[F-16 Fighter Aircraft Operations Manual] 7. Explanation of the Fuel System and Related Control Panels

Fire and Overheating Detection System

The system comprises two independent parallel sensing circuits, dedicated to fire detection and overheating detection respectively. The fire detection circuit is installed within the engine compartment, whilst the overheating detection circuit covers the engine compartment, the main landing gear wheel wells, the environmental control system compartment and the emergency power unit compartment. The activation temperature of the overheating detection circuit is approximately 100°C lower than that of the fire detection circuit. A fire signal triggers the engine fire warning light to illuminate, whilst an overheating signal triggers the overheating warning light to illuminate. When the temperature of the sensing element falls below the threshold value, the signal ceases and the corresponding indicator light goes out. The detection circuit is powered by the No. 2 critical AC busbar and the No. 1 battery busbar (excluding the battery busbar).

Test switch panel

The test switch panel is located at the bottom left of the left-hand console

1. MAL & IND LTS Test Button: Main warning light and indicator light test button

A central test switch used to quickly check that all major warning and indicator lights in the cockpit are functioning correctly. Pressing this button illuminates all relevant warning and indicator lights, allowing the pilot to verify that these critical visual warning devices are in good working order before or during flight.

2. OXY QTY Indicator Test Switch

A dedicated test device used to verify the accuracy and reliability of oxygen level indicators (readings). Its core function is to simulate a known oxygen pressure signal in order to check that the entire measurement chain—from the oxygen sensor to the cockpit instrument display—is functioning correctly.

3.FLCS PWR Lights (Green)

A set of green status indicators used to confirm to the pilot that power is being supplied to each channel of the aircraft’s fly-by-wire flight control system and that they are ready for operation.

4.FLCS PWR TEST Switch

A dedicated test switch used to manually check whether the flight control system power indicator lights (FLCS PWR Lights) and the underlying circuitry are functioning correctly. Its primary function is to verify that the green indicator lights, which indicate the power status of the flight control system, are working properly.

5.STICK CONTROL Switch

Switches the electronic control authority for the primary flight controls (control stick) between the front and rear cockpits of a two-seater aircraft (such as the F-16B/D).

6.EPU/GEN Test Switch

A dedicated test switch used to periodically check that the emergency power unit and its generator are functioning correctly. Its primary function is to verify that the electronic control circuits of the start-up system, fuel system and power generation system are ready for operation without actually starting the EPU.

7.PROBE HEAT Switch

Electrically heated elements used to simultaneously activate or deactivate all critical atmospheric data sensors and associated probes on an aircraft, in order to prevent them from freezing or accumulating ice in icing conditions.

8.FIRE & OHEAT DETECT Test Button

A central test switch used to verify that the entire fire and overheating detection system (including detection circuits, control logic and warning indicators) is functioning correctly.

Fuel system

The fuel system is divided into seven major functional modules: fuel tank system, fuel delivery system, fuel tank venting and pressurisation system, engine fuel supply system, fuel quantity/level sensing system, Fuel tank explosion suppression system and Fuel filling/draining system.

The fuel transfer system employs two independent methods for fuel transfer. The primary method utilises the siphon effect created by the risers connecting the various fuel tanks; the effectiveness of this method depends on the absence of air within the receiving tank. The ejectors in each collection tank automatically expel any air.

In the event of a siphon system failure, the system will activate the power fuel pump to continuously transfer fuel from the internal tank to the collection sump. This power transfer system also utilises an electric pump and a pump unit driven by fuel pressure from the engine manifold to drain the tank, thereby minimising the amount of unusable fuel. The two transfer modes operate simultaneously and independently, working together to ensure fuel transfer throughout the entire system.

The fuel transfer system is divided into two independent tank systems: the forward and rear systems. The forward system comprises the right external fuel tank (if fitted) and the right wing internal fuel tank; the rear system comprises the left external fuel tank (if fitted) and the left wing internal fuel tank. If a centre-line fuel tank is fitted, it is part of both the forward and rear systems.

The fuel transfer path is as follows: wing-mounted external fuel tanks → corresponding wing internal fuel tanks → fuselage fuel tanks → forward and aft collection tanks → engines. To automatically maintain the centre of gravity, the forward and aft systems operate in a synchronised transfer mode.

If external fuel tanks are fitted, fuel will be transferred to the in-wing tanks via pneumatic pressure. When the external fuel transfer switch is in the NORM position, the fuel transfer sequence is as follows: centre line tank → in-wing tanks → once the external fuel tanks are empty, the fuel flows into the corresponding in-wing tanks.

The transfer valves on the external fuel tanks of each wing are designed to cut off the fuel supply to prevent overfilling. Should any valve fail, the float switch will detect the fuel level and cut off the transfer to all external fuel tanks, thereby preventing fuel from spilling out of the aircraft. If the external fuel transfer switch is set to the ‘WING FIRST’ position, the wing external fuel tanks will be drained first; in this case, should a transfer valve fail, the float switch will be unable to prevent fuel from spilling out.

The automatic forward fuel transfer system supplements the function of the forward fuel pump by preventing an unfavourable rear centre of gravity. The system is activated only when the fuel selector is in the NORM position and the forward fuselage total fuel indication is below 2,800 pounds (approximately 820 kg).

Under specific conditions, forward fuel transfer is initiated when the forward-to-aft heavy fuel oil differential pressure drops below 300 lb (100 kg) and ceases when it reaches 450 lb (150 kg); or it is initiated when the forward-to-aft heavy fuel oil differential pressure exceeds 900 lb (300 kg) and ceases when it drops to 750 lb (250 kg). This system only performs fuel transfer from the aft to the forward tanks and does not correct any imbalance in the forward fuel supply.

Fuel Tank Ventilation and Pressurisation System

The system draws cooled, pressurised air from the environmental control system to force fuel from the external fuel tanks into the wing tanks and to power the injection pumps when the fuel source selector is set to the NORM or DUMP position, whilst preventing the fuel in the internal tanks from vaporising at high altitudes. The external fuel tank venting and pressurisation valve regulates the air supply pressure to the external fuel tanks.

If combat mode is activated via the fuel tank inerting switch, halon gas will be mixed with the air; at this point, the pressure is controlled by the internal fuel tank vent and pressurisation valve.

When the air source selector knob is set to the OFF/RAM position or the environmental control system fails, the fuel tank pressurisation function ceases to operate and external fuel transfer is interrupted. However, in the event of a multi-engine failure, fuel tank pressurisation continues and external fuel transfer remains unaffected.

The system operates as follows:

1. Normal boost mode

• Air source: Cooling air drawn from the environmental control system
• Function: Maintains fuel transfer from the auxiliary tank + prevents fuel vapourisation
• Control component: External fuel tank vent and pressure relief valve

2. Battle Mode

• Medium: Halon-air mixture (to reduce the risk of fire and explosion)
• Pressure control: managed by the internal tank vent and pressure relief valve
• Trigger condition: Fuel tank inerting switch activated

3.Fault handling logic

• Environmental control system failure: Fuel tank pressurisation interrupted (external supply stopped)
• Common generator faults: continuous boost (as the bleed air system is independent of the generator)

Engine fuel supply system

When the engine fuel supply selector is in the NORM position, the boost pumps in the front and rear fuel summing tanks pump fuel through the fuel supply lines into the fuel flow proportioner. Within the FFP (Fuel Flow Proportioner), a fixed-displacement pump driven by hydraulic system A draws equal quantities of fuel from the front and rear summing tanks to maintain balance. Two fuel lines fitted with check valves can form a bypass in the event of an FFP failure, ensuring that fuel flow is not interrupted.