Engine Fuel and Control

The APU fuel system answers two questions: where the fuel comes from, and how it is metered precisely into the combustor. But APU fuel has one feature that sets it apart from the main engines — it is not only burned. The same stream of fuel, raised to high pressure by the Fuel Control Unit (FCU), also acts as the hydraulic working fluid for the Inlet Guide Vane (IGV) actuator, the Surge Control Valve (SCV) actuator, and the oil-pump-module de-oil valve. One stream, four jobs: it burns, it drives the IGVs, it drives the SCV, and it powers the de-oil valve. This is the clearest expression of the APU's "self-contained, no separate hydraulic source" design philosophy.

This article covers the full APU fuel path from the FCU to the atomizers: the trim-tank fuel source and the 3-minute re-start rule, the FCU's high-pressure gear pump and torque-motor metering valve, the four pressure-control valves, the flow divider feeding 12 dual-orifice atomizers, the ecology drain that recovers residual fuel at shutdown, and the FUEL LO PR warning. The ECB's fuel-scheduling algorithm (min/max fuel schedule, start acceleration curve) belongs to ECB Control and Monitoring; the trim-tank line and the APU feed pumps themselves are ATA 28.

1. One stream, four jobs

The defining fact of the APU fuel system is stated plainly in the AMM:

"The FCU supplies filtered, pressure controlled high-pressure fuel to the fuel distribution system. It also supplies filtered, pressure controlled fuel to the SCV actuator and the IGV actuator (the actuators use the hydraulic energy of the fuel). Filtered high-pressure fuel is also used for the operation of the de-oil valve on the oil pump module."

So a single pressurised fuel stream leaving the FCU splits into two functional families:

• a metered path — fuel sent through the torque-motor metering valve, the flow divider, and the atomizers, to be burned; and
• a regulated hydraulic path — fuel held at constant pressure to power the IGV actuator (see Load Compressor), the SCV actuator (see Bleed and Surge Air), and the de-oil valve (see Oil System).

The APU carries no dedicated hydraulic system of its own; the fuel it already needs for combustion does double duty as actuation power. That is the heart of the self-contained philosophy: every consumer the APU has is fed from systems the APU already runs.

2. Where the fuel comes from — and the 3-minute rule

The APU is fed from the aircraft's trim-tank transfer line, not from a dedicated APU tank. Per FCOM DSC-49-10-20:

"The APU is supplied with fuel through the trim tank transfer line."

The supply is pressurised by the APU feed pumps (FWD and aft), which belong to ATA 28. The FCOM note that follows captures the operational consequence:

"The APU fuel feed line is pressurized when the APU MASTER pb-sw is set to ON. If the initial start attempt fails, the crew may wait 3 min before setting the APU START pb-sw to ON again. This delay ensures the APU fuel feed line is sufficiently pressurized."

The reasoning behind the 3 minutes is worth spelling out. A start attempt often fails because the feed line is not yet fully pressurised — there may be air in the line, or the pumps may not have built pressure after MASTER ON. Retrying immediately presents the FCU with insufficient inlet pressure and fails again. Waiting 3 minutes lets the pumps build line pressure and purge air, so the next START is far more likely to succeed.

[!warning]- The 3-minute wait and the 60-minute wait are different things

Two distinct waiting periods exist and they are easy to confuse:

• 3 minutes after a failed start — to allow the fuel feed line to pressurise (this article, FCOM DSC-49-10-20).
• 60 minutes after more than three consecutive start attempts — for starter / battery thermal recovery (see Starting; FCOM DSC-49-30 states "After more than three consecutive APU start attempts, the flight crew should wait 60 min before a new start attempt.").

Different purpose, different trigger. The 3-minute wait is about pressure; the 60-minute wait is about heat.

3. Architecture — the full fuel path

The full chain, from the aircraft fuel feed to the combustor, with the parallel hydraulic and ecology-drain branches:

 Aircraft fuel  (trim tank transfer line, ATA 28-22)
        │  low-pressure fuel (FWD + aft APU feed pumps, MASTER ON)
        ▼
 ┌───────────────── FCU  Fuel Control Unit 59KF19 ──────────────────┐
 │  inlet filter (10 micron) ─► HP gear pump ─► HP filter (70 micron)│
 │                              2250 lb/hr at 800 psi                │
 │             │                                                    │
 │             ├──► TM metering valve ◄ ECB current                 │
 │             │     (lowest position = min fuel if de-energised)   │
 │             │     + fuel shutoff solenoid (opens at 7% speed)    │
 │             │                                                    │
 │             └──► four pressure-control valves:                   │
 │                   HP relief 950 psi  |  diff-P 50 psi            │
 │                   pressurising 125 psi  |  actuator reg 325 psi  │
 └────────┬──────────────────────────────────────┬─────────────────┘
          │ metered HP fuel                       │ regulated fuel (325 psi)
          ▼                                       ├──► IGV actuator  (art 02)
 ┌─ Flow divider 59KF25 ────────────┐            ├──► SCV actuator  (art 03)
 │  inlet filter (100 micron)       │            └──► de-oil valve  (art 10)
 │  secondary sequencing valve      │
 │    (opens at ~81 lb/hr)          │     FUEL LO PR switch 59KF17
 │  primary line / secondary line   │     (1.03 / 1.34 bar) ─► ECB ─► ECAM
 │  primary + secondary drain valves│
 │  fuel temperature sensor 59KF6   │
 └───┬─────────────────┬────────────┘
     ▼ primary mfld     ▼ secondary mfld
     └──► 12 dual-orifice atomizers 5100KF1 ◄──┘
                │ injected
                ▼
         Reverse-flow annular combustor (art 01)

 Ecology drain (at shutdown):  PCD 2 air ─► drain-air filter ─► accumulator
                ─► drain valves ─► pushes residual fuel into combustor to burn

Reading this diagram, three points stand out: (1) one stream entering the FCU splits in two — a metered path (through the TM metering valve, the flow divider, the atomizers, to be burned) and a constant-pressure hydraulic path (to the IGV, SCV, and de-oil valve); (2) the flow divider then splits the metered fuel again into primary and secondary lines feeding the 12 dual-orifice atomizers; (3) at shutdown the ecology drain uses stored high-pressure air to blow residual fuel back into the combustor rather than dumping it overboard.

The internal arrangement of the FCU, showing where the four pressure-control valves sit in the fuel circuit:

 LP fuel in  (APU feed pumps, ATA 28-22)
     │
     ▼
 ┌──────────────────────────────────────────────┐
 │  inlet filter (10 micron) + red ΔP pin        │  bypass opens at ΔP > 8 psi
 │  FUEL LO PR switch 59KF17 at the inlet        │  (supply over filtration)
 └───────────────────┬───────────────────────────┘
                     ▼
 ┌─ HP gear pump ────┐     ┌─ HP filter ────┐
 │  2250 lb/hr       ├────►│  70 micron     ├───► metering circuit
 │  at 800 psi       │     └────────────────┘
 └────────┬──────────┘
          │  HP relief valve 950 psi  ─► returns excess to pump inlet
          ▼
 ┌──────────────────────────────────────────────┐
 │  differential-pressure regulation valve       │  holds 50 psi across the
 │  locks 50 psi across the metering valve        │  metering valve
 └───────────────────┬───────────────────────────┘
                     ▼
 ┌─ TM metering valve ◄ ECB current ─┐
 │  lowest position = minimum fuel    │
 └───────────────────┬────────────────┘
                     ▼
 ┌─ fuel shutoff solenoid (opens 7%) ─┐──► pressurising valve (125 psi)
 └───────────────────┬─────────────────┘    ──► flow divider + drain valves
                     │
 ┌─ actuator pressure regulation valve (325 psi) ─┐
 │   constant pressure ─► IGV + SCV actuators      │◄── return from actuators
 └─────────────────────────────────────────────────┘

This second view fixes the "geography" of the four valves: the inlet carries the filter (with red ΔP pin and bypass) and the low-pressure switch; the HP relief valve sits just after the gear pump (950 psi over-pressure protection); the differential-pressure regulation valve straddles the metering valve (locking 50 psi so metering stays accurate); a separate branch carries the actuator pressure regulation valve (325 psi) feeding the IGV/SCV actuators and collecting their return; and finally the shutoff solenoid plus pressurising valve send metered fuel onward to the flow divider.

4. The FCU — pressurise, meter, and supply hydraulic fuel

The FCU (Fuel Control Unit, FIN 59KF19) is the heart of the system. It does three things at once: raises fuel pressure, meters the burn quantity on ECB command, and supplies regulated fuel to the actuators. It is attached to the front flange of the oil pump module and held by a V-clamp.

High-pressure gear pump

The high pressure comes from a two-gear rotary pump driven mechanically by the accessory gearbox, through the central pump shaft of the oil pump module. Its capacity is considerable:

"At 100 % APU speed the gear pump supplies approximately 2250 lb (1020.5820 kg) of fuel per hour at the outlet pressure of 800 psi (55.1581 bar), and 2300 lb (1043.2616 kg) per hour at 500 psi (34.4738 bar)."

The pump deliberately supplies far more fuel than combustion actually needs — this guarantees that fuel is available under any operating condition. The surplus is returned by the pressure-control valves (see §5).

The torque-motor metering valve and its fail direction

Burn quantity is set by the torque-motor (TM) metering valve. Increasing the ECB current to the torque motor opens the valve and increases the metered flow area; decreasing the current closes it. The fail direction is a key teaching point:

"When no electrical power from the ECB 59KD is supplied, the torque motor metering-valve moves to the lowest position to allow the minimum fuel flow."

De-energised does not mean fuel cut off — it means minimum flow (enough to keep idle-level combustion). The actual fuel shut-off is a separate device.

The fuel shutoff solenoid — the real fuel "off" switch

"The fuel shutoff solenoid-valve is a normally closed, electrically opened solenoid valve. During the APU start sequence, the fuel shutoff solenoid-valve receives a command signal from the ECB 59KD to open at 7 % APU speed. On normal or fault shutdowns, the fuel shutoff solenoid-valve gets a 'close' signal from the ECB 59KD."

This is the true master fuel valve: normally closed, opened by power at 7% speed during start, and commanded closed by the ECB on a normal or fault shutdown.

When metering current stops — not only in a fault

[!warning]- "Metering valve drops to minimum fuel" is not only a failure behaviour

The de-energised-to-minimum behaviour is a fail-safe direction, but the metering valve also drops to minimum as part of normal governing. Per AMM:

"The electrical supply to the torque motor metering-valve stops when the ECB 59KD receives one of the subsequent signals: - the protective shutdown, - the APU shutdown, - the governor speed signal."

The first two are shutdowns (current naturally falls away as the APU stops). But the third — the governor speed signal — is a normal speed-control event: when the APU reaches its governed speed, the ECB withdraws metering current to stop adding fuel. So "drop to minimum fuel" is both the fail-safe direction and the everyday tool of constant-speed control (add current to add fuel, withdraw current to cut fuel — this is how the ECB holds N at 100%). The ECB speed-control law is detailed in ECB Control and Monitoring.

The three fail-safe directions together

Reading the metering valve's fail direction alongside the IGV and SCV fail directions reveals the APU's overall failure bias:

All three bias toward "APU unloaded, not surging, idling" — the overall failure-safe direction is the lightest-burden, safe-to-shut-down state. This is the master principle behind APU fault behaviour.

5. The four pressure-control valves

The FCU contains four pressure-control valves:

"The FCU has four valves which control the fuel pressure: - the high-pressure relieve valve, - the differential-pressure regulation valve, - the pressurizing valve, - the actuator pressure regulation-valve."

Two of them are especially valuable for understanding why APU fuel metering is both accurate and ice-free.

Differential-pressure regulation valve — making metering depend only on valve opening

"It keeps the pressure of the fuel which flows through the metering valve (independently from the fuel quantity) constantly at 50 +5 psi or -5 psi (3.4474 +0.3447 bar or -0.3447 bar)."

Flow through an orifice depends on both the opening area and the pressure drop across it. By locking the pressure drop across the metering valve at a constant 50 psi, this valve makes the metered flow depend only on the valve opening — so the ECB current (and hence the opening) maps directly to fuel quantity, and metering stays accurate. The surplus fuel (more than momentarily required) is returned to the FCU inlet filter, and that return path carries a useful side-benefit:

"Because of the increase of the fuel pressure in the high-pressure gear pump, the temperature of the fuel increases. The quantity of the warm fuel which flows back to the inlet of the FCU inlet fuel-filter is sufficient to prevent the formation of ice in the filter and the FCU inlet."

So pump work heats the fuel, the warm surplus recirculates to the inlet filter, and that warmth prevents ice forming at the filter and inlet. Accurate metering and anti-icing fall out of the same recirculation.

Actuator pressure regulation valve — constant force and speed for IGV/SCV

"The actuator pressure regulation-valve controls the outlet pressure of the fuel which is used to power the IGV actuator and the SCV actuator. The valve keeps the pressure at 325 +25 psi or -25 psi (22.4080 +1.7237 bar or -1.7237 bar). Thus the actuators operate with constant force and speed."

This is why the IGV and SCV stroke times (given in articles 02 and 03) are stable and predictable: the actuation pressure is held at a regulated 325 psi independent of what the rest of the fuel system is doing.

The other two

• High-pressure relief valve — a spring-loaded check valve set at "950 +50 psi or -50 psi (65.5002 +3.4474 bar or -3.4474 bar)". It protects the components downstream of the pump: if a downstream blockage drives pressure above the set point, it opens and returns fuel to the pump inlet.
• Pressurising valve — ensures that "sufficient fuel pressure for the operation of the actuators (IGV and SCV) is available when minimum fuel is supplied (during start and shutdown and in high altitude conditions)." It opens when the pressure difference reaches "125 +25 psi or -25 psi (8.6184 +1.7237 bar or -1.7237 bar)."

Inlet filter — supply over filtration

One inlet-filter detail is worth a pilot's awareness. The FCU inlet fuel-filter element is "specified to 10 micron nominal, 40 micron absolute." If it begins to clog, a red differential-pressure indicator pin pops out when the pressure difference exceeds "5 +0.5 psi or -0.5 psi (0.3447 +0.0345 bar or -0.0345 bar)"; if it clogs fully, "the bypass valve opens and the inlet fuel does not go through the filter" — and the bypass also opens when the difference exceeds "8 +0.5 psi or -0.5 psi (0.5516 +0.0345 bar or -0.0345 bar)." The design prefers dirty fuel to no fuel: supply has priority over filtration.

6. The flow divider and the 12 dual-orifice atomizers

The combustor carries 12 dual-orifice atomizers, evenly spaced:

"The APU has 12 dual orifice fuel-atomizers. They are installed in equal distances around the outer case of the combustion chamber."

Each atomizer has two orifices working different parts of the flow range:

"Each fuel atomizer has a primary and a secondary fuel orifice. The primary orifices give accurate fuel atomization at low fuel quantity and pressure."

"The secondary orifices get the fuel when the secondary sequencing valve is open, they provide accurate fuel atomization at high fuel consumption."

Why two orifices? The APU's fuel demand ranges enormously, from a trickle at start to many times that at high power, and no single orifice atomizes well across the whole range (a large orifice atomizes poorly at start; a small one cannot pass enough fuel at high power). The solution: use only the primary orifice first (small, good atomisation at low flow), then automatically add the secondary orifice (large, for high flow) once demand grows.

The secondary sequencing valve — automatic, pressure-driven

The switchover is handled inside the flow divider (assembly 59KF25) by the secondary sequencing valve — and it is fully automatic, opened by pressure alone:

"The secondary sequencing valve is a poppet valve which is spring-loaded closed. It opens when the fuel flow is approximately 81 lb (36.7410 kg) per hour. At this quantity the primary fuel manifold and atomizer orifices cause a backpressure of approximately 125 psi (8.6184 bar). This pressure is sufficient to open the secondary sequencing valve."

So: at low flow (start), primary backpressure is low, the spring holds the sequencing valve closed, and only the primary orifices spray. As flow rises to ~81 lb/hr, the primaries build ~125 psi of backpressure, which pushes the sequencing valve open, and the secondaries begin to spray too. The whole transition needs no ECB command — it happens hydraulically.

A useful analogy: it works like a showerhead with a "boost" setting — at low flow only the central jets run; open it up and the rising water pressure itself pushes open the outer ring of jets.

The per-atomizer flow split, quoted for reference: "At a fuel pressure of 100 psi (6.8948 bar) the primary flow path of one atomizer has a fuel flow of 6 lb (2.7216 kg) per hour. The secondary flow path of one atomizer has a fuel flow of 20 lb (9.0718 kg) per hour."

The drain valves and the sensors

The flow divider also carries the primary and secondary drain valves — "of the three-way poppet type" — each with a fuel inlet and a drain-air inlet. During start, when fuel pressure reaches "5 psi (0.3447 bar)" the poppet moves over to seal the drain-air inlet and connect the flow-divider line to the manifold; at shutdown, when pressure falls below 5 psi, the spring resets the poppet to seal the fuel inlet and connect the drain-air line to the manifold (priming the ecology drain — see §7).

Two sensors sit on the flow divider:

• The fuel temperature sensor 59KF6, a nickel-wire RTD, sends an analog temperature signal to the ECB.
• A fuel flow meter (rotor and electrical pickup, signal frequency 50–1500 Hz proportional to flow) is fitted on certain FSN groups; together with the temperature signal it forms the closed loop the ECB uses to calculate accurate start fuel quantity. On other FSN groups the flow meter is not fitted and the ECB instead computes start fuel from the temperature signal and ambient (altitude) pressure.

7. The ecology drain — residual fuel is burned, not dumped

This is one of the more environmentally and fire-conscious touches in the APU fuel system. At shutdown, fuel remains in the atomizers and manifolds; letting it drip overboard would be both a pollutant and a fire hazard. The ecology drain instead blows the residual fuel back into the combustor to be burned:

"The ecology drain system removes the remaining fuel from the primary and secondary fuel atomizers and manifolds. The drain fuel is burned in the combustion chamber."

The motive power is stored compressor-discharge air:

"Discharge air from the second stage of the engine compressor (PCD 2 air) flows through the air ecology drain-filter, into an accumulator."

The accumulator is "a tube which has the contour of the turbine plenum and goes along in parallel to the fuel manifolds", with check valves at its inlet and outlet to prevent backflow. The drain itself is started by the ecology drain solenoid-valve 59KF28, which is "a normally closed shutoff-valve which is in the open position when it is energized."

The sequence at shutdown: the ECB closes the fuel shutoff solenoid (cutting the burn supply), then energises the ecology drain solenoid 59KF28, which releases the stored PCD 2 air; the air opens the drain valves and flows through the manifolds and atomizers, pushing the residual fuel into the combustor where it burns off.

So a brief sense of "afterburn", or short-lived EGT activity, after you shut the APU down is the ecology drain burning off residual fuel — normal, and part of the shutdown/cooldown cycle.

[!warning]- Cross-layer cooldown timing — flagged, not reconciled here

The shutdown "cooldown" period appears with two different values in two manuals. In the ecology-drain context the AMM states:

"Wether APU bleed air was selected or not, a cooldown cycle of 100 seconds, plus the 15 seconds for the no-break power transmition are necessary before the over-speed test can start."

The FCOM (pilot layer) instead states:

"If the APU BLEED was selected, the APU performs a cooling period of 85 s at 82 % speed"

The figures and conditions differ (FCOM's 85 s applies only when bleed was used; the AMM's 100 s is unconditional and is the maintenance-level pre-overspeed-test cooldown). These look like different manual layers / different events rather than a contradiction, and they are not merged here. The authoritative reconciliation is left to the shutdown articles (Emergency Shutdown / Automatic Shutdown). For pilots, the FCOM's 85 s is the figure to carry.

8. The FUEL LO PR warning

Inlet low-pressure protection is provided by the fuel low-pressure switch 59KF17, a differential-pressure switch installed at the FCU inlet:

"The switch closes when the pressure decreases to 1.03 +0.6 bar or -0.6 bar (14.9389 +8.7023 psi or -8.7023 psi). The switch opens and stops the fuel low-pressure signal when the pressure increases to 1.34 +0.6 bar or -0.6 bar (19.4351 +8.7023 psi or -8.7023 psi)."

When closed, the switch supplies a ground signal (and a 28 V DC signal when there is no low-pressure condition) — and that signal is routed to two computers:

"The ECB 59KD and the Fuel Control and Monitoring Computer (FCMC) monitor the signals from the fuel-low pressure switch."

Both the ECB and the FCMC watch the switch; the ECB then transmits the condition over an ARINC bus to the EIS for display. A closed switch puts an amber FUEL LO PR indication on the ECAM APU page. The two thresholds (1.03 / 1.34 bar) give hysteresis to prevent chatter, and on recovery to 1.34 bar the ECB "also stops the signal to the aft fuel feed-pump" (the extra boost pump is no longer needed).

The low-pressure switch also interfaces with the APU Emergency Shutdown (49-62) — low fuel pressure is one of the inputs the shutdown protection chain can reference (see Emergency Shutdown).

9. Pilot scenarios

APU start, fuel on. At 7% speed the fuel shutoff solenoid opens and the FCU meters fuel on ECB command. At low flow only the primary orifices spray (sequencing valve closed); once flow passes ~81 lb/hr the secondary orifices open automatically. The FCU simultaneously supplies regulated fuel to the IGV and SCV actuators.

Large electrical load applied, EGT rises. The ECB raises the metering-valve current, more fuel is sprayed to hold constant speed, and EGT rises (the energy chain of article 01). Metering stays accurate because the differential-pressure valve has locked 50 psi across the metering valve.

Failed APU start. Wait 3 minutes (to let the trim-tank feed line pressurise) before pressing START again — do not retry repeatedly. This is distinct from the 60-minute wait after three consecutive failed starts (thermal recovery, article 06).

ECAM FUEL LO PR (amber). Fuel pressure at the FCU inlet has fallen below ~15 psi (1.03 bar). Investigate the trim-tank supply and the APU feed pumps (ATA 28).

Normal shutdown, brief "afterburn". The ecology drain is using PCD 2 air to blow residual fuel back into the combustor to burn it off — a normal, deliberate part of the shutdown/cooldown cycle, not abnormal combustion.

APU control power lost. The metering valve drops to minimum fuel, the IGVs go fully closed, and the SCV goes fully open (the three fail-safes acting together) — the APU falls back to its lightest-burden, safe-to-shut-down state.

Self-test

[!note]- Q1. Why must the crew wait 3 minutes after a failed APU start, and how is this different from the 60-minute wait?

The APU draws fuel through the trim-tank transfer line, pressurised by the APU feed pumps once MASTER is ON. A start often fails because the feed line is not yet fully pressurised (air in the line, pumps still building pressure). The 3-minute wait lets the pumps build line pressure and purge air, so the next START succeeds. This is entirely separate from the 60-minute wait, which follows more than three consecutive start attempts and exists for starter/battery thermal recovery — pressure versus heat.

[!note]- Q2. The same high-pressure fuel stream does four jobs. What are they, and how much can the gear pump deliver?

The four jobs: (1) it is burned in the combustor; (2) it powers the IGV actuator; (3) it powers the SCV actuator; (4) it powers the oil-pump-module de-oil valve — the actuators using the hydraulic energy of the fuel. The high-pressure gear pump supplies approximately 2250 lb/hr at 800 psi at 100% APU speed. It deliberately delivers more than combustion needs so fuel is always available; the surplus is returned by the pressure-control valves.

[!note]- Q3. What is the loss-of-power direction of the metering valve, the IGV actuator, and the SCV, and what overall failure bias do they show together?

De-energised, the TM metering valve drops to its lowest position = minimum fuel (idle-level combustion, not a fuel cut — the cut is done by the shutoff solenoid). The IGV actuator goes fully closed (sheds the pneumatic load). The SCV goes fully open (vents air, prevents surge). Together they bias the APU toward "unloaded, not surging, idling" — the lightest-burden, safe-to-shut-down state. That is the master failure-safe direction of the APU.

[!note]- Q4. How does a single set of atomizers cover the flow range from start to high power, and what opens the secondary path?

Each atomizer is dual-orifice: a small primary orifice gives good atomisation at low start flow, and a large secondary orifice carries the high-power flow. At start only the primaries spray. As fuel flow rises to approximately 81 lb/hr, the primary manifold and orifices build about 125 psi of backpressure, which pushes open the spring-loaded secondary sequencing valve in the flow divider, and the secondaries begin to spray. The switchover is fully automatic and pressure-driven — no ECB command is involved.

[!note]- Q5. What does the ecology drain do, what powers it, and what are the FUEL LO PR switch thresholds?

At shutdown the ecology drain removes residual fuel from the atomizers and manifolds and burns it in the combustor (rather than dumping it overboard — pollution and fire prevention). It is powered by stored second-stage compressor-discharge air (PCD 2) held in an accumulator; the ecology drain solenoid 59KF28 releases the air, which opens the drain valves and pushes the residual fuel into the combustor. The FUEL LO PR switch closes (warning) when inlet pressure falls to 1.03 bar (15 psi) and opens again when it rises to 1.34 bar (19.4 psi).

Key takeaways

References

Per FCOM DSC-49-10-20 (trim-tank fuel supply, MASTER-ON line pressurisation, 3-minute re-start rule); FCOM DSC-49-20 (85 s / 82% cooling period, pilot layer) and FCOM DSC-49-30 (60-minute wait after three start attempts). Per AMM D/O 49-30-00 (engine fuel and control overview: FCU and flow divider, metered fuel in proportion to ECB command, the three sub-systems). Per AMM D/O 49-32-00 (FCU: high-pressure gear pump 2250 lb/hr at 800 psi, torque-motor metering valve and its de-energised-to-minimum behaviour with the three current-stop signals, fuel shutoff solenoid open at 7%, the four pressure-control valves at 950 / 50 / 125 / 325 psi, the 10-micron inlet filter with red ΔP pin and bypass). Per AMM D/O 49-31-00 (flow divider 59KF25, 12 dual-orifice atomizers, secondary sequencing valve opening at ~81 lb/hr and ~125 psi backpressure, three-way poppet drain valves at 5 psi, fuel temperature sensor and flow meter, ecology drain using PCD 2 air via accumulator and solenoid 59KF28, the unconditional 100 s + 15 s pre-overspeed-test cooldown). Per AMM D/O 49-34-00 (fuel low-pressure switch 59KF17 thresholds 1.03 / 1.34 bar with hysteresis, ECB + FCMC monitoring and ARINC transmission to the EIS, interface to Emergency Shutdown 49-62, aft feed-pump signal stop on recovery). The "one stream, four jobs = self-contained philosophy", the "three fail-safes together", the metering-accuracy-from-locked-ΔP reasoning, and the showerhead analogy are integrative synthesis built on these sourced facts.

Independent study material, not an Airbus publication. Refer to current operator FCOM, FCTM, AMM, and QRH for operational use.