Outflow Valves — Engineering Details

Cabin Pressure Controller gave the brain. This deep-dive opens up its hand — the two outflow valves that take the controller's command, execute it mechanically, feed position back, and carry their own hardware-level protection. There are two valves below the flotation line, six motors, an in-actuator pressure switch that overrides the controller, a two-dimensional-nozzle gate geometry that recovers thrust, and a fail-to-closed design. The safety / negative-relief valves and the RPCU are ata-21-13.

1. Location — below the flotation line

Two outflow valves are located below the flotation line. ... The outflow valves 313HL (315HL) are installed in the lower fuselage (313HL is installed forward and 315HL is installed aft). — FCOM DSC-21-20-20 + AMM 21-31-00 §3.C

[!note]- "Below the flotation line" — the outflow valves sit below the waterline on purpose

The flotation line is the waterline when the aircraft floats after a ditching. Putting the outflow valves below it looks wrong (water would flood in), but it is intentional: every below-the-line opening (the outflow valves, the negative-relief valve, the avionics drain) is closed by the DITCHING pushbutton before contact, so water cannot enter; above-the-line parts (such as the safety valves, mounted above the flotation line) need not close. This automatic above/below-the-line differentiation is the design logic. The DITCHING pb is the pilot's "close all below-the-line openings" switch (see Ditching).

2. Two part numbers — the full component list

NOTE: This description is applicable only to Outflow Valves Part Number 7012-17971-4 and previous. ... Each outflow valve has: two outflow valve electronic actuators (electronic boxes) 313HL1 (315HL1) and 313HL2 (315HL2), two automatic motors 313HL3 (315HL3) and 313HL4 (315HL4), a manual motor 313HL5 (315HL5), a feedback module 313HL6 (315HL6), an outflow valve body 313HL7 (315HL7), a gearbox 313HL8 (315HL8). NOTE: This description is applicable only to Outflow Valves Part Number 20792-01AA. ... a gearbox and feedback assembly 313HL6 (315HL6) and, an outflow valve body 313HL7 (315HL7). — AMM 21-31-00 §6.C

   ┌──────────[ OUTFLOW VALVE 313HL (fwd) / 315HL (aft) ]──────────┐
   │  cast aluminium-alloy housing, flush with the skin            │
   │                                                               │
   │  ┌─ electronic actuator 1 (313HL1) ─┐  → CPC #1               │
   │  │  actuator electronics + pressure │                        │
   │  │  switch (14,550 ft override)     │                        │
   │  └──────────────────────────────────┘                        │
   │  ┌─ electronic actuator 2 (313HL2) ─┐  → CPC #2               │
   │  │  (same; pressure switch)         │                        │
   │  └──────────────────────────────────┘                        │
   │  ┌─ auto motor 1 (313HL3) ─┐  DC brushless + e-m brake        │
   │  ┌─ auto motor 2 (313HL4) ─┐  DC brushless + e-m brake        │
   │  ┌─ manual motor (313HL5) ─┐  DC brush, slower, RPCU-fed      │
   │                                                               │
   │  ┌─ feedback module (313HL6) ─┐                               │
   │  │  RVT → CPC #1 + #2 (via actuators)                         │
   │  │  potentiometer → CPC #1 manual-backup section              │
   │  └────────────────────────────┘                              │
   │  ┌─ gearbox (313HL8) ─┐  gear train + e-m brake  (old PN only)│
   │                                                               │
   │  ┌─ valve body (313HL7): rectangular frame, two gates ───────┐│
   │  │   forward gate (opens OUTWARD) ── mechanically linked ──┐  ││
   │  │   aft gate (opens INWARD) ── linked to actuator + fwd ──┘  ││
   │  │   low angle → the two gates form a 2-D nozzle (thrust rec.)││
   │  └────────────────────────────────────────────────────────────┘│
   └───────────────────────────────────────────────────────────────┘
   Old PN 7012-17971-4: 6 items (separate gearbox 313HL8 + feedback 313HL6)
   New PN 20792-01AA:    5 items (gearbox + feedback merged into 313HL6)

[!note]- Old vs new part number (6 vs 5 items) — simplified, same function

The old PN (7012-17971-4) keeps the gearbox (313HL8) and the feedback module (313HL6) as separate LRUs; the new PN (20792-01AA) merges them into one "gearbox and feedback assembly" (313HL6, no 313HL8). Spares implication: an old-PN fleet stocks the two pieces separately; a new-PN fleet stocks the combined unit. The two part numbers are not interchangeable (different FINs, possibly different fit) — spares must be held to the effectivity, and a "this aircraft uses the new PN, we must wait for the new LRU" delay is possible.

3. Six motors — the command path

   CPC 1 (311HL) active            CPC 2 (312HL) backup
        ▼ ARINC 429                     ▼ ARINC 429
   actuator 1 (313HL1)             actuator 2 (313HL2)
        ▼                               ▼
   auto motor 1 (313HL3)           auto motor 2 (313HL4)
   DC brushless, primary           DC brushless, backup
        └──────────────┬───────────────┘
                       ▼ mechanical drive → valve gates

   Manual mode: MAN V/S CTL 5HL → manual motor (313HL5), DC brush, slower
   RPCU (dual-CPC fail + ground + stationary + all engines stopped):
     28 V DC essential bus 301PP → relays 22/23HL → manual motor 313HL5/315HL5 direct

Each valve carries two automatic + one manual motor = three; two valves = six motors. Auto motor 1 (313HL3) is used by the active controller; auto motor 2 (313HL4) by the backup controller (it takes over at a changeover). Each motor is on an independent supply, so a single motor failure does not lose the function. The manual motor is driven by the pilot via the MAN V/S CTL toggle, or by the RPCU directly.

4. The valve body — rectangular frame, two gates

The outflow valve bodies 313HL7 (315HL7) have two gates, one forward and one aft. The gates are installed in a rectangular frame. The forward gate opens outwards and is mechanically connected to the aft gate. The aft gate opens inwards and is mechanically connected to the actuator and the forward gate. — AMM 21-31-00 §6.C (5)

[!warning]- The outflow valve is not a simple butterfly valve

A pilot may picture a round butterfly valve (like a car throttle). It is in fact a rectangular frame with two gates opening in opposite directions: the forward gate opens outward (pushed out of the fuselage), the aft gate opens inward (drawn into the cabin), and the two are mechanically linked so the actuator moves one and the other follows. Why two opposed gates — see the two-dimensional nozzle below.

5. Two-dimensional nozzle + thrust recovery

At low valve angles the valve gates make a two-dimensional nozzle that directs and increases the speed of the outflow of air. This also gives thrust recovery to the valve. In the fully open (high valve angle) position, the forward gate moves into the airflow. — AMM 21-31-00 §6.C (5)

   Low valve angle (typical cruise):
      cabin (high P)
        forward gate slightly out ↘
                          ╲╲╲   ↓ accelerated flow
                            ╲╲╲  ← convergent 2-D nozzle
        aft gate slightly in ↗
              → outflow accelerates to ambient → small forward reaction (thrust recovery)

   Fully open (ground mode after landing, or before 14,550 ft):
      forward gate fully out → into the airflow
      ████████████████  ← external airflow
      aft gate fully in
              → large free outflow

[!note]+ Thrust recovery (integrative, on the manual's "thrust recovery" wording)

The pilot never notices it, but the engineer intended it. In cruise the cabin pressure exceeds ambient, so air flows out naturally. A plain butterfly valve dissipates that energy in turbulence. The two-dimensional convergent nozzle instead increases the flow speed and momentum → the exit momentum exceeds the inlet momentum → a small forward reaction on the valve. The magnitude is a few kilograms per valve (the manual gives no figure); over an 8–14 h oceanic flight the cumulative fuel saving is real. Knowing this helps a pilot see the outflow valve as a precision engineered part, not a "simple bleed flap".

6. Electronic actuators + the 14,550 ft pressure-switch override

The outflow valve electronic actuators 313HL1 (315HL1) and 313HL2 (315HL2) contain the actuator electronics. They send and receive signals to and from the cabin pressure controllers 311HL and 312HL. A pressure switch is installed in each outflow valve actuator. It overrides the signal from the CPC and/or e-box CPU and closes the applicable outflow valve 313HL (315HL) if the pressure in the fuselage is less than the atmospheric pressure at an altitude of 14550 -450 ft. or +450 ft. — AMM 21-31-00 §6.C (1)

[!warning]- The 14,550 ft pressure switch is a hardware protection layer — independent of, and earlier than, the controller's 15,000 ft software close

This is a separate protection chain from the CPC's 15,000 ft software auto-close (ata-21-11 §16):

Layer	Threshold	Mechanism	Depends on
Controller software	15,000 ft	CPC logic commands the motor closed	CPC health + ARINC link
Valve hardware	14,550 ±450 ft	the in-actuator pressure switch overrides directly	nothing — works even with both CPCs failed

Typical sequence: cabin altitude rises → near 14,550 ft → the hardware switch closes the valve first (the actual point varies ±450 ft per unit, i.e. 14,100–15,000 ft); if the hardware layer fails, the 15,000 ft software is the backstop. The crew need not close the valves manually — the hardware does it. The ±450 ft is a manufacturing tolerance, so one airframe may trip at 14,100, another at 15,000.

7. Automatic motors — DC brushless + electromechanical brake

The automatic motors 313HL3 (315HL3) and 313HL4 (315HL4) are DC brushless type. They are used in automatic operation. Each automatic motor has an electromechanical brake. — AMM 21-31-00 §6.C (2)

DC brushless = no brush wear (long life), high efficiency (~90 %+), electronic commutation (precise). The electromechanical brake locks the motor shaft (and, through the gearbox, the gate position) when de-energised — preventing the gate free-wheeling under airflow — and releases when powered.

8. Manual motors — DC brush + slow

The manual motors 313HL5 (315HL5) are DC brush type. They are used in manual operation. To allow an easy and smooth control of the cabin's vertical speed in manual mode, the outflow valves move at a slower speed than in automatic mode. — AMM 21-31-00 §6.C (3) + FCOM DSC-21-20-20

[!note]- Automatic = DC brushless (advanced); manual = DC brush (classic) — different philosophies

Why is the manual motor the "older" DC-brush type? Because the manual motor is the last-resort path (dual-CPC failure, RPCU), and there the priority is most stable + simplest: a DC-brush motor commutates mechanically — no complex electronic commutator — so it runs stably even when the RPCU supply is marginal. The automatic motor optimises performance (efficiency, life, precision) for full-time cruise use. Same philosophy as the controller's "digital automatic part + analogue manual backup": digital for performance, simple for fallback. And the manual motor is deliberately slower so the pilot can smoothly control cabin vertical speed.

9. Feedback modules — RVT + potentiometer (dual path)

The feedback modules 313HL6 (315HL6) are rotary variable transformers. They send position data to the cabin pressure controllers 311HL and 312HL through the outflow valve actuators 313HL1 (315HL1) and 313HL2 (315HL2). Potentiometers send position data to the manual backup circuit of the cabin pressure controller 311HL. — AMM 21-31-00 §6.C (4)

[!note]- Two feedback paths — the RVT to the main controllers, the potentiometer to controller #1's manual backup

This is the physical realisation of controller #1's backup section (ata-21-11 §8):

  gate position change → rotating shaft
       ├─→ RVT          → actuators → CPC #1 + #2 (digital main, ECAM in AUTO)
       └─→ potentiometer → CPC #1 manual-backup section (analogue, ECAM in MAN)

The RVT (rotary variable transformer) — a transformer coil + rotor; rotor position changes the output voltage/phase → digitised to a precise position; no contact, no wear, long life. The potentiometer — a resistance + sliding contact; simple, analogue, no dependence on electronics → compatible with the analogue manual-backup circuit. In manual mode the ECAM outflow-valve position comes from the potentiometer → CPC #1 backup → ECAM: slightly less precise than the RVT, but works on the simplest possible circuit.

10. Gearboxes (old vs new part number)

The gearboxes 313HL8 (315HL8) are gear trains. Each gearbox has an electromechanical brake. (old PN 7012-17971-4) Gearbox and Feedback Assemblies 313HL6 (315HL6). ... The gearboxes are gear trains. Each gearbox has an electromechanical brake. The feedback modules are rotary variable transformers ... (new PN 20792-01AA) — AMM 21-31-00 §6.C (6)+(7)

The gear train translates motor rotation into gate travel; the brake holds position when de-energised. Old PN: a standalone gearbox 313HL8. New PN: gearbox + feedback merged into 313HL6 (with the same gear train + brake + RVT + potentiometer).

11. 14,550 ft vs 15,000 ft — two independent protection layers

   cabin altitude rising
      ~14,100 ft
        │ (per-unit hardware switch trips, ±450 ft tolerance)
      ~14,550 ft ← valve HARDWARE pressure switch overrides → close
        │            independent of the controller
      15,000 ft  ← controller SOFTWARE auto-close (if hardware failed)
        │            needs CPC health + ARINC
      after 15,000 ft → the valves are closed (one layer has tripped)
        → cabin pressure retained → time for the occupants + crew

What a pilot must remember: between 14,000 and 15,000 ft the outflow valves will begin to close — regardless of controller state.

12. Outflow-valve behaviour across the modes

13. Fail-to-closed design

They are of failsafe design: If there is a failure in the valve drive, the differential pressure between the inside and outside of the aircraft in combination with the external airflow closes the valve. — AMM 21-31-00 §3.C

[!warning]- A motor failure does not leave the valve at a random position — the airflow pushes it toward closed

In cruise the cabin is 4–8 PSI above ambient. The gates are force-loaded on both faces: inside cabin pressure pushes the forward gate open; ambient pressure + the forward airflow push it back (and the aft gate inward). With the motor controlling, the opening is held. On a drive failure → the external airflow becomes dominant → the forward gate is pushed closed, the aft gate follows. The electromechanical brake locks at the de-energised position, but the airflow force exceeds it, pushing the valve near closed. So a failure leaves the valve near closed (fail-to-closed) — retaining cabin pressure and buying the crew time, not a random uncontrolled position.

14. ECAM PRESS field, "OFV not open", and the RPCU

FWD (AFT) OFV Not Open On Ground. If an outflow valve 313HL (315HL) is less than 100 degrees open for more than 70 seconds on the ground ... — AMM 21-31-00 §7.D (2)

The gate opening is measured in degrees: 0° = fully closed, 100° = fully open. On the ground the valves should reach 100°; if one is < 100° for > 70 s → CAB PR FWD (AFT) OFV NOT OPEN. Possible causes: a RAM AIR or DITCHING pb inadvertently pressed, an actuator/motor fault, a stuck brake/gearbox, a jammed gate, or the pressure switch tripping. Do not force a door open (residual pressure may remain) — let the RPCU or maintenance act.

The RPCU opens the outflow valves automatically after landing if: The outflow valve is not fully open; The CPCS is in manual mode or there is a failure of the two CPCs; The crew did not open the outflow valves in the manual mode. — AMM 21-31-00 §3.F

The RPCU drives the manual motors directly (28 V DC essential bus 301PP → relays 22/23HL → 313HL5/315HL5, bypassing the controllers and actuators) to release residual pressure on the ground — so a door does not open against a pressure difference.

Self-test

[!note]- Q1. Is "below the flotation line" a design error?

No — it is intentional. Every below-the-line opening (outflow valves, negative-relief valve, drain) is closed by the DITCHING pb before water contact; above-the-line parts (safety valves) need not close. The DITCHING pb is the pilot's one-press "close all below-the-line openings". This automatic above/below-the-line differentiation is the design logic.

[!note]- Q2. Two opposed gates forming a 2-D nozzle with thrust recovery — can a pilot feel it?

No — the magnitude is a few kilograms per valve. Why: a plain butterfly valve dissipates the outflow energy in turbulence; the convergent 2-D nozzle accelerates the flow → increases exit momentum → a small forward reaction on the valve. Over a long oceanic flight the cumulative fuel saving is real. The value is to see the outflow valve as a precision part, not a bleed flap.

[!note]- Q3. Automatic = DC brushless, manual = DC brush — why different?

Different philosophies. Automatic (DC brushless): high efficiency / long life / precise, for full-time cruise; needs an electronic commutator. Manual (DC brush): most stable / simplest, for the last-resort path (dual-CPC failure / RPCU); mechanical commutation runs stably even on a marginal supply. Same as "digital automatic part + analogue manual backup" — digital for performance, simple for fallback. The manual motor is also deliberately slower for smooth cabin-V/S control.

[!note]- Q4. Is the 14,550 ft pressure switch the same protection as the controller's 15,000 ft?

No — two independent chains. Hardware (14,550 ±450 ft): the in-actuator pressure switch overrides the controller and closes the valve — independent of CPC health. Software (15,000 ft): controller logic commands the motor closed — needs CPC health + ARINC. Typical sequence: hardware trips first (per-unit tolerance 14,100–15,000); if it fails, the 15,000 ft software is the backstop.

[!note]- Q5. What happens to the valve on a motor failure?

Fail-to-closed. The gates are force-loaded both faces; with the motor lost, the external airflow dominates and pushes the forward gate closed, the aft following. The brake locks at the de-energised position but the airflow force exceeds it, pushing the valve near closed. So a failure leaves the valve near closed (retaining cabin pressure), not at a random position.

Key takeaways

Common misconceptions

Scope — what this deep-dive covers and defers

References

A330 specifics per FCOM DSC-21-20-20 (the two valves below the flotation line, fwd/aft FINs, three motors each, the single-pack ΔP > 4 PSI aft-close, the RAM AIR 50 % open, the 15,000 ft software close, the fail-safe close on a drive failure) and AMM 21-31-00 §3.C/§6.C/§7.D (the lower-fuselage location, the two part numbers with their full component lists, the rectangular-frame two-gate body, the two-dimensional nozzle and thrust recovery, the in-actuator pressure switch overriding at 14,550 ±450 ft, the DC-brushless automatic motors with electromechanical brakes, the DC-brush slower manual motors, the RVT + potentiometer dual feedback, the gearboxes, the fail-to-closed physics, the 100° / 70 s "OFV not open" indication, and the RPCU driving the manual motors directly from essential bus 301PP). The thrust-recovery magnitude, the brushless-vs-brush philosophy, the two-independent-layer relationship between 14,550 ft and 15,000 ft, and the fail-to-closed force analysis are integrative syntheses. All engineering detail is from the A330 knowledge base; no cross-type comparison is made, and no fleet tail numbers appear.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.