Table of Contents Fuel in the oil

 Fuel in the oil

Gets fuel into the lubrication oil, several problems develop:

  • With higher amounts of fuel, ignition of fires in bearing chambers with the danger of a shaft fracture and catastrophic damages with the exit of fragments.
  • Incompatibilities with the fuel with accelerated oil aging and declined lubrication capability.
    Shortening of the bearing lifetime.
  • Promoted cavitation and formation of vapour bubbles. Failures of pumps, face seals and problems at control units.

Thereby it must be remarked, that untill now merely fires in vearing chambers emerged (Ill. About time dependent effects nothing is reported. This can have several causes.

  • Contamination of oil by fuel are identified in time because of the smell („sniff test“).
  • The relatively frequent oil change prevents negative, time dependent effects of the fuel like aging
  • The deteriorated oil properties are tolerated by the bearings.
  • Bearing failures are not traced back at contaminations of the oil by fuel, because the connection isnot realised. This may depend from longer time periods between the oil contamination and the bearing failure.

Obviously there is only one possibility how markedly amounts of fuel can get into the lubrication oil. This exists in the heat exchanger at the side of the aeroengine (preheating of the fuel/oil cooling). Does here a leakage occur, dependent from the pressure distribution, fuel can enter into the oil circuit. Mostly this is the case, because the pressure of the fuel is markedly higher than the oil pressure.

 Fuel smell and mist in the cabin

Ill. (Lit. and Lit. During rolling of the aircraft to the runway intense fuel smell and mist formation in the passenger cabin occurred. After that the aircraft was parked. From a following aircraft, fire was observed at the right aeroengine. In spite of this, the cockpit instruments of the concerned aircraft showed no fire warning. The at once arrived fire brigade tried to extinguish the fire with water from the rear side of the aeroengine. White smoke formed, which however turned fast to black. Then about 3 meters long flames escaped at the rear of the aeroengine. After few seconds the fire went out. The following inspection showed:

  • At the exhaust cone ond the jet pipe sooty residues of fuel and oil.
  • The whole region of the fan and the bypass have been contaminated with oil and fuel. These contaminations came from different vents.
  • During opening of the cover from the oil tank a continous stream of fuel oozed out.This suggested a large internal leak. At once the oil/fuel-heat exchanger (fuel cooled oil cooler = FCOC)was suspicious (sketch above right). In it also during idle, the fuel pressure is markedly higher as that of the oil. So a leak will always lead fuel into the oil.

During the earlier flight the oil volume was already increased by about 3,5-liters. This shows, that already a small leak was existing. The heat exchanger was already a year before exchanged with an other, which also leaked. At that time intense fuel smell of the oil revealed the leakage.

 Fuel and oil heat exchanger

The heat exchanger consists of a high strength aluminium alloy (Lit. Its strength reacts very sensitive at a temperature influence during the production process (e.g., brazing). The exchanger is longitudinal devided from an axial orientated partition wall (devider plate). It is brazed into the casing. With this, high thermal stresses can develop between casing and axial wall. Into the crosswise oriented covering plates, the fuel guiding exchanger pipes are also brazed. The other cross walls serve only for positioning. They are fixed in the casing, however the pipes can slide in them. This enables a dangerous friction wear (fretting) at the contact zones.
Already during X-raying of the heat exchanger displacements of the pipes could be seen.

A Borescope inspektion revealed, that the pipes have been broken and the axial deviding plate showed a several centimetres long crack. The microscopic analysis of the fracture surface (SEM, volume 4, Ill. 17.3.2-7) suggested a LCF-fatigue crack. It developed under cyclic thermal stresses (thermal fatigue) in the region of the stress concentration (change of stiffness, notch) from the transition of the deviding plate into the casing. Here at the braze the material strength is lower. This is caused by the production (brazing), but inevitable (weak spot/flaw, no failure!).
After the exchanger was opened, in the pipes and the axial deviding plate cracks and fractures could be seen. Intense fretting had heavily weakened the walls of several pipes.
Obviously already formerly fuel leaks at cracked pipes had been repaired. This showed the glued plugs.

History: At the OEM of the aeroengine similar cases of a fuel leak to the oil have been already known. In all cases the cause was a crack formation in the heat exchanger. Already about 20 years before, a fire in the oil sump of the low pressure turbine bearing („1” in the detail below left) occurred.
At least in one case, the low pressure shaft fractured by overheating. It came to an overspeed and the centrifuging of the blades from two stages (volume1, Ill. 4.5-4).
In 11 incidents fire broke out in the middle turbine bearing („2“ in detail below left).
It was tried to solve the problem with different measures and relating service bulletins.

  • Improvement of the vent system with an additional pipe line (extension tube).
  • Introducing a flame trap, to prevent a dangerous expansion of the flames from the middle bearing chamber foreward.
  • Several informations to the operators.
  • Sniff test during the checks of the oil level.
  • Borescope inspections of the brazings at the axial deviding plate.
  • Introducing of an improved exchange version. At this the pipes have been pressed into the covering plates instead of brazed.

Note: Does the oil level/the oil volume rise, basically the cause must be identified. To this belongs the sniff test and an oil analysis for contaminations by fuel.

References AAIB Bulletin No: 4/99 Ref: EW/A97/5/1 Category: 1.1, „Mc. Donnell Douglas DC-10-30 Incident 11 May 1997”, page 1-12. I.E.Traeger, „Aircraft Gas Turbine Engine Technology“, Second Edition, Glencoe, ISBN 0-07-065158-2, 1994, page 509 and 514.

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