The fuselage as a pressure vessel
A transport fuselage (ATA 53) is built in sections — nose, forward, centre (containing or sitting on the centre wing box and the main wheel wells), aft and tail cone — joined by circumferential butt joints with splice straps, while skin panels are joined lengthwise by lap or butt splices. Inside the sections, frames, stringers, longerons and floor beams form the semi-monocoque structure described in Chapter 3.
The pressure vessel is the part of the fuselage that holds cabin pressure. It is closed by:
- the forward pressure bulkhead behind the radome (which carries the weather-radar antenna outside the pressure vessel);
- the aft pressure bulkhead, usually a dome (a spherical shape carries pressure as membrane tension, so it can be light), with the unpressurised tail cone and APU compartment behind it;
- pressure decks and walls around the nose and main wheel wells, which are unpressurised cut-outs in the lower lobe.
Pressurisation sealing
Every joint and penetration of the pressure boundary is sealed:
- Faying-surface (interfay) sealant between mating parts, fillet seals along joint edges, and fasteners installed wet with sealant.
- Pressure seals and grommets where wiring, pipes, ducts and control cables pass through bulkheads — control-cable seals let the cable move while stopping the air.
- Door and hatch seals, usually hollow rubber profiles that cabin pressure presses harder against the frame.
- Drain valves that close under pressure (Chapter 3) and outflow valves that control the leak deliberately (Chapter 6).
Leaks are found with a fuselage pressure (leak-rate) test: the aircraft is pressurised from a ground source and the rate of pressure decay is compared with the AMM limit, with leaks located by sound, ultrasonic detectors or leak-check fluid.
In 1988 a B737 (Aloha Airlines flight 243) lost a large section of cabin roof after many small fatigue cracks along a lap joint linked up — multiple-site damage made worse by disbonding and corrosion in the joint. It led to the aging-aircraft programmes, corrosion prevention and control programmes and the widespread-fatigue-damage rules that drive many heavy-check tasks today.
Attachments: wings, stabilisers, pylons and landing gear
- Wings attach to the centre wing box, which passes through or under the fuselage and is joined to heavy fuselage frames; the wing skins and spars are spliced to it with large numbers of high-strength bolts.
- Horizontal stabiliser — on jets with a trimmable stabiliser, the stabiliser centre box pivots on hinge fittings at its rear spar, and its front spar is moved by the trim screwjack attached to a reinforced frame. The fin is bolted to fuselage frames through root fittings.
- Pylons carry the engines from wing fittings (often with fuse pins, Chapter 3); rear-mounted engines hang from stub pylons on reinforced fuselage frames.
- Landing gear — main gears are hinged on trunnion fittings at the wing rear spar and a gear beam or support beam; side and drag braces take the loads into the wing and fuselage. The nose gear hangs on trunnions in the nose wheel well.
Seats and cargo
Seat installation
Passenger seats are attached to seat tracks — extruded rails running along the floor beams with a hole or slot pattern at a 1-inch (25.4 mm) pitch — by track fittings with locking plungers or studs, plus anti-rattle devices. The seat positions follow the approved Layout of Passenger Accommodation (LOPA), which also protects the required access to emergency exits. Transport-category seats are dynamically tested (CS 25.562, the "16 g" seat), and seats, galleys and cargo restraints must hold the emergency-landing inertia loads of CS 25.561 — 9 g forward, 3 g upward, 3 g sideways (4 g for seats), 6 g downward and 1.5 g rearward.
Cargo-loading systems
- Unit load devices (ULDs) — containers (for example the LD3) and pallets with nets — are loaded through a large outward-opening cargo door.
- Ball transfer units in the door area let a ULD be turned in any direction; roller trays and conveyors carry it fore and aft.
- Power drive units (PDUs) — electrically driven rubber rollers that lift against the ULD base when selected — move it from a control panel by the door.
- Side guides, end stops and restraint locks hold each ULD against flight and emergency loads; bulk compartments use nets and barrier nets. A missing or unserviceable lock reduces the permitted load in that position.
Doors
- Plug doors — passenger and service doors on most airliners. Because the door is larger than its opening and seats against stops on the frame, cabin pressure locks it shut: it cannot be opened while the cabin is pressurised. To open it, the mechanism first lifts or pulls the door inward to clear the stops, then swings it outward on its hinge arm.
- Non-plug (outward-opening) doors — most cargo doors, because an inward-opening door would take up valuable hold space. Pressure tries to push them open, so they rely on latches (carrying the load), locks (preventing the latches from back-driving), and indication. Many have vent doors or pressure-relief flaps that stay open, preventing pressurisation, unless the door is properly latched and locked. Several accidents (DC-10s in 1972 and 1974, a B747 in 1989) followed cargo doors that were not fully latched.
- Emergency exits — floor-level doors and over-wing hatches, removable or hinged, with external handles and markings.
- Flight-deck door — reinforced to resist intrusion and small-arms fire, with electrical locking, keypad emergency access and decompression panels that open if the cabin or flight deck loses pressure.
Door mechanisms and safety devices
| Device | Purpose |
|---|---|
| Latches, locks and lock sectors | Hold the door closed and stop the latches being driven open; position checked by proximity sensors |
| Door warning / indication | Proximity sensors tell the flight crew (ECAM / EICAS door page, master caution) if any door is not closed, latched and locked |
| Slide arming lever and girt bar | "Armed": the girt bar of the escape slide is engaged in floor brackets, so opening the door pulls the slide out and fires its inflation bottle. "Disarmed" for normal ground use |
| Slide-armed indication | Warns anyone about to open the door that the slide will deploy |
| Residual-pressure warning | A light that flashes if the handle is moved while the cabin is still pressurised — opening against residual pressure can blow the door and the person out |
| Emergency power assist | A gas-charged actuator that drives the door fully open in the armed (emergency) mode on many types |
| Flight lock | On some types an electro-mechanical lock that prevents the door being opened in flight or at speed |
| Gust lock / door stay | Holds the open door against the wind |
An inadvertent slide deployment in the hangar can injure people and costs a slide. Before opening any door from inside, check that it is disarmed, check the residual-pressure warning, and follow the AMM; from outside, check the "slide armed" warning through the window.
Emergency exits and evacuation
| Type | Minimum opening | Location | Passengers per pair |
|---|---|---|---|
| A | 42 in × 72 in (1.07 × 1.83 m) | Floor level | 110 |
| B | 32 in × 72 in | Floor level | 75 |
| C | 30 in × 48 in | Floor level | 55 |
| I | 24 in × 48 in | Floor level | 45 |
| II | 20 in × 44 in | Floor level or over the wing | 40 |
| III | 20 in × 36 in | Over the wing (step-up inside ≤ 20 in) | 35 |
| IV | 19 in × 26 in | Over the wing | only for aircraft with ≤ 9 passengers |
- CS 25.803: the aircraft must be shown to be evacuable in 90 seconds with only half the exits available.
- Where a floor-level exit sill is more than 1.83 m (6 ft) above the ground (gear extended), an assisting means — normally an inflatable slide or slide/raft — is required. It deploys automatically when the armed door is opened and must erect within 6 seconds at Type A and B exits (10 seconds at others). Over-wing exits have off-wing ramps or slides and escape-route markings on the wing.
- Slides are inspected for gauge pressure, seals and service life, and are overhauled on a schedule; a slide that is unserviceable can restrict the number of passengers or the use of that door under the MEL.
Windows and windscreens
Flight-deck windscreens
- Laminated from chemically tempered glass and/or stretched acrylic structural plies bonded with vinyl or polyurethane interlayers, with a thin outer glass ply as a hard, abrasion-resistant face.
- A transparent conductive film (gold or indium-tin oxide) near the outer face is electrically heated: it prevents ice and fog and keeps the interlayer pliable, which greatly increases impact strength. Temperature sensors in the panel feed the window-heat controllers, which protect against overheating (Chapter 15).
- CS 25.775 requires the windscreen panels in front of the pilots to withstand a 1.8 kg (4 lb) bird at cruise speed (VC) without penetration. Windscreens are attached by bolts through their edge members into the window frame, with sealant to hold pressure.
- Some side windows open (sliding direct-vision windows) for ventilation, ground communication or as a flight-crew emergency exit.
Cabin windows
Each window has an outer pane that carries the cabin pressure, a middle pane able to take the full load if the outer pane fails (fail-safe), and an inner, non-structural scratch pane. A small breather hole in the middle pane equalises the gap with the cabin, so the outer pane normally takes the whole pressure load and the space between them does not fog.
Maintenance of transparencies
- Look for crazing (networks of fine surface cracks in acrylic caused by stress, solvents or UV), scratches, chips, delamination and bubbles in laminated panels, moisture ingress at the edges, and burned or arcing areas of heating film.
- Compare damage with the AMM/SRM limits; some scratches may be polished out with approved compounds.
- Clean acrylic only with approved cleaners — mild soap and water and soft cloths; never solvents such as acetone or fuel, which cause crazing.
Airborne towing devices
Some aeroplanes are equipped to tow gliders, advertising banners or aerial targets (syllabus level 1). A tow hook or release on the tail or under the fuselage holds the tow rope and can be released from the cockpit; glider-tow ropes include a weak link rated to break before the structure is overloaded, and the release must work under load and at any rope angle. Banner-towing installations use a hook and release handle with a pick-up hook technique, while target-towing aircraft use powered winches to reel a drogue or target out on a long cable. All are approved modifications with load limits and inspection requirements of their own.