9.2 Oil Fires
The temperature levels and thus the risk of an oil fire in an engine increase along with output concentration. The risk of oil sump fires is increased by high operating temperatures in flight (Fig. "Operation data influencing oil fire risk") in connection with the shaft seals of the main bearing chambers ( ). If the oil temperatures rise, a small unplanned temperature increase is sufficient to ignite an oil fire or oilsump fire. Synthetic oils self-ignite from 260°C (Ref. 9.2-1). Oil fires can have different forms of appearance (Fig. "Detecting and verifying an oil fire") and temporal progressions (Ref. 9.2-5).
Most oil fires are due to one of two major causes:
- Oil fires in the bearing chamber region due to overly high temperatures in the oil sump, vents, and/or return oil tubes.
- Leakages in the oil- or hydraulic systems after seal failure or line leaks (Example "Oil fire by hot air incursion").
These fires occur in ignitable oil/air mixtures in bearing chambers. The oil can be present as either oil vapour, oil mist, or in drop form. Both short and long duration oil fires have been observed in bearing chambers. Primarily, coke deposits that can cause blockages of return oil tubes and filters and damage bearing tracks (impressions and locking-up in the tracks) were created.
The burning of oil results in very intensive fires that can overheat large cross-sections in short periods of time (minutes; Fig. "Oil fires in low pressure turbines").
Trials in an oil fire testing rig show the conditions under which an oil fire ignites:
- The oil temperature is a deciding factor for a spark to trigger ignition. For typical synthetic engine oils, for example, it is around 217°C (Ref. 9.2-5). Ignition is also dependant on oil vapour pressure. Ignition will not occur if oil vapour pressure is below or considerably above the flammability limit.In order for an ignited fire to continue burning without a constant ignition source, the temperature of the oil/air mixture must be above the combustion point (e.g. 285 °C).
If the temperature is high enough for self-ignition to occur (e.g. 432 °C), no ignition source is necessary to start a fire.
- If the oil/air mixture is not suitable for ignition (either too rich or too poor), no fire can occur.
- If temperatures are below self-ignition levels, a sufficiently powerful ignition source must be present (Fig. "Oil fire risk by bearing chamber temperature"). Remedies can be based on these observations. Usually, the oil/air mixture in bearing chambers is too rich for an oil fire to ignite.
Figure "Main bearing chambers air systems" (Ref. 9.2-5): The main task of air systems in bearing chambers of the hot-part regions is to protect the bearing sump from the hot surroundings. Conditions surrounding an especially flammable main bearing chamber are the temperature and pressure at the compressor exit. The bearing chamber`s seals should ensure that the oil temperature stays at safe levels (Fig. "Oil fire at a supersonic aircraft bearing chamber"). The following example of a typical main bearing chamber with labyrinth seals emphasizes this:
The inner chamber, which contains the main bearing, is filled with an oil/air mixture (temp. about 180°C, pres. about 0.14 bar).
The bearing chamber is enclosed by the first mantel and is loaded with air from the high-pressure compressor (temp. about 200°C, pres. about 5.5 bar), which serves as blocking and cooling air for the bearing. The high pressure levels create a leak flow to the vent and prevent hot gases encroaching into the bearing. This also prevents foreign objects from entering the bearing from outside. Only a relatively cool leak flow from the blocking air reaches the bearing across the labyrinth seals.
The second mantel<U></U>is created by the vent, which blows out the leaked air from the labyrinths and prevents hot gases from reaching the bearing.
Around the bearing chamber, conditions are the same as those of the high-pressure compressor exit (temp. about 650°C , pres. about 25 bar).
It is worth noting that bearing chambers with a combination of labyrinth and washer bushing seals also exist. With these, very tight washer bushings are used around the bearing to seal off the oil/air mixture in the inner chamber.
Figure "Causes of oil fires in bearing chambers" (Ref. 9.2-5): The frequency of oil fires in the bearing chamber region and the oil sump system in the rear compressor and hot-part areas corresponds to the frequency of fires in an engine. This frequency rests in no small way on the multitude of potential causes (top diagram):
- Problems in the bearing chamber area
- Oil mist being released in and between concentric shafts.
- Leakages in the inner oil system and ignition of the leaked oil through hot gases (Example "Oil fire by hot air incursion").
- Leakages in the outer oil system.
- Perforation of oil lines and ignition of an oil fire by a titanium fire.
- If bearing chamber seals are poor, pressure waves in oil mists can create conditions suitable for ignition (diesel effect).
Oil fires occur most frequently in main bearing chamber regions. The factors that promote oil fires and their causes are shown in the bottom diagram in Ill. 9.2.-2:
Hot gas incursion into the bearing casing after crack initiation (Fig. "Oil fires by oil leaks in bearing chambers"). Bearing casings are subjected to high dynamic stresses from bearing thrusts, rotor vibrations, and cyclical heat strain. If fatigue cracks form, there is a danger that hot air (hot gas) could flow into the bearing chamber due to the usual pressure gradients which run towards the bearing (Fig. "Main bearing chambers air systems").
The same is true for return oil- and vent lines (Example "Oil fire by hot air incursion"). There are frequent reports of cracks and fractures of these lines (Fig. "Oil fire by cracks in oil lines and vent lines"). These low-pressure lines typically run through hot gas regions with high pressure levels (e.g. compressor exit conditions), which promotes the incursion of hot gases (Fig. "Main bearing chambers air systems").
Dangerous hot gas incursions into the bearing chamber at the shaft seals can be expected if the seals become very worn by friction.
Pressure waves can have an igniting effect on this type of leakiness (e.g. during a compressor stall).
A damage sequence can result in pressure gradients that promote the escape of oil from a leaky bearing chamber. This oil can then be ignited by the hot periphery of the bearing chamber.
Oil leakages in shaft seals: a dangerous oil leakage can be expected only if the seal fails. However, there are special cases where a change in temperature gradients (Fig. "Oil fire between shafts (inter shaft)") leads to overpressure in the bearing chamber and the escape of a highly flammable oil mist.
Sparks and heating-up during rubbing in seals (labyrinths) with an unfavorable tribo-system (see chapter 7) can ignite flammable oil mixtures.
Overheating of a bearing can cause a flammable air/oil vapour mixture to form and ignite. Since it follows bearing failure and the axial and radial shaft movements it causes, this type of fire can escape through the countersunk labyrinths and heat the rotor to dangerous levels (Fig. "Oil fire overstressing rotating engine parts").
If the temperature levels in the bearing chamber are too high (
Example "Oil fire by hot air incursion" (Ref. 9.2-2):
Excerpt: “During climb a bang was heard and the aircraft shuddered. The crew stated that the number three engine oil pressure and oil quantity indicated low. The fire warning light and bell activated. The engine was shut down and the aircraft landed without further incident. Postflight inspection revealed that the outboard cowling of the number three engine had separated. The cowling was found in a suburb ten miles southwest of the airport. Engine teardown revealed a crack in a vent line strut in the diffuser area allowing bleed air to ignite oil in the Number three bearing area. Pressure forced the fire to vent through the compartment. Breather air tube disintegrating the main bearing breather tube elbow. The force of the explosion at the disintegrating elbow blew the outboard cowling off.”
Comment: A common cause of oil fires is hot air incursion into the vents. Vent lines that run through hot engine braces are an especially weak point. Heat strain, twisting due to stress on the mountings, or vibrations of these lines tend to lead to fatigue fractures through which hot air (hot gas) can enter into the oil system.
Figure "Operation data influencing oil fire risk" (Ref. 9.2-4): The danger of an oil fire in the bearing chamber and return oil regions rises with the engine temperature levels and the number and type of components connected to it (e.g. tanks). The situations in which this danger is present can easily be understood by looking at the aircraft`s flight envelope. When tactical aircraft (bottom right corner) fly at high speed and low altitude, they require high engine output and create high impact pressures (Fig. "Oil fire at a supersonic aircraft bearing chamber"). The top right corner shows the situations in which supersonic commercial aircraft are susceptible to oil fires. High speeds are critical, since they can heat up the fuel in the wing tanks to the point that the lubricating oil in the oil cooler (Fig. "Oil fire danger by aerodynamic fuel tank heating") is no longer sufficiently cooled (Ills. 9.2-5 and 9.2-6).
Figure "Oil fire danger by aerodynamic fuel tank heating" (Ref. 9.2-4): In this supersonic commercial aircraft, the engine oil is cooled by the fuel, which is the coldest liquid medium available for the task. The fuel tanks are located in the wings. It has become clear that during longer flights (about 2 hours) at speeds around Mach 2.0 the wing temperature at the leading edge and at the engine intake rises from an initial 92°C to 127°C. Also, before the fuel enters the engine`s fuel system, its temperature increases from 45° C to 115°C. Consequentially, the oil temperature also rises from 180°C to 230°C. The oil comes into contact with engine surfaces of up to 310°C, which the oil is intended to cool. According to document sources, from roughly 350°C upwards the oil being used is at risk of igniting.
Figure "Oil fire risk by bearing chamber temperature" (Ref. 9.2-5): Ignition of a sustained fire of a oil vapour/air mixture is only possible under certain pressure and temperature conditions (left diagram). If the oil vapour is very concentrated, it may be too rich for ignition, if it is not very concentrated, it may be too lean a mixture for burning.
The flow rate of the oil/air mixture can have a strong influence on the intensity and spread of an ignited fire (right diagram). The intensity of a fire is measured by the temperature increase after ignition. The results are registered in an oil fire testing rig with a bearing chamber. Evidently, there are limits of bleed air flow rates, under which no ignition (Fn) resp. self-sustaining fire (Fe) can occur.
The highest burn speed is reached if there is a stoichiometric relationship between the oil vapour and oxygen.
The flammability, however, is not determined by the flow rate of air and liquid oil, if one ignores their influence on temperature and thus on vapour pressure.
Figure "Oil fire ignition dependent on operation data" (Ref. 9.2-5): The left diagram shows the influence which oil flow in a bearing chamber testing rig has on the temperature of an oil/air mixture and the ignition at different oil inflow temperatures. The higher the oil flow from the injection nozzle is, the lower the temperature of the oil/air mixture in the bearing chamber drops, which in turn reduces the risk of a fire. The oil tested was type II Ester, the bleed air temperature was about 530 °C, and the ignition source was an electric spark.
As predicted, no ignition took place when the mixture was below ignition temperature (TL). Even when the temperature was considerably higher than the ignition temperature (arrow), it was not always possible to ignite a fire. There is an upper flammability limit (TU), above which ignition does not occur, because the necessary pressure and temperature conditions in the oil vapour are no longer present (Fig. "Oil fire risk by bearing chamber temperature", left diagram, intercept point of the upper concentration limit with the oil pressure/temperature curve).
The right diagram shows the influence which the oil inflow temperature (the oil stream from the injection nozzle) has on the temperature of the oil/air mixture and thus on flammability. Trial conditions: bleed air temperature of about 540°C, fires only occurred when the oil inflow temperature rose above 146 °C. This shows the importance of maintaining sufficiently low oil temperature.
Figure "Detecting and verifying an oil fire": Detection and verification of oil fires is required in order to take targeted countermeasures. There are signs that indicate oil fires while burning, and characteristic traces that can be used to identify oil fires afterward.
During an oil fire:
- CO2 increase in the vent and/or return oil systems.
- Temperature increase (bearing chamber, oil)
- Impaired oil flow (filter blockage, coke build-up, and/or damaged oil lines)
- Imbalances, consequential damages (not necessarily specific to oil fires)
- Drop in pressure (leak in the pressure oil feed)
After an oil fire:
Without dissection (maintenance):
- Changes in the oil (chemical/oxidation, soot content)
- Soot deposits in the filter
- Unusual oil consumption
- Unusual soot traces in the engine exhaust area
- Traces of overheating on the bearing chamber (tarnishing, oxidation, deformation, crack initiation).
- Overheated parts, especially rotor components (plastic deformation, loss of hardness).
- Characteristic soot deposits and soot traces on parts outside of the oil circulation area; e.g. around labyrinth seals on shafts and discs. -Soot and coke deposits inside the bearing chamber. Coke is formed by localized, longer duration, and recurring oil fires.
Figure "Oil fire at a supersonic aircraft bearing chamber" (Ref. 9.2-4): During the development and adaptation of a military engine variant (top diagram) for a supersonic commercial aircraft, problems arose with the temperature levels of the lubricating oil and the indicated bearing chamber. The high supersonic speeds combined with the impact pressure caused the temperature levels of the engine to increase and air friction heated up the wing tanks. As a result, the fuel-cooled heat exchanger was unable to cool the lubricating oil ( ). The problem was solved through appropriate measures.
Figure "Oil fire between shafts (inter shaft)": Intermediate-shaft oil fires in the intermediate-pressure turbine region of a tactical aircraft with multiple concentric shafts occurred during the development stage (top diagram, Fig. "Oil fire overstressing rotating engine parts"). During high-speed flights at low altitude (Fig. "Operation data influencing oil fire risk"), the pressure gradients around the main bearing chamber changed in such a way that oil vapour flowed from the bearing chamber into space between the shafts and ignited. The shafts were about 550°C. In a few minutes, this had softened the shaft and caused plastic torsion (bottom diagram).
Figure "Oil fire by cracks in oil lines and vent lines": This shaft-power engine from a large military helicopter (bottom diagram) had several oil fires occur in a ripped vent line (top detail). It was discovered that the line was subjected to high cyclical heat strain that its fasteners were not able to cope with. Corresponding prestressing occurred, promoting vibration-induced fatigue fractures in combination with stiffness and form notching (welding).
Excerpt from Ref. 9.2-7: “…(the OEM) is testing modifications…for its engine…to halt the occurrence of cracking of one of the four steel alloy bearing sumps that has led to excessive oil consumption on the affected engines.
… Modifications will include increasing material thickness, reducing thermal stresses and changing certain manufacturing processes.
…senior vice president for maintenance operations said the cracking always seems to occur on the No.2 engine, the one mounted in the tail. He said that tests to determine what unique conditions might be experienced by the tail-mounted engine had so far been unsuccessful.'We don't know what's causing it,' he said.
…(the OEM) sent out a service bulletin asking airlines to initiate an inspection routine to examine the C sump visually with a boroscope. Eight of the sump-caused engine removals occurred in May and June, and in all but one of these cases, the cracking was caught on the ground through the inspection program.
…A (OEM) spokesman said there appeared to be no correlation between the number of hours of operation of an engine and the occurrence of the cracking. The C sump located between the high- and low-pressure turbine sections, like the other three sumps is pressurized and cooled by fan discharge air.
Excerpt from Ref. 9.2-8: ”(the OEM) recommended airlines…sharply increase their inspection frequency in the wake of a second massive failure of a low-pressure turbine…in flight. The company says the low-pressure turbine failure of May 2 also was caused by the C sump on the engine cracking, and suspects that the C sump cracking was the cause of the july 27 engine failure.“
Excerpt from Ref. 9.2-9:”…The engine modification program is being accompanied by prescribing engine operation procedures and inspections to prevent recurrence in unmodified engines of two incidents…in which parts of the engine separated in flight. These were believed caused by oilfires in the hot cavity around the C sump due to oil seepage from cracks or loosening mechanical joints in oil lines.
The changes to prevent cracking of the C sump and other related problems include:
- Providing a new, heavier-walled C sump, increasing the wall thickness from 0.06 in. to 0.10 in., plus additional tolerance in the No. 5 bearing for extra dampening.
- Changing fore and aft air seals in the C sump area to provide air tubes for bringing more air to reduce temperatures in the C sump cavity. This is expected to alleviate the possibility of an oilfire from any seepage.
- Replacing the present C sump oil scavenge line, which has a bolted flange design, to one that has a brazed flange. In this case also, the modification represents a return to a design that was incorporated in the early production models, in this case about 80 engines. Again maintenance reasons were cited for the change of the initial design.
… (airline)engineers contend, however, that the tail position does produce a different environment for the No. 2 engine than the Nos. 1 and 3 experience, even though this difference may not be causative factor…In the tail-position an adaptor weighing about 40 lb. accomodates the movement between the airframe and the engine. In the No. 1 and 3 positions, the inlet cowl, weighing about 500 lb., is mounted on the engine….that extra mass of the cowl in the No. 1 and 3 positions dampens vibrations, and as a result No.2 engine recieves more stress.“
Comment: This case was given in detail because it illustrates the causes of an oil fire, the complexity of damage analysis, and the difficulty of developing appropriate countermeasures in an exemplary manner.
Evidently, the oil fires are primarily caused by fatigue fractures in the bearing casing and the vent- and return oil lines. The engine maker gave its own plausible explanation of the causes of the heavy fatigue stress in the engine`s middle region (thermal rotor bow), which is given in Example "Late start and early shutdown of tail mounted engine".
Figure "Oil fires by oil leaks in bearing chambers": Example "Oil leaks in bearing chambers" describes the background and probable causes of oil fires in the main bearing region of the turbine (schematic diagram bottom left, structural design see detail) of a large turbofan engine type. Fatigue cracks and fractures in the walls of the bearing chamber and lines seem to be the primary cause of oil leakages and/or hot gas incursions (return oil, vent).
Figure "Oil fires in low pressure turbines" (Ref. 9.2-10): This diagram shows an oil fire in the low-pressure turbine of a large turbofan engine. Evidently the low-pressure turbine is susceptible to oil fires in some engine types (see also examples 9.2-1 and 9.2-2, Fig. "Oil fires by oil leaks in bearing chambers"). There is also the danger of disks bursting and/or expanding and releasing the entire blading.
Figure "Oil fire overstressing rotating engine parts": A typical example of an oil fire in a low-pressure turbine (top diagram, Fig. "Oil fire between shafts (inter shaft)") after the failure of a labyrinth seal in the chamber of the main bearing. This type of oil fire in low-pressure turbines is especially dangerous to the integrity of the rotor (see also Fig. "Oil fires in low pressure turbines" and Example "Ruptured oil pressure line"). Within minutes, the oil fire heated up the shaft/disk flange to the point of softening. This resulted in noticeable plastic deformations as shown in the bottom diagram.
Example "Ruptured oil pressure line" (Ref. 9.2-3):
Excerpt: “As the aircraft (4 engine type) climbed through 11,500 feet the number 2 engine sustained an uncontained failure of the low pressure section. Following engine shutdown, the crew dumped 147,000 pounds of fuel and returned… for an uneventful landing. Postaccident inspection of the aircraft revealed a 2-foot hole in the number 2 engine cowl turbine area at the 9 o'clock position. Minor dents were noted in the number 1 engine cowl, pylon and underside of the left wing. Several fan blades from the number 1 engine were found with nicks and leading edge damage. Disassembly of the engine revealed that the oil pressure line to the number 4 bearing had fractured, with evidence of oil spray and fire signatures on the 6th stage low pressure turbine disk. Measurements of the 6th stage disk disclosed disk growth about 1.06 inches over blue print maximum, which liberated all 6th stage blades from the disk. Metallurgical examination of the tube fracture revealed a fatigue crack through about 25 percent of the tube circumference, with origins at spiral scratches found on the inside diameter of the tube. Periodically the tube must be cleaned of coke build-up….(the Airline and the OEM) maintenance/overhaul documents relating to the overhaul and cleaning of the number 4 bearing internal pressure tube assembly specify an oven baking procedure. Cautionary notes state that wire brushes or reamers are not to be used for internal cleaning of the tubes. Probable cause: The fatigue fracture of the number 4 bearing oil pressure tube due to an improper maintenance cleaning procedure by company maintenance personnel.
Comment: This fracture of an oil pressure tube due to internal scratches, along with the overhaul instructions, show how powerful the dynamic loads this line was subjected to, and that this fact was known. The heating-up of the turbine disk followed by a release of the blading can be expected from oil fires in oil mist escaping from a pressure line (compare Fig. "Oil fire overstressing rotating engine parts").
Example "Blocked oil filter causing minor fire" (Ref. 9.2-10):
Excerpt: “The U.S. National Transportation Safety Board is investigating two incidents…Both incidents occurred on July 9…As the aircraft approached 3,000 ft., a fire erupted in the nascelle of the No. 1 ….engine (of a 2 engine aircraft), apparently on the left side of the case. Activation of the fire suppression system eventually extinguished the blaze….'preliminary inspection of the engine indicates the fire was not caused by a failure in the turbine section, but occurred between the engine case and the nacelle/cowl assembly'…In the second incident, (an aircraft of the same aircraft type and the same airline)..was focused to return shortly after takeoff….'a warning light illuminated, indicating a blocked engine oil filter in the No.2' (engine)…The pilots secured the powerplant and landed safely…“
Comment: The successful extinguishing and the blocked oil filter in the second incident indicate limited oil fires outside of the engine. In this instance, the most likely cause is an oil leak in the mounted components of the oil system. Unfortunately, no further information was given concerning the causes of these fires.
Example "Smoke entering cockpit" (Ref. 9.3-10):
Excerpt ”…at an altitude of 600 m a seal failure in the hydraulic system I above the engine caught fire. Considerable amounts of smoke entered the cockpit.”
Example "Smoke entering cockpit II" (Ref. 9.3-11):
Excerpt: “During a strafing run on the range an engine fire due do leaked hydraulic oil occurred. The pilot exited the aircraft after the cockpit filled with smoke…“
Comment: both of these incidents occurred in an old single-engine tactical aircraft. The hydraulic oil was a synthetic engine oil.
Preventive Measures Against Oil Fires:
- Use of fire-proof and fire-resistant lubricants.
- Identification of the data that influence oil fires (oil flow rate, oil inlet temperature, bleed air rate in the sump, bearing loads, intensity of the ignition source, etc.) through use of computer calculations (if possible, supported by measurements) in order to ensure conditions under which fires cannot occur ().
- The temperature of the oil/air mixture must be kept below ignition temperature throughout the bearing chamber (Ref. 9.2-5).
- Sufficiently low oil injection temperatures can lower the temperature levels in the bearing chamber far enough that fire danger subsides. If a suitable heat management system is designed (e.g. through measures such as the use of heat exchangers), the oil inlet temperature can be kept low enough that the temperature of the oil vapour drops below ignition temperature (Ref. 9.2-5).
- Lowering the temperature in the bearing chamber by increasing the oil flow rate (Fig. "Oil fire ignition dependent on operation data").
- Since it is difficult to guarantee that the oil/air mixture is always too rich for burning, designs should aim at keeping the mixtures too lean for burning to occur (Ref. 9.2-5).
- Not changing to or mixing with oils that have not been proven to be completely safe for the engine type.
- No seal coatings that tend to dangerously heat-up or create sparks in case of rubbing.
- Coordinating and tuning the temperature, pressure, and flow rates of the cooling and blocking air.
- Appropriate thermal insulation of the bearing chamber.
- Fracture- and crack-proof inflow lines (see Fig. "Oil fire by cracks in oil lines and vent lines").
9.2-1 J.Auger, “Les Risques Affectant la Resistance Structurale et la Securité des Propulseurs Modernes”, Proceeding AGARD-CP-215, of the AGARD Conference “Power Plant Reliability”, pages 6-1 to 6-14.
9.2-2 NTSB Identification CHI86IA034, microfiche 30398A, 1985.
9.2-3 NTSB Identification LAX96IA087, 1996.
9.2-4 E.W. Doherty, “Lubricant Experience and Duties in a Civil Supersonic Gas Turbine Engine”, Proceedings AGARD-CP-84-71 of the AGARD Conference “Aircraft Fuels, Lubricants, and Fire Safety”, 1984, pages 30-1 to 30-12.
9.2-5 W.R. Loomis, “Aircraft Engine Sump-Fire Studies”, ca. 1974, pages 443-456.
9.2-6 J.W. Rosenlieb, “Aircraft Engine Sump Fire Mitigation”, report NASA CR-121158, SKF AL73T007, 1973, pages 1-59.
9.2-7 “GE Tests Sump Cracking Fix”, periodical “Aviation Week & Space Technology, July 10, 1972, page 25.
9.2-8 “CF6-6D Inspection Frequency Increased”, periodical “Aviation Week & Space Technology”, August 7, 1972, page 23.
9.2-9 “GE, Airline Users Press Fixes for CF6”, periodical “Aviation Week & Space Technology”, September 11, 1972, pages28 and 29.
9.2-10 E..H. Phillps, “CF6 Engine Incidents Prompt NTSB Inquiry”, periodical”Aviation Week & Space Technology”, July 20, 1998, page 39.
9.2-11 E.A. Witmer, T.R. Stagliano, J.J.A. Rodal, “Engine Rotor Burst Containment/Control Studies”, AGARD-CP-248, 1978, page 15-15 Fig. 9.