Thrust Reverser

"The aircraft performs engine reverse thrust via four pivoting blocker doors on each engine to create a deflected fan airstream. A hydraulic door jack positions each door. ‐ The blue circuit powers the doors on engine 1 ‐ The yellow circuit powers the doors on engine 2."

Only the fan (cold) stream is reversed — article 01 established that most of the thrust lives in the bypass anyway (bypass ratio 4.66), so deflecting the cold flow forward reverses most of the thrust while the core stream keeps flowing aft. And note the hydraulic supply: the doors run on aircraft hydraulics — blue for engine 1, yellow for engine 2 — not on engine-internal fuel-hydraulics.

The engineering problem of a reverser has never been "how to reverse" but "how to guarantee it can never open itself in flight" — which is where the three lock levels, the four-condition interlock and the two independent command chains come from. Half this article is about opening; half is about staying shut.

1. Two command chains that refuse to trust each other

 thrust-lever sensors ── TLA (ch A) ──► FADEC 1(2) Channel A ┐
      │                    ┌──────────────┐                  │
      │      N3 ≥ 50 % ────┤     AND      │                  │ (channel B identical)
      │   LGCIU¹ 1(2)─EIVMU┤              │──────────────────┴──► valve control
      │                    └──────────────┘                          │
      │                                                              ▼
      │            blue (ENG1) / yellow (ENG2) hydraulics ──► ISOLATION VALVE
      │                                                              │ ⓟ "reverser pressurised"
      │                       INHIB relay switch ──► DIRECTIONAL CONTROL VALVE   → FADEC
      │                                                              │
      │                                            4 door hydraulic locks + actuators → 4 doors
      │
      └── TLA ──► PRIM 1 ── or ──► TERTIARY LOCKS ×4   (never passes through the FADEC)
               └► PRIM 3 ──┘          (PCM, 104 V DC)
 ¹ LGCIU 1 has priority when valid

Three reading points. First, the two command chains run in parallel: the FADEC chain (TLA + N3 ≥ 50 % + LGCIU-on-ground, ANDed, controlling the two hydraulic valves) governs whether hydraulic power may arrive; the PRIM chain (PRIM 1 or 3 reading the lever sensors directly, commanding the tertiary locks) governs whether the door pins may release — and it never touches the FADEC. No single computer's misbehaviour can assemble the full set of opening conditions. Second, a pressure switch downstream of the isolation valve reports "reverser system pressurised" back to the FADEC — the sensing source of the REV PRESSURIZED alert (article 32). Third, the INHIB relay switch sits in the directional-valve command path — the EIVMU's 28 V DC permission (relay R4, article 06) acting as a third, electrical "soft lock".

The component roster (FCOM, in full)

"The thrust reverser system for each engine includes all of the following: ‐ Four pivoting blocker doors each activated by an hydraulic actuator ‐ A hydraulic isolation valve controlled by the FADEC ‐ A directional control valve, which directs hydraulic pressure to 4 hydraulic locks and to the deploy or stow line ‐ Four independent electrical locks controlled by the FADEC ‐ Four stow proximity switches and four lock proximity switches … ‐ Four tertiary locks (one for each of the pivoting doors) that enable the reverse to be maintained in the locked position via a door pin. During engine reverse thrust, a solenoid enables the door pin to be released."

The three lock levels, sorted:

For the record, the reverser cowl doors themselves (the pylon-mounted pair from article 02): five hinges per door — the front two on a floating hinge beam tying left and right doors together, the rear three on the pylon, the last of fail-safe construction — seven latches along the bottom, and a V-blade/V-groove engagement with the fan case rear edge.

2. The four-condition interlock

"Deployment requires: ‐ TLA reverse signals from FADEC and from the flight control primary computer (PRIM 1 or PRIM 3) ‐ One FADEC channel that operates with its associated throttle reverse signal ‐ Aircraft on ground signal from at least one LGCIU."

Add the N3 ≥ 50 % input visible in the schematic's AND gate (the engine must genuinely be running — no deploying hydraulic doors on a parked aircraft) and the interlock reads: you pulled the levers (FADEC sees it) × you really pulled the levers (PRIM sees it independently) × the aircraft is on the ground (LGCIU) × the engine is running (N3). The design philosophy (synthesis): every condition is supplied by a different sensing or computing chain — FADEC and PRIM corroborate without trusting, and no single-point failure opens the door. The in-flight prohibition on reverse (article 00) is enforced in hardware by the LGCIU vote.

The wiring and hydraulic schematics (read directly) add precision the text alone lacks. The throttle control unit's reverse-lever criterion is annotated at RLA > 34.5° — only past that angle does the "crew genuinely selected reverse" condition on the R4 relay line come true (the reverse-side graduation of article 08's angle ledger). The tertiary-lock command end is labelled FCPC 1(2,3) — the flight-control primary computers under their formal name, confirming "never through the FADEC" — plus a ground safety button vote that lets maintenance physically break the deploy chain during work. The hydraulic sheet names every executing part: four actuators, four primary locks, four RVT position transducers, four stow proximity switches, the directional control unit — and two items of operational interest: the isolation valve carries a MANUAL INHIBIT position (the physical embodiment of a reverser locked out per the MEL — the REV INHIBITED state of article 32), and the isolation pressure switch that feeds REV PRESSURIZED sits immediately downstream of it.

3. The hydraulic deployment sequence: load first, unlock second, in order

"High pressure fluid … passes through the T/R ICU, when the Thrust Reverser Isolation Solenoid Valve is energized via the EEC, and pressurizes the rod end of the pivoting door actuators loading the door in an overstow direction. … The Thrust Reverser Direction Control Solenoid Valve (T/R DCSV) is energized … allowing high pressure fluid to pass to the primary locks. This opens each primary lock in turn and then move the Thrust Reverser Direction Control Unit (T/R DCU) to the deploy position. This is to allow high pressure fluid into the head end of the actuators. The hydraulic pressure releases the secondary locks in each actuator. Then, provided the tertiary locks have released, and due to differential area advantage, the actuators extend to deploy the doors."

Note the counter-intuitive first step: the first hydraulic action of a deployment pushes the doors toward closed (the overstow preload). A loaded lock pin cannot be pulled; pressing the doors hard shut unloads the locks so they can open (synthesis). Then the primary locks open one by one, the directional unit shifts to deploy, head-end pressure releases the secondary locks, and — provided the PRIM chain has already released the tertiary locks — the actuators extend on their head/rod area difference. Stowing runs the sequence in reverse, and carries the rule that defines the whole system's temperament:

"The lock system does not require electrical power to allow the door to restow."

Opening needs everyone's signature; closing is never blocked — the safe direction always has the lower threshold. One obscure insurance deserves its line: between the directional unit and the first lock sits a restrictor bypass (3.5–612 cc/min) to the case-drain line, so that even a fully blocked return line (maintenance error included) cannot trap pressure in the lock circuit.

4. The tertiary lock up close: a latch that carries no load

Everything about the tertiary lock is in the word independent: command-independent (PRIM 1/3, never the FADEC), power-independent (a dedicated power control module converting 115 V AC to 104 V DC), and mechanically independent —

"The system comprises one lock per door and is not loaded in normal operation." — "When the system is stowed and locked: the hook covers but does not touch the door pin."

Zero load, zero contact in normal service — it accumulates no fatigue, and goes to work only if primary and secondary have both failed and the door pin actually comes calling (the same clearance-standby philosophy as the engine-mount fail-safe pins of article 02). The release chain: PCM energises the solenoid → the shoot bolt (a safety pin) withdraws → a spring rotates the hook clear → the door pin may pass. On restow the pin pushes the hook back itself, the PCM de-energises, the shoot bolt springs home. Maintenance can unlock by hand with an unlock shaft for cowl work. The EEC, throughout, only monitors lock position and reports — execution authority and monitoring authority ride two separate chains.

5. Thrust's two gates: 70 % and 90 %

"During deployment, engine reverse thrust is limited to idle, until the reversers are deployed by more than 70 %. Then, FADEC will command full reverse thrust to be available when reversers position are more than 90 % deployed."

Power against a half-open door means flow loads on structure and a surge risk — so thrust waits for the doors. This closes a loop with two earlier articles: reverse idle's "start low, rise later" behaviour (article 05 — deliberately low while the doors travel, rising to approach-idle level if max reverse is not selected within about 5 seconds), and the lever story of article 08: how fast you pull the reverse levers does not set the thrust — door position (measured by the RVTs) is the key that releases it.

6. AUTO RESTOW: the 5 % line, and idle for the rest of the flight

"The FADEC will automatically command the engine to idle and the reverser to stow if at least one door is unstowed by more than 5 % and reverse thrust is not selected while engine is running. The affected reverser will remain pressurized after affected door is locked back. If the door is still detected unstowed, the engine will remain at idle for the remainder of the flight."

Read it as three escalating moves (synthesis). Cut the thrust — reverse moment leaking through a door gap at high power is a lethal yaw source, so power goes first. Press the door home and keep pressing — "remains pressurised after locked back" means hydraulics become a standing fourth lock. And if it will not go home, concede: that engine spends the remainder of the flight at idle. The 5 % figure is the door-gap tripwire, adjudicated by the proximity switches and RVTs together. This automatic script is the system-side foundation under the REV UNLOCKED procedure of article 32.

The indication chain, briefly: four stow and four lock proximity switches watch the two discrete states; four RVTs (rotary variable transformers) measure door position continuously — the data source for the 70/90 % gates; the E/WD's green/amber REV legend timing belongs to article 15; and the ground safety button (one per engine, behind door 415BL) lets maintenance isolate the hydraulics — worth a glance on the walkaround after any reverser work.

7. Where the reverser meets the failure chapters

Self-test

[!note]- Q1. Name the four interlock conditions — and which chain never passes through the FADEC? FADEC's TLA reverse signal × PRIM 1/3's independent TLA signal × on-ground from at least one LGCIU × N3 ≥ 50 %. The PRIM → tertiary-lock chain (via the 104 V DC PCM) never touches the FADEC — the schematic labels it FCPC 1(2,3), with the lever criterion at RLA > 34.5°.

[!note]- Q2. What is the first hydraulic action of a deployment, and why? Rod-end pressure loading the doors toward overstow — pressing them harder shut. A loaded lock pin cannot be withdrawn; unloading the locks first is what lets the primary locks open one by one.

[!note]- Q3. What does "not loaded, not touching" mean for the tertiary lock? In normal service the hook covers the door pin without contact — zero load, zero fatigue. It is pure standby, engaging only if primary and secondary have both failed. And neither stowing nor locking requires electrical power.

[!note]- Q4. Why does reverse thrust arrive in two steps after you pull the levers? Thrust is held at idle until the doors pass 70 % deployed, and full reverse is released only beyond 90 % — door position (RVT-measured) is the key, not lever position or speed.

[!note]- Q5. In cruise the ECAM reports a door unstowed 6 %. What has the system already done? Commanded that engine to idle, commanded the reverser to stow, and — once the door locks back — kept the system pressurised as a standing fourth lock. If the door still reads unstowed, the engine stays at idle for the remainder of the flight.

Key takeaways

References

• FCOM DSC-70 (thrust reverser) — door arrangement and hydraulic allocation, full component roster, the three deployment requirements, the 70/90 % thrust gates, the AUTO RESTOW paragraph, LGCIU priority.
• AMM 78-30 (reverser general, D/O) — three-lock overview, cowl hinges/latches/V-groove, the no-power restow rule.
• AMM 78-31 (hydraulic control, D/O) — the deployment sequence verbatim, restrictor bypass figures, RVTs, ground safety button.
• AMM 78-37 (tertiary lock, D/O) — PCM and 104 V DC, hook-clear-of-pin geometry, shoot-bolt release chain, manual unlock shaft.
• FCOM reverser schematic + ASM control/hydraulic schematics (read directly) — the two parallel command chains, the AND gate with N3 ≥ 50 %, the INHIB relay, the pressurised-system switch, RLA 34.5°, FCPC labelling, MANUAL INHIBIT position.
• Integrative synthesis (marked in text): the four-votes-four-sources philosophy; the loaded-pin reasoning; the clearance-standby echo of article 02; the door-position-as-key formulation; the three-escalation reading of AUTO RESTOW.

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.