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ATR 42/72-600 Systems Guide — Chapter L: Fuel System

2026-06-23

Chapter L. FUEL SYSTEM (FCOM DSC 28)

1. GENERAL DESCRIPTION & STORAGE ARCHITECTURE (Page 131)

The fuel system stores, manages, and delivers fuel directly to the engine control units. Storage is entirely contained within two independent structural wing tanks (Left Wing Tank and Right Wing Tank).

1.1. System Capacities and Refueling Properties

  • Total Usable Fuel Capacity: $5,000\text{ kg}$ maximum ($2,500\text{ kg}$ usable per wing tank).
  • Tank Ventilation: Symmetrical NACA vent scoops located near each wing tip protect structural integrity by balancing internal tank pressures during high-rate fueling, climb, and descent profiles.
  • Refueling Logistics: Driven via a centralized ground pressure refueling panel located under the right wing root trailing edge. Fuel lines feed both tanks simultaneously using automated high-level volumetric shutoff sensors.

1.2. FUEL CORE SYSTEM SCHEMATIC (Page 132)

The baseline delivery architecture employs a split-isolated left and right feed manifold layout connected via a cross-feed line.

       LEFT WING TANK                                RIGHT WING TANK
   ┌───────────────────────┐                    ┌───────────────────────┐
   │ [ ELEC PUMP 1 ]       │                    │       [ ELEC PUMP 2 ] │
   │          │            │                    │              │        │
   │    [ JET PUMP 1 ]     │                    │        [ JET PUMP 2 ] │
   └──────────┬────────────┘                    └──────────────┬────────┘
              │                                                │
              ▼                                                ▼
     [LP SHUTOFF VALVE 1]                             [LP SHUTOFF VALVE 2]
              │                                                │
              ├───────────────► [ CROSSFEED VALVE ] ◄──────────┤
              │                   [X-FEED VLV]                 │
              ▼                                                ▼
     [ENG 1 MECH PUMP]                                [ENG 2 MECH PUMP]
              │                                                │
              ▼                                                ▼
         TO ENGINE 1                                      TO ENGINE 2

1.3. Delivery Components:

  • Electrical Fuel Pumps (ELEC PUMP 1 & 2): Submerged DC-powered auxiliary pumps used primarily for engine starting, cross-feed maneuvers, and fallback pressure generation during mechanical system failures.
  • Motive Flow Jet Pumps: Main delivery pumps driven entirely by fluid velocity vectors (motive flow return loops) tapped from the high-pressure engine-driven mechanical pumps.
  • Low-Pressure (LP) Shutoff Valves: Hardwired motorized valves that isolate fuel flow at the structural wing-to-fuselage boundary layer during emergency fire shutdown events.

2. SYSTEM INDICATION AND ELECTRICAL PUMP OFF STATE (Page 133)

Under normal steady-state cruise operations, the electrical fuel pumps are unpowered to prolong component life.

  • Jet Pump Dominion: Once an engine stabilizes at idle configurations, return flow loops generate sufficient motive pressure across the venturi nozzles of the main jet pumps to maintain engine feed demands completely independently.
  • Overhead Panel Logic: Symmetrical indicators monitor system line pressure. Selecting an electric pump to OFF isolates its DC power line, changing its panel state indicator to a white OFF text field. The surrounding system routing lines on the MFD synoptic display remain green as long as engine-driven suction loops maintain normal line boundaries.

3. ENGINE STARTING PROCEDURE PROFILE (Page 134)

Prior to rotation of the high-pressure engine core ($N_H$), line pressure must be generated manually to fill the fuel manifolds and protect the mechanical high-pressure fuel control units from running dry.

3.1. Starting Sequence Timeline Laws

  1. Electric Pump Engagement: The pilot manually selects the respective ELEC PUMP to ON. This closes the auxiliary power relays, starting the submerged pump.
  2. Pressure Validation: The FWS senses fuel manifold delivery line pressure rising above a minimum functional threshold ($>8.5\text{ psi}$). The white RUN indicator illuminates on the overhead panel button.
  3. Engine Ignition Entry: Once delivery line integrity is satisfied, the Electronic Engine Control (EEC) permits the start sequence. Advancing the Condition Lever to the START gate opens the high-pressure fuel scheduling valves to initiate light-off.
  4. Motive Flow Takeover: As engine RPM accelerates past $61\%\ N_H$, the mechanical high-pressure pump output stabilizes. This automatically initializes the jet pump motive flow loop to assume primary delivery overhead.

4. NORMAL IN-FLIGHT BALANCED MANIFOLD OPERATION (Page 135)

During normal operations, the fuel system relies on split-isolation loops. Left and Right wing networks run fully isolated from one another.

  • Cross-Feed Contactor State: The central X-FEED valve is commanded to the fully CLOSED position.
  • Symmetrical Feed Constraints: Engine 1 draws fuel exclusively from the Left Wing Tank, while Engine 2 draws exclusively from the Right Wing Tank. This maintains lateral trim and balances the aircraft's center of gravity without pilot intervention.
  • Automatic Backup Scheduling: The automatic monitoring logic stands ready to instantly re-energize the auxiliary electric fuel pumps if line pressure drops below safety limits ($<5\text{ psi}$), safeguarding the continuous fuel feed.

5. CROSS-FEED (X-FEED) PROCEDURE EXECUTION (Page 136)

Cross-feed operations are manually initiated to resolve lateral fuel imbalances or to feed both engines from a single wing tank following an engine failure.

  CROSS-FEED ACTIVE FLOW MATRIX
  ┌────────────────────────────────────────────────────────┐
  │  LEFT WING TANK  ──► [ ELEC PUMP 1 ACTIVE ]            │
  │                           │                            │
  │                           ▼                            │
  │                     [ X-FEED VALVE ] ──► (OPEN STATE)  │
  │                           │                            │
  │             ┌─────────────┴─────────────┐              │
  │             ▼                           ▼              │
  │     [ ENG 1 FEED LINE ]         [ ENG 2 FEED LINE ]    │
  └────────────────────────────────────────────────────────┘

5.1. Operational Workflow Steps:

  1. Auxiliary Pump Activation: Turn the ELEC PUMP on the high-quantity wing tank side to ON. This ensures positive line pressure vectors before opening the cross-feed lines.
  2. Valve Selection: Rotate the cockpit X-FEED control switch to the OPEN selection block. The inline motorized selector valve turns 90 degrees.
  3. Opposing Pump Deactivation: Turn the ELEC PUMP on the low-quantity (receiving) wing tank side to OFF. The higher pressure output from the active tank's electric pump dominates the manifold. This forces both engines to draw fuel from the high-quantity tank, resolving the asymmetry.

5.1. CROSS-FEED VISUAL INDICATIONS & DISPLAY STATUS (Page 137)

System state changes are verified via integrated visual feedback loops on both the overhead controls and the MFD synoptics.

  • Overhead Valve Advisory Label: Opening the cross-feed valve illuminates a cyan X-FEED ON status memo line on the PFD/MFD alert column.
  • MFD Synoptic Redraw Logic: Selecting the SYS / FUEL synoptic display page redraws the line map. The horizontal cross-feed manifold link changes from a broken white line to a solid green path, confirming flow continuity between the left and right engine delivery networks.

6. JET PUMP LOW PRESSURE (FEED JET PUMP LOW PR) CONDITIONS (Page 138)

If a mechanical component fails or a line line break disrupts motive flow feedback loops, the primary jet pump will experience a loss of delivery pressure.

  • Trigger Limits: If fuel manifold delivery line pressure falls below a safe baseline threshold ($<5\text{ psi}$):
  • The FWS triggers an automatic Master Caution single chime.

  • Generates the amber flashing FUEL RUN PUMP 1(2) or FUEL FEED JET PUMP LOW PR warning on the EWD.

  • Automated System Response: The Multi-Function Computer (MFC) bypasses standard system logic to immediately command the standby auxiliary electric fuel pump inside the affected tank to start.

  • Crew Verification Actions: The pilot must confirm that the white RUN light illuminates on the respective electric pump button. This verifies that the standby pump has successfully restored pressure to protect the high-pressure engine pumps from cavitation damage.

7. TANK FUEL LOW LEVEL ALARM LOOPS (Page 139)

The wing tanks are equipped with independent, shielded float sensors to guard against fuel exhaustion during extended flight profiles.

  • Low-Level Threshold: The warning system triggers automatically if the usable fuel quantity inside either wing tank drops below $160\text{ kg}$.
  • Cockpit Alert Annunciations: Reaching the low-level threshold activates multiple flight deck notifications:
  • Triggers the Master Caution alert loop and single chime acoustic warning.
  • Displays the amber flashing FUEL L(R) LO LEVEL message on the EWD screen.

  • The digital fuel quantity readout for the affected tank on the EWD changes color from white to Amber.

  • Warning: This is a critical structural alert indicating low fuel reserves. Pilots must execute immediate diversion procedures to land at the nearest suitable airfield before engine flame-out thresholds are reached.

8. ENGINE FIRE EMERGENCY FUEL ISOLATION LAWS (Page 140)

If an uncontained engine fire occurs, the primary safety goal is to isolate all highly volatile fuel lines at the wing boundary before thermal tracking breaches the main tank cells.

8.1. Low-Pressure (LP) Valve Mechanical Operations

  • Actuation Input: Pulling the overhead Engine Fire Handle sends an isolated electrical command directly to the hardwired low-pressure fuel shutoff valve motor.
  • Valve Isolation Mechanics: The LP valve motor drives its internal gate blade shut, mechanically blocking the main fuel supply line between the wing root dry bay and the engine nacelle feed lines.
  • Synoptic Status Update: The MFD FUEL synoptic display maps this isolation instantly: the green engine delivery line breaks, and the valve icon transitions to an inline amber crossbar configuration. This confirms that fuel flow to the engine compartment has been fully cut off.

Chapter L. FUEL SYSTEM (CONTINUED)

9. FUEL PANEL OVERHEAD CONTROLS & DIAGNOSTICS (Page 141)

Located on the central overhead panel assembly, the fuel control panel manages the electrical booster pumps and cross-feed logic gates.

  FUEL OVERHEAD CONTROL PANEL
  ┌────────────────────────────────────────────────────────┐
  │                         FUEL                           │
  │                                                        │
  │         ELEC PUMP 1      X FEED      ELEC PUMP 2       │
  │          [  RUN  ]      [ Cyan ]      [  RUN  ]        │
  │          [ FAULT ]                    [ FAULT ]        │
  │          [  OFF  ]                    [  OFF  ]        │
  └────────────────────────────────────────────────────────┘

9.1. Annunciator Light Logic Matrix

Control Label Indication State System Telemetry / Trigger Source
ELEC PUMP 1 / 2 RUN (White)

FAULT (Amber)

OFF (White) | Electric auxiliary pump is active and generating line pressure ($>8.5\text{ psi}$).

Manually selected ON but manifold pressure drops below safe threshold ($<5\text{ psi}$).

The pump line contactor is open; auxiliary pump is completely unpowered. | | X FEED | Inline Bar (Green)

Cross Bar (White) | Cross-feed valve is fully open, unifying left and right engine delivery manifolds.

Cross-feed valve is fully closed, maintaining split-system isolation boundaries. |

10. FUEL QUANTITY INDICATION & MEMO SYSTEM INTERFACES (Page 142)

Fuel monitoring systems compile data from multiple capacitive tank probes to feed continuous mass readouts to the flight deck.

10.1. MFD & EWD Fuel Data Presentation

  • Normal Readout (White): Displays individual tank quantities and total combined fuel mass in kilograms (e.g., 1840 kg).
  • Degraded Sensor Alert: If a capacitive measuring probe delivers corrupted telemetry, the associated digital digits cross out with an amber strike line on the MFD display page, and the FWS references fallback volumetric estimates.
  • XFEED Memo Logic: When the cross-feed valve leaves its closed seat, a cyan FUEL X FEED memo note automatically appends to the PFD/EWD alert column, alerting the crew that split-system isolation is compromised.