19.2.2 Foreign objects and contaminations.
Differing not foreseen objects can get into an aeroengine during operation and stand still. They necessitate specific maintenance activities. Foreign objects also include in this chapter, besides massive sucked foreign objects, contaminations of the air with natural and technical origin (Fig. "Typical corrosion media for aeroengines" up to Fig. "Inserting fire extinguishant into aeroengines"; volume 1 chapter 5.2).
It can be supposed that in most of the cases the foreign objects and contaminations are sucked in by the aeroengine. But there are definitely situations in which the shut off aeroengine is concerned. Examples are dust deposits on parking aircrafts or extinguishing media (Fig. "Inserting fire extinguishant into aeroengines").
The experience teaches, that during the maintenance as well as the assembly, foreign objects can remain in aeroengines. Typical indicator of such an effect is the so called bath tub curve (volume1, chapter 22.214.171.124). Very differing foreign objects are possible:
- Tools like screw wrenchs (Fig. "Rebalancing as a measure") , e.g., wrench sockets.
- Auxiliary means like pocket lamps, writing utensils, cleaning rags, Schreibgerät, Putztücher, inspectiondevices like mirrors (Fig. "FOD recommendations for maintenance").
- Connecting elements like bolts, nuts, shims and clamps.
- Media like cleaning/washing agents/fluids or spilled oil, fuel and hydraulic fluid.
- Consumable supplies like plastic foils/bags.
Figure "Typical corrosion media for aeroengines": Certain aeroengine components (frame below) are endangered by corrosion (volume 1, chapter 5.4.) from specific media ( Lit. 19.2.2-2) . This picture shows a survey of especially dangerous corrosion systems. Quite generally it can be determined, that the exposure to corrosion increases with the down time. This explains, why especially aeroengines in military use show aggravated corrosion. In addition there are operation conditions like low-level flight and/or sea atmosphere. Such conditions must be expected are intensified on aircraft carriers.
In the foreward compressor on the blading region `insect roughness' can occur during operation near the ground (rolling, start, landing, test rig). This can also promote corrosive deposits and considerably deteriorate the operation characteristic of the compressor.
In aeroengines of helicopters, agro planes and fire fighting planes a low flight altitude promotes corrosion in contaminated air. Acording to the use ash, water/sea water/condensate, fertilizer, herbizides, insecticides and industrial exhaust gas can be ingested.
Not to underestimate is the danger, that during a certification/approval runs and test runs detrimental materials can be sucked in. Resides in the proximity of test rigs as galvanic, chemical or cleaning shop from experience especially attention is necessary (volume 4, Ill. 126.96.36.199-2.2). Especially military `field test facilities' (not inside a building) are potential endangered by damaging media. To these belong agro dusts but also industrial media like paint mist (volume1, Ill. 5.5-1). Thereby it must not be forgotten, that aeroengines can intake media over far distances. In such a case a connection may not be at once identifiable. This is especially true if the damage/deterioration shows observable not until longer times.
But also aeroengines of airliners can be submitted especially intensely acting corrosion media. To those belongs the not so seldom intake of volcanic ash and/or of aggressive gases (Fig. "Volcanic ash" and volume 1 Ill. 5.3.2-15). Even if a diminishment of the flow cross sections and blocked cooling air holes (by overheating) dont occur as a spontaneous damage, those media act in the hot parts over a long time especially damaging. For ash deposites typical high phosphor and sulfur content acts especially through hotgas corrosion respectively sulfidation shortening for the lifetime ot he hotparts.
Not to underestimate is the damaging effect of fire extinguishants (volume 1, Ill. 5.5-2). Are those ingested by the aeroengine, often a disassembly and an extensive cleaning is inevitable (Fig. "Inserting fire extinguishant into aeroengines"). In a few hundred operation hours aggressive decomposition products (halogens like chlorine and fluorine) of the fire extinguishant can damage hot parts irreparable. Is fire extinguishant blown from behind into the low pressure turbine of a shut down aeroengine (flames after an aborted start; tail cone fire, volume 2, Ill. 9.0-3), the washing of the blading my be sufficient for cleaning. However thereby the valid specifications must be met very exactly.
Unsuitable auxiliary media like not authorised/certificated cleaning agents or lubrication grease can trigger corrosion (Fig. "Approved lubrication media"). Thereby operation conditions like temperatures of the hot parts and/or the simultanous acting of other media can play a role.
Figure "Volcanic ash" (volume 1, chapter 5.3, Lit. 19.2.2-4): In volcanic ash, depending from the site of formation, it must be reckoned with individual compositions respectively ingredients which together with humidity or hot parts typical temperatures form aggressive corrosion media. Thereby not only the intake during the flight is problematic. Also into not sufficient covered intake of aeroengines of airplanes which stand in the region of an „ashfall“, dangerous amounts of durst can enter. This will be supported by wind. Also an exchange of air inside during the change of the outdoor temperature at day and night, transports dust and aggressive gases into the aeroengine or other aggregates. If there is a suspicion that dust entered, extensive maintenance work with high costs can be necessary (volume 1, Ill. 5.3.2-15).
Components at the outside of an aeroengine like accessories and actuator systems of variable compressor guide vanes are also potential endangered from a deterioration. In Generators with the cooling air also dust can enter. This can trigger electric shorts (conductive moistly ash) and mechanical damages (e.g., by erosion). Enters ash in electronic devices, in case of an bridge circuit it must be also reckoned with a malfunction. Are filters in the cooling air supply of electronic devices blocked, the danger of overheating exists. For control units this can mean the outage.
Volcanic ash is not only able to trigger corrosion. A further problem is the clogging of cooling air guidances in hot parts with the danger of a creeping overtemperature and markedly shortening of the lifetime. Do deposits narrow flow cross-sections like in turbine vanes/nozzles behind the combustor, the operation behaviour of the aeroengine can be dangerous influenced, e.g., by an early compressor surge (volume1, Ill. 5.3.2-15). Also deposites which change the profiles of the blading in compressors, deteriorate the efficiency and the surge behaviour. So the likelihood if a vibration overload increases (e.g., during surge pulses) with catastrophic secondary failures. With this the fuel consumption rises. Then a sufficient performance demands higher gas temperatures.
Gets volcanic ash into oil or fuel, wear damages in the systems must be expected. In anti friction bearings dust particles can trigger fatigue failures on the races and rolling elements.
As a first measure to evaluate the danger of an contamination by ash and/or the reduction of risks in the available/cited literature for military aircraft is recommended:
- Before a washing process the ash must be blown off or suction cleaned. This sequence must be kept to avoid that ash or washing fluid will enter into intakes (ports, holes), vents or movably elements.
- Rinsing of remaining rests. No scraping or sweeping.
- Washing of the landing gear, the lower side of the fuselage, the air intakes and aeroengines.
- Check of the pH value for acid reactions of the airplane fuselage upper side and the aeroengines. Is this the case, the increased danger of an stand still corrrosion of susceptible materials like Al-alloys and Mg-alloys, polyester rub in coatings filled with Al-powder and steels (e.g., blading) of elder aeroengine types exists (volume 1, Ill. 188.8.131.52-2).
- For „acid ash” a neutralisation is recommended. An admixture on the basis of petroleum is recommended for the washing fluid.Basically also here counts: Valid are always the aeroengine specific instructions respectively manuals of the responsible OEM, operator and authorities. In doubt here the approval must be obtained.
Figure "Paint particles from overhaul processes" (Lit. 19.2.2-3): At this supersonic airliner aeroengines had to be exchanged after during the abrasive finshing/grinding of the aircraft, before painting dust sedimented on the compressor blading (`fouling', chapter 19.2.3). The four aeroengines were indeed covered during the painting work. Before painting obviously entered grinding dust during remove of the old ground coat (primer) the aeroengines. Worrying, that this dust (harmful to health?) could get from the compressor into the cabin air system, extensive cleaning work on the aeroengines had to be carried out.
Figure "Inserting fire extinguishant into aeroengines" (Lit. 19.2.2-3 and Lit. 19.2.2-4):
Occures fire on ground it can get necessary to activate fire extinguishers. The used extinguishing substances normally contain components, which act during decomposition highly aggressive (volume1, chapter 5.5). These are halogens loke bromine, chlorine anf fluorine.
They damage the protecting oxide layer of the hot parts and trigger hot gas corrosion. Also modern extinguishant are problematic because of their sulfur content. Together with chlorine of the sea atmosphere they promote sulfidation.
Halogens in the exinguishing agent can also be dangerous for seemingly corrosion resistant materials like titanium alloys at elevated temperatures. Above about 450°C and sufficient tensile stresses stress corrosion cracking can occur (volume1, Ill. 184.108.40.206-8).
In the disassembled condition deposits of extinguishants can be removed if in time without damage by mild abrasive blasting.
It is especially problematic if fire extinguishant gets into a running aeroengine. Such a situation develops for example, when a fire at a landing gear (overheated brakes) is extinguished and during this the extinguishant is sucked in (lower sketch). In this case much more components understandably are concerned as if a fire must be extingushed at the exhaust of a stand still aeroengine (tailcone fire, volume 2, Ill. 9.3-1; upper sketch).
In every case the measures comply with the valid specifications and/or instructions of the responsibles.
Figure "FOD recommendations for maintenance" (Lit. 19.2.2-1): Few minutes after the start there was a loud bang together with the outage of the aeroengine and warning signals in the cockpit. The pilot initiated the autorotation and tried fly over a forest to attain a street for a landing. Thereby the rotation speed of the main rotor dropped markedly. Anyway the landing succeeded. In fact the tail boom was damaged by the main rotor, however the passengers remained unhurt.
The following investigation showed:
A cardboard tube, which was used to hold the inspection door open (sketch in the middle), jammed before the compressor intake. It blocked almost the whole cross section of the entrance. Apparently the tube has slipped into the inlet duct. It was left in this difficult observable position, inspite of the obligatory check for foreign objects. This can be seen in connection with a short-term exchange of the responsible engineer. Thereby a personally work transfer to the succesor did not take place. Promotive may have act the lack of a warning flag and a tool table (`shadow board') without markings for devices/tools.
Figure "Rebalancing as a measure" (Lit. 19.2.2-6): During the airplane rolled to the runway for start, all cockpit displays have been normal. Three seconds after the start a hissing sound occurred, followed by a „hollow bang“. In the same moment the airplane swung to the right and vibrated heavily. The pilot counter steered and initiated the rejection of the start. During the braking at one aeroengine the rotation speed of the low pressure rotor attended 0, the high pressure rotation speed 107,5% and the exhaust gastermperature 657 °C. Although no warning light indicated a failure of the aeroengine, the pilot shut down the engine. Visitors from outside saw smoke and vapour coming out of the engine. Hereon the informed pilot activated the fire extinction system of the aeroengine.
Escaped fragments had damaged the wing and torn off parts of the pylon. Tires and wheels of both main landing gears were damaged, two tires flat. Also the electric and hydraulic ststem to the fuselage was affected by impacts.
The following disassembly and investigation showed:
The casing of the low pressure turbine (LPT) had loosened along the flange of the turbine intermediate casing over the whole circumference (sketch in the middle).
The first stage of the LPT rotor was separated from the 2nd stage and was missing. Associated fracture fragments of the 1st stage secured in the surrounding.
34 studs and nuts of the connection between 1st and 2nd LPT rotorstage and the interstage labyrinth (lower frame) were found. They showed impacts with rust like discolorations.
All in the flange of the 2nd stage remained studs were deformed in the rotation direction deformed and showed impact damages.
The flange was cracked at four locations and the neck of the shaft (spacer arm) at the inside surface showwed countless impacts.
An analysis of the rustlike disoloured locations showed, that this is about abrasion of a high strength tool steel type M 50. This is used for assembly tools (e.g., screw wrench) and anti friction bearings. Additional several small pieces of the same material with about 4 mm wall thickness were found inside the LPT rotor drum in the area of stage 1 and 2 and at the runway.
History: about 2 years before the accident the 1st LPT rotordisc was exchanged by the operator during an overhaul. All documents correct compiled and available. From this moment the LPT rotor was no more opened. The following testbed run showed even acoordint the at this time relatively wide standard no inadmissible vibrations. Half a year later the aeroengine has to be removed because of inacceptable heavy vibrations, according an restricted standard. A new balancing decreased the acceleration of the vibration to 1/25 of the rejected value. At the day of the accident the disk of the 1st LPT stage had about 2 500 operation hours with about 600 starts-stop cycles after the installation.
The logbook of the airplane showed four items of the pilot about vibrations of which only two concerned the failed aeroengine. In both cases the aeroengine was inspected without finding something that could assigned directly to the vibrations.
Failure cause: The steel parts could only get into the LPT rotor drum during the assembly. They hitover a longer period of time in the rotor against the bolted connection of the flange and damaged it. Then the flange connection failed during the start load.
Alhough the restpieces of the foreign object consist of tool steel the identification of the tool was not possible. But the suspicion is obvious, that it's a forgotten assembling tool. Parts of a bearing could be ruled out. Thinkable scenario: A tool socket, sticking on a nut would not attract attention by noises during spinning of the rotor, as long as it does not com loose. Also visual it would be difficulf to identify. Obviously the balance tolerances at this time have been large enough, to cast the unbalance during the approval run into doubt. Obviously later this limits were reduced and these three times exceeded by unbalances. A secondary balancing was no sufficient measure. Instead it would have been absolutely necessary, to clear the cause of the vibrations respectively of the unbalance.
Figure "Foreign objects through oil refill" (Lit. 19.2.2-1): The seals in the closing caps of plastic containers are a danger. Do they not stay in the dropped closing cap but stick at the container (upper sketches), they can get into the oil filler if there is unsufficient attention.
Also film sealings (lower sketches) are problematic. There are reports of several cases , where pieces of such sealing film came into the oil system. Especially dangerous is the bad habit simply to ram the film. This for example is the case if the film cover is rammed with a tool into the container. So it can get unnoticed into a tank without filling screen. This risk is avoidable if really the whole film will be pulled off.
Figure "Cotter pins and fixings of splints" (Lit.19.2.2-5): Cotter pins are especially noticed as foreign objects caused by maintenance and/or assembly. Related to the aircraft this damages can be called OOD (own object damage). For the aeroengine they can count as FOD. Do fixings/securings fail with the drop out of a cotter pin, loosened components like nuts or bolts (upper sketch) can trigger considerably secondary damages (Fig. "Considerations for installation of a hing").
Figure "Identifying foreign objects" (Lit.19.2.2-5): Did a foreign object damage already occur, in the most favorable case the failure can be repaired with an approved rework, according the aeroengine specific instructions. However mostly this is not possible. This means expensive repairs.
To prevent further failures, the foreign object must be identified. Did this succeed, often can be concluded also at an origin and with this, at the cause. Then targeted preventive measures get possible.
The identification of a foreign object demands investigations of the impact.
Not seldom little foreign objects or fragments of it already stick in the blade of the compressor. In these cases, the identification by means of the geometry and material should be possible (volume1, Ill. 220.127.116.11-6).
Is merely the impact analysable/evaluable there are conclusions from the shape (a circle hints at a cylindric part) and if applicable patterns of the indentation (e.g., thread) possible (upper sketch, (volume1, Ill. 18.104.22.168-3 and Ill. 22.214.171.124-4)).
Soot plumes in flow direction (volume1, Ill. 4.2-7) behind curled burrs and bulges show, that the impact occurred some time ago. Also erosion marks/traces of an usual dust load on the impact surface gives in the scanning electron microscope (SEM) a hint for the age.
In the SEM are micro analysis of the impact surface possible which can be compared with the unaffected neighbouring surface. Because from experience, traces of the foreign object remain on the impact surface (smearing by galling, sticking), the material of the foreign object can be identified (volume1, Ill. 126.96.36.199-4).
19.2.2-1 Transportation Safety Board of Canada, Aviation Occurrence Report Number A96A0099 „Power Loss/Hard landing, 16 June 1996” page 1-3.
19.2.2-2 D.Pollard, Newsletter „Aviator Aviation“,Volume 4 Issue 4, April 2001, page 3. (3459.2)
19.2.2-3 „Airwise News”, January 31, 2000, page 1.
19.2.2-4 J.R.Labadie, „Volcanic Ash Effects and Mitigation“, Auszug aus einem Bericht für das „Air Force Office of Scientific Research” und „Defense Advanced Research Projects Agency“, 1983, page 1-15.
19.2.2-5 U.S.Airforce, Safety Agency, Gale Group, „Flying Safety: Maintenance matters”, April 2004, page 1-3.
19.2.2-6 National Transportation Safety Board, Aircraft Accident Report NTSB-AAR-82-3, „ Air Florida Airlines, Inc., McDonnell-Douglas, Inc., DC-10-30CF, N101TV, Miami, Florida, September 22, 1981“, page 1-32.