12.5 Helicopter airframe structures (ATA 51)

Scope

12.5(a) General concepts (A 2, B1.3 2): structural strength requirements, structural classification, fail-safe/safe-life/damage tolerance, zonal and station identification, types of stress, drains and ventilation, system installation provisions, lightning strike protection. 12.5(b) Construction methods (A 1, B1.3 2): stressed skin, formers, stringers, longerons, bulkheads, frames, doublers, struts, ties, beams, floors; pylon, stabiliser and undercarriage attachments; seats; doors; windows; fuel storage; firewalls; engine mounts; riveting, bolting, bonding; surface protection; cleaning; symmetry checks. About 10–13 questions on the B1 paper — the largest single block of Module 12.

Structural classification

Class	Definition	Helicopter examples
Primary	Failure would endanger the aircraft (carries flight, ground or pressure loads)	Main frames carrying the transmission deck, tail boom, engine mounts, landing-gear attachments
Secondary	Carries some load; failure would not immediately endanger the aircraft	Fairings, some panels, cowlings
Tertiary	Lightly loaded; failure has negligible effect	Interior trim, small access covers

Design philosophies

Philosophy	Principle	Example
Safe-life	Part is retired after a fixed number of hours, cycles or landings, before a fatigue crack can occur	Rotor blades, rotor hub parts, transmission components with life limits
Fail-safe	Multiple load paths: if one member fails, the remaining structure carries the load until the failure is found	Redundant frames, multi-bolt attachments, crack-stopper straps
Damage tolerance	Structure designed and inspected so that cracks/damage are detected before reaching critical size; based on crack growth rates and inspection intervals	Fuselage, tail boom inspections in the maintenance programme

Key fact: Many dynamic components on a helicopter are safe-life (life-limited) because they experience very high numbers of load cycles.