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Internal Fuel System Print E-mail
(12 votes, average 3.83 out of 5)
Written by Sabc   
Article Index
Fuel Transfer System
Pilot Controls
Fuel Feed System
Cautions and Warnings
Sources
Forum Discussion
Related Articles

Internal fuel of the F-15E is stored in four fuselage tanks and two wing tanks (made by Goodyear). The fuselage tanks are interconnected and are located right behind the cockpit and under the speedbrake, between the two engine air intake ducts (see figure below).

argaiv1683

internal_fuel_tanks

1
2
3
- Left wing tank
- Auxiliary tank
- Left engine feed tank
4
5
6
- Right wing tank
- Right engine feed tank
- Tank 1 (main tank)

Fuselage tanks are the main tank called Tank 1 (6), left and right engine feed tanks (3,5) and an auxiliary tank (2). Tank 1 and the wing tanks (1,4) are transfer tanks, which means that they are used to transfer fuel to and from other tanks through them to compensate from any aircraft imbalances originating from consuming fuel during flight. Fuel from CFT's are transferred to the internal tanks by electronically operated fuel transfer pumps, while fuel from external tanks are transferred to the internal tanks by regulated engine bleed air pressure (this latter provides positive pressure on all internal tanks during flight). The location and shape of internal fuel tanks are designed in such a way, that even if all fuel transfer pumps fail, all internal fuel will transfer automatically (by the force of gravity) towards the engine feed tanks, although at a reduced rate.

In case of emergency, all internal, external and CFT fuel can be dumped overboard via the Fuel Dumping System. All internal fuel tanks are vented through fuel vent outlets located at the trailing edge of each wingtip.

Tanks can be refuelled from a single point either from the ground or from the air. On the ground a single pressure fueling connection is used, while in the air an air refueling receptacle is used.

Survivability was a key issue during design. All fuel tanks are located in front of the engines, so they are not affected in case of an engine fire. All tanks contain special fire and explosion supression foam slabs, which prevent the buildup of explosive fuel vapours and the buildup of static electricity generated by the sloshing movement of fuel. As a result, the tanks won't explode even if they take a direct hit. The tanks are coated from the outside by a self-sealing material which swells upon contact with fuel, so in case a tank gets punctured (by shrapnel for example) the fuel itself seals the hole. Fuel transfer ducts are routed inside the tanks whereever possible and are coated with the same self-sealing material when outside the tanks. Empty spaces inside the aircraft structure around the fuel tanks are filled with explosion supression polyether foam (from below and from the sides).

A pressurization and vent system provides regulated engine bleed air pressure to all internal tanks to prevent fuel boiloff at higher altitudes with lower external air pressure. This same system provides air pressure to the internal tanks for fuel transfer purposes. This system also provides pressure relief for fuel tanks during climb and vacuum relief during descent. Tank pressurization works only when the landing gear handle is UP. WHen it is moved into its DOWN position, all internal and external tanks will be depressurized.

Fuel Transfer System

The task of the fuel transfer system is to transfer fuel to and from internal and external tanks to prevent imbalances caused by different fuel quantities in different tanks. The system should keep fuel imbalances between left and right sides below 200 pounds in case of internal wing tanks and below 1000 pounds in case of CFT's. If the aircrew experiences any imbalance greater than these and lasting for more than 5 minutes, then it's a malfunction and should be reported.

Internal fuel transfer is accomplished by electrically operated fuel transfer pumps for the three transfer tanks (two wing tanks and tank 1), plus a fuel ejector pump for the auxiliary tank. These pumps automatically transfer fuel towards the engine feed tanks, once these tanks are ready to accept fuel. If the fuel in the engine feed tanks decrease below a set limit, level control valves automatically open, thus accepting fuel from transfer tanks.

Transfer pumps run continuously if there is electrical power to drive them and the engine master switch is ON. Note that during aerial refueling tank 1 transfer pumps run only if the aerial refueling door is closed - even if a FUEL LOW condition occurs (see article Fuel Quantity Indicating System).

During normal operation 200 pounds of tank 1 fuel is transferred before the internal wing tanks start transferring. This is called " CG kick" and the goal of this procedure is to place the aircraft's center of gravity a bit more aft. Once transfer from wing tanks kicked in, all three transfer pumps operate simultaneously during either flight or ground operations. Although tank 1 fuel capacity is higher than the capacity of any of the wing tanks, the pumps' fuel transfer rate is set such a way, that tank 1 and the wing tanks empty within 200 pounds of each other.

Check valves prevent fuel to flow back from engine feed tanks to transfer tanks. The fuel ejector pump in the auxiliary tank transfers fuel toward tank 1 when its transfer pump is operating.

Pilot Controls

The aircraft's fuel management systems can be controlled by switches on the fuel panel, located forward on the pilot left side console (see figure below). The switches and their functions are discussed below.

internal_fuel_panel

1
2
3
- Fuel control switch: external wing tanks
- Fuel control switch: centerline tank
- Fuel control switch: conformal tank
4
5
6
7
- Fuel dump switch
- CFT emergency transfer switch
- External transfer switch
- Slipway switch

The three fuel control switches (1, 2, 3) control transfer and refuel procedures of all external tank types: external wing tanks (WING), centerline tank (CTR) and conformal fuel tanks (CFT). Their functions are discussed in details in article External Fuel Tanks.

The fuel dump switch (4) is used to dump fuel. Its functions are discussed in details in article Fuel Dumping System.

The CFT emergency transfer switch (5) is used in emergency situations to transfer CFT fuel. Its functions are discussed in details in article Conformal Fuel Tanks.

The external transfer switch (6) is used to schedule external fuel transfer. Its functions are discussed in details in article External Fuel Tanks.

The slipway switch (7) controls the aerial refueling receptacle door. Its functions are discussed in details in article Air Refueling System.

Fuel Feed System

The F-15E engines require pressurized (or "boosted") fuel feed. For this reason two main fuel boost pumps are installed in a Strike Eagle (one for each engine), plus a third emergency boost pump. The main boost pumps are powerful enough to be able to continuously feed their engine (left pump feeds the left engine, right pump feeds the right engine) at all power settings throughout the flight envelope. To keep fuel temperature on an optimal level, fuel is constantly recirculated to the internal wing tanks where it can cool down - in other words the integral wing tanks serve as fuel heat exchangers. The left and right engine feed tanks contain baffles in order to provide a limited amount of fuel to the boost pumps during inverted flight or during negative G maneuvers.

Survivability being a key issue, the fuel feed system is designed to be able to function when one or more of its components are broken or inoperative.

Below 1000 pounds of total feed tank fuel, feed tanks may not feed simultaneously. If either or both main boost pumps fail or, either or both main generators are inoperative or both main transformer-rectifiers fail, the emergency boost pump is activated and a system of tank interconnect and crossfeed valves allows the remaining operative pump(s) to supply all usable fuel in the feed tanks to both engines.

If only one main boost pump and the emergency pump operating, the fuel feed system is capable of feeding both engines with pressurized fuel at all non-afterburner power settings throughout the entire flight envelope. If any two boost pumps fail (out of the three), the remaining single pump is capable to feed both engines at all non-afterburner power settings from sea level to 30,000 feet of altitude.

If all three boost pumps fail, fuel is available to the engines by suction feed only. Since the engines require pressurized fuel under most flight conditions (to prevent fuel vaporization), in case of triple boost pump failure dual engine flameout may occur.

During single engine operation, the feed tank of the inoperative engine will not feed to the operative engine until fuel level in the operative feed tank drops well below the FUEL LOW limit.

Cautions and Warnings

L/R BOOST PUMP: This caution is displayed on MPD/ MPCD if the associated boost pump pressure is low.

EMER BST ON: This caution is displayed on MPD/ MPCD and a caution light illuminates on the caution panel, if the emergency boost pump starts operating and provides sufficient fuel pressure.

BST SYS MAL: This caution is displayed on MPD/ MPCD and a caution light illuminates on the caution panel, if emergency fuel boost pump output is insufficient.

XFER PUMP: This caution is displayed on MPD/ MPCD if a failure of a CFT or a wing fuel transfer pump occurs. There is no differentiation between left or right sides or between external wing tanks and CFT.

TRANSFER PUMP: This is a voice warning and is played in the aircrew's headset if a failure of a CFT or wing fuel transfer pump occurs. Bitching Betty repeats the following message twice: "Warning, transfer pump".

Sources

  • F-15E Flight Manual (TO 1F-15E-1) C 15 August 1990, courtesy of eFlightManuals

Steve Davies:

Boeing F-15E Strike Eagle - All-Weather Attack Aircraft

2003, Airlife Books, ISBN 1840 373 784

Hardcover, 7.7" x 10" (19.5 cm x 25 cm), 208 pages, over 250 images

It has taken over 18 months to research and write, and the author estimates that as much as 70% of the text is new information that has yet to reach the public domain. It is, without question, the most detailed, well-researched and authoritative analysis of the F-15E Strike Eagle ever written. It is an absolute must-have for all F-15E enthusiasts, many info within this site comes from this book.

F-15E.info rating: HIGHLY RECOMMENDED

Forum Discussion

You can discuss this article in the Strike Eagle Forum with the latest posts appearing here below as well. Clicking on the 'Discuss' button takes you to the SEF, while clicking on the 'Quick Post' text enables you to make a post here right away.

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Last Updated on Monday, 30 May 2011
 


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