Airframe & structures

021 Aircraft General KnowledgeChapter 1

Structural design philosophies

Key fact

Safe life: Component replaced after a set number of cycles/hours regardless of condition — one load-carrying member is sufficient if strong enough. Does not guarantee no failure within the safe-life period.

Fail-safe: Redundant parallel load paths — more than one load-carrying component. Does not guarantee the system will never fail despite backups.

Damage tolerant: Withstands a defined amount of structural weakening (e.g. cracking) without catastrophic failure — accounts for crack growth.

Key fact

Maintenance types: Hard time — overhaul/remove after set hours/cycles regardless of condition. On condition — monitor critical parameters; replace when limit exceeded.

Stress and strain

Key fact

Stress types: Tension (resists pull), compression (push), shear (parallel to cross-section), torsion (twist), buckling (combined forces). Strain = deformation as % change from original dimension.

Elastic deformation returns to original shape when load removed. Aircraft flown beyond certified load factor may suffer permanent deformation.

Formula — Design ultimate load

DUL = 1.5 × DLL (Design Limit Load)

Classification	Probability	Crew workload
Minor	10⁻³	Slight increase
Major	10⁻⁵	Significant increase
Hazardous	10⁻⁷	Excessive / physical distress
Catastrophic	10⁻⁹	Fatality / hull loss

Fuselage construction

Key fact

Truss type: Frame carries load; skin covers only — light training aircraft. Monocoque: Stressed skin carries all load — aluminium/magnesium alloy; difficult to add doors. Semi-monocoque: Transport aircraft standard — skin, frames, stringers; majority of stress on internal structure.

Key fact

Fuselage shell structures transmit normal bending, tangent bending, tension, and torsional stresses — not shear. Pressurisation load = tension on fuselage skin. Hoop stress from cabin differential pressure acts in tension on frames.

Torque links (nose gear) experience greatest stress during tight turns on the ground.

Exam trap

Long-haul aircraft cannot be used for short-haul without affecting fatigue life — safe life is determined on a load spectrum. MZFM = Maximum Zero Fuel Mass — max mass without usable fuel.

Wing structure

Key fact

Cantilever wing: Attached at root only — no struts. Maximum vertical loads and bending moments at the wing root.

Torsion box: Spars, ribs, reinforced skin/stringers. Spar bears most load (web + girders, I-beam). Ribs maintain aero shape; stringers help skin absorb longitudinal compressive loads.

Condition	Upper surface	Lower surface
In flight (lift)	Compression	Tension
On ground (weight)	Tension	Compression

Key fact

Wing bending relief: Use fuselage fuel first; keep wing fuel as long as possible; upfloat ailerons reduce root bending moment. From root to tip along leading edge: first compression, then tension.

Flutter and materials

Key fact

Aerodynamic flutter: Caused by torsion + bending when COP is ahead of COG. Prevented by torsional stiffness, mass balance forward of hinge, correct mass distribution. If wing bends down and aileron COG is behind hinge, upward aileron deflection can trigger flutter (and vice versa).

Key fact

Sandwich structure: Two thin sheets + light core — high stiffness, low mass; unsuitable for concentrated loads; does not use resin. Composite: Matrix + fibres — strength tailored by direction; higher strength-to-weight than metal.

Engine decking/firewall: Stainless steel or titanium sheet.

Material	Use
Aluminium/magnesium alloy	Semi-monocoque skin, spars
Titanium	High-temp areas, firewalls
CFRP/GFRP/KFRP	Tailored composite structures
Steel	Bolts, high-stress fittings

Primary flight controls

Key fact

Primary controls: Ailerons (longitudinal axis), elevators (lateral axis), rudder (normal/yaw axis), roll spoilers.

Yoke rearward + left: Right aileron down, left aileron up, elevator up. Yoke forward + right: Right aileron up, left aileron down, elevator down.

Control type	Gust lock	Pilot feel	Trim method
Manual (cable)	Required	Aerodynamic feedback	Trim tabs
Power assisted (reversible)	Required	Partial feedback	Trim tabs — no Q-feel
Powered (irreversible)	Not required	Artificial Q-feel	Adjust zero-force point

Exam trap

Pitch trim = moving the tab. Roll/yaw trim on powered aircraft = moving neutral point of Q-feel (yoke/pedals). Irreversible controls: trimming adjusts zero force point of feel unit, not the surface directly.

Rudder ratio changer / variable stop: Reduces rudder deflection for given pedal input as IAS increases (not decrease).

Secondary controls and FBW

Key fact

Secondary controls: Speed brakes, lift augmentation (slats/flaps), trimmable stabiliser. Fowler flap: Moves aft then turns down. Krueger: Leading edge, near root. Auto slat: Extends at set AoA from stall warning.

Fly-by-wire modes: Normal → Alternate → Direct → Mechanical (most basic). Advantages: weight saving, improved handling, gust load alleviation.

Key fact

Cockpit windows: Glass + inner polycarbonate laminate; electrical heating (de-ice) improves strength. Side windows: defoggers only, not de-ice. Some types have speed restrictions with window heat inoperative (bird impact).