Tank Venting

Fuel tanks are not sealed drums — they must breathe. The venting system's whole job is one sentence in the AMM:

"The tank venting system keeps the air pressure in the fuel tanks near to the external air pressure. This function prevents a large difference between these pressures which could cause damage to the fuel tank/aircraft structure. This function is specially necessary: ‐ during the refuel or defuel operations ‐ when the aircraft climbs or descends. The operation of the tank venting system is fully automatic. There are no manual controls."

Refuelling pushes air out as fuel comes in; defuelling pulls air in; climb thins the outside air, descent thickens it. The tank wall is wing skin (tanks and storage) and will not tolerate a large differential. So every tank gets an airway to ambient — and the airway gets a set of guards: against flame travelling up it, against overpressure, against fuel escaping through it, and against fuel pooling inside it.

1. The airway — surge tank, stack pipe, NACA duct

     ambient air
       ↕  NACA duct (intake in the lower wing / THS surface)
   ┌───┴───────────────────┐
   │  stack pipe            │  ← flame arrestor lives here:
   │  (with flame arrestor) │    blocks flame, passes air & fuel freely
   └───┬───────────────────┘
       ↕
 ┌────────── SURGE TANK (vent buffer, wing tip / RH THS) ─────────┐
 │ · two-way airflow + temporary holding for fuel out of the vents │
 │ · capacity ≥ 5 × the vent-pipe volume before any spill          │
 │ · overpressure protector (burst disk)                           │
 │ · 19.01 mm drain pipe — gravity-returns pooled fuel inboard     │
 └───┬────────────────────────────────────────────────────────────┘
       ↕ vent pipes (inner 88.9 / outer 50.8 / centre 88.9 / trim 38.1 mm)
 ┌───┴──────────────────────────────────────────┐
 │  each tank:                                   │
 │   · vent (float) valve — fuel rises → closes  │
 │   · breather — one-way slit, drains the vent  │
 │     pipe back into the tank, never the reverse│
 └───────────────────────────────────────────────┘

"The surge tank is open to the air through a stack pipe which is connected to a NACA duct... The surge tank lets air flow through it in each direction. It is also a temporary reservoir for fuel that can come into it from the vent pipes. The stack pipe gives the surge tank a capacity (before fuel can flow overboard) that is approximately five times larger than the fuel capacity of the vent pipes."

The flame arrestor blocks exactly one thing — flame:

"If a ground fire occurs, it prevents the ignition of the fuel vapor in the surge tank... It also lets air flow freely through it in two directions, and prevents ice. If a failure occurs in the fuel system (which causes large quantities of fuel to enter the surge tank) then the vent protector lets the fuel flow freely overboard."

Inside it is a stack of metal-mesh cartridges: a flame front passing through is quenched by heat absorption, while air and fuel flow on unimpeded.

2. Burst disks — and the 145 psi asymmetry

The surge-tank overpressure protectors are two-directional burst disks — they protect against both bursting (internal overpressure) and crushing (internal underpressure):

"The burst disk has a white cross painted on it. When you can see the cross from the ground, the burst disk is serviceable."

On six-tank aircraft there is one more disk, between centre tank and inner tank — with an extreme asymmetry:

"If a center tank-to-inner tank overpressure occurs, the burst disk breaks and lets fuel flow from the center tank to the inner tank... If an inner tank-to-center tank overpressure occurs... the shear rivets break, the fuse moves in and the burst disk breaks."

[!warning]- Why 5–7 psi one way and 145 psi the other The centre tank has no independent airway — it breathes through the inner tank's vent path. A small centre overpressure must relieve early into the inner tank to protect the centre structure (low threshold). The reverse direction is set twenty times higher so that inner-tank fuel cannot push into the centre tank under any normal pressure — only an extreme inner overpressure (shear rivets first) opens that door. The asymmetry is deliberate design, not a typo. (Rationale is integrative synthesis; the values are AMM verbatim.)

3. Keeping fuel out of the airway — and getting it back

Two guards work as a pair:

"The float assembly usually hangs down which keeps the poppet valve open. When the fuel lifts the primary floats, the float arm moves up and closes the valve. The secondary float prevents incorrect operation of the vent valve when the fuel level decreases to below the shut-off level."

"When fuel comes into the breather (from the vent pipe) it is held in the breather until the pressure is sufficient to open the slit. The fuel then flows into the fuel tank. The usually closed slit prevents a flow of fuel back through the breather into the vent pipe."

The vent valve guards the entrance (float closes it as the level rises); the breather drains whatever fuel did get into the pipe back to the tank through a one-way slit; and the surge tank's gravity drain pipe returns its pooled fuel inboard. Three layers: don't let fuel in, return what gets in, never let it back out the wrong way.

The trim tank, living in the THS through large attitude changes, uses multiple vent valves assigned to different attitudes:

"The vent valve 5208QS... makes sure the RH part of the trim tank is open to the surge tank in level flight... The vent valve 5209QS1(5209QS2)... make sure the trim tank is open to the surge tank when the aircraft climbs."

4. Two special cases

Centre-box venting (five-tank aircraft only). Where the six-tank aircraft has a fuel tank, the five-tank aircraft has a dry bay — which must not accumulate vapour:

"The center box venting and drainage system prevents the collection of fuel vapor in the center box area if minor fuel leaks occur... The air supply pipe... supplies pressurized air from the air conditioning system to the center box. The check valve... does not let fuel vapor go into the air conditioning system... a leak monitor 5400QS... Maintenance personnel use the leak monitor to do a check that there is no fuel leakage."

Same structural bay, opposite logic by configuration: tank (vent it) versus void (purge it with conditioned air).

Trim-pipe shroud (all aircraft). The long fuel pipe running aft to the THS is double-skinned:

"The trim fuel pipe has an outer shroud. If the trim pipe has a fuel leak, the shroud contains the fuel leakage. The shroud has a fluid drain at its lowest point (FR 47)... connects the shroud to a drain mast."

A leak in the trim line shows itself at the drain mast on the lower fuselage — a ground-check point.

5. Operations, alerts, dispatch

The crew never operates the venting system, but its failure signature is visible: fuel climbing into a surge tank wets the overflow sensor and raises FUEL WING TK OVERFLOW in flight (handled by stopping transfers, not by touching the vents — see quantity and level faults); on the ground the same sensors drive the refuel panel's amber OVERFLOW lights.

Dispatch shows a three-tier pattern in some operators' MEL:

• Wing surge-tank overpressure protectors — one or both may be damaged or missing (the normal airway is the primary path; the disk is a last line).
• Trim surge-tank protector — may be damaged or missing, same logic.
• Centre-tank protector — a tier stricter, and the price is striking: fuel loading per a maintenance procedure, total fuel on board capped at 70 000 kg, both centre pumps operative, centre/inner FQI operative, crossfeed indication checked. Every condition serves one idea: if the centre tank can no longer relieve itself, you must be able to see its fuel, move it out fast, and not fill it up in the first place.

Position decides risk: the disk buried in the centre wing box carries the heaviest structural consequence — the same logic as the 145 psi asymmetry.

Self-test

[!note]- Q1. When does a fuel tank most need to breathe? During refuel/defuel (liquid displacing air and vice versa) and during climb/descent (ambient pressure changing). The system is fully automatic with no manual controls.

[!note]- Q2. What three things does a flame arrestor do — and what does it never block? It quenches a flame front (metal mesh heat absorption), passes air freely both ways with ice protection, and lets large fuel quantities flow overboard unimpeded. It never blocks air or fuel — only flame.

[!note]- Q3. Why is the centre-to-inner burst disk rated 5–7 psi but the reverse 145 psi? The centre tank breathes through the inner tank's airway, so small centre overpressures must relieve early; the reverse path is held shut (shear rivets first) so inner fuel cannot push into the centre under normal pressures.

[!note]- Q4. How does fuel that entered a vent pipe get home? The breather holds it until pressure opens its one-way slit back into the tank; the surge tank's drain pipe gravity-returns pooled fuel; the slit and check valves prevent reverse flow.

[!note]- Q5. What does a serviceable burst disk look like from the ground? A visible white cross painted on the disk — see the cross, the disk is intact.

Key takeaways

References

• AMM 28-12-00 Description and Operation (airway, flame arrestor, burst disk tables, vent valves, breathers, trim venting).
• AMM 28-13-00 (centre-box venting, five-tank aircraft); AMM 28-14-00 (trim-pipe shroud drainage).
• Some operators' MEL, vent-system items (three-tier dispatch pattern).
• The 145 psi rationale and the three-layer guard framing are integrative synthesis from the cited sources.

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.