Table of Contents
22.2 Fuel related problems of the fuel system
The fuel (chapter 22.2.1) itself and/or contaminations in the fuel (chapter 22.2.2) influence alternately the fuel system (Fig. "Fuel influence at components" and Fig. "Clocked or blocked fuel nozzles") with its components like
- Pumps (Ill. above and Fig. "Fuel influencing fuel pumps", volume 1, Ill. 5.3.1-11.2).
- Control units/governers (indirect influence Fig. "Fuel influencing cavitation behaviour", Fig. "Combustion process influenced by change of fuel properties").
- Seals (Fig. "Fuel composition and influence at materials", volume 1, Ill. 5.3.1-11.3),
- Injectors (steam formation, Fig. "Combustion process influenced by change of fuel properties", Fig. "Problems by overheated fuel 1", Fig. "Problems by overheated fuel 2", volume 3, Ill. 188.8.131.52-3 and Ill. 184.108.40.206-5).
The picture above shows a multistage fuel pump with a turbo component and gear components and typical deteriorations/damages which can be connected to the pumped fuel. They can lead to malfunctions which for their part can triggre failures at other aeroengine components. To these belong
- Overheatings /shortening of the lifetime of hot parts (Fig. "Terms to fuel properties", Fig. "Critical fuel properties"):
- Failures of combustion chambers (Fig. "Hot parts influenced by fuel production", Fig. "Critical fuel properties", viscosity and volatility Fig. "Properties of fuels 2").
- Overheating of the turbine blading (Fig. "Problems by overheated fuel 1" and Fig. "Problems by overheated fuel 2").
- Explosively failing of the combustor casing (volume 3, chapter 11.2.2).
- Fractures of lines/tubes by vibration fatigue (chapter 23.5).
- Start problems (Fig. "Properties of fuels 1" and Fig. "Problems by overheated fuel 2").
- Ignition problems in the combustion chamber or in the after burner (volume 3, Ill. 220.127.116.11-14 and volume1, Ill. 5.1.5-3.2).
- Corrosion (Fig. "Surface attack by fuel contamination"):
- High temperature corrosion at hot parts (Fig. "Surface attack by fuel contamination", volume 1, chapter 5.4.5).
- Corrosion in casings of Al alloys or Mg alloys.
- Failures of fuel pumps by reaction of sulfur with silver platings (Fig. "Failures by water in fuel").
Fig. "Fuel influence at components" (Lit. 22.2-3, Lit. 22.2-8 and Lit. 22.2-12): This picture shows the typical components of
an aeroengine fuel system with problems and damages/failures, influenced by the
fuel. Do these systems function with displacement pumps (gear, axial pistons, Fig. "Fuel influencing fuel pumps"), mean downtimes
(MTBF) of 175 000 operation hours and an unplanned exchange
(MTBUR) after 75 000 operation hours can be expected. With this inflight shut downs (= IFSDs) are lowered to 0,6 per
106 flight hours with overhaul interval times of about 14 000 hours. Especially reliability requirements in connection with an
ETOPS approval/certification are for a fuel system very demanding.
Most of the mentioned component specific failures/damages are discussed in detail at other
citations. In the following the necessary cross references to the
illustrations are given.
Fuel properties 22.2-1, 22.2-2, 22.2-3, 22.2.1-2, 22.2.1-3
- Ebullition/boiling behaviour/cavitation tendency 22.2-12, 22.2-16.
- Calorific value
- Density 22.2-12
- Viscosity/settling rate 22.2.2-13, 22.2.2-14, freezing/solidification 22.2.1-6
- Water solubility/water absorption 22.2.2-14
- Lubrication property 22.2.1-6, 22.2.2-4
- Colour 22.2.2-3, 22.2.2-5
- Thermal stability 22.2-13, 22.2.1-5, 22.2.2-6soot formation, coke formation 22.2-2, 22.2-3, 22.2-4
- Contaminations 22.2.2-2.1, 22.2.2-3, 22.2.2-6, 22.2.2-12.1, 22.2.2-13
- Gases 22.2.2-3
- Water 22.2.2-3, 22.2.2-4, 22.2.2-19
- Gum 22.2.2-2.1,
- Rust/iron oxides 22.2.2-13
- Sulfur (micro organisms) 22.2.2-4, 22.2.2-5
- Corrosive effect 22.2.1-6, 22.2.1-6, 22.2.2-5, 22.2.2-13
- Attack at elastomeres 22.2-8, 22.2.1-4, 22.2.2-9.1
- Flame temperature 22.2-2, 22.2-3, 22.2-4
- Safety 22.2.1-7
Aeroengine components influenced by the problems of the fuel system:
Fuel pumps 22.2.1-6
- Cavitation 22.2-9, 22.2-10
- Galling/seizing/jamming/blocking 22.2.2-4, 22.2.2-10
- Corrosion 22.2.2-5
Fuel filter/screen 22.2.1-4
- Clogging/blocking/icing 22.2-19, 22.2.2-7, 22.2.2-10
- Contamination 22.2.2-10
Fuel control unit/governer 22.2.1-4
- Icing 22.2.2-11
- Vapour bubble 22.2.1-6
- Wear, abrasion 22.2.2-4
- Galling/seizing/jamming/blocking 22.2-14, 22.2.2-4
Fuel injector/injector nozzle 22.2.1-4, 22.2.1-6
- Overheating 22.2-14, 22.2-15
- Clogging/blocking 22.2-15, 22.2-18
- Coking 22.2-13, 22.2.2-14
- Spray behaviour/atomising 22.2.1-6
Combustor/combustion chamber 22.2-4, 22.2.1-4
- Ignition problems 22.2.1-1, coldstart/restart problems22.2.1-8, see also 22.2-12
- Combustion instabilities
- Gas pressure pulsations 22.2-1, 22.2-11, 22.2.1-5
- Combustion characteristic (volume 3, Ill. 18.104.22.168-4.1), evaporation behavior 22.2.1-6, quenching 22.2.2-9.2
- Thermal load 22.2-3, 22.2-14
- Thermal fatigue 22.2-11
- Overheating 22.2-3, 22.2.2-11
High pressure turbine (HPT) 22.2.1-4
- Erosion 22.2-7, 22.2-15- OOD = own object damage (carbon/coke impact)
- Sulfidation 22.2-7
- Thermal load 22.2.2-11
- Vibrations (by combustion instabilities) 22.2.1-4
Low pressure turbine (LPT) 22.2.1-4
- Sulfidation 22.2-7
- Vibrations (from after burner instabilities) 22.2.1-4
- Ignition problems 22.2-12, 22.2.1-1, restart 22.2.1-8
- Pressure pulsation/deflagration (volume 3, Ill. 22.214.171.124-14 and Ill. 11.2.4-1.2), 22.2-12.
- Combustion instabilities (buzz, screetch, volume 3, Ill. 11.2.4-11 and Ill. 11.2.4-13), combustionm characteristic 22.2.1-5 , ignition point 22.2.1-7.
Fig. "Influence of fuel deviations" (Lit. 22.2-13): It can be expected, that fuelspecific problems (Fig. "Fuel influence at components", Fig. "Problems by softening of fuel specification" up to Fig. "Influence of the fuel lubrication effect") will rise. Problems of the availability and legitimate demands of the environment protection may cause this. Already little deviations of the fuel properties (chapter 22.2.1) within the specifications (Fig. "Hot parts influenced by fuel production" and Fig. "Problems by softening of fuel specification") can aggravate problems in modern aeroengines. They can be already caused by different production processes (Fig. "Hot parts influenced by fuel production"). An example are flame instabilities in low-NOx-combustion chambers (Fig. "Life time of hot parts influenced by the fuel"). Also damages from overheating and erosion can accumulate on hot parts. In Fig. "Combustion process influenced by change of fuel properties" problems connected with the vaporisation behaviour and the temperature depending viscosity of the fuel are compiled. These properties are of importance for the start/ignition behaviour and the combustion properties (Fig. "Properties of fuels 1" and volume 3, Ill. 126.96.36.199-4.1 and Ill. 188.8.131.52-4.2). The influence of contents of different molecular structures (chains of different length, ring) in the fuel at the heat radiation load on hot parts by the combustion (flame temperature, temperature of the combustion chamber wall) are shown in the diagrams below. The lower the hydrogen content/ratio respectively the higher the carbon content/ratio, the higher are the radiation caused component/part temperatures (volume 3, Ill. 184.108.40.206-6 and Ill. 220.127.116.11-7). With this the lifetime drops (Fig. "Hot parts influenced by fuel production"), Overhaul intervals will shorten and costs will rise (volume 3, Ill. 18.104.22.168-5).
Fig. "Hot parts influenced by fuel production" (Lit. 22.2-12): The scatter of the fuel properties within the specification limits can be failurerelevant (Fig. "Problems by softening of fuel specification" and Fig. "Problems by overheated fuel 1"). The flame temperature and with it the heating of the illuminated hotpart walls can, depending from the fueltype (hydrogen content/ratio), scatter about 40 % (diagram left). Even within a fuel type (here JP5) can, depending from the origin (geological, geographical, refinery, syntheses process, catalyser), the flame temperature can scatter 20%. With this the content/ratio of ring hydrocarbons (aromatic, diagram in the middle) and so the hydrogen/carbom weight ratio (diagram right).
Fig. "Problems by softening of fuel specification" (Lit. 22.2-10 and Lit. 22.2-13): Forced by the availability a widening of the fuel specification can get necessary. Thereby many possible consequences must be considered and may be suitable measures are necessary. The susceptibility of an aeroengine depends from the design and the operation parameters as well as from the total pressure ratio. For example, modern aeroengines with high pressure ratio show with „sensitive“ fuels not so pronounced formation of soot and coke. A higher freeze point can rise the viscosity (Fig. "Clogging cold fuel") during extremely low fuel temperatures, that with the pumps a sufficient delivery is no more possible. With this the danger of an accident/crash exists. A higher content/ratio of aromatics means less hydrogene, more carbon and a widened boiling range (e.g., diesel fuel). The consequence is a formation of more smoke, soot and coke with a higher failure/damage potential (Fig. "Life time of hot parts influenced by the fuel" and volume 3, Ill. 22.214.171.124-13, Ill. 126.96.36.199-6.2 and Ill. 188.8.131.52-3). The increased formation of soot in the flame leads to an more intensive heat radiation. This rises the hot part temperature and shortens the lifetime (Fig. "Life time of hot parts influenced by the fuel"). Higher upper boiling point: With this the evaporation of the fuel and so the ignitability is worsened (Fig. "Combustion process influenced by change of fuel properties"). During idle the imperfect combustion of the not evaporated fuel and with this the emissions (soot, CO) is especially pronounced. The reason is the low temperature level, which additionally diminishs the evaporation of the fuel (volume 3, Ill. 184.108.40.206-3 and Ill. 220.127.116.11-12). A higher percentage of nitrogen compounds in the fuel similar to a higher aromatic ratio also affects the chemical/thermal stability. This can lead to decomposition and coking before the fuel nozzle with dangerous consequences (volume 3, Ill. 18.104.22.168-3 up to Ill. 22.214.171.124-6.2). Also emission of NOx (nitrous gases) is increased. Though these mainly originate from the oxidation in the combustion chamber of the nitrogen in the air (volume 3, Ill. 126.96.36.199-12).
Fig. "Clogging cold fuel" (Lit. 22.2-13): Fuel screen during a low temperature test. The usual temperature limit (freezing point) to avoid a intlolerable high viscosity of cold fuel is at -40°C. Such fuels need under usual conditions in an airliner for the aeroengine no fuel heating. Even for polar routes also after several flight hours it must not be reckoned with the drop of the fuel temperature in the wing tanks below -30°C. However is the temperature limit increased in the specification to -29°C a slight preheating, depending from the aircraft type and the mission type/operation conditions, can get necessary. Temperature limits of -19°C demand in every case a preheating of the fuel.
Fig. "Specific fuel cunsumption and overhauls" (Lit. 22.2-13): A decrease of the efficiency of an aeroengine and with this an increase of the specific fuel consumption (SFC, volume 2, Ill. 7.0-2 and Ill. 7.0-3) can also be in connection with the fuel properties. The upward sawtooth curve is the result of the overhauls. In spite of the exchange of the hot parts, obviously the level of the efficiency of the previous overhaul can no more fully reached. Such an effekt can be expectedi, if for example every repair of the aerodynamically and thermodynamically active geometry worsens the efficiency of the hot parts. This may be in connection with repairbrazed blades/vanes (chapter 21.2.2) or with the adjustion of vanes/nozzles (chapter 21.2.7). Dimensional changes at parts like turbine casings which position the blading/stators may act similar. This trend can be intensified, inforced by the availability, by widened fuel specifications with a higher content of aromatic hydrocarbons (Fig. "Influence of fuel deviations" up to Ill. 22.2-4) and contaminations like vanadium and sulfur (Fig. "Surface attack by fuel contamination").
Fig. "Surface attack by fuel contamination" (Lit. 22.2-13): Contaminations in the intake air and in the fuel can act together and so intensify the deterioration of the hot parts (volume 1, chapter 5.4.5). This combination takes place in the combustion chamber during the combustion process. Especially effective are ingested compounds which contain natrium, sulfur and chlorine. They react with calzium (Ca), potassium (K) and magnesium from the fuel (sketch above left and middle). This deterioration is supported by erosive effects (sketch above right) with the formation of fresh, reactive surfaces (diagram below). To those belongs an intensified formation of coke particles (volume 3, Ill. 188.8.131.52-6.2) or the spalling of sticking fused dust particles (volume 1, Ill. 5.3.2-12.1).
Fig. "Fuel composition and influence at materials" (Lit. 22.2-6, Lit. 22.2-11 and Lit. 22.2-13): Elastomers like ruber blends can loose their, for the function necessary properties, through the influence of specific fuels. This applies if the composition of the fuel is changed. It is possible, that the durability of the elastomeres, which are not tested and approved for this, is not sufficient. Thereby especially influential is obviously the content of aromates (ring hydrocarbons). Usually a deterioration arises as a reduced elastic deformability. This leads to relaxation, not seldom to crack formation and early fracture. Loose seals like O-rings (chapter 23.4.1), rotary shaft seals (chapter 23.4.2) or flange seals in such a manner its effect, this can trigger dangerous failures. Shrinking is a further deterioration. It is caused by the leaching of components like plasticisers from the elastomere. With this the hardness and the brittleness rise (drop of the plystically deformability). Adittionally the tension stress in the expanded elastomer region (e.g., sealing lip ) can increase. Both promote a crack formation. Also permanent plastically deformation like a flattening can be the result of a reaction with the fuel and cause a leak. Susceptible rub coatings made of elastomeres, which came unaware in contact with fuel can rip during shrinking and embrittling and lose their functionality (volume 2, Ill. 7.1.3-21) . Does in contrast the elastomere swell as a result of a absorption of fuel respectively of its components, a pressure overload of the elatomere can be caused (squeezing of an O-ring). Parts of the O-ring are pressed through the sealing gap and affect the sealing function. Are extruded seal particles transported by the fuelflow, the danger exists, that narrow cross sections like injectors/nozzles/jets are clogged/blocked. Swelling can also increase necessary actuating forces (e.g., of actuator pistons) unacceptable. With this the function of whole systems can be prevented. Besides at function caused fuel wetted elastomeres, also the danger of deterioration exists during accidentally fuel exposure. Typical example is spilled fuel during assembly/disassembly (loosening/leaking of a pipe connection). This can damage a rub coating rubber in the fan casing. A swelling of the elastomere triggers loosening and „rising” of the rub coating, especially if also the adhesion is worsened (Fig. "Problems due to change of the oil type"). In an extreme case, a heavy rubbing with a dangrerous damage of the blades can occur (volume 2, Ill. 7.1.3-22). Do fuels, depending of the refining process, lack antioxidants, peroxides can form which deteriorate rubber (neoprene and nitrile) with pores, cracks and blisters. This applies especially for rubberised fabric. A yellow discoloration of the fuel in presence of nitrile points at dissolved flexibiliser. Other fuels obviously discolour orange and form thick brown deposits by a chemical reaction with the elastomere.
Fig. "Fuel influencing cavitation behaviour" (Lit. 22.2-4): An aeroengine type, used in high numbers of different fighters had problems with cavitation. With this fuel pumps with special design features have been concerned. 20 samples of JP-4 fuel (MIL-T-5624L) have been investigated for chemical ond physical properties. All data have been within the specification. This pointed at properies which are not specified.
Fig. "Fuel influencing fuel pumps" (Lit. 22.2-1, Lit. 22.2-2, Lit. 22.2-11 and Lit. 22.2-17): The components of turbo pumps, gear pumps (sketch above) and axial piston pumps (sketch below) are lubricated by the fuel. Depending from relatively little differences ion the fuel composition, problems can arise: Lubrication effect of the fuel: Several times the jamming (galling, seizure) of friction bearings in pumps arose. That is also true for pistons in axial piston pumps (Fig. "Failures by water in fuel"). The majority of those failures could be allocated hydrogenated fuels. It was supposed, that from the hydrogenation a deficit of components which support the lubrication effect, exists. Components of the fuel are obsorbed at the boundary layer of the matallic surface and influence the sliding behaviour. Operation conditions with high radial bearing loads and low sliding speed promote failures by seizing. Bearing failures or increased wear/abrasion like at the ball heads to the sliding shoes of the axial piston pumps („1“ in the lower sketch), could in some cases assigned to an additive for corrosion protection (corrosion inhibitor). Corrosion by the fuel: Dangerous are fuels with unusual high corrosive acting sulfur due to hydrosulphide (H2 S). This must not mean, that the total amount of sulfur; to which also not corrosive acting sulfur compounds count; is very high. At silver plated sliding surfaces silver sulphides form. With this a volume expAnsion takes place. Does the silver sulpHide flake/wear the fresh silver surfaces will be attacked. The silver sulphide has a higher friction coefficient compared with metallic silver. This increased friction as the case may be, with a clamping effect (e.g., at pistons) caused by the volume expansion increases movement forces and friction. This promotes a seizing of the sliding surfaces. Such failures often occur at axial piston pumps without warning and lead to a total damage. Dangerous sulfur compounds can develop in the tank during storing of the fuel (Fig. "Danger by microorganisms in fuel"). Cavitation of the fuel (Fig. "Fuel influencing cavitation behaviour" and Fig. "Combustion process influenced by change of fuel properties"): Gear pumps are endangered at higher temperatures by cavitation. Thereby arising high frequency pressure pulses are suspicious to excite dangerous vibrations in the control unit, pipe lines and the fuel injectors/nozzles.
Fig. "Life time of hot parts influenced by the fuel": Even not contaminated fuel, fulfilling the specifications, can act deteriorating respectively shorten the lifetime of hot parts. Influence at the lifetime of hot parts: Type and percentage of different hydro carbon molecules influence the crack formation (volume 3, Ill. 184.108.40.206-12 and Ill. 220.127.116.11-13). This rises with the carbon ratio respectively drops with a higher hydrogen content (Fig. "Influence of fuel deviations" and volume 3, Ill. 18.104.22.168-5). In the combustion chamber the forming of soot is responsible for the intensity of the heat radiation of the flame (volume 3, Ill. 22.214.171.124-7). From this the heating of the combustion chamber walls and especially the leading edges of the turbine nozzles (1st stator, nozzle guide vanes) are concerned. This causes an increased thermal fatigue load (volume 3, chapter 12.6.2, diagram above left, volume 3, Ill. 126.96.36.199-6). Typical operation caused damages at combustion chambers shows the sketch above right (volume 3, Ill. 188.8.131.52-1). Formation and deposition of soot can also trigger failures. The fuel can already desintegrate (crack) in the feeding pipes. Does it come to a change of the spray cone (deflection), a dangerous overheating of the combustion chamber wall is possible (volume 3, Ill. 184.108.40.206-5 and Ill. 220.127.116.11-9). Develops soot at the combustion chamber wall or at the outside surface of the fuel nozzle and chips off, the turbine blading can erode in an estonishing short time (volume 1, Ill. 5.3.2-12.1 and Ill. 5.3.2-12.3; volume 3, Ill. 18.104.22.168-6.1, Ill. 22.214.171.124-6.2 and Ill. 126.96.36.199-3). Influence at the combustion stability: Also the combustion behaviour of a fuel depends from its composition. Even within the fuel specifications, dangerous combustion instabilities can occur in „sensitive” combustion chambers , e.g., with low NOx properties (frame below). Those depend from the atomisation behaviour and evaporation behavour (volume 1, Ill. 5.1.5-3.1 and volume 3, Ill. 188.8.131.52-4.1). The consequences are fatigue cracks/fractures of the combustion chamber wall (volume 3, Ill. 184.108.40.206-2), fretting wear at all contact surfaces respectively plug connections (volume 2, Ill. 6.2-11) and the turbine blading.
Fig. "Combustion process influenced by change of fuel properties" (Lit. 22.2-3): Combustion processes in combustion chambers and after burners
during ignition and operation (Fig. "Hot parts influenced by fuel production") are influenced markedly by the fuel properties (Fig. "Life time of hot parts influenced by the fuel").
Also problems at other aeroengine components, which at the first side hardly are connected with fuel
properties, can be triggered.
To the problem relevant properties of a fuel belongs its
composition. Primarily it's the mass ratio
of chain hydrocarbons(aliphatic) and ring hydrocarbons (aromatic, Fig. "Problems by softening of fuel specification"). From this depend
important features like evaporation behaviour/boiling
trend (diagram above right) and oxidation
behaviour. Kerosene F34 and F35 (Fig. "Wide cut fuel"), begins to evaporate at about 180°C. In contrast to this F40
(JP4) evaporates already at comparatively low temperatures of 65°C. The curves are shifted with
increasing atmospheric pressure to higher temperatures.
The temperature depending viscosity of the fuel influences the
shape of the spray cone and the droplet
Additionally surrounding/environment parameters like temperature and
pressure are of importance for properties like
density (diagram above left). This influences the fuel volume and with this
the fueling process.
Start problems: The start characteristic
of the early evaporating JP4 is distinguished by a shorter
time till igniting (volume 1, Ill. 5.1.5-3.1). This accordingly influences the time period of the starting
process. So the starter will be preserved (volume 1, Ill. 5.1.5-3.2). In this connection the high viscosity of
the cold oil plays a intensifying role (Fig. "Lubrication oils").
Also problems during the start of the
afterburner in great hights and at low flight speeds (volume 3,
Ill. 220.127.116.11-14 and Ill. 11.2.4-4; diagram below right) can be in connection with the ignition properties
of the fuel. Does a delayed ignition lead to a strong pressure impulse, this can trigger a dangerous
flow separation (surge, scetches in the middle) in the compressor.
Problems by cavitation: An early evaporation at low temperatures (e.g., JP4) promotres the
formation of vapor bubbles in the fuel stream. When the vapour bubbles implode at locations of higher
pressure, high frequency vibrations can occur.
Fatigue failures and damages on the surfaces of fuel
guiding components are possible consequences. Endangered are
pumps, seals and control units (volume 1,
Ill. 5.3.1-11.2 and Ill. 5.3.1-11.3). Above this,
vibrations/pulsations in the fuel flow can influence
the function of the control unit and the injection
The restrained fuel supply by the formation of vapor bubbles (vapor lock, Fig. "Terms to fuel properties") in front of the exit of a fuel nozzle (frame below left) is a related problem.
Fig. "Failures by thermal overload of fuel" (Lit. 22.2-9, Lit. 22.2-14 and Lit. 22.2-17): Fuels can decompose at operation temperatures for which their thermal stability is not sufficient. We distinguish in the specialist literature between thermal oxidation stability at oxygen access and thermal stability if the fuel has no dissolved oxygen. In this case fine coke particles and/or coke deposits in different forms develop (sketch above). Those are
- soft rubber like are called „gum“,
- hard lacquer like (fuel lacquering, varnishes),
- brittle cokes .Usually all accumulations are of small beads (microspheres), not so often are platelets or little
rods. Compared with fuel, their hydrogen content is lower, but the oygen concentration and the percentage of nitrogen and sulfur compounds is extremely higher. A thermal decomposition of the fuel can occur in overheated fuel or at hot walls. An example are feed lines to nozzles/jets/injectors and these themselves (volume 3, Ill. 18.104.22.168-5). Also the function of other components of the fuel system like heat exchangers (fuel/oil) or valves can be deteriorated (Ill. 22.2-15). By the fuel transported particles can additionally block filters. At the transition from the fuel system of the fuselage/wings to the aeroengine the fuel temperatures lay between 80°C and 120°C. At the fuel nozzles the fuel temperature is hold below 163 °C. The highest fuel temperatures, which must be expected in the aeroengine generation of today, lay at about 200 °C. The maximum accepted temperature must lay below 205°C. In subsonic airliners the thermal durability of the present fuels may be no problem. However fuels with extended properties/specifications can get problematic. To these belong also such which don't base on petroleum like coal, tar sands and shale oil. This is due to the higher contents of alkene (olefins, hydrocarbons with double bond), compounds with nitrogen, oxygen or sulfur and traces of metals (especially copper). The changes of the fuels under elevated operation temperatures occures chemically and physically (shown sequences). This permits the suggestion of an important role during the thermal decomposition process of aromatic compounds and traces of organic nitrogen compounds or sulfur compounds. The thermal stability of the fuels can be markedly increased by additives. These are so called antioxidants (Lit. 22.2-17). They prevent the formation of peroxides (with -O-O cluster). Deposits will be lowered fuel depending at 250°C up to an order of magnitude. Such additives (e.g., „+100”) are in the position to dissolve already existing coke deposits up to 90%. With this the a cleaning effect for the fuel system, especially pipelines and nozzles/injectors exists. In special test facilities (thin heated tubes, Thermal Stability Rig = TSR) the investigation of the thermal stability takes place. Thereby temperatures of 250, 350 and 400 °C are used to shorten the testing time.
Fig. "Problems by overheated fuel 1"
Fig. "Problems by overheated fuel 1" (Lit. 22.2-1, Lit. 22.2-10 and Lit. 22.2-12): Already in the early fifties problems caused by too high fuel temperatures arose in the first military aeroengines with higher pressure ratios. This was caused by the design because the fuel distribution to the nozzles was arranged in a primary system and
a secondary system at the region of the compressor exit (Fig. "Problems by overheated fuel 2"). The secondary system had contrary to the primary system only a low fuel flowrate. That permitted an increase of the fuel temperature over a longer operation time period. In less than 100 operation hours, fuel unsoluble deposits formed which clogged feeds and nozzles. The result was an nonuniform temperature distribution and heavy overheating of the combustion chambers.
Fig. "Problems by overheated fuel 2" (Lit. 22.2-10): In this early civil aeroengine type it came to dangerous erosion of airseals and turbine blading. So at the 1st stage high repair costs arose. Cause have been deposits inside the fuel nozzles (detail above right) which changed the spray picture of the jets. The fuel distribution at the compressor outlet (frame middle and below) promoted a dangerous heating. Probably erosive acting coke particles formed in the combustion chamber (Fig. "Problems by overheated fuel 1" and volume 3, Ill. 22.214.171.124-3 and Ill. 126.96.36.199-6.2). Cause have been copper containing contaminations in the fuel from the refinery process. problems have also been noticed in modern fanengines at certain operators. Thereby slight deviations of the fuel specification played a role. Already after 3000 operation hours consequences of a derioration of the function from the fuel control unit arose. This have been erratic rotation speed/power fluctuations, hotstarts, compressor surge and high gas temperatures. The damaging high fuel temperatures could be traced to high heat input from the oil in the heat exchanger for the fuel deicing.
Fig. "Influence of the fuel lubrication effect" (Lit. 22.2-5 and Lit. 22.2-7): It came to the flight accident, shown above, after about 650 operation hours. Cause was the failing of the spline coupling from the drive shaft of the fuel pump due to wear. In one week two further cases, both at the ground, have been occurred. Here the failure arose in each case after an operation time of about 800 hours. After the disassembly of the pump in the area of the worn coupling, a red powdery coating in the flutes and seals could be seen. This was a wear/fretting prodcuct (volume 2, Ill. 6.1-3) of the shaft spline. The fuel filter after the pump contained small spline fragments. A mechanical cause for the wear could not be found. Noticeable was, that only the pumps of one producer showed during operation an especially rough run and vibrations. They operated with a certain fuel (Jet B) markedly louder as in Jet A. According the specification Jet B has a higher naptha percentage (lower boiling destillate). This explains a worse lubrication effect and with this the accelerated wear. The subsequent described problem concerns several failures of different aeroengine types which could be assigned a special refined fuel (Jet-A1). In the fuel, lubricated control units heavy wear of the spline toothing from the shaft linkage between fuel pump and control unit had been occurred (frame below left). This was traced back to a poor lubrication ability of the fuel. The problems arose after in the refinery with a new process the sulfur percentage and other components in the fuel have been reduced. As a remedy material changes in the coupling spline toothing and additives in the fuel have been carried out.
Fig. "Clocked or blocked fuel nozzles" (Lit. 22.2-15): The following investigation unfolded as accident cause a clogging of the screen of the, for this aeroengine type only one fuel nozzle (frame above). This is not a fail-safe-design. Also the screen of the fuel control unit (FCU) and the filters of the fuelpump have been contaminated (frame below). The contamination consistet of foreign material, which contained sodium chloride (common salt). It can be expected in sea atmosphere, in which the aeroengine was operated. Periodical washing of the compressor is also an indication for a considerable air pollution. Obviously these contamionations have been inserted intp the fuel tank. An explanation could provide Ill. 22.2.2-9.1. Prehistory: A 100/300 hours inspection took place about 20 hours before the accident. This did not apply for the screens of the fuel nozzle and the FCU. These must be only checked when the bypass warninglight of the der pump filter flashs or these are contaminated. However the filters have been exchanged. In the many already occurred cases the fuel nossles have not been exchanged as from the OEM intended. Instead then maintenance personnel proceeded at power drop after its experience. As a measure periodic inspections also of the screen from the fuel nozzle are sheduled. Comment: Unfortunately in the available literature it can not be seen, which foreigen material was concerned and how it could get in th numerous cases into the fuel system. Because the incidents only arose in helicopters without optional filter in the fuel tank, it can be assumed, that the contaminations got over a longer time during the filling of the fuel (already contaminated?) or during a tank air vent.
Fig. "Icing of the fuel system" (Lit. 22.2-16): The investigation of the helicopter at the accident site showed no evidences of the power drop of the aeroengine. During a following test run all specification data have been achieved. The electronically stored aeroengine data indicated no ovreload of the aeroengine. As well the fuel system showed no contaminations. This is also true for water and ice. Before the flight the helicopter was refueled with a sufficient amount of 130 liters Jet A-1 fuel. At external temperatures below 0°C an anti icing additive is obligatory prescribed. This additive combines below the freezing point with free water in the fuel. So no ice crytals can for, which could block the filter. With anti icing additive no fuel was available. The fuel bill let suggest, that also no additive has been directly put into the tank. The crew neither demanded an additive nor cared for it themselves. Although no direct proof was possible, the cause of the accident was obvious, that the fuel was used without anti icing additive during low temperatures. The distant position of the aeroengine from the cockpit and the concentration of the pilot at the landing process may have influenced the accident occurrence. This would explain, why the power drop of the aeroengine was not indicated at the operation noise, but first at the warning signals.
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