Table of Contents
22.2.2 Contamination of the fuel.
The contaminations in fuels can be solid (Fig. "Fuel contamination by auxilary materials"), liquid or gaseous (Fig. "Identification of water in fuel"). Its
causes are very different. Normally they com from
in front the intake of the aeroengine. Possibilities
for contaminations exist during transport and storage (Fig. "Good house keeping of fuel storage") as well as during refueling (Ill.
22.2.2-2.1 and Fig. "Contaminators of fuel") and in the tanks (Fig. "How fuelgathers water in tanks" and Fig. "Risk of an untight tank filter cap") of the aircraft (Ill.
22.2.1-3). Also inside the aeroengine respectively the fuel system, contamionations can occur. Thereby changes
in the fuel caused by operation influences (e.g., coking) play an important role (Fig. "Failures by thermal overload of fuel").
Problems due to water absorption are especially frequent. Comes fuel in contact with
air, its humidity can be „hauled“ into the fuel (Ill. above). As driving force of this
„pumpeffect” temperature changes
act . In warm fuel more water dissolves than in cold fuel. An air exchange by so called
„breathing“ of the tank (Fig. "How fuelgathers water in tanks"), „supplies” with humid air. During cooling, water discards and settles at the bottom
of the tank (Fig. "Failures by water in fuel"). However a considerable part of the water will stay in the fuel (Fig. "Identification of water in fuel")
and represents a danger during icing
conditions. In such cases a disruption of the fuel supply causing
the failure of the aeroengine can occur (Fig. "Icing of the fuel system" and Fig. "Accidents by icy fuel systems", example 22.2.2-3).
Bacteria and fungi can accumulate during contact with water sulfur compounds in the fuel (Ill.
22.2.2-5). Does so temporary increase the sulfur
concentration in the fuel, heavy damages
can be triggered. These occur in the
fuel system (Fig. "Failures by water in fuel" and Fig. "Danger by microorganisms in fuel") and/or
hot gas corrosion (sulfidation,
Fig. "Surface attack by fuel contamination") at the hot parts, especially the turbine blading (volume 1, Ill. 22.214.171.124-3).
Also free, fine dispersed water (Fig. "Identification of water in fuel") can damaging influence the
function of the control unit or trigger aqueous
corrosion (volume 1, chapter 5.4.1).
There are effective measures against dangerous amounts of water in the fuel:
- Avoiding of the water absorption (Fig. "Prevention of fuel contaminations" and Fig. "Separation of water and particles from fuel").
- Separation of the water before refueling (Fig. "Good house keeping of fuel storage").
- Water separation between aircraft tank and aeroengine (Fig. "Prevention of fuel contaminations" and Fig. "Separation of water and particles from fuel").
The effectivity of these measures understandably depend from the specified use.
Fig. "Sedimentation of a fuel" (Lit. 22.2.2-4):
Contaminations occur in fuels for aeroengines in
different form (frame below). The higher the the viscosity of the fuel (Fig. "Properties of fuels 2") the easier it can float and transport
Water (Fig. "Identification of water in fuel") is one of the most frequent contaminations and can block the fuel supply by ice formation (Fig. "Failures by water in fuel", Fig. "Unsufficient fuel storage" and Fig. "Accidents by icy fuel systems").
Further danger is the biological enrichment of sulfur containing compounds (Fig. "Danger by microorganisms in fuel" and Ill. 22.2.2-9).
Products of aqueous corrosion in pipe lines, casings (Ill. 22.2.2-12) and tanks like rust and oxides of light metals. Also the mixing with other petroleum products can deteriorating influence the operation behaviour. To these also belong other fuels, oils or greases. In an extreme case the aeroengine can fail (Fig. "Critical fuel properties").
Obviously, special situations can force to mix aviation petrol/gasoline/avgas with turboengine fuel. Lead from the additive, which increases the knock resistance of the gasoline (tetra-ethyl lead = TEL), can be transported by the hot gas on the turbine blading and deteriorates it over a long period of time.
Also contaminations from residues of washing/cleaning fluids and dissolvers/solvents in tanks and pipe lines are problematic.
Foreign particles can deteriorate the function of components (Fig. "Fuel influence at components") of the fuel system from
an aeroengine.To these belong pumps, metering valves, filters, sieves/screens and nozzles/jets.
Concerned is the blocking/galling/seizing of movable parts as well as wear and clogging of openings/cross sections.
Thereby contaminations occur frequently as sediments (sketch above). This can be every material which got in contact with the fuel.
- Sand or dust show itself in fuel with a crystalline, grained or glassy structure. It will be gathered by „breathing“ either in the aircraft tank (Fig. "How fuelgathers water in tanks") or it is caused of unsufficient care during storage, production and transport.
- Compounds aluminium and magnesium by the attack of mikroorganisms. These corrosion products (Fig. "Danger by microorganisms in fuel") form a light grey powder. With water a very sticky mass, similar to gelatine (hydroxides?).
- Iron as oxide/rust (e.g., corrosion products from the tanks, Fig. "Danger by microorganisms in fuel") typically derives from wear particles (e.g., from gear pumps of spline toothings). Rust can be magnetic (read rust) or non-magnetic (black rust), depending from the oxide type. Its form can be fine dispersed or grained.
The sedimentation (deposits, Fig. "Separation of water and particles from fuel") of fuel contaminations points at type and causes. So
it helps to identify risks.
Sediments can consist as well of anorganic material as of organic
Distinguishing of sediments by their size (detail above):
Course Sediment consists of particles larger than 0,01 mm. It is visible in the fuel and sets easily.
Such particles can clogg control units measuring
openings and measuring edges as well as
dimensioned sliding guidances. This affects directly the function of the aeroengine. The particles are
in the position to restrict fine seaves/screens (e.g., in front of nozzles/jets, Fig. "Clocked or blocked fuel nozzles").
Fine sediments have a size below 0,001 mm. These particles are not visible with the naked eye.
They show merely as tiny light reflections or streaks. From these 98 % can be removed by filtering,
sedimenting and separation. Therefore after filling of the tank minimum setting
times till the withdrawal must be kept. Remained particles deposit at the sliding surfaces of valves and components from the control
unit as a dark coating, similar to lacquer. Under centrifugal forces, they can arise
slurry like and cause a dull fuel
The sediments appear in the form of dust, fibres, grains, flakes od discolorations. Visible are particles with a size of more than 0,04 mm. They indicate in markedly amounts (not only few particles) the malfunction of a filter, separator system or a following source of comtamination. Common filters can not catch very fine particles. These can pass the filter and deposit as colourant, extremely fine powder or sludge. Thereby usually rust or dust is concerned.
Markedly amounts of fibre material hint at the failing of a filter or certain water seperators (Fig. "Accidents by icy fuel systems").
Larger metallic particles, especially chips or fatigue platelets point at a mechanical failure of a component from the fuel system. Probably it is a (gear) pump (Fig. "Fuel influencing fuel pumps" and Fig. "Gear pump failures"). Recurrent deposits are alarm signals. This demands a detailed investigation of the fuel system for the origin and cause of the particles. Fuels can oxidise during long time contact with air of environment temperatures. Concerned are chemical reactions of the aerial oxygen with components of the fuel. This process is supported by metallic surfaces (Lit. 22.2.2-18). Such conditions reside in storage tanks. Thereby so called gum forms (existent gum, Lit. 22.2.2-17). This is a low-volatile residue of sticky or lacquer like material. Is the sticky insoluble variant, fine dispersed in the fuel in the tank, higher amounts can clogg components of the fuel system like filters and sieves/screens. Occures gum in soluble form (soluble gum) it can pass these filters and form hard deposits on hot surfaces (Lit. 22.2.2-19). These are in the position, to reduce nozzle/jet openings or to jam sliding guides.
In the expert literature also the term potential gum (accelerated gum), is known. This is a test feature for the oxidation stability of a fuel. It designates the tendency of the fuel to form gum during long periods of time. The insensitiveness against this is in contrast called fuel tability. There are chemical inhibitors, which indeed delay the forming of gum but can not dissolve already formed gum.
Fig. "Fuel contamination by auxilary materials" (Lit. 22.2.2-14): The danger of
work at the open fuel system can quite be compared
with the handling of blood preservation and surgery at the blood circuit.
Whatever during transport or storage happens, in aircraft tanks or at the aeroengine, always the danger of a possibility of a contamination during an inattention or an incorrect behavior exists. Therefore it is necessary to mind, that only for the particular application processes, devices and auxilary material which are approved/certified by the responsible authorities are used. In the shown case in the wing tank of a business jet an adhesive tape was found.. With this the danger of blocking the fuselage/wind sided fuel guiding cross sections and filter sieves existed. Does this prevent an intertank transfer during flight, the probable consequence is a failure of an aeroengine.
A further danger was seen, when not fuel resistant materials of the adhesive tape are dissolved and get into the fuel system of the aeroengine. With this the function of sieves/screens, control units and fuel nozzles as well as the display of the flow rate would be endangered. As remedy an AD-Note was edited. This demands extensive control work at all airplanes of the concerned type. It can be supposed, that this means high costs and a markedly lack of use.
Fig. "Identification of water in fuel" (Lit. 22.2.2-4, Lit.22.2.2-9 und Lit. 22.2.2-16):
An identification of contaminations in the
fuel as soon as possible, is safety relevant.
Coarse sediment can be seen in fuel with the naked eye. Requirement is a clear fuel without free water.
Such a water shows itself as clouds, streaks or a
sedimentation. Anyway, even seemingly totally
clean fuel can contain more than the triple of water.
Two types of water in fuel are possible (Fig. "Identification of water in fuel"):
- Dissolved form: In aviation fuels, the amount of water, absorbed and hold by the fuel, depends from the temperature (the higher the more dissolved water) and the contained hydrocarbons. At 16°C
the water content lays between 0,003 and 0,010 weight %. During cooling, free water forms.
- Free water as fine distributed small droplets (suspension). These can develop from absorbed
air humidity during cooling of the fuel (entrained
water, Fig. "How fuelgathers water in tanks"). It can also be during pump
down, whisked settled water (Fig. "Failures by water in fuel"). In a higher concentration the fuel turns dull or cloudy
(sludge). Clouds in the fuel can also be influenced from air. For this clouds near the surface are a sign for this.
Liquid water can act deteriorating:
- Disturbance of the fuel control by short-circuiting the electric sensor of the flow rate.
- High amounts of water can trigger the blackout of the aeroengine (quenching, Fig. "Consequence of water in fuel").
Contains free water salt (e.g., sea salt, Fig. "Clocked or blocked fuel nozzles"), it will act corrosive. That is applied for steels and light metals (Al-, Mg-alloys), as they are used in tanks and casings (Fig. "Sedimentation of a fuel"). In coaction with water and microorganisms, fuel also acts corrosive (Fig. "Danger by microorganisms in fuel"). There are several methods to check the water content. In one case a food colour which dissolves only in water is used. In a chemical reaction the added powder discolours at more than 30 ppm of water. Similar sensible is a special chemical activated method. In this case a testfilter discolours. Is the fuel temperature lower than 0 °C (after a flight in great hights or a cold night at the ground) ice crystals can falsify the identification of the water. Did ice form at other locations of the tank, than at the bottom and was not identified, it will stay not detectable till thawing. Therefore the maintenance must take place till the ice is thawed. Then the water must be discharged by the drainage before during the pump down swirling (whisking) will occur again.
Fig. "Failures by water in fuel" (Lit. 22.2.2-5, Lit. 22.2.2-6 and Lit. 22.2.2-7): Water can
collect during storage in the fuel tank at the
bottom over a longer period of time. During pump down this water forms fine
droplets which are whisked by the pumps into the fuel (detail right, Fig. "Influences of fuels at quality and failures").
At the border between settled water and fuel, bacteria get effective, which already from the low amount of sulfur of the fuel, produce sulfur enrichments. Are these entrained during pump down and get into the aeroengine, serious failures are possible:
- Spontanous failing („galling/seizing”, mechanical blocking/jamming, example 22.2.2-1) of
silver plated sliding surfaces. These are usual in axial piston pumps with swashplates (Fig. "Fuel influencing fuel pumps" and frame below).
- Clogging of measuring orifices, nozzles/jets, filters and jamming of control pistons Reglerkolben
- Over longer periods of operation at hot parts hot gas corrosion (sulfidation) especially at the
turbine blading develops (Fig. "Surface attack by fuel contamination"). For the proof of water in the fuel periodic tests and/or if concerns exist, checks must be carried out (Fig. "Identification of water in fuel").
Example 22.2.2-1 (Fig. "Failures by water in fuel"): At the
fuel pumps of the axial piston type (Fig. "Fuel influencing fuel pumps") from an
elder military aeroengine type, heavy damages with a
total destruction occurred. Investigations showed that
obviously suddenly acting extremely high forces triggered
overload failures in the region of the axial pistons. Because
of the extensive secondary failures, it was not clear if primarily at the
silver plated slide shoes of the pistons at the swash plate extremely high friction forces developed, or if the pistons began to jam (Fig. "Failures by water in fuel").
The failures only occurred at aeroengines whose fuel originated from a certain tank. In it
the fuel was stored over a long time (Fig. "Unsufficient fuel storage"). So the suspicion was obvious, that a failure causative connection with
this fuel exists. To get clarification, an aeroengine war taken on the test rig and the suspicious fuel was provided
by road tankers. There are pump failures like in the aeroengines which occurred before. This confirmed
the suspicion but the real cause stayed unclear. For further tests the fuel stock was finshed.
Later releases of other authorities (Australia, see also example 22.2.2-2), that the cause must be searched in a biological caused (bacteria, fungicide) sulfur enrichment in the fuel (Fig. "Danger by microorganisms in fuel"). This sulfur obviously reacts with the sliding silver platings where the coefficient of friction raise markedly. The increased friction forces rise the heat generation of the sliding process. With this the sliding conditions at the priston surfaces my also detweriorated (thermal expansion, vapour formation). With this self-energising process the spontaneous failure of the pump can be explained casual.
Later it became clear, that in the case on hand during the long storing time in the not full tank, obviously water/humidity from the air was absorbed by the fuel (Fig. "How fuelgathers water in tanks"). This settled at the tank bottom. With this the preconditions for a sulfur enrichment would have been given.
Example 22.2.2-2 (Lit. 22.2.2-5): During the usual sampling of fuel before the first flight of an Australian
air liner the maontenance personnel noticed a red brown
fluid in the sample. The examination of the fuel
system e.g., filters) showed no alarming indication. Also an investigation in the laboratory could rule out fungal
infestation. However a high amount of bacteria in a film between settled water and
fuel was found. This contamination was not assigned to the airplane but to the fuel supply. A n inquest together with the fuel supplier
(refinery) showed no explaining anomalies.
Similar cases in Australia, which already date from 1962 and obviously happen till now, pointed at a connection with sulfur compounds in the fuel and traces of metals. Involved are componds (complexes) of naphtha (Fig. "Wide cut fuel"), sulphurous acid and transition metals, as well, as iron (Fig. "Unsufficient fuel storage"). Also in this case, as often, the actually sourc of the contaminations could not be found.
Fig. "Danger by microorganisms in fuel" (Lit. 22.2.2-4, Lit. 22.2.2-5 and Lit. 22.2.2-16): In fuel tanks,
different microorganisms (bacteria, fungi) can appear. In fact they are disturbed by
frequent circulation but if they occur
corrosive compounds develop. These attack the walls of the fuel tank. That can lead in
wing tanks to a dangerous weakening of the
structure. Therefore tank walls are provided with a protective coating
against corrosion. However some organisms are in the position to penetrate in a dangerous manner
former used coating systems. The corrosion products
in the tank (Al componds and Mg
compounds) can enter and deteriorate the fuel system of the aeroengine (Fig. "Sedimentation of a fuel"). The requirement for the growth
of organisms is given if the tank (storage or in the aircraft) is resting over a long period of time.
Slimy red up to brown deposits develop (detail above right, Fig. "Unsufficient fuel storage"). However for this,
water is needed at which transition to the
fuel the microorganisms grow (detail below right). This water is absorbed
from the air over a longer period of time and can then settle at the bottom of the tank (Ill. 22.2.2-0).
An 'attacked' tank can be identified at the unusual colour and/or the unnormal smell of the fuel.
To avoid those contaminations, appropriate specifications/instructions about the handling of fuel must be attended strictly by the maintenance personnel.
Fig. "Unsufficient fuel storage" (Lit. 22.2.2-6): At this aeroengine type the fuel gets into the combustion chamber
through a hollow shaft. The centrifugal forces arrange the distribution by centrifuging/spraying. The
investigation of the accident aeroengine showed, that
5 of the 6 fuel injection openings in the shaft have
been blocked and one was restricted. In the
distribution space in front and the related labyrinth seals a
red brown mass has accumulated (Bild 22.2.2-5). This lead to a power loss of the aeroengine.
Samples from the fuel system of the helicopter contained two contamination
phases. Above was a clear, slightly discoloured fuel with a water content of about 90
ppm (Fig. "Separation of water and particles from fuel"). The dark heavy substance below consisted of an
iron containing compound (probably rust) and also of about 90 ppm water.
The OEM of the aeroengine permits maximum 10
ppm water in the fuel.
After drying the substance discolours into a light grey with red brown embeddings. Such deposits have been also foud in fuel filters, fuel control units and its inlet line. Evaporates the water, a dark brown, brittle and transparent film develops, corresponding to the deposits of the samples.
The operator of the helicopter had a buried tank with a volume of about 25 m3. This was verifiable since 14 years not maintained, exept a single filter exchange about 10 years before. Fuel samples from this tank showed a high degreee of rust and water. Laboratory tests showed, that the thermal properties (thermal stability, Fig. "Failures by thermal overload of fuel" and Fig. "Critical fuel properties") had been deteriorated.
Further enquiries allowed the conclusion at a bad supervision in the organisation of the operator. Fuel management training or safety training of the personnel did not take place.
The pilot got in the last 5 years only one training of two days with one training flight. This can explain, why after the failing of the aeroengine no autorotation was carried out.
Fig. "Accidents by icy fuel systems" (Lit. 22.2.2-8): Resides water in the fuel, the formation of ice in the fuel system is a
particular danger. In the late fifties more than 10% of all flight accidents of military aircraft was traced doun
to the icing of the fuel (Lit. 22.2.2-7).
Already 1958 a crash of a big military airplane occurred (sketch above left) after three of the four double aeroengines had dropped out because of ice. An investigation pointed at an icing caused blocking of the fuel filters in the fuelage. Typical are collapsed filters (sketch right).
Also today mistakes can trigger catastrophic fuel icing (example 22.2.2-3).
Example 22.2.2-3 (Fig. "Accidents by icy fuel systems", Lit. 22.2.2-8): After several aeroengines failed, it came 10 seconds after
the take off to a crash of a big transport aircraft. Aeroengine No. 3, No. 2 and No. 1 failed one after the
other. Also the power of aeroengine No. 4 was concerned but recovered for a short time.
A causative influence is supposed in the mixing of summer fuel and winter fuel. The airplane came before from a warmer region and had landed with a larger amount of this fuel mixture.
The outside temperature was during the start below -20°C. In this case a suitable fuel must be refueled, to prevent the icing of the water in the fuel. For this a fuel mix is unsuitable. It is obvious, that the filters have been blocked fast by ice (Lit. 22.2.2-7 and Lit.22.2.2-9) and so prevented the fuel flow to the aeroengines. Thereby also the control system of the aeroengine may have played a role.
Fig. "How fuelgathers water in tanks" (Lit. 22.2.2-9): The proof of the sufficient safe behaviour of the fuel system from an aeroengine is the duty of the manufacturer (OEM, Lit. 22.2.2-9). This is true as well for civil application as for military use. Anyway naturally the icing behaviour of the fuselage/wing sided fuel system is also relevant for the aeroengines safety (example 22.2.2-3). For the check/inspection of the fuel system, not belonging to the aeroengine, there are own specifications/instructions and recommendations. Water can ways get into the fuel in different:
- During the flight by absorption of the air humidity (Ill. 22.2.2-0).- Suction of free water from the tank sump.
- Settled water in filters is transported by the fuel.
- Not drainable water (e.g., when there is too much water).For the fuel system, an especial danger of ice formation by water in the fuel exists during a flight caused pressure change and temperature change (Lit. 22.2.2-9).
Especially in the wing tanks during climb the fuel is cooled down with the dropping outside
temperatures. At the same time the surrounding pressure drops and
the pressure balance causes a blowing of air
into the tank. During the descend the surrounding pressure rises. Now the warmer air from outside is
ingested during the pressure balance, the tanks
„breath“. Above the cold fuel and the cold tank
walls, condensation occurs and caondensate forms. This can combine with the
pumping effect of the fuel (Ill. 22.2.2-0). With this the water content in the fuel rises.
A further possibility for the air exchange in fuel tanks are temperature cycles at the ground. Cold nights and hot days respectively heating by solar radiation cause volume changes of the air inside the tanks and so also „breathing”.
Fig. "Risk of an untight tank filter cap" and Fig. "Consequence of water in fuel" (Lit. 22.2.2-10): The helicopter should make transport flights. It was on the evening before parked on a suitable meadow near the site of operation. Than a refueling from a 200 l barrel with Jet A1 (Fig. "Wide cut fuel") took place before the helicopter stood in covered condition on site over night. After the accident the fuel sample from the concerned b arrel showed no dissolved water. Also after the accident no water in the fuel of the helicopter could be detected. Anyway the investigation of the crashed helicopter showed the following remarkable indications:
- The hoses of the fuel pump on the side of the fuselage and the filter casing have been widely empty.
- There was a water bubble of about 1 cm3 and dark deposits as well as a fuel rest of a few
cm3 in the appendant filter casing at the fuselage side. This was as a sample investigated and showed besides free water about 120 ppm water content (=,0,012 %, Fig. "Separation of water and particles from fuel"). This surpasses the dissolubility of water in fuel markedly (Fig. "Identification of water in fuel").
- A fuel sample from the filter at the aeroengine side showed a watercontent of more than 140 ppm (!). Also this surpasses by far the dissolubility of the fuel.
- In the fuel pipeline (9,5 mm diameter) to the aeroengine was a sand grain with a size of 2 x 3 mm. It may have disturbed the flow rate. However this is no sufficient explanation for the failure of
the aeroengine, but sheds light on maintenance deficits.
- The O-ring seal (rubber) of the tank cover has been rippled and cracked (chapter 23.4.2). This
may expect a leakage. With this is an absorption of water by the fuel („breathing“, Fig. "How fuelgathers water in tanks") or entering rainfall together with former, remaining settled water quite understandable.
- The tank consists of two volumes (sketches below). They are connected with a tube. The openings of the tube in the right tank are positioned about 15 mm above the tank bottom. So in the
horizontal position about 30 cm3 fuel could be no more pumped down. In the left tank the „rest volume” was about 90 cm3. This volume increased markedly during the tilted position. This enabled the accumulation of enough free water at the bottom of the tank. It can be suddenly suck in during a maneuver.
- An investigation of the cockpit displays (features of the warning lamps caused by the impact) let suggest at a too low aeroengine speed.
- This relatively small aeroengine (sketch above) has a central combustion chamber with only a single fuel nozzle. From such a configuration an especially
sensibility for fuel contaminations can be expected. Few cm3 free water may be already enough for extinction of the combustion chamber („flame out“). These indications show as most probable accident cause: The special tank configuration enabled during a maneuver the sudden suction of settled water in the tank. The `flame out' was the result.
Fig. "Blocked fuel system by water" (Lit. 22.2.2-11): Fuel samples where taken from the accident helicopter and of other locations:
- From the filler,
- the filter casing at the fuselage side,
- the road tanker,
- an other helicopter which was refueled with the same facility.
The samples of the filler region have been nearly
saturated with water (Fig. "Identification of water in fuel"). An anti
icing additive was added by the supplier.
The aeroengine was investigated with the following results at the OEM under supervision of
the responsible aeronautical authority.
It run erratic at the test rig during acceleration. After this, the aeroengine was disassembled and suspicious areas closely investigated.
At this aeroengine type, the fuel is supplied through a central injection tube in the hollow shaft (see also Fig. "Unsufficient fuel storage"). From there the fuel is sprayed into the combustion chambre by a distributing wheel. X-ray pictures showed inside the tube extensive deposits. From the fuel in the aeroengine fuel system altogether 0,4 gram unmagnetical, brown coloured deposits could be extracted. Consistency and distribution allowed the conclusion, that they had formed during centrifugal forces acted.
During the investigation it got known, that at the same operator again an aeroengine of the same helicopter type failed. This time a flight accident could be avoided. Here also the same deposits existed. A fuel sample from the tank showed a jelly substance.
Such a gel could be targeted produced by dissolving the brown deposits of the fuel tube. The analysis showed, that it's cellulosic resin. Further analysis identified the material as carboxiymethyl cellulose (CMC). This is a frequently used water soluble powder. It influences the humidity as a stabiliser and so increases the viscosity of watery media.
This product was used in a certain type of fuel filters. For safety reasons the aeronautical authorities had edited about 3 months before the accident a warning letter (aviation maintenance alert) and urged the withdrawal of this product. Such filters consist of a polymer that absorbs free water, similar baby diapers (Fig. "Separation of water and particles from fuel"). With this the content of free water drops below 2 ppm. Accumulates water in this material, the flow resistance in the filter cartridge rises. From a certain resistance the cartridge must be changed. An unnormal amount of free water in the fuel (slugs) causes a fast pressure rise and cuts off flow. The producer of the filters stated, that an unnamed number of filter cartridges blocked in this manner under normal operation conditions. The water absorbing material is then pressed through the filter and transported by the fuel into the aeroengine system. In the filter of the accident helicopter such in water swelling material (polyacrylate) resided. Obviously it did not correlate the designated product. With this the following accident cause arose:
The aeroengine of the helicopter lost power because „incorrect ” filter material escaped from the filter and clogged/blocked fuel lines as well as the injection. The blocking of the filters obviously was promoted by an unusual high water content of the fuel.
Example 22.2.2-4 (Lit. 22.2.2-21, volume 1, example 5.1.4-7):
There have been failures of both aeroengines of a big airliner with ETOPS approval. The lack of fuel is explained with a blocking of the oil/fuel heat exchanger by ice. This obviously derives from the pipe line system and was deposited by the fuel.
A hint a t this failure sequence give cavitation marks in the high pressure pump behind. The type of pump can not be seen from the available literature. Ill. 126.96.36.199-1 and Fig. "Gear pump failures" deal with cavitation in gear pumps.
Fig. "Fuel supply problem by low temperature" (Lit. 22.2.2-12): The flight took place in
Alaska. Before (-15°C) and during (about
-40°C) the flight it was extremely
cold. Equivalent the fuel cooled down. During the
drainage of the sump in the aircraft tanks before and after the refuelling no free water was identified. Such a
visual check can indeed see free water but not dissolved
water (Fig. "Identification of water in fuel"). Anyway the danger of
icing exists.The pilot according to the instructions, shut down the fuel heating during the descent from
about 800 meters. In this moment the right aeroengine showed an unusual low rotation
speed and a low
torque moment at the propeller. Corresponding the airplane pulled to the right. The aeroengine
didn't respond to the power leverage and was shut down with the feathered
Shortly after this, the pilot noticed during approach for landing similar symptoms at the other aeroengine. After deploying the landing gear he heared some `bangs' and noticed the smell of smoke. The airplane began pivoting to the left. The instrument for the fuel flow of the left aeroengine indicated markedly. The aeroengine lost power and the pilot started the right aeroengine again. He realised, that he would no more reach the airport. So he decided for an emergency landing at a near ice plain. With retracted landing gear the airplane slided about 1 km on the ice. Dangerous injuries did not occur. Both aeroengines have been dismounted and investigated at the OEM under attendance of the responsible authorities.
In the left aeroengine the stator and rotor blades of all 3 turbine stages are molten and „burned“ (volume 3, Ill. 188.8.131.52-9). Injection nozzles and combustion chamber showed features of overtemperature. Fuel pump and control unit functioned sufficient. In the control unit, there was some free water. The magnesium casing was corroded.
The right aeroengine showed no damages. At the test rig, its behavior was normal.
Samples had been taken from the filters of both aeroengines and the sump of all fuselage tanks during an environment temperature of -15°C. There have also been samples from the filling opening of the tank from the starting airport (Jet A). All samples showed water in different amounts. The highest value of 150 ppm (= 0,015%) showed the water test from the fuel filter (Fig. "Separation of water and particles from fuel"). The values of the other samples lay under 40 ppm. In all samples markedly amounts of solid contaminations like fibres and crystals have been found (Fig. "Sedimentation of a fuel").
The OEM took further fuel samples from the pump and the control unit of the left aeroengine which he investigated. They showed extreme 30 percent (!) volume of water. The water contained 10-15 percent chlorides and so it could be classified as extremely corrosive.
From former aeroengine tests of the OEM it was known, that at 100 ppm watercontent the fuel system is blocked in intervals.
Every aeroengine is supplied from the fuel tank of the related wing (sketch above). With switched on fuel, heating bleed air of the aeroengine flows in (schematic sketch in the middle). During the landing approach, according to the manual 5 minutes before the landing, the fuel de-icing must be activated only 2 minutes.
According the investigation report the most probable cause of the accident
is: During the flight in the cold, fuel dissolved water settled as free
water (frame right). This free water was fine dispersed
in the fuel as droplets or ice crystals. The fuel heating may
warmed the very cold fuel only little above the freezing
point. After the specified switch off of the fuel
heater for the landing procedure, the water can form ice very fast in sieves/screens, narrow openings and the filter.
The disturbed flow lead to the drop of power and the shut off from the right aeroengine. The
overheating of the remained left aeroengine
can be explained with fuel pulsations during low rotor speed and with this too
lille compressor air, caused by the icing of the
Comment: The repeated `bangs' during the landing approach may be caused by a compressor surge at the left aeroengine. This may be caused from the low compressor exit pressure correspondent with the low rotor speed. Such a surge leads for the mechanical fuel control units of elder aeroengine types (sketch below, development in the fifties) frequently to extreme overtemperatures with the described failure mode (volume 3, Ill. 184.108.40.206-7). Especially if during surge the rise of the power/rotor speed with an increased fuel supply is tried.
Fig. "Contaminators of fuel" and Fig. "Prevention of fuel contaminations" (Lit. 22.2.2-2 and Lit. 22.2.2-4): It is questionable to
come from, that the producer always provides clean
fuel. So at least attention is recommended. Thereby the
measures of the producer against
contaminations are of high importance. To these also belong the modernity
and the condition of the facilities. A further aspect is to pay attention to the
advice of the producer, to look especially for contaminations when there are fuel problems.
After the production of the fuel there are from experience three potential regions where aviation fuel could be contaminated:
- Decanting and refilling.
To minimise these risks suitable procedures offer themselve (Fig. "Prevention of fuel contaminations", Lit. 22.2.2-4).
Comments and recommendations:
- Basically applies the principle of „good housekeeping” (Fig. "Good house keeping of fuel storage"). It is crucial determined from the expertise/education, experience and not at least from the motivation of the personnel.
- To this belong to close at once openings of the fuel systems with suitable closing caps during assembly and disassembly against entering contaminations like dust and insects. * Are aircraft tanks opened for maintenance manifold possibilites for a contamination arise.Attention must be payed especially at remained cleaning rags and paper towels. Therefore it is the best, to check the tank before closing by a second person.
- To minimise the water absorption from the air (Ill. 22.2.2-0), tanks should as possible filled respectively filled up (top off). So the air cushion can be minimised. * Filter and sieves/screens must be periodical inspected and cleaned, according to the specifications/instructions.
- Is a bypass opened during a blocking, contaminations can pass undisturbed. With this the danger of further failures in the following systems exist. So this is no suitable remedy.
- Above this, possibilities of contaminations which can be caused as secondary failures, must be avoided. To these belong problems with the growth of bacteria and corrosion (Fig. "Sedimentation of a fuel" and Fig. "Danger by microorganisms in fuel"). This demands knowledege of the contamination mechanisms and the causes.
- An important role to avoid contaminations plays the inspection/examination of the fuel. Instructions in detail about the sampling and testing can be found summarised in the Aerospace Recommended Practice ARP 1401 (Lit. 22.2.2-9). It is important to realise the limitations and the conclusions of the tests (Fig. "Identification of water in fuel" and Fig. "Fuel supply problem by low temperature").
- As remedies can also be seennot at least a „contamination robust“ (e.g., against icing) design of the fuel system and its components (Fig. "Separation of water and particles from fuel").
Fig. "Good house keeping of fuel storage" (Lit. 22.2.2-2): Just for smaller aircrafts like
private airplanes and helicopters, the
fuel is often stored in barrels and/or
With this many possibilities for a fuel contamination arise. They are tightly connected with the
„house keeping” respectively with the
behaviour of the personnel.
Disorder, dirt and lying waste are alarming signs. For example these should prompt to check the the closings/seals of the tank vessels for tightness. Also the filling level should be inspected more closely (Fig. "Risk of an untight tank filter cap"). Air admission is always a possibility, that fuel absorbs unacceptable much water. This is promoted by large temperature changes of free standing thin walled containers, respectively the fuel inside (Ill. 22.2.2-0).
Water can enter into outdoor containers without a covering. For example if it gathered upon a barrel.
Also the possibility is problematic, when fuels with different additives are mixed uncontrolled. Is the corrosion protection at the inside of the of the barrels damaged, rust must be expected. From the outside visible dents/deformations point at a damage with crack formation in the coating on the inside. Gets so developing rust into the aeroengine this can have dangerous consequences (Fig. "Sedimentation of a fuel").
Unsufficient labeling/marking can lead in an extreme case to a confusion. This can have catastrophic consequences. So a case arose, when it came to the crash of a fully occupied airliner. Canisters of a drained water-fuel mixture have been mistaken with water-methanol canisters which stood by for the injection into the compressor during hot day conditions. During the start the additionally injected fuel triggefred the damage of the hot parts of both aeroengines (volume 1, Ill. 5.5-4).
Fig. "Separation of water and particles from fuel" (Lit. 22.2.2-1, Lit. 22.2.2-4 and Lit. 22.2.2-9):
That dangerously fuel contaminations can get into an aeroengine shold be prevented with suitable cleaning procedures.
It is important to keep sufficient time periods for the settling of contaminations (diagram above left, Fig. "Prevention of fuel contaminations"). These depend as well from the fuel propperties like viscosiy, i.e., from the composition and the temperature, as also from the type and size of the particles. The settling time must be adjusted to the settling depth.
There are tanks which contain fibrous material like cellulose (sketcjh below, see also Fig. "Blocked fuel system by water"). This transforms small droplets dispersed water (entrained water) into lager droplets. These then settle faster. The so generated „free water“ can be drained from the sump.
The content of dissolubled water depends, besides from the fuel, primarily from the fuel temperature (diagramm above right). This can not be proved visual, but with samples (Fig. "Identification of water in fuel"). This is a precondition to initiate pointed counteractions.
Also a suitable design can make a fuel system robust against contaminations. This adds to the safety. To this also belong measures on the side of the aeroengine and the airplane like:
- Larger sieves/screens,
- additional bypassvalves,
- coarser filters,
- heating of the fuel (heat exchanger) before the aeroengine.
- Ice preventing additives (anti icing additives).Similar measures can also applied for the fuel system.
Fig. "Silver-strip-test for reactive sulfur in fuel" (Lit. 22.2.2-20): Elder aeroengine types use
axial psiton fuel pumps (Fig. "Failures by water in fuel").
The sliding faces of the sliding shoes from the pistons are silver plated for the improvement of the
sliding behaviour (dry running conditions/emergency operation). Higher contents of
reactive sulfur in the fuel (Fig. "Danger by microorganisms in fuel") react with the silver coating
(„sulphidization”) and increae the coefficient of
friction markedly. This leads to mechanical and thermic overload of the silver coating/plating. The result
is flaking and abrasion/wear. These particles accelerate the failure process up to the catastrophic
failing of the pump by galling/seizing/jamming. Above this the fuel system is contaminated and its
components are deteriorated.
This is the cause why aviation authorities demand a „silver strip-test“ at the fuel providers. From every delivery a sample was taken. In it a metalstripe with a silver surface (silver foile) was immerged. A dark discolouration of the wetted stripe indicates a dangerouds concentration of reactive sulfur. This test was suspended, because of the rejection of the concerned aeroengine types, as far the pump producer doesn't demand this further.
As a further remedy the coating/plating of the pumps had been modified.
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