In the following, a metal fire is defined as a fire resulting from the burning of metals under aerial oxygen. During this steady burning, there is a balance between the heat created and the heat lost into the surrounding area. Technical circles have known of metal fires for a long time. In a suitable environment, all metals except for gold, silver, and platinum can be made to burn (Ref. 9.1.1-4). The often explosive burning of metal dusts (e.g. magnesium, Ref. 9.1-1, Example "Magnesium dust catching fire") is a typical example. This property made magnesium suitable for use as an early flash for cameras. Under certain conditions, however, even massive metal cross-sections can ignite and burn continuously (Ref. 9.1.1-2, Example "Combustion of magnesium hub during rollout accident"). In the torch cutting process, the burning of metals (usually steels, in this case) is used technically. The danger level is increased in pure oxygen, which has lead to extensive damages in oxygen compressors. Metal structures with thin cross-sections and large surface areas promote metal fires (e.g. Lungstroem preheater made from steel sheeting).
The ignition temperature must be reached in a suitable atmosphere for a metal fire to start. A metallic (reactive) surface is required. Ignition can be described as a thermal instability that occurs when the amount of heat created surpasses the amount of heat removed and leads to a thermal “runaway” in the form of a chemical reaction. High ignition temperatures are usually reached due to rubbing occurrences such as blade rubbing. Due to the high pressure and/or high air speeds in engines, the rear compressor region and labyrinths contain enough oxygen to ignite and sustain a metal fire. The most commonly ignitable materials in engines are titanium alloys (titanium fire, chapter 9.1.2-2). The ignition of magnesium in engines is covered in the text (Ref. 9.1.1-2).
Up until the 1970`s, titanium fires were mentioned in various texts, but detailed descriptions of the cases were not known in freely accessible documents (Ref. 9.1.1-4). Only in the last few years was this phenomenon of titanium fires in aircraft engines described more closely and with examples at hand (Examples 9.1.2-1 and 9.1.2-2).
Example "Magnesium dust catching fire" (Ref. 9.1.1-1):
Excerpt: “While engaged in a log sliding operation over montainous, densely forested terrain the rotor craft suffered a total power loss forcing the pilot to execute an autorotative landing into high conifer trees. Disassembly of the engine revealed evidence that the spacer between the No. 5 compressor and the impeller had not been position seated during the last overhaul and that during the accident flight the spacer slipped into its proper position thus releasing the compressive torque load under which the assembly had been subjected during buildup. The compressor assembly subsequently became uncoupled forcing the impeller forward into its magnesium housing. This action resulted in a overtemperature condition and breakup within the turbine assembly brought on by the combustion of Magnesium Particles.”
Comment: Apparently, metal abrasions created during the rubbing occurrence ignited. This fire obviously led to extensive consequential damages.
Example "Combustion of magnesium hub during rollout accident" (Ref. 9.1.1-2):
Excerpt: “On Rollout, while the pilot was attempting to correct a drift to the right, the aircraft suddenly veered to the left and the nose gear collapsed. During the resulting slide, the aircraft began burning and was destroyed.The pilot reported that the magnesium hub of the nose wheel began burning and could not be extinguished by using portable dry chemical and CO2 extinguishers.”
Comment: Surprisingly, a magnesium fire started in normal atmospheric conditions. It is typical, though, that the ignition temperature was reached through a rubbing occurrence. This was obviously so intense, that the relatively high heat conductivity of the magnesium alloy was not sufficient for avoiding a dangerous overheating. The portable extinguishers were, as would be expected, not able to put out the mag-fire (see chapter 9.1.3).