9.3 Fuel Fires
In the following text, the term fuel fire applies to all fires that involve the ignition and burning of fuel and those that can be traced back to an escape of hot flue gases not necessary for engine operation (e.g. hot gases escaping sideways from a perforated combustion chamber housing). Causes (Fig. "Fuel fires by leaks in lines"), manifestations (Fig. "Fuel flame types"), and consequences of fuel fires are many and varied. It is possible that no engine damages are found after a fire has been extinguished (e.g. after a tailcone fire). In standing engines, only small amounts of residual fuel are burned off (e.g. after an aborted engine start, examples 9.3-3 and 9.3-4). If larger amounts of fuel in the form of clouds or direct streams are ignited, extensive damages can be expected. Flames or hot gases escaping from pressure housings (e.g. combustion chamber) are especially dangerous (Fig. "Catastrophic combustion chamber failure").
Prevention of fuel fires must begin with the seals of the fuel system.
- Tubing is a typical weak point. Tubing made of nickel alloys or high-alloy steels is preferable to that made of titanium alloys (Fig. "Fuel fires by leaks in lines").
- In areas in which there is a possibility of a titanium fire, proper preventive measures should be taken (chapter 9.1.3).
- Firewalls or protecting walls only offer limited protection, but definitely reduce the risk of catastrophic damage occurring (chapter 9.5).
- Precise mounting in line with regulations and careful handling of tubing are prerequisites for any fuel-leak-proof system.
Example "Leaking fuel nozzle" (Ref. 9.3-2):
Excerpt: Left engine fire warning light illuminated shortly after reaching cruise altitude. Engine fire bottle was discharged and the fire light went out… The leaking fuel nozzle and portions of the engine wiring harness were subsequently replaced.“
Comment: It can be assumed that the fuel nozzles leaked outside of the housing, because otherwise the fire would only have been noticed after larger damage. This small of a fire is more likely to have been caused by a dripping leak than a stream of fuel.
Example "Tailcone fire" (Ref. 9.3-3):
Excerpt: ”…During the taxi to the passenger terminal, a fire erupted in the tailpipe of the left engine. The flight crew executed the emergency evacuation procedures. Fire was extinguished. With no damage to the aircraft or engine. Post incident examination revealed a malfunctioning fuel flow regulator.“
Comment: This typical “tailpipe”- or “tailcone”- fire occurred during taxiing and can most likely be traced to the burning off of fuel collected in the engine outlet. This notion is supported by the fact that the engine was not damaged and the hot parts did not overheat, as they would be expected to after a fire in the combustion chamber or turbine.
Figure "Fuel flame types": Escaping fuel can cause fires with various manifestations and consequential damages.
Fuel escaping from pressure lines of the fuel- or hydraulic systems (diagram A) causes directed bundled flames (darting flame) or explosive fires of clouds of small fuel drops (Example "Catastrophic fuel fires in supersonic tactical aircraft"). Darting flames are very intense. They are capable of melting through titanium firewalls between two parallel engines.
A dripping leak typically occurs at seals or screw joints (diagram B, Example "Leaking fuel nozzle"). Unless a large amount of fuel has collected, this unintensive undirected type of fire (provided no strong air flow is present) can be controlled by the engine`s fire extinguishing system (chapter 9.5).
Flames and/or hot gases escaping from pressure housings (diagram C) are explosive. The directed hot gas stream or darting flame is very energy intensive (Fig. "Danger by hot gases / flames under pressure", Example "Burnt through combustion chamber") and can penetrate normal firewalls.
The escape of a dark stream of exhaust gas with flames (D) after a compressor stall. This involves a fuel cloud several meters long that burns from the end. The fuel could not burn in the engine due to the temporary lack of air in the compressor.
Flames escaping from the exhaust pipe while the engine is stopped (E). If fuel has collected in the engine after a failed start, failure of the ignition system, or a regulator malfunction, the fuel can ignite and burn off undirectedly (examples 9.3-3 and 9.3-4).
Flames escaping from the exhaust pipe while the engine is running (F): if fuel has collected inside the engine (examples 9.3-2 and 9.3-4) and ignited while the engine is running, it exits as a directed flame along with the exhaust stream.
Figure "Fuel fires by leaks in lines": Fuel that causes a fire can be released in various different ways (top diagram). The most common cause is a leak in a fuel line. Fires have been known to result from leaks in fuel pumps, in the combustion chamber and afterburner`s injection systems, and in fuel-related hydraulic systems (e.g. exhaust nozzle offset, Example "Catastrophic fuel fires in supersonic tactical aircraft"). This can result in the injection stream, i.e. the flame of a damaged fuel nozzle, escaping sideways (e.g. after erosion damage to the nozzle mouth or overheating).
The bottom diagram shows typical causes of leaks in fuel lines:
Leakages due to problems with the mountings: insufficient pick-up, strained tubing, missing or improper retention straps, missing, improper, or damaged seals, and mechanical damage (dents, scratches). In cases involving damage, insufficient fastening, or strain, the fuel leak is indirectly caused by a fatigue fracture. Strain, for example, can lead to a higher mean stress and thus a lowering of the withstandable dynamic loads. Notches create stress points where fatigue fractures can initiate. Bulges can cause a jump in stiffness and thus increase stress levels. Titanium alloy tubing is especially prone to this type of notching, most of all sharp scratches, and after a spectacular damage sequence occurred in the design phase of a tactical aircraft type, its use as fuel tubing has been very limited.
Scuffing due to fretting can perforate the tube wall or weaken it excessively. In titanium alloys, fretting damage is accompanied by a dangerous loss of resistance to dynamic loads (Fig. "Decreasing dynamic strength by fretting").
Crack initiation or fractures in tubing-related screw connections (retention straps, clamping nuts) occurs primarily through crack corrosion in steels and aluminum alloys (chapter 5.4.2).
If fuel-carrying lines are melted-through by burning melt drops from a titanium fire, ignition of the escaping fuel is to be expected (Fig. "Pipelines penetrated by titanium fire").
Example "Failed engine ignition leading to fire" (Ref. 9.3-4):
Excerpt: ”…during an attempted start the No. 1 engine failed to gain ignition during the prescribed 25 sec limit and ..(the crew) aborted the start. Shortly thereafter, ground persons reported that the engine had developed a. Following emergency procedures, the fire persisted and the captain ordered the aircraft evacuated…Investigation revealed that the “B” ignition system was inoperable…Examination revealed a short in the “B” system harness between the exciter and the ignition plug.“
Comment: Evidently the unburned fuel from the failed start burned off. The report does not mention any lasting damage to the engine.
Example "Tailcone fire managed by motoring engine" (Ref. 9.3-5):
Exerpt: “The No.3 engine torched during the start attempt and resulted in a internal engine tailcone fire. Several passengers panicked upon seeing the torch flame/tailcone fire…The flight crew was at first neither aware of the engine torch and subsequent tailcone fire...
When ..(the crew) learned of the tailcone fire they motored the engine in order to extinguish the fire which they believed was under control and therefore requested no emergency assistance.”
Comment: The only danger in this case was the panicked reaction of the passengers. This reaction indicates the spectacular appearance of the fire. No information regarding the cause of the fire and its possible effects on the engine was given in the report at hand.
Figure "Danger by hot gases / flames under pressure" (Ref. 9.3-1): If the wall of an internally pressurized housing (e.g. combustion chamber) heats up to a high temperature (e.g. due to a damaged injection system and/or combustion chamber), it will bulge out (top diagram, Ill. 9.1.2-8). If the wall breaks open, a nozzle-shaped opening is formed and the hot gas stream escapes through it at up to supersonic speeds. In the worst case, a stream escapes from the combustion chamber housing (Example "Rupture of combustor air casing", Fig. "Catastrophic combustion chamber failure"). In modern engines, the pressure inside is 20-40 bar. This means that the flame temperature can be in the region of stoichiometric conditions (about 2000°C). If this kind of hot gas stream strikes a wall (e.g. a firewall, Fig. "Loads at housing walls by hot gas jet"), a good high-temperature heat transfer occurs (bottom diagram) under high gas loads. This leads to correspondingly high thermal and mechanical stresses, as well as a heating-up and a drop in strength. Normal firewalls, such as thin titanium sheets, cannot resist this type of stress for a sufficiently long time. Because of this, surface temperatures of 1500°C were recorded in trials where a plate was struck perpendicularly by a hot gas stream from about 0.5 meters, as would be expected escaping from a combustion chamber housing. This is above the melting point of both titanium and nickel alloys. This means that the airframe, pylon, parallel engine in the hull, etc. are endangered despite the presence of this type of firewall (partition wall; chapter 9.5).
Example "Catastrophic fuel fires in supersonic tactical aircraft".1 (Ref. 9.3-8):
Excerpt: ”…a fire occurred in the engine due to a failed fuel flange. After an arrester hook-assisted emergency landing, the rear hull burned out.“
Example "Catastrophic fuel fires in supersonic tactical aircraft".2 (Ref. 9.3-9):
Excerpt : “After a fire warning, the aircraft shook violently and there was an explosion in the engine. The aircraft went out of control and crashed.”
Example "Catastrophic fuel fires in supersonic tactical aircraft".3 (Ref. 9.3-12):
Excerpt: ”…Fuel escaped due to an overheated afterburner fuel pump and caused a fire in the engine..“
Comment: These incidents occurred in an older model single-engine supersonic tactical aircraft. The examples show how varied the causes of fuel leakages followed by catastrophic fires can be.
Figure "Loads at housing walls by hot gas jet" (Ref. 9.3-1): If a stream of hot gas or flames shoots from a nozzle-shaped opening (Fig. "Danger by hot gases / flames under pressure") and strikes a plate, complex aerodynamic and thermodynamic processes occur (top diagram). In the gas stream, speeds reach supersonic levels and pressures are above those of the surroundings (underexpanded jet). After the gas stream has struck the plate, it spreads concentrically across the surface as a “wall stream”. The expansion of the unreleased pressure accelerates the wall stream to supersonic speeds. The speed differences (subsonic at the stagnation point; supersonic in the stream) cause circular temperature maxima around the point of impact. The vortex-induced total temperature separation is responsible for this phenomenon. This results in the plate being subjected to large temperature gradients with corresponding thermal strain.
In general, two different flow patterns are observed:
The pressure peak is in the geometric center of the impacting stream (bottom left diagram).
A stagnation bubble is created above the plate. In this case, the highest pressure is in a ring-shaped pattern around the impact point in the stagnation bubble. This means that a larger plate surface is under high pressure than in the first scenario of a single pressure peak. The hot gas is trapped in the stagnation bubble and recirculates due to shear stress in the gas stream. This recirculation then acts to stabilize the stagnation bubble.
Example "Fuel nozzle nut lock ring mis-assembled" (Ref. 9.3-7):
Excerpt: “During the climb-out, a 15% decrease in the right engine power was noted by the flight crew. This was followed by a loud bang & failure of the No.2 engine….An initial examination revealed that the right engine top cowling had separated and the lower section ot the combustion case was split. Engine removal and teardown revealed that the No.5 fuel nozzle nut lock ring was mis-assembled. This detracted from the torque on the fuel nozzle nut and inadequately compressed the No.5 fuel nozzle seal. Subsequently, the fuel nozzle leaked and allowed local overtemperature of the outer combustion chamber case at the 6 o'clock position.”
Comment: This example clearly shows the danger posed by a leaking fuel nozzle, which created a fuel stream or fuel cloud that heated up the combustion shamber housing until it failed explosively.
Example "Burnt through combustion chamber" (Ref. 9.3-6):
Excerpt: “While on walk around inspection the flight crew noticed a burned through area on the number one engine outboard cowling. The engine was removed from service and disassembled and examined. The fan duct and the engine combustion outer liner were also burned through. The number 7,8,9 combustion chambers were found to be partially burned away and distorted. The combustion chambers had been previously removed and weld repaired about 16000 hours prior to this failure. The chambers exhibited thermal fatigue failures.”
Comment: It is not clear whether hot gas escaping from a damaged combustion chamber caused the failure of the combustion chamber housing or if it was due to a fuel stream from the injection nozzle being diverted by hot gas.
Figure "Catastrophic combustion chamber failure" ( Example "Rupture of combustor air casing", Ref. 9.3-13): This spectacular case describes the creation of a hot gas stream after an overheated pressure housing burst. The explosive final damage stage was not due to the explosion of a fuel cloud, but due to the expansion of the hot gases in the combustion chamber, which were under high pressure.
Excerpt (Ref. 9.3-13):”…the UK Transport Department's Accident Investigations branch blamed an explosive rupture of the combustor air casing.…as the initiating cause of the disaster. Several combustor cans were completely exposed, which resulted in distortion and failure of the fan duct casing. A small part of this casing struck a fuel tank access panel immediately outboard of the engine, punching an 8-inch hole and allowing fuel to pour out…
Cracking of the …combustor cans resulting from thermal fatigue is regarded as the most likely cause of the combustor rupture of the accident engine (the front one-third of the No.9 can in the top position was completely missing.“
Excerpt (Ref. 9.3-14):”…Before the … incident there has been 12 reported cases of combustion-chamber explosive rupture, of which seven were attributed to a primary defect. Of the remaining five cases, two were attributed to problems with the fuel nozzle and/or support and three to combustion-can problems. In two of these latter cases…parts of the can responsible had been expelled, causing minor airframe damage, but with no fire.
There were also 16 recorded previous cases of combustion chamber burnthrough. Four were attributed to combustion can failure, five to can shift (locating-pin failure), and the remainder to fuel nozzle or fuel system failure.
The AAIB report into the …incident finds that the left-hand engine failure was caused by an explosive rupture of the combustion chamber. The rupture immediately caused the engine to run down. The forward (dome) part of the No. 9 combustion can ejected radially from the engine. The combustion-chamber outer case rupture was caused by localised overheating in the area adjacent to the No. 9 combustion can, which caused a reduction in material strength over a critical length of casing…
(The OEM) had advised all … operators of a 1979 incident in which the No. 8 combustion can ruptured, resulting in a hole in the aircraft's fin. The incident was initiated by the complete fracture of one of the chamber seam welds joining two liner sections. Resultant misalignment of the chamber segments caused combustion within the chamber to impinge on the combustion case wall, softening the case to the point of rupture, a mechanism similar to that…“
Comment: If material escapes in the combustion chamber`s primary zone, one can expect the flame to exit sideways and the combustion chamber to overheat to dangerous levels.
9.3-1 M.L. Messersmith, S.N.B. Murthy, “Thermal And Mechanical Loading On A Fire Protection Shield due To A Combustor Burn-Through”, AGARD-CP-587, Proceedings of the Agard Conference “Aircraft Fire Safety”, 14-17 October 1996, pages 22-1 to 22-13.
9.3-2 NTSB Identification: CHI88IA045, microfiche number 37738A, 1988.
9.3-3 NTSB Identification: DEN861A235, microfiche number 32896A, 1986.
9.3-4 NTSB Identification: FTW861A078, microfiche number 32092A, 1986.
9.3-5 NTSB Identification: NYC86FA076, microfiche number 33858A, 1986.
9.3-6 NTSB Identification: MIA86IA072, microfiche number 30551A, 1986.
9.3-7 NTSB Identification: ATL84IA246, microfiche number 26009A, 1984.
9.3-8 G. Fischbach, “916 Deutsche F-104 Starfighter, ihre Bau- und Lebensgeschichten”, page 127.
9.3-9 G. Fischbach, “916 Deutsche F-104 Starfighter, ihre Bau- und Lebensgeschichten”, page 159.
9.3-10 G. Fischbach, “916 Deutsche F-104 Starfighter, ihre Bau- und Lebensgeschichten”, page 216.
9.3-11 G. Fischbach, “916 Deutsche F-104 Starfighter, ihre Bau- und Lebensgeschichten”, page 304.
9.3-12 G. Fischbach, “916 Deutsche F-104 Starfighter, ihre Bau- und Lebensgeschichten”, page 519.
9.3-13 “Turbine DMS Intelligence”, “Pratt & Whitney Threatened With Legal Action on UK 737 Disaster”, Volume 10, Number 23, September 23, 1985.
9.3-14 “Report highlights JT8D problems”, Zeitschrift “Flight International”, 18 March 1989, pages 6 and 7.