It is possible to minimize the danger and damages from titanium fires with the help of varoius different strategies, which can also be combined with one another (Fig. "Measures against ignition and damages of titanium fires"):
Figure "Measures against ignition and damages of titanium fires" (Ref. 9.1.3-1 and Ref. 9.1.3-2):
Design and configuration: the probability of a titanium fire igniting can be considerably reduced if certain basic principles are (able to be) followed.
The development of a coating for the blading that would hinder ignition and/or sustained burning has already been attempted. These coatings must not have an inadmissible influence on blade strength (e.g. dynamic load resistance, ductility, foreign object damage resistance). Aluminium has been shown to resist sustained burning when used as a coating (Ref. 9.1.3-2). However, its melting point is too low, the corrosion and erosion resistance is insufficient, and it forms brittle phases through diffusion with the titanium base material. To date, no use of fire-protection coatings on titanium alloy compressor blades in serial production has been reported.
“Passive Measures”: These are based on the unavoidability of occasional titanium fires. They serve to limit consequential damages. Primarily, this means that the titanium fire is contained in the engine. This can be accomplished in various ways:
Fire extinguishing mechanisms (see chapter 9.5, also Refs. 9.1.3-1 and 9.1.3-5): It is not possible to extinguish the fire during ignition or while it is burning. To date, there is no practically applicable measure that reacts with sufficient speed and effectiveness. Effectively, only the minimizing of consequential damages, such as the spread of a fuel- or oil fire, can be feasibly addressed. Naturally, fuel flow should be halted before the fire extinguishing system is activated.
Early detection of titanium fires by pilots: This possibility is very limited. However, it depends on avoiding or taking appropriate countermeasures against consequential damages such as fuel- or oil fires. This includes halting the fuel flow. Usually the fire will be noticed as a result of damages with extreme imbalances and corresponding vibration indications as well as extreme exhaust gas temperatures (Example "Magnesium dust catching fire"). Limited fires will usually result in stalls (lock- in surge) and temporary increases in gas temperatures.
Air frame protection: Firewalls at critical air frame regions and between parallel engines (bottom diagram). In some tactical aircraft, these are made of titanium sheeting, which should be sufficient in the case of fuel- and oil fires, but not in the case of a titanium fire. For this reason, firewalls are often retrofitted with coated fibre mats (Example "Most common cause of titanium fires").
Figure "Containments of titanium fires" (Ref. 9.1.3-1): It has been shown that, specific to engine type, compressor areas are more prone to uncontained titanium fires. In this case, local measures can improve the containment. This can be accomplished by suitable selection of materials, coatings, and appropriate design directly at the housing`s point of ignition (top detail). If this is not sufficient, fire-retardant coatings or inserts (ceramic, fibre-technical) can be used locally outside of or between housings (bottom detail). If possible, these measures should not be taken in the gas duct, in order to keep the compressor blading as safe as possible from separated coating layers or inserts that could damage it.
Figure "Protecting air vents against titanium fire": Air outlet openings on the housing are designed in such a way that the air flows out as favourably as possible with regard to the air stream. This means that in the case of a titanium fire, burning drops of titanium carried by the air flow directly strike the edges and walls of these openings. The rapid heating-up of the edges and the intense air flow provide optimal conditions for ignition and sustained burning (Fig. "Ignition mechanisms in metals"). The protection of these regions through selection of heatproof base materials (detail at top) and/or sufficient protective layers (detail at bottom) should be undertaken.
Figure "Titanium fire at edges of radial gaps": Edges of radial clearance gaps in the housing region (Fig. "Weak points at compressor housings for titanium fires") can easily ignite through contact with burning melt flowing along the inner housing wall. If the design of the engine necessitates this type of edge, it should at least be made of a unburnable, heat-proof material. Edges made of materials that tend to ignite and/or react with melt (e.g. Ni-Graphite abradable coatings) are to be avoided.
Figure "Titanium alloys preventing titanium fire": It has been reported that a titanium material suitable for use in compressors (alloy C) that prevents sustained burning of titanium fires anywhere in the typical flight envelope of modern tactical aircraft (bottom diagram) was developed in the USA (Ref. 9.1.3-4). Previous titanium alloys (see also Fig. "Titanium fire burn test") could only achieve this result in a small part of the flight envelope (dark grey region). As far as is known, this material is not freely available for commercial use. Therefore, this material has only seen serial application in engines of the latest models of US tactical aircraft. One disadvantage of this material would be its higher specific gravity compared
to the currently available titanium alloys.
Intermetallic phases such as TiAl have low flammability and good fire resistance (Ref. 9.3.1-2). However, their extreme brittleness at temperatures up to a few hundred °C limit their use.
Russian sources (Ref. 9.3-6) indicate that there, as well, burn-resistant titanium alloys are being intensively researched. Trials have shown that the behaviour of these alloys is considerably better than that of the standard material Ti-6-4. However, these alloys are more brittle and tend to notch easily. Thus the problems are similar to those of intermetallic phases.
For this reason, use of these materials can be expected primarily in housing components.
9.1.3-1 T. Uihlein, H. Schlegel, “Titanium Fire in Jet Engines”, Proceedings AGARD-CP-587 of the AGARD conference “Aircraft Fire Safety”, 14-17 October 1996, chapter 25-1 to 25-12.
9.1.3-2 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, about 100 pages.
9.1.3-3 “Navy Grounds F/A-18 Aircraft Following Engine Fire Incidents”, magazine “Aviation Week & Space Technology”, November 23, 1987, page 31.
9.1.3-4 E.C. Bryan, “Using Advanced Technology to Achieve Reliability as well as High Performance”, Proceedings of the “Aero Engine Relability, Integrity and Safety”, Thursday 17-Friday 18 October 1991, Royal Aeronautical Society, ISBN 0 903409 70-4, pages 12.1-12.12.
9.1.3-5 NTSB Aviation Accident/Incident Database Report LAX 84LA035, 1983.
9.1.3-6 V.V. Tetyukhin, “Titanium Alloys in the USSR”, Proceedings of the “Titanium 1990, International Conference”, Volume 1, Published by Titanium Development Association, page 56-51.