Accessory Drive & Nacelle

Article 01 covered the rotating machine itself. This article covers everything that connects that machine to the aircraft — four distinct interfaces, each with its own hardware and its own failure logic:

1. the mechanical interface — two mounts that hang several tonnes of engine, and tens of thousands of pounds of thrust, on the pylon;
2. the power interface — the accessory gearbox that splits HP-shaft mechanical power among six accessories;
3. the aerodynamic clothing — the four-piece nacelle (air intake cowl, fan cowl doors, thrust reverser cowls, common nozzle);
4. the liquid outlets — the drains system that collects unburned fuel and seal leakage, recycles what it can, and preserves evidence of what leaked.

The AMM's definition of a powerplant introduces one concept worth keeping for line operations:

"A power plant is a Quick Engine Change (QEC) unit which is installed on the wing with all engine and aircraft systems connected and panels attached."

The QEC unit includes the air intake cowl, the CNA and the accessories — but not the fan cowl doors and not the thrust reverser cowls, which are hinged to the pylon and stay with the aircraft. So after an engine change, the cowl doors you walk under are usually the same pair as before: they belong to the airframe, not to the engine.

1. The accessory drive train — three components, one diagonal shaft

              core engine (6 o'clock, below the HP compressor case)
                   ┌──────────────────────────┐
                   │ EGD — External Gearbox    │   takes power off
                   │ Drive (bevel gear pair)   │   the HP rotor
                   └──────────┬───────────────┘
                             ╱
                HS GEARBOX DRIVE SHAFT
               (steel, splined both ends, shrouded —
                runs diagonally forward and down)
                          ╱
                   ┌─────┴─────────────────────┐
                   │ IDBG — input bevel gear    │ (on the EGM rear face)
                   └─────┬─────────────────────┘
   ┌─────────────────────┴────────────────────────────────┐
   │  HS EXTERNAL GEARBOX MODULE (EGM)                     │
   │  6 o'clock under the fan case, cast aluminium,        │
   │  hung on two clevis fittings                          │
   │                                                       │
   │  FRONT face: starter │ EEC alternator │               │
   │              hand-turning breather │ hyd pump No. 1   │
   │  REAR face:  IDG │ IDBG │ oil pump │ fuel pump │      │
   │              hyd pump No. 2                           │
   └───────────────────────────────────────────────────────┘

The drive shaft is diagonal by necessity: the take-off point (EGD) sits under the slim core engine, the gearbox sits under the much larger fan case, and a single shrouded high-speed shaft slides the power down and forward between the two. The AMM states the chain plainly:

"The Input Drive Bevel Gear (IDBG) installed on the rear face of the EG, transmits the mechanical power from the External Gearbox Drive (EGD) installed below the HP compressor case at 6 o'clock. A high-speed gearbox drive shaft is installed between the EGD and the IDBG to transmit the mechanical power."

"The accessory drives assembly has a High-Speed External Gearbox Module (HS EGM) installed below the rear case of the fan case at 6 o'clock. It has a gear train that decreases and increases the speed to meet the specified drive requirements of each accessory."

"Decreases and increases" deserves a pause. Inside one casting, every customer gets a bespoke gear ratio: the IDG wants a high drive speed, the fuel pump wants speed proportional to N3, and the starter works the train backwards — feeding power into the gears to crank the HP spool. Each accessory mounts on its own circular pad with a quick-attach/detach (QAD) ring, so any single accessory can be changed without disturbing the gearbox.

The full face-by-face allocation, per the AMM:

"The EGM housing front face has installation faces for the accessories and equipment that follow: starter / EEC dedicated alternator / hand turning breather / hydraulic pump No. 1. The EGM housing rear face … has installation faces for the equipment that follows: IDG (Integrated Drive Generator) / input drive bevel gears / oil pump / fuel pump / hydraulic pump No. 2."

This matches the FCOM's six-item drive list from article 01 exactly — the AMM simply adds the spatial information of which face carries what.

One sealing detail completes the picture: each gearshaft bearing chamber is sealed not by rubber but by a labyrinth air seal — sealing fins on the static housing run at minimum clearance against a rotating bush, and IP-compressor air blows continuously from the static side into the chamber, holding the oil in with air pressure. No contact, no wear: the standard sealing philosophy for high-speed rotating machinery, and one you will meet again in the internal air system (article 03).

2. The nacelle and the two mounts

"A nacelle gives the engine an aerodynamic shape. The nacelle is an assembly of these items: the air intake cowl / the fan cowl doors / the thrust reverser / the CNA."

   air intake cowl    fan cowl doors     thrust reverser cowls      CNA
 ┌───────────────┐ ┌───────────────┐ ┌─────────────────────┐ ┌────────────┐
 │ on the ENGINE  │ │ on the PYLON  │ │ on the PYLON        │ │ on the     │
 │ drooped 6 deg  │ │ wrap LP case  │ │ wrap the core       │ │ ENGINE     │
 └───────────────┘ └───────────────┘ └─────────────────────┘ └────────────┘
                  ▲ forward mount                  ▲ aft mount
                  (top of intermediate case)       (top of exhaust case)
                  carries: THRUST + vertical       carries: TORQUE + vertical
                           + side loads                     + side loads
                  does NOT carry torque            (the torque the fwd mount won't take)

Both mounts sit along the top of the core engine, and their division of labour is elegantly complementary:

"The forward mount transmits engine thrust, vertical and side loads to the airframe structure. The aft mount transmits vertical, side and torsional loads to the airframe structure." — "The engine torque loads are not transmitted through the engine forward mount to the aircraft pylon."

Thrust goes only through the forward mount; torque only through the aft mount. The synthesis behind that split: a statically indeterminate structure would let load shares drift with temperature and wear, whereas a clean division gives each path a definite, checkable design load. Both mounts use spherical bearings to absorb thermal growth and relative motion, and both are fail-safe:

"The vertical load fail-safe link will engage with the rear horizontal trunnion (on the outer support) if there is a failure of a primary component in the vertical load routing." — "The failsafe link pin is installed in a clearance hole. Thus, the pin will only transmit loads if a failure occurs."

"A pin installed in a clearance hole" is the essence of fail-safe design: the backup carries zero load in normal service — so it never accumulates fatigue — and goes to work only when the primary breaks. The aft mount layers this more than once (fail-safe link → pin; link lugs → torque-stop pads), and the forward mount's outer support is built in two halves so thrust and side loads are born dual-path from the start. The same "clearance-gap standby" philosophy reappears in the thrust reverser's tertiary lock (article 13).

3. The air intake cowl — a 6-degree droop and three tenants

"The cowl structure has an almost cylindrical shape with its center at the cowl longitudinal axis. The cowl longitudinal axis is pitched down at an angle of 6 degrees from the engine longitudinal centerline."

Why the droop? In cruise the wing flies at an angle of attack, the pylon-hung engine pitches up with it, and the free stream actually arrives slanting downward relative to the engine axis. Drooping the intake lip 6° means that in cruise the inlet faces the oncoming flow squarely — minimum spillage drag, cleanest fan inflow. On the ground the cowl looks like it is hanging its head; at altitude it is exactly face-on to the wind.

Structurally the cowl is four pieces — an aluminium C-section lip with the anti-ice distribution ring inside it (HP3 hot air arrives through a ball-jointed duct — ATA-30), a load-carrying inner barrel with three composite acoustic panels, a two-half composite outer barrel, and a rear bulkhead with a second identity worth quoting:

"The rear bulkhead is a fire seal between the cowl and the engine Zone 1."

Three "live" tenants occupy the cowl:

1. The P20/T20 probe — intake pressure and temperature for the EEC, foundation data for thrust computation:

   "A P20T20 probe gives the engine air-intake air-flow conditions to the EEC. The probe is installed in the cowl at top right of the engine vertical centerline."

   The access panel on the right side of the nacelle (411AR) exists precisely for this probe.

2. The Zone 1 cooling air inlet at the top of the cowl — ram air in, zone ventilation out (article 03).

3. The electronics-box cooling loop — air taken from the intake duct flows through the protective box that houses the EEC (and its companions) and returns to the duct. The FADEC's "air conditioning" is a borrowed breath of intake air; when that breath weakens, the FADEC HI TEMP / OVHT story of article 20 begins.

4. Fan cowl doors — the walkaround map

Each composite door hangs on four hinges (the rear three rigid to the pylon, the foremost on a floating beam) and closes with four latches along the bottom; struts hold it open at 44°/55° for maintenance. What matters to a pilot is the map of small doors and ports — every one you pass on the walkaround is named in the AMM:

"You can find these access doors on the left side of the nacelle: a starter control valve and thrust reverser ground safety button access door, 415BL (which is also a pressure relief door) / an IDG (Integrated Drive Generator) oil fill, sight glass and reset lever access door, 415CL (425CL) …" — "These doors and panels are on the right side: a P20/T20 probe access panel, 411AR / an oil fill and sight glass access door, 416CR / a hydraulic filter contamination indicator and master Magnetic Chip Detector (MCD) access door, 416BR …"

A workable memory handle (synthesis): left doors serve "air and electricity" (starter air, reverser safety, IDG); right doors serve "oil and evidence" (oil fill, MCD, P20/T20 data). And the dual identity of 415BL is explained in the AMM itself:

"One of these also provides a blow off door that will open to prevent over pressurisation in the event of a pneumatic duct failure."

If a bleed duct bursts inside the cowl, high-pressure hot air would inflate the doors like a balloon — the blow-off door is the relief valve for exactly that breath. A door standing slightly proud on the walkaround is a sign that something has already happened inside.

5. The drains system — the engine's evidence locker

The drains system has three official jobs:

"To collect fuel which has not burned because of an engine shutdown or failure to start / To remove and discard fuel and/or oil if a leak occurs across an internal seal in specified primary components (thus the system can also be used to monitor the condition of these seals) / To remove unwanted liquids which can collect in the pylon, cowls and fairings."

The parenthesis in the second item is the soul of this section: the drains are not just plumbing — they are a diagnostic instrument. The mechanism lives at the drains mast, the lowest point of the installation:

"There are six outlets at the drains mast. … The related components for each outlet are identified on the side of the drains mast. A small sump is installed in many of the component drain tubes. The sump will hold some of the fluid, if a leak occurs from a component. This held fluid can then be used to identify the defective component."

"One of the outlets is a wet drain. If one of the dry drain outlets become wet, then this is a sign that there is a defective component."

Six outlets, five dry and one legitimately wet — and each outlet is labelled with the components it serves. The components plumbed into the system (i.e. whose seal failures will show themselves here) include the AOHE, both hydraulic pumps, the LP/HP fuel pumps, the FMU, the starter, the IDG, the VSV and VIGV actuators, the collector tank and the oil tank. Each accessory's mounting interface is even built with a deliberately dry cavity between two shaft seals — so the accessory's fluid and the gearbox's oil can never meet, and a leak from either is separately attributable.

The collector tank closes the loop on job one: after a shutdown or failed start, manifold fuel drains into the drain collector tank (front face of the EGM, beside the starter). On the next start, LP fuel flowing through an ejector creates suction that draws the tank's contents back into the fuel system to be burned — nothing wasted, nothing dumped on the ramp; only an overfull tank spills overboard through the mast. And one "legal wetness" must be recognised:

"One outlet is related to the oil tank scupper/fuel drains tank overflow. This outlet can become wet during usual conditions. This outlet will release oil if the oil tank is filled incorrectly."

So a dripping mast on the walkaround is not, by itself, a finding. First identify which outlet is wet: the scupper/overflow outlet may simply reflect a recent oil uplift; a dry outlet labelled FMU or fuel pump turning wet is the genuine red flag, and the sump above it holds the sample that tells maintenance which seal let go.

6. Fire zones — the walls that define the words

"The fire seal system prevents the movement of flames through the engine if there is an external engine fire. The fire seal system also divides the engine into accessory cooling zones."

Fire protection and ventilation share one set of walls: the zone boundaries are simultaneously fire barriers and duct walls for cooling flow (article 03). The boundary hardware: two annular fire bulkheads (the intake cowl's rear bulkhead in carbon composite at the cool end; the CNA front bulkhead in titanium, because it lives against the exhaust), the carbon-fibre electronics box (doubling as the fire enclosure for the EEC and friends), P-section fire seals along the reverser cowl upper edges, insulation blankets on the reverser fixed structure, and the pylon firewall. The fire-detection loops are routed within these zones — where the walls are determines what an ENG FIRE warning literally means (article 17).

7. Electrical harnesses — channel separation down to the wire

Besides forces (mounts) and fluids (ducts), the engine exchanges signals with the aircraft. The AMM's harness overview:

"There are 27 external harnesses on the Trent engine (this does not include the IDG cables). … Most of the harnesses … are related to the FADEC system. Thus their routing … connects them with one or more of the three primary FADEC units that follow: the EEC, the PCU, the OPU. The core engine FADEC harnesses connect with the related LP compressor case harnesses at the bifurcation panel. The FADEC system harnesses are divided into Channel A and Channel B harnesses."

Three interface facts worth retaining. First, FADEC channel separation extends to the harness level: channels A and B are not merely two circuit sets inside the EEC (article 04) — the entire physical path from sensor to computer runs in two independent harness sets, so mechanical damage to one loom cannot take out both channels. Second, looms approach their connectors through a drip loop — routed down toward the engine's low point and back up — so moisture and fluids fall away at the loop instead of tracking along the cable into the connector. Third, every electrical interface with the aircraft passes through the pylon: forces, fuel, air and wiring all share that one corridor. (Thrust-reverser harnesses are documented separately with the reverser itself — article 13.)

8. One apparent contradiction worth resolving

Two AMM statements describe the rotation direction differently — one section says counterclockwise seen from the rear, another says clockwise seen from the front. These are the same physical direction expressed from two vantage points, not a conflict. It makes a tidy teaching example: when two manual statements appear to disagree, check the observation reference before concluding anything.

Self-test

[!note]- Q1. Trace the power path from the HP spool to the IDG. HP drum front stubshaft → internal gearbox (inside the intermediate-case vanes) → EGD bevel set at 6 o'clock under the HP compressor case → diagonal high-speed drive shaft → IDBG on the EGM rear face → gear train → IDG pad (rear face). During start, the same path runs in reverse from the starter (front face) to crank N3.

[!note]- Q2. Which loads does each engine mount carry — and which load does the forward mount deliberately not carry? Forward mount: thrust + vertical + side. Aft mount: torque + vertical + side. Torque does not pass through the forward mount — the clean split keeps every load path determinate and inspectable. Both mounts carry fail-safe elements that sit in clearance (zero load, zero fatigue) until a primary member fails.

[!note]- Q3. Why is the intake cowl axis drooped 6° relative to the engine axis? Because in cruise the engine rides nose-up with the wing's angle of attack. The droop pre-aligns the inlet with the actual oncoming flow, so the lip meets the free stream squarely — least spillage drag, cleanest fan inflow.

[!note]- Q4. On the walkaround you find a wet outlet on the drains mast. What decides whether this is a finding? Which outlet it is. One outlet (oil-tank scupper / collector overflow) is legitimately wet — recent oil servicing can explain it. A dry, component-labelled outlet (e.g. FMU, fuel pump) turning wet indicates a failed internal seal; the sump in that drain line holds the fluid sample that identifies the component.

[!note]- Q5. What happens to the fuel that drains from the manifold after a failed start? It collects in the drain collector tank on the EGM front face. On the next start, LP fuel through an ejector draws it back into the fuel system to be burned. Only an overfull tank overboards through the mast — which is also why repeated failed starts can legitimately wet the overflow outlet.

Key takeaways

References

• AMM 71-00 (power plant general, D/O) — QEC definition, nacelle four-piece list, mount load split, access door map.
• AMM 71-20 (mounts, D/O) — forward/aft mount load paths, fail-safe link and clearance-hole pin design.
• AMM 71-61 (air intake cowl, D/O) — 6° droop, structure, P20/T20 location, electronics-box cooling loop, rear bulkhead fire seal.
• AMM 71-13 (fan cowl doors, D/O) — hinges, latches, hold-open struts, blow-off door.
• AMM 71-71 (drains, D/O) — three functions, six outlets, sump evidence mechanism, ejector recovery, legal-wet outlet.
• AMM 71-30 (fire seals, D/O) — zone division, bulkhead materials, seal hardware.
• AMM 71-50 (electrical harness, D/O) — 27 looms, channel split, bifurcation panel, drip loops, pylon interface.
• AMM 72-60 / 72-62 / 72-33 (accessory drives, D/O) — EGD/shaft/EGM chain, face allocations, labyrinth air seals.
• FCOM DSC-70 — accessory gearbox drive list (cross-referenced from article 01).
• Integrative synthesis (marked in text): the determinate-load rationale for the mount split; the cruise-alignment reading of the 6° droop; the left/right walkaround memory handle.

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.