Article citation information:
Wojdat, C. Security of military aviation flight operations concerning
the quality of fuel supplied to aircraft. Scientific
Journal of Silesian University of Technology. Series Transport. 2017, 94,
239-247. ISSN: 0209-3324. DOI: https://doi.org/10.20858/sjsutst.2017.94.21.
Czesław WOJDAT[1]
SECURITY OF
MILITARY AVIATION FLIGHT OPERATIONS CONCERNING THE QUALITY OF FUEL SUPPLIED TO
AIRCRAFT
Summary. The specificity of military
flights operations imposes a number of requirements on aircraft. One of the
main factors concerning the realization of the air task is the reliability of
the engine. The most common cause of aircraft engine malfunctioning is the
quality of the fuel supplied. This paper presents the factors affecting the
quality of fuel supplied to aircraft and the procedures preventing the delivery
of aircraft fuel, which could interfere with the operation of the aircraft
engine.
Keywords: aviation, aviation fuel, fuel quality
1. INTRODUCTION
As the saying goes, to build an
aircraft, we only need a good engine and something to place this engine on,
such as the door of the hangar. The main element in determining the performance
of the aircraft is the aircraft engine. Thanks to its construction, efficiency
and reliability, we accept a certain level of operation costs so that it is
suitable for performing specific tasks. In addition to the construction of the
engine, along with proper workmanship and materials needed to take full
advantage of the modern combat aircraft engines, it is necessary to provide the
appropriate fuel. Currently, liquid fuels are used in this context,
particularly those with the prescribed chemical composition and calorific value
of strictly defined, adequate physico-chemical characteristics, a high level of
purity and with freedom from water. Due to the large quantity of fuel consumed
per unit of time, the contents of even small amounts of impurities or water may
interfere with the correct operation of engine units, and hence the entire
engine. This is particularly important in the case of engines in military
aircraft, due to the much stricter work procedures involved, compared with
civilian aircraft.
Among the causes of failure relating
to ab engine fuel system, a majority have been caused by the poor quality of
the fuel supplied to the engine fuel system. The quality of the fuel results
from its entire distribution system, from its production to its delivery to the
aircraft tank. Of importance are the storage conditions at every stage of the
distribution and all kinds of manipulations and procedures of the fuel [2, 7].
2. TYPES OF FUEL USED IN POLISH
MILITARY AVIATION
The Polish Armed Forces mainly use
two types of aviation fuel: NATO-coded F-34 and F-44. The F-34 fuel is known as
kerosene and complemented by an inhibitor of water crystallization (FSII),
which is coded S-1745. It is intended for the aircraft air units stationed on
land. It is also known under the symbol JP-8 and AVTUR/FSII.
F-44 fuel is a type of fuel oil with
a high flash point and a water crystallization inhibitor (FSII), which is coded
S-1745. This fuel is used in naval aviation on the decks of warships in order
to limit the fire risk.
Occasionally, the Polish Armed
Forces use two other types of aviation fuel:
§ F-35 fuel is equivalent to fuel employed by civilian
airlines and known under the trade name Jet A-1 or AVTUR. It can be used
instead of F-34 fuel and as emergency fuel instead of F-44.
§ F-40 fuel is a fuel enriched with a water
crystallization inhibitor (FSII code S-748) used in aircraft based on land. The
fuel is also known as JP-4 or AVTAG/FSII. It can be a substitute fuel for F-34
and F-35 fuels [3].
Currently, the primary type of fuel
for turbine aircraft engines in the Polish Armed Forces is F-34 fuel,
which is produced in domestic
petrochemical plants according to the requirements contained in Defence
Standard NO-91-A258-2.
In order to achieve the adequate
operation characteristics, we introduce into the fuel a number of individual
additives in F-34 fuel, such as:
§ an additive preventing the crystallization of water in
the fuel - (S-1745) = 0.10 ÷ 0.15 (mg/dm3)
§ an additive for anti-corrosion and lubricity -
(S-1714) ÷ 9.0 = 23.0 (mg/dm3)
§ an anti-static additive in fresh fuel, whose contents
should be no more than 3.0 mg/dm3; when prompted to add the total
amount, the additive may not exceed 5.0 mg/dm3
§ deactivator metals, but not more than 2 mg/dm3
§ an oxidation inhibitor, which is added only in the
case of the fuel produced on the basis of hydrorafinate in an amount of up to
2.0 mg/dm3, and not more than 5.7 to the next refilling of the
additive [3]
The NATO countries’ standardization
documents (STANAGs) for quality and
physicochemical requirements define the minimum requirements to be met by
petroleum products, including fuels for aviation technology. A basic document setting
out minimum requirements for the aviation fuel is STANAG 3747 [6]. In a significant number of documents drawn up by
individual member states, these requirements are much stricter in order to
achieve acceptable safety thresholds for these products. Table 1 contains
selected quality requirements for the basic types of aviation fuel used in the
Polish Armed Forces.
In addition to the physico-chemical
requirements, a very important issue when supplying fuel to aircraft tanks is
that it has a high level of purity. The
document STANAG 3149 includes
provisions on the need to remove impurities from the fuel and for it to be
water-insoluble (free water). Fuel supplied to the aircraft must have a light
colour, be clean, must not contain visible dirt and be water-insoluble. The
apparatus removing the water and the pollution should be placed close to the
refuelling aircraft [5]. The efficiency of these devices should be checked at
least once every three months. Increasingly, these devices have their own
automatic control system efficiency.
Table 1. Selected physical and
chemical requirements of aviation fuel for turbine aircraft engines used in the
Polish Armed Forces.
|
Characteristics |
Aviation fuel |
||||
|
F-34 |
F-35
|
F-40 |
F-44 |
||
|
Exterior view at ambient temperature |
Clear, transparent, light |
||||
|
Aromatics content, % (v/v), not more than |
25.0 |
||||
|
Mercaptan sulphur, % (m/m), not more than |
0.003 |
||||
|
Sulphur total, % (m/m), not more than |
0.30 |
0.30 |
|||
|
Fractional composition (distillation normal) |
|
|
|
|
|
|
Temperature
of the beginning of distillation, °C |
Score |
||||
|
10% distilled to a temperature, °C, not more
than |
205 |
Score |
205 |
||
|
20%
distilled to a temperature, °C, not more than |
Score |
145 |
Score |
||
|
50%
distilled to a temperature, °C, not more than |
Score |
190 |
Score |
||
|
90% distilled to a temperature, °C, not more
than |
Score |
245 |
Score |
||
|
Final
boiling point: temperature, °C, not more than |
300 |
270 |
300 |
||
|
Rest,
% (v/v), not more than |
1.5 |
||||
|
Loss,
% (v/v), not more than |
1.5 |
||||
|
Flash
point, °C, not more than |
38 |
|
60 |
||
|
Vapour
pressure at 38°C, kPa |
|
|
14-21 |
|
|
|
Density at 15°C, kg/m3 |
775-840 |
751-802 |
788-845 |
||
|
Freezing point, °C,
not more than |
-47 |
-58 |
-46 |
||
|
Kinematic
viscosity at -20°C, mm2/s, not more than |
8.0 |
|
8.5 |
||
|
Calorific value, MJ/kg, not more than |
42.8 |
42.6 |
|||
|
Copper
corrosion, 2 h/100°C, not more than |
1 |
||||
|
Thermal stability - pressure drop, mm Hg (kPa), not
more than -
deposits on the tube, degree not more than |
25.0 (3.33) <3 |
||||
|
Existent gum, mg/100 cm3,
not more than |
7 |
||||
|
Water
reaction interface rating not more than |
1 b |
||||
|
Separation of water not more than |
85 |
||||
|
Content of metal deactivator, mg/l, maximum |
5.7 |
||||
|
Icing inhibitor content % (v/v) |
0.10-0.15 |
|
0.10-0.15 |
0.10-0.20 |
|
|
Electrical
conductivity at the point of delivery to the user, pS/m |
50-600 |
50-450 |
150-600 |
150-600 |
|
Different countries demand different
levels of purity in aviation fuels. The level of purity is defined as the
amount of impurities and water per unit volume of the fuel. According to the
NATO minimum, the refuelling of aircraft must be stopped when the impurity
content exceeds 1 mg/l and the water content is higher than 30 ppm. The
research on the impurity content is defined in NATO standardization document STANAG 3149. Due to the internal
requirements of the associated countries, national normalization is much
sharper, for example, in the case of the French Army: the mechanical impurities
in the fuel at the moment of refuelling the aircraft cannot be greater than
0.5g/dm3, and the water content of the dispersed may not be greater
than 5 ppm.
In the Polish Armed Forces, the
requirements for cleanliness and water limits are set out in Defence Standard
NO-91-A800 [4]. Apart from the quantitative requirements, the standard contains
the methodology of inspection regarding the cleanliness of the aviation fuel.
The selected requirements are presented in Table 2.
Table 2. Acceptable amounts of the
mechanical impurities and water in fuel intended for filling aircraft tanks
|
Specification |
Unit of measure |
Value |
|
Content of micropollutants and the size
distribution of solid foreign objects: 1) Symbol pattern micropollutants, not more
than 2) Symbol pattern of the grain-size
composition, not more than |
- - |
A-6, B-6, C-6 D |
|
Content of the mechanical impurities, not
more than: 1) in a single sample 2) on average in mg/dm3 |
mg/dm3 |
1.0 0.1 |
|
Grain-size distribution of foreign bodies,
not more than: 1) in the interval dimension >5 μm 2) in the interval dimension >15 mm
pieces/100 cm3 |
Pieces/100 cm3 |
78,000 14,000 |
|
Presence of free water in the fuel
(by visual inspection) |
- |
None |
|
Presence of foreign bodies and
free water in the fuel samples (visually) |
- |
None |
|
The water content of the dispersed fuel, not
more than: 1) in a single sample 2) on average |
% V/V |
0.003 0.001 |
3. CHANGES IN THE QUALITY OF AVIATION FUEL
AFFECTING FLIGHT SAFETY
Research tests conducted by military
research centres and laboratories lead to the conclusion that changes in the
quality of aviation fuel, which occur during storage on airbases, are mainly
due to the contamination of fuel by water or solid contaminants. The
deterioration in the physico-chemical characteristics as a result of the
contamination by microorganisms present in the aviation fuel is extremely rare,
in particular, during long-term storage. On airbases, aviation fuel is kept for
a relatively short period, usually no longer than a year. During such a period,
the ageing processes that occur in the aviation fuel may not cause change in
the quality of flight safety. The process of supplying the military airbases in
the aviation fuel is mainly due to the supply of fuel directly from production.
Although there is a need to rotate (“refresh“) fuel stocks held in depots, fuel
is sometimes stored for longer than ideal because of possible acts of war or
other events causing protracted disruption in the production or supply of
liquid fuel to military units. This happens periodically, depending on the
adopted refreshing plan. The fuel is stored for a minimum period of four years
and transported from large fuel depots
to the airbases. Currently, tank trucks are usually involved in transportation,
although, in previous years, this would have been done by rail as well. A long
period of storage could mean that the process of ageing has begun and physical
and chemical changes have occurred. Before being directed to the airbases, the
fuel is subjected to laboratory research testing related to the B-2 test or the
A test, depending on the storage period of the batch of fuel.
Any manipulation of the fuel between
tanks, particularly when it involves the transport of fuel from the pipeline or
flexible hoses, which are not constantly filled with fuel, creates a risk of
fuel contamination during pumping. This is due to the exposure of the surface
of the inner pipe to the atmosphere. Transmitted by air, mechanical dirt and
moisture deposit on the inner surfaces of the pipe (hose), causing its
pollution or the formation of the corrosion processes. When pumping the next
batch, the fuel is likely to include particles of dirt, water and other
corrosive elements. Depending on the length of the contaminated section of
pipeline and the time that it remained unfilled, the extent of the pollution
located on the inner surfaces can vary.
Depending on the amount of the
pollution stored in an airbase’s individual storage, there may be a variety of
the threats to the functioning of the distribution system of aviation fuel, as
well as for the safe operation of flights using military aircraft.
The majority of the impurities,
including free water, are removed from the fuel with the use of distribution
procedures. The first procedural step to improve the quality (purity) of the
fuel is a detachment of the fuel and a pooling of the embedded contaminants
from the lowest point of the transport rail tank or the transport car tank.
Depending on the size of the tank transport (a column of liquid), the
detachment time can vary from a period of 30 min/1 m column of fuel. It is also
the first stage in the purity control of aviation fuel supplied to the airbase
[4]. Detached fuel testifies to its purity. In the absence of mechanical impurities,
and free or dispersed water, the detachment of the fuel will be clear,
transparent, light in colour and with no sediment on the bottom of the vessel
control. After a positive result of the detached fuel, there is a fuel quality
control measure related to the C test, which consists of checking the
conformity of the fuel with the received laboratory documents. Then, the fuel
is pumped into a storage tank depot at the airbase.
The detachment of fuel procedure is
repeated many times in the process of distributing aviation fuel. This allows,
in a simple way and without significant investment, the isolation of any
mechanical impurities and water. Depending on the construction of the storage
tank, wherein the fuel is currently stored, detached fuel is removed by the
gravity or by means of special pumps. In addition, the construction of the
storage tanks and cisterns of airport distributors is designed, so that the
fuel is taken from the bottom of the tank. This prevents the distribution
system from sucking up any of the water or dirt accumulated on the bottom of
the tank. The removal of impurities and free water from the bottom of the
reservoir is necessary not only to avoid the possible penetration further into
the distribution system, but also because the sludge and water can become a
breeding ground for microbial growth. It may cause a corrosion of the metal
parts of the tank and reduce the surface tension of the fuel. Consequently, it
reduces the efficiency of water filtration from the fuel. In addition, the
growth of colonies of microorganisms increases the amount of impurities
resulting from the decaying microorganisms. A large number of microorganisms
can cause valve crashes, blocking of filters and accelerated wear of the pumps.
A shorter time storage of aviation
fuel at airbases is not conducive to the occurrence of microorganisms. Another
factor in preventing the growth of microorganisms is adding a crystallization
inhibitor to water, which is also a potent biocide, on the airbase.
During the storage and distribution
of fuel, changes in fuel quality occur, caused by reactions between various
less stable components of the fuel, such as sulphur compounds, olefins, and
oxygen and nitrogen compounds, particularly when contact is made with the
dissolved oxygen in the fuel. These processes are much faster in the case of
fuel contact with metals such as copper and its alloys, zinc and cadmium. These
processes cause a reduction in the fuel’s thermal stability, characterized by
an accelerated ageing process through the formation of high molecular weight
hydrocarbons. These compounds are not sprayed and settle on the engine
components to form hard deposits, causing interference with the work of the
injectors or vaporizers; in extreme cases, the deposits can cause the complete
blockage of the openings of the injectors. The forming sediments reduce the
heat dissipation from the engine components, which can cause them to overheat
or burnout, which, in turn, may cause improper engine operation or the engine
failure.
The alloys of copper, zinc and
cadmium have been the main construction materials for the internal coat
protection of fuel installations for many years [1]. Zinc has been the main
component. The use of these metals in distribution installations has caused them
to come into contact with the fuel, such that alloys of these metals chemically
or electrochemically corrode to form a volatile layer of corrosive product on
the alloys’ surface, which has also been damaged by the flowing fuel
contaminating them. When the corrosive product is dissolved in the fuel, it the
causes the speeding-up of the fuel’s ageing process.
Since Polish accession to the NATO
structures, the process of upgrading fuel systems for the storage and
distribution of aviation fuel has taken place. This process involves, among
other things, the removal from the aviation fuel distribution system of the
metals that can cause changes in fuel quality. It also involves a process of
technological change in the aviation fuel distribution system used by the
Polish Armed Forces.
On most airbases, it has been
necessary to remove the devices, assemblies and components that used metals or their alloys that were not
appropriate for aviation fuel. In addition, the existing distribution system was
based on the so-called “open distribution system of aviation fuel”, which was
particularly sensitive to the impact of weather conditions. The filling of
railway cisterns, distributors’ airport tanks and most aircraft took place
using an open container, which was penetrated by mechanical impurities,
moisture from the air and even precipitation.
Fig. 1. Portions of the corroded
filter on the measuring tube of the storage fuel tank
The higher requirements of modern
aircraft in relation to the purity of aviation fuel demanded the introduction
of another system of fuel pumping between the different parts of the
distribution system, enabling the maintenance of high-purity fuel all the way
to its distribution point. The pressure fuel distribution system has
substantially reduced the risk of impurities and water from the atmosphere
entering into the fuel. Thus, it reduces the levels of corrosive product and
the risk of microorganisms, as well as the intensity of the fuel’s ageing
process. The pressure system is based on hermetic connectors (socket-head),
which transfer the fuel to the aircraft’s tank in virtually all weather
conditions without fear of contamination of the pumped fuel.
This system allows for a greater
intensity of fuel pumping into the aircraft’s tank, which, in turn,
significantly reduces the risk of fire and the possibility of environmental
pollution due to fuel spillage.
Fig. 2. Refuelling of the aircraft
with a pressure head
4. CONCLUSION
The introduction of the pressure
pumping aviation fuel system allows for maintaining the high quality of fuel
supplied to aircraft tanks. Changes in the distribution system and material
components, which create a risk of deterioration in the aviation fuel, are
necessary. That said, this should not be regarded as the end of any action to
address the quality control of aviation fuel supplied to the aircraft.
The intensive exploitation of the
fuel infrastructure on airbases makes it necessary to constantly monitor the
filter equipment, the transport equipment and the fuel storage equipment. A
very important factor, which is often taken into account as it has a
significant impact on the safety of the flights operated by the Polish Air
Force, is the degree of training and the level of practical skills among
service staff implementing the distribution of aviation fuels, lubricants and
other materials. These skills are particularly important for conducting quality
control of fuel, manipulating the product and conducting laboratory tests. A
failure to follow procedures can result in erroneous measurements, which could
lead to poor quality fuel being used to refuel aircraft, thereby threatening
flight safety. This risk should be heeded by all professional staff on an
airbase, namely, technicians, operators, laboratory technicians and engineers
operating the exchange of the filters of stationary equipment. Improper
handling of the filter cartridge can result in the inefficient operation of the
entire filter and the supply of poor quality fuel.
References
1.
Bonczek Jerzy.
2002. Eksploatacja lotniskowych systemów
dystrybucyjnych paliw z wewnętrznymi powłokami cynkowymi do turbinowych
silników lotniczych. [In Polish: Operation
of airport fuel distribution system with internal zinc coatings for turbine
aircraft engines]. PhD thesis.
Warsaw: Air Force Institute of Technology.
2.
Karp Grażyna, Wojciech Dzięgielewski. 2011. “Course of chosen properties
of aviation fuels in the time of the storage and on trade channels”. Journal of KONBiN, Vol. 1(17): 121-134. Warsaw: Air Force
Institute of Technology, Poland. ISSN 1895-8281.
3.
NO-91-A258-2: Materiały Pędne i Smary - Paliwo
do Turbinowych Silników Lotniczych kod NATO F-34. [In Polish: NO-91-A258-2.
Petroleum, Oil and Lubricants – Aviation
Turbine Fuels – Part 2: Fuel of NATO Code F-34]. Warsaw: Ministry of National Defence.
4.
NO-91-A800: 2003. Materiały Pędne i Smary -
Magazynowa i Lotniskowa Kontrola Zystości Paliw do Turbinowych Silników
Lotniczych. [In Polish: NO-91-A800. Petroleum,
Oil and Lubricants – Store and Airbase Inspection of Aviation Turbine Fuel
Purity]. Warsaw: Ministry of National Defence.
5.
STANAG 3149: Edition 10 - Minimum Quality Surveillance for Fuels.
Brussels: NATO Standardization Agency.
6.
STANAG 3747: Guide Specifications (Minimum Quality
Standards) for Aviation Turbine Fuels (F-24, F-27, F-34, F-35, F-40, and F-44). Brussels: NATO Standardization Agency.
7.
Havenga Jan H.,
Anneke de Bod. 2016. “Freight corridor performance measurement system: A
framework for South Africa”. Journal of
Transport and Supply Chain Management, Vol 10, No 1: 1-11. DOI: http://doi.org/10.4102/jtscm.v10i1.252.
ISSN: 2310-8789.
Received 10.12.2016;
accepted in revised form 02.02.2017
Scientific Journal of Silesian University of
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