Every year, aircraft accidents are reported to occur due to the fuel exhaustion, starvation or contamination reason. Most of them stem from the pilot error and can be easily avoided if the pilots know the fuel systems properly. The reasons for such eventualities range from the failure to monitor consumption by the aircraft during the flight to the inadequate knowledge of the fuel system.
Fuel Tank Vents
The airplane contains fuel in seven tanks positioned between the rear and the front spars. They are vented to the surge tanks, one on every side of the airplane. The surge tanks are also vented via a flush to the atmosphere. Moreover, the airplane wing underside has the round vent outlet. This location of the outlet gives pressure to the right tank during the flight maneuvers. The wings of the plane are vented through the dive ports positioned at the tank’s outboard end and a climb port at the inboard end (Langton, 2008). In addition, there are also two climb ports, which are open ended, at the tank’s forward end. The dive ports have float valves that prevent fuel spillage during climbing and rotation with a full tank. The number 1 and 4 (outboards) tanks are fitted with extra float valves-equipped vent ports located centrally in each tank.
Engine Fuel Feed System
Fuel is supplied directly from every main tank to the corresponding engine by the fuel feed system. The system has two centrifugal boost pumps powered electrically, boost pump bypass, and a fuel shut-off valve of the butterfly type for every fuel tank. The components are regulated by the control panel of the flight engineer. Furthermore, the check valve is integrated with each boost pump’s inlet port to prevent tank draining during the pump removal. Two boost pumps have discharge lines connected in parallel so that when one is not operative, fuel will be supplied at the required pressure and flow rate by the remaining pump for all the conditions of the operating engines (United States Navy, & USN, 2007). The connection between the discharge lines and the bypass boost pump is made parallel. Moreover, in emergency situations when two boost pumps are defective or malfunctioned, the bypass enables the engine to draw fuel at the pressure and the rate adequate to initiate takeoff thrust from the tank. The check valve found on the bypass avoids reverse flow during the normal boost processes.
Four main tank-to-engine systems of fuel feed are interconnected by the cross-feed system enabling fuel to be supplied to any engine from any main tank. Besides the tubing, the system has four fuel cross-feed valves of butterfly types. The two jettison/override pumps located in the tank at the center wing discharge into the cross-feed manifold. Hence, they allow to supply fuel to one or more engines from the tank (Langton, 2008). The pump’s pressure of discharge is designed to be high to supersede the main tank’s boost pumps for the situations of the operating engine flow of fuel. There are also check valves on the boost pumps for the main tanks designed to avoid reverse at the times of fuel feed. The outboard (Fuel number 1 and 4) reserve is indirectly distributed to the engines by force of gravity where it enters the next outboard tanks. During the process, the flow is checked by the shut-off valves mainly of butterfly types.
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Jettison Fuel System
The airplane has a fuel jettison system that ensures a quick discharging during emergencies. The system comprises of six jettison pumps two of which are jettison/override pumps. All the pumps can be safely removed without draining (Condon, 2007). Four main inboard tanks offload into the manifold while two jettison/override pumps positioned at the center wing discharge into the jettison manifold. Besides, the fuel in the outboard tank is jettisoned indirectly by the main tank transfer valves allowing the valve to open. Similarly, letting the reserve tank’s gravity transfer valve to open, fuel is ejected indirectly, which enables fuel to move by force of gravity into the outboard main tank coming from the reserve tank.
Airplanes can be refueled through the fueling receptacles. The process needs a hydrant car or refueling tanker to serve as a reservoir. The jettison system enables defueling to the level of reserve fuel. Moreover, gravity transfer valves between the outboard and the inboard main tank should be opened. The nozzle valves of jettison should remain closed to avoid fuel spillage via jettison nozzles, which can be achieved through interconnecting lines and the cross-feed manifold by opening the defuel valves.
Fuel Quantity System
The fuel amount that can be used at any time in the tank is shown on the gage panel of the flight engineer every time the airplane is powered. Besides, there is a totalizer gage which adds the individual gage readings and provides the total fuel quantity in the tank. The compensator and the tank units that detect the fuel amount give the input indications to the corresponding indicators (Langton, 2008). In addition, the varying capacitance corresponds to the level of fuel in the tank. The compensators are always put in the fuel and submerged to provide variation correction of the fuel density. Finally, the level of fuel in the tank can also be obtained on the ground with the measuring sticks (four levels).
The tanks of the airplane are filled using the pressure system. Fueling the plane can also be achieved by applying the overwing ports method. Overwing fueling port of the airplane’s main tanks is designed and positioned in such a way that the tank cannot be over fueled. The reserve of the airplane tanks and the plane’s center wing tanks (outboard, the number 1 and 2) can be filled through number two and three (inboard) filler ports. As a result, this is implemented through the transfer of fuel from the inboard tanks to the preferred destination tanks using the pressure jettison/fuelling manifold and the system’s jettison tanks (Nagabhushana & Sudha, 2010).
Volumetric Shutoff System
The system utilizes the signal sensors of the tanks that makes fuel amount automatically stop the fueling process by closing valves of the fueling shutoff when the tank units reach the desired level. The compensator and the control units are among the major components of the system of shutoff. The compensator units of the airplane are fitted in the number one and four (outboard) tanks, number one (outboard) and the main tank. An electric circuitry card is the regulatory unit of the entire system for every tank. When the plane is fueled using pressure system, the volumetric shutoff’s control units energize the system’s solenoid valves, which results in the valves’ opening. When a tank is full, it is indicated, and the solenoid stops receiving electrical power leading to the closure.
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Fueling Control Panel
The panel involves all the necessary controls for pressure fuelling operation. Its components include an indicator test switch, seven fuel quantity indicators, a refusal power selector, ten valve switches for fuelling shutoff, and ten position indicator valve lights for fueling shutoff. The quantity indicators receive their signals from the corresponding quantity gages on the engineer’s panel. The test switch indicator gives a method of confirming the quantity indicator at the panel for operation.
In conclusion, the fueling system for B747 is elaborate and designed to function with minimal human interference. Moreover, it is a perfect and rarely failing system which has interfaces that can be easily interpreted by the engineers and pilots.