed there was a false fire warning indication due to shortened fire detector …“
Example "Melted fire warning circuit" (Ref. 9.5-8):
Excerpt: ”…en route (the aircraft) had fire warning No. 3 engine. Crew performed emergency procedure but light remained on. Crew made emergency landing…Fire warning caused by separated bleed air duct. Due to fatigue which melted fire warning circuit…“
Example "Intermittent fire warning" (Ref. 9.5-9):
Excerpt: ”…(the aircraft) was on final approach in the vicinity of the outer marker when an intermittent fire warning indication on the Number one engine started. The engine was shut down and the fire bottle was discharged; however, the fire warning continued. When the airplane landed and cleared the runway it was stopped. The fire warning continued and the second bottle was discharged. When the fire warning continued, a precautionary emergency evacuation was initiated. Post incident examination of the airplane disclosed a chaffing fire loop on the number one engine“
Example "False alarm by faulty detection element" (Ref. 9.5-10):
Excerpt: “While passing 10,000 feet msl on the decent for a night landing, the number two engine fire warning light and bell activated, following accomplishment of all checklist procedures, the fire warning indication remained on. The captain ordered an emergency evacuation on the runway after the airplane landed….A faulty fire detection element on the number two engine, and no evidence of inflight fire was observed.”
Example "Shorted fire detection loop" (Ref. 9.5-11):
Excerpt: “Aircraft made a precautionary landing after getting a fire warning indication in the No. 2 engine. Initially the indicatings, with the fire warning light and bell, were intermittent; however, they then went to steady and the crew secured the engine and discharged both fire bottles with no effect. Investigations revealed that there had not been a fire or over temperature condition and that the engine fire detection loop was shorted on both elements. Contaminants were found on the connectors and pins of one end of the loop during initial inspection. After handling and shipment to the operator's maintenance facility, the short dissapeared and could not be duplicated… Contaminants appeared to be engine oil and hydraulic fluid.
Figure "Extinguishing titanium fire?": Titanium fires inside engines cannot be extinguished due to their spontaneous ignition, high intensity, and short duration (4-20 s, Ref. 9.5-2). If a titanium fire is detected, this means that extensive internal damage has already occurred.
Even if an escaping titanium fire is noticed early enough that extinguishing measures could be taken, an appropriate extinguishing material must be decided on (Ref. 9.5-1).
Halon (also known as CB, chemical composition CF2ClBr), Freon and CF3Br are typical extinguishing materials used in engines ( ). While they are effective, they can be toxic and extremely corrosive. If temperatures exceed 450°C these compositions become instable and break down into the Halogenic acids HBr, HF and HCl. At the extreme temperatures of a titanium fire, these compositions further break down to form dangerous gaseous halogens. If these should enter the pilot`s breathing air or the cabin, they would immediately be most damaging to the health of persons exposed to them.
CO2 is also unusable, since it provides the titanium fire with oxygen and promotes burning more than the air itself. For example, if the amount CO2 in air is 23 % by volume, it increases the burn rate by 50 %; in pure CO2, the burn rate increases by 300% (Ref.9.5-2).
Although a mixture of air/60% Argon would be suitable for extinguishing titanium fires, the large amounts of this mixture that would be necessary coupled with the short time available for identifying and extinguishing a titanium fire make its application unfeasible.
In summary, to date, no serially applicable extinguishing method for titanium fires has been documented.
Powder and CO2 extinguishers have not been effective in extinguishing magnesium fires (Ref. 9.5-4).
Figure "Principle of fire warning system in engine nacelle": Fire warning systems consist of a system that detects fires and an actual warning system. These systems must have the following properties:
- fastest possible detection of a fire
- fastest possible fire warning signal
- high reliability against malfunctions (see examples 9.5-1, 9.5-2 and 9.5-3).
Fire detection and warning must happen as soon as possible after a fire starts in order to prevent the engine shut-down regulator and the extinguishing system itself from being damaged. The detection system is made up of several individual detectors or a long electrical (semiconductor), pressure sensitive (gas filled), or capacitive sensor that can be formed to fit the contours or fastenings to be monitored (Ref. 9.5-6) and reacts to high temperatures. In spot sensors, thermoelements or semiconductors are used. The detectors and/or sensors should be mounted in such a way that they detect fires in potential fire areas reliably and with as little delay as possible.
Figure "Principle of engine fire extinguishing system" (Refs. 9.5-5 and 9.5-6): All sources of flammable liquids must be sealed off before the extinguishing system can be activated. This means that the engine must be shut down and the vents of the (low pressure) fuel supply system must be closed.
In order to avoid a fire starting again after the extinguishing material has been used up, one must not attempt to restart the engine.
Figure "Firewall safety": Engine fires must be contained inside the engine (Ref. 9.5-6) in order prevent the fire from spreading to other parts of the aircraft. The cowling is usually made of aluminium sheeting that can be melted and burned by fires that occur while the aircraft is stationary. In flight, the powerful air flow is used to cool the cowling so that the fire cannot penetrate it. Cowling areas that cannot be sufficiently cooled and could therefore act as flame holders are made of steel or titanium alloys.
Figure "Engine behaviour during fire" (Ref. 9.5-6): The behaviour of the engine during a fire is also influenced by cooling and ventilation. In order to remove flammable or explosive vapours, the area surrounding the engine is usually cooled and ventilated by atmospheric air that is subsequently blown away. The air flow must be designed with regard to the fire extinguishing system and the amount of extinguishing material available. The diagram shows a three-zone cooling in a modern turbofan engine, in which every zone is individually regulated.
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9.5-2 T.R. Strobridge, J.C. Moulder, A.F. Clark, “Titanium Combustion in Turbine Engines”,Report Nos. FAA-RD-79-51 and NDSIR 79-1010 , July 1979, pages 24 and 25 of
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9.5-8 NTSB Identification: FTW851A341, microfiche number 31517A, Sept 03 1985.
9.5-9 NTSB Identification: NYC911A165, micrfiche number 43746A, Jul 02 1991.
9.5-10 NTSB Identification: ATL90FA135, microfiche number 42911A, Jun 21 1990.
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