Aeroengine Safety

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.

• Shifting areas where loss of axial clearance may cause rubbing into colder areas.
• Titanium surfaces should not be involved in rubbing occurrences. Those parts that could be involved should be made of materials with sufficient heat resistance and suitable tribologic behaviour.
• If titanium surfaces are exposed to potential rubbing, they must be appropriately coated (e.g. spray-on ceramic layers). These rubbing surfaces are created by rotor offset due to main bearing damage or clearance losses due to imbalances.
• No thin wall cross-sections in housings in areas where rubbing may occur.
• Because fatigue fractures in compressor blades are the most common cause of titanium fires, these should be made with as large a leading-edge radius as possible. This makes the blades more resistant to foreign object damage.
• As large as possible spacings between blades and flow cross-sections. Experientially, this reduces the probability of a titanium fire.
• Robust blading in the front compressor to minimize damage from bird strikes.
• Secure main bearings with (magnetic) chip detectors
• Appropriate clearances between housing and rotor (see chapter 7)
• Compressor guide apparatus should be outfitted with the most massive possible inner shroud. Avoid guide vanes fixed by a single edge. These tend to cause failed blade leaf fragments to jam (Fig. "Titanium fire ignited by blade fracture") and/or dangerously heat-up the tips during rubbing.
• Intermediate stage labyrinths or rotor drums made of titanium alloys with smooth surfaces where guide vanes might rub must be suitably coated. These coatings should insulate against heat and be cuttable or abradable. The labyrinth tips must also be properly coated. Unprotected labyrinth tips made of titanium must be avoided.
• Labyrinth rings, intermediate stage labyrinths, and labyrinth rings must be constructed so solidly that self-increasing rubbing following an uncontrollable expansion due to heating-up is safely avoided (Figs. "Labyrinth rings bulging by local heating" and "Damage mechanisms of titanium fire ignition").
• The regulator should be designed in such a way that a damage-induced drop in the compressor rotor`s RPM (e.g. after a blade failure) does not result in increased fuel flow. Otherwise, the probability and intensity (uncontainability) of titanium fires are increased.
• No dehiscing radial clearances on the inner walls of housings. Extra caution is necessary behind a “swan-neck” duct. If edges are unavoidable, these should be made of fire-proof materials (e.g. Ni alloys, ceramic spray-on coatings; Fig. "Weak points at compressor housings for titanium fires"). Air outlet openings on the casing (Fig. "Protecting air vents against titanium fire") must be especially well protected, since the burning titanium melt is directed here.
• If possible, the use of flammable or reactive (with titanium melt) coatings in housing regions where rubbing may occur should be avoided.The use of fire-proof or fire-resistant materials: wherever possible, the guide vanes should not be made from titanium alloys. Titanium fires in engines with compressor guide vanes made from Ni alloys or steels have not been reported. The ignitibility and flammability of titanium alloys used in modern engine construction do not vary enough for a specific selection of these to noticeably increase security. Although there are several fire-resistant titanium-based materials (Ref. 9.1.3-2, Fig. "Titanium alloys preventing titanium fire"), these have seen only very limited use in serial production to date. This is due partly to insufficient strength (e.g. intermetallic phase NiAl) and partly to their insufficient avaliability as “strategic materials” (e.g. “Alloy C”, Ref. 9.1.3-4).
  

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:

• Housings made of fire-resistant, sufficiently heat-proof materials.
• Coating the inside of compressor housings with fire-resistant materials (Fig. "Containments of titanium fires").
  
• Sheathing or coating the outside (Example "Most common cause of titanium fires") of the housings or sensitive regions such as parts of the fuel systems or regulators. Both coated and uncoated fibre mats can be used in this capacity (Example "Most common cause of titanium fires").
• Protecting the engine mounts from fire damage.

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.