Chapter 5: Wings, Stabilisers, Control Surfaces and Nacelles

Wings (ATA 57) — construction

All transport wings are cantilever wings: they carry their loads to the fuselage internally with no external struts (braced, semi-cantilever wings belong to light aircraft). In flight the wing bends upwards, twists, and is sheared by lift, drag, the weight of fuel and engines, and gusts. The structure that carries all of this is the wing box.

• Spars — the main spanwise beams, usually a front and a rear spar (some designs add a centre spar inboard). Each has upper and lower caps (booms, flanges) that carry the bending loads as compression and tension, and a web, stiffened vertically, that carries the shear. Spars are built up from extrusions and plate, machined in one piece, or made of carbon-fibre composite.
• Ribs — chordwise members that hold the aerofoil shape, support the skin and stringers against buckling, and feed concentrated loads into the box. Types include plain ribs with lightening holes, heavier compression ribs, sealed tank-end (bulkhead) ribs, baffle ribs with flapper check valves to stop fuel surging, and light nose (false) ribs in the leading edge.
• Stringers and skins — the upper skin (compression in flight) is thicker and more closely stiffened than the lower skin (tension, fatigue-critical). Large panels are often machined from thick plate with integral stringers, or made of composite.
• Wing root — the box is joined to the fuselage centre wing box by splice plates and terminal fittings on the spars and skins. Wing tips are light structure carrying navigation and strobe lights, static dischargers and winglets.
• Leading and trailing edges — removable panels in front of the front spar and behind the rear spar enclose slats and their tracks, anti-ice ducts, flap mechanisms, spoilers, hydraulic and control runs.

Fuel storage in the wing

• Integral tanks ("wet wing") — the wing box itself is the tank. Every joint is sealed with interfay and fillet sealant, fasteners are installed wet or with cap seals, and tank-end ribs close the compartments. Access is through manhole (access) panels in the lower skin; some are structural and all have seals that must be correctly installed and leak-checked.
• Baffle ribs with one-way flapper valves let fuel flow inboard towards the pumps but stop it surging outboard in manoeuvres.
• Surge (vent) tanks near the tips collect fuel that enters the vent system and allow the tanks to breathe; dry bays are kept empty over the engine and other hazard zones.
• Other tank types: bladder (bag) tanks — flexible cells fitted into a structural cavity — and rigid removable tanks, mainly on smaller aircraft or as auxiliary tanks. A centre tank in the centre wing box is common on airliners.

Wing attachments

Stabilisers (ATA 55)

The horizontal and vertical stabilisers use the same principle as the wing — front and rear spars, ribs and stiffened skins forming a torsion box — but smaller, and today mostly carbon-fibre composite. The leading edges are removable and may be protected against bird strike (CS-25 requires the empennage to withstand an 8 lb (3.6 kg) bird at cruise speed). The fin is bolted to fuselage frames through root fittings; tailplanes are mounted on the fuselage (conventional), part-way up the fin (cruciform) or on top of it (T-tail).

• A trimmable horizontal stabiliser (THS) pivots on two fittings at its rear spar; its front spar is moved by a ball-screw jack (screwjack) driven by electric or hydraulic motors through a gearbox with no-back brakes, or mechanically by the manual trim wheel.
• Control surfaces (elevators and rudder) hang on hinge fittings along the rear spar, with their power control units attached to fittings on the same spar.

Flight control surfaces (ATA 55/57)

Construction and attachment

• Light aircraft: an aluminium spar or torque tube with ribs and skin, sometimes fabric-covered.
• Transport aircraft: a front spar, ribs and skins — very often honeycomb sandwich or carbon-fibre composite panels — with metal hinge and actuator fittings and a trailing-edge wedge.
• Surfaces hang on several hinge brackets with self-aligning (spherical) bearings so that wing flexing does not bind them; the hinge line must be straight, and hinge bolts, bushings and bearings are checked for wear and free play.
• Bonding jumpers bridge every hinge to carry lightning current and static (Chapter 3).

Balancing — mass and aerodynamic

Control surfaces are balanced aerodynamically (horns, inset hinges, balance panels, tabs — Chapter 2) to set control forces, and by mass to prevent flutter. Static balance is checked whenever a repair adds weight fore or aft of the hinge line and whenever the surface is repainted.

1. Remove the surface and support it at its own hinge points on a level balance stand, in still air, free to rotate.
2. Lock any tabs in their neutral position.
3. Observe the trailing edge: if it drops the surface is underbalanced (tail-heavy, usually given a plus sign); if it rises it is overbalanced (nose-heavy, minus sign).
4. Measure the out-of-balance moment with a scale or weights at a known distance, and compare with the SRM limits.
5. Correct by adding or removing balance weight ahead of the hinge line as the manual directs. Most manufacturers require a slight nose-heavy condition; tail-heavy is normally not permitted.

Nacelles, firewalls, pylons and engine mounts (ATA 54)

Nacelles and cowlings

• Inlet cowl — the aerodynamic lip and duct in front of the fan, with an anti-iced lip and acoustic lining.
• Fan cowls — hinged at the pylon and latched along the bottom; held open by hold-open rods for access. Fan cowls left unlatched have been lost in flight, so latches have visual indicators and closing them is a critical, independently checked task.
• Thrust-reverser cowls (C-ducts) — hinged at the pylon, opened by hydraulic actuators for engine access.
• Core cowls and exhaust — around the hot section; exhaust nozzle and plug.
• Cowlings are aluminium, composite sandwich or titanium (hot areas), and contain the fire zones of the engine, with fire detection and extinguishing (Chapter 11).

Firewalls

A firewall isolates a designated fire zone — engine, APU, combustion heater — from the rest of the aircraft (CS 25.1191). It must be fireproof: able to withstand a flame of about 1 100 °C (2 000 °F) for 15 minutes while still doing its job, compared with 5 minutes for a merely fire-resistant part. Firewalls are typically stainless steel or titanium; every penetration by pipes, wiring or controls uses fireproof fittings, grommets or sealants, and firewall sealing is inspected whenever anything passing through it is disturbed.

Pylons (struts)

The pylon is a box beam of spars, ribs and bulkheads — titanium or steel where it is hot, aluminium elsewhere — that hangs the engine from the wing (or the rear fuselage on rear-engined types). It carries the engine loads into the wing box through its attachment fittings and fuse pins, and houses fuel, hydraulic, bleed-air, electrical and fire-extinguishing lines. Its lower spar and aft fairing form a firewall and heat shield between the engine fire zone and the wing.

Engine mounts

• A turbofan normally has a forward mount (on the fan case or fan frame) and an aft mount (on the turbine rear frame). Between them they carry vertical, side and torque loads; thrust is taken at the forward mount or through thrust links from the fan frame to the aft mount.
• Mounts include fail-safe features: "waiting" links or catcher pins that carry load only if a primary link fails.
• Turboprop and many rear-mounted engines sit on tubular steel mount frames with elastomeric vibration isolators.
• Mount bolts and links are inspected for cracks, corrosion and correct torque; disturbed mount bolts are often single-use or have special torque and locking requirements.