Describe what is meant by fuel waxing based on the freezing point of fuel and why this could not have been the cause in this particular accident. Briefly explain the differences between the static and...

Aircraft Reliability and Maintenance
1. Fuel path way in Boeing 777 airline
There are three fuel tanks in Boeing 777 airliner
A. Left main tank
B. Right main tank
C. Central tank
· Fuel is stored in above tanks
· Central fuel tank consist two ove
ide/ jettison pumps
· Right main tank and left main tank consist two –two boost pumps
· For safety point of view for pumps ,they have provided ¼” mesh at the suction
· In order to avoid the back flow of fluid they have also provided non return valve at discharge port
· A pressure switch is also used between the pump’s impeller and non return valve, Pressure switch monitor the fuel pressure and if the pressure rise across the pump falls below between 4 and 7 psi then it will provide a warning signal in flight deck.
Fuel flow line diagram:-
Figure shows the fuel flow line diagram for Boeing -777 air liner
There are following components used over here as mentioned below
1. Fuel tank
2. Water scavenges 9. Fuel /oil heat-exchanger
3. Ove
ide/ jettison pumps 10. Fuel filte
4. Jettison -check valve 11. Fuel metering unit
5. Boost pumps 12. Flow mete
6. Cross feed valves 13. Fuel feed manifold
7. Spar valve
8. Low Pressure Pump & High Pressure Pump
1. Fuel tank :
There are three fuel tanks in Boeing-777. Central fuel tank, right fuel tank & left fuel tank. Pump withdraws the fuel from the bottom of tank and it is very necessary to maintain the fuel level in each fuel tank so that pump can’t be able to build-up the air.
2. Water scavenges:
In order to avoid the build-up of large quantity of free water there is provision provided in the fuel flow system and that system is called as water scavenges
3. Ove
ide/ jettison pumps:
Used to deliver the fuel
4. Jettison -check valve
It allows flowing of fuel in only one direction and avoiding the flow in backward direction
5. Boost pumps
Boost pump is an auxiliary fuel pump which is basically used in fuel system for aircraft engine to provide the fuel at a pressure required for the ca
uretor or to the fuel-metering unit of a fuel injection system
6. Cross feed valves
Function of Cross feed valves in fuel system in aircraft engine is to deliver the fuel from tank to tank and also to transfer the fuel from engine to engine
7. Spar valves
Spar valves as the same function as the fuel shut-off valve
8. Low Pressure Pump & High Pressure Pump
Function of fuel pump is to increase the fuel pressure & fuel temperature (Temperature will be increased in small amount) and to move the fuel to the burner through a fuel/oil heat exchanger.
Lower pressure pump is also used to prevent cavitations that occur in the high pressure pump.
9. Fuel /oil heat-exchange
The Fuel /oil heat-exchanger Has to ca
ied out two main work
(1). Cooling the engine oil
(2). To increase the fuel-temperature so that ice may not affect the downstream components such as LP filter and the Fuel Metering unit.
Fuel /oil heat-exchanger is a shell tube cross flow type of heat exchanger that requires the routine cleaning of its tube for its better effectiveness
10. Fuel filte
There are two fuel filters in fuel system of aircraft engine low pressure filter and high pressure filter. When low pressure fuel filter will be chocked due to dirt or containments then fuel will bypass that filter and contaminated oil will flow to the control unit. The function of high pressure filter is to capture the wastage or unwanted particle from the high pressure pump.
11. Fuel metering unit
Fuel metering unit delivers the fuel to the fuel injectors and avoid the wastage of fuels and hence increase the power and decrease the emission and fuel consumption and cost also.
12. Flow mete
Flow meter is device used to measure the quantity of fuel flowing to the system per unit time. Flow meters provide us the information regarding fuel consumption.
13. Fuel feed manifold
The function of fuel feed manifold is to run across the aircraft and to connect the fuel feed lines of engine. Fuel feed manifold is divided in two parts right and left system with the help of cross feed valves
Part 1
Answer 2.
Fuel waxing:
Before going for the study of fuel waxing based on the freezing point of fuel, we must know about the freezing point
Freezing point: - Freezing point is a temperature at which a liquid changes its phase and converted to solid phase. It is observed that freezing point is directionally proportional to the su
ounding pressure. It is also observed that freezing point will be lower when some solid particle will be added.
When fuel is cooled beyond the freezing point of the fuel then the first wax crystal that will be formed is the long straight chain hydroca
ons. These wax crystals will remain suspended in the fuel because they have same density as of liquid fuel. Wax crystal & fuel mixture could be pumped to the fuel-system with the help of boost pumps until the temperature of fuel reaches to its pour point, which is much colder as compared to the fuel freezing point
It can’t be the cause in particular accident- Explanation
There were sixty-six number of sample of fuel provided by the aircraft and the engines Some of these selected samples were investigated and some critical properties for example density, freezing points, flash points, fuel additives ,viscosity, contamination, and presence of water were investigated. The result of the investigation indicated that the fuel samples contained such specification that completely satisfies the fuel specification required for Jet A-1 and simultaneously secured the quality assurance certificate.
The freezing point of fuel sample of G-YMMM was -570C .Fuel sample of G-YMMM was investigated by manual and automatic method also but it was observed that both tests did not identify any crystal of wax at a temperature warmer than -57 degree centigrade.
Conclusion: - after all inspection and data analysis it was concluded that fuel did not reach to a temperature required for fuel to form wax during particular accident.
Further investigation were ca
ied out of that particular accident and it was concluded that the design of Fuel /oil heat-exchanger should be changed in order to avoid the accumulation of ice on its inlet face so that it may ensure fuel flow will not be restricted to achieve commanded thrust
Part 1
Answer 3.
Static and Total Air Temperature:
Static air temperature is normally refe
ed to outside temperature or temperature of ambient air around the aircraft
Total air temperature is a term used in aviation technology. There is a provision i.e. Temperature probe which is situated on aircraft-surface in order to calculate the temperature
Total air temperature is measured with the help of temperature probe. Temperature probe’s design is in a way that it will
ing the air in rest w. r. t air craft. When air will come in rest condition the kinetic energy will be converted in to internal energy and hence air will be compressed and will feel adiabatic increase in temperature and due to this reason total air temperature will be higher as compared to static air temperature
In case of long flights the fuel temperature in the main wing tanks will tend towards the temperature of the boundary layer around the wing that could be up to 30C than that of total air temperature.
On the accident flight the minimum total air temperature was -450C. Due to the position of the centre fuel tank, the fuel temperature in this tank will be warmer than that fuel which is in the main tanks.
Part 1
Answer 4
Combination of Water and Fuel:
Air craft fuel system always contains some amount of water and could be introduced during refueling.
Water will be in terms of as mentioned below:
1. Dissolved water –
It can be happen when a water-molecule sticks itself with a molecule of hydroca
on. The Quantity of dissolved water in the fuel will be a function of mainly humidity, temperature & fuel’s chemical composition. When Warn fuel is cooled; the dissolved water will be released and will take the form of entrained water or free water.
2. Entrained Water-
Entrained water is molecule of water which will be suspended in the fuel in the terms of very small droplets & would not be seen by the naked eyes in concentrations less than 30 PPM
3. Free water-
Free water is the molecule of water that is not dissolved and not entrained. Free water can take the form of droplets because its density is high as compared to fuel density. Free water could be found in fuel filters.
Content of water will must be present in aviation fuel around 35 -40 PPM. The total content of water determined in the fuel-samples provided by the G‑YMMM was same as that in the samples given by another B777, G‑YMMN.
Part 1
Answer 5
Icing inhibitors-
Icing inhibitors are normally used by those aircraft that does not have any fuel heaters. Icing inhibitors works as anti icing agent and avoid the formation of ice crystal in fuel tank. Icing inhibitors is normally used by military aircraft but not in large public transport aircraft because large public transport aircraft consist with fuel heaters hence there is no use of icing inhibitors in these types of aircraft.
Part 1
Answer 6
During the time of accident, it was not possible to maintain the condition of the fuel in the centre tank because that was contaminated with fire fighting foam/water.
In order to disregard the foam/water that was sprayed by the fire fighters, they have used a technique and that was the Karl Fischer test. In the Karl...