Module 6.6-6.8 Pipes and unions, springs, bearings

Knowledge levels

Item	A	B1 / B3	B2 / B2L
6.6(a) Identification and types of rigid and flexible pipes and connectors	2	2	2
6.6(b) Standard unions for hydraulic, fuel, oil, pneumatic and air pipes	2	2	1
6.7 Springs: types, materials, characteristics, applications	—	2	1
6.8 Bearings: purpose, loads, material, construction, types, applications	1	2	2

6.6 Pipes (rigid lines)

Material	Typical use
Aluminium alloy (5052-O, 6061-T6)	low/medium pressure: fuel, oil, instrument, return lines
Corrosion-resistant steel (stainless 304, 321)	high-pressure hydraulic lines, areas of heat or abrasion, fire zones
Titanium (3Al-2.5V)	high-pressure hydraulic lines where weight matters
Copper	old instrument and oxygen lines (largely replaced)

• Size is the outside diameter in sixteenths of an inch (a -8 line = 8/16 = 1/2 in OD) and wall thickness.
• Ends: flared (37° AN flare, single or double flare for soft aluminium), flareless (MS bite-type sleeve), beaded (for hose clamps on low-pressure lines), welded/swaged permanent fittings.

Flexible hoses

Hose	Construction	Use
Low pressure	synthetic inner tube, single fabric braid	up to about 250 psi (instrument, vacuum)
Medium pressure	one wire braid	up to about 1 500-3 000 psi depending on size
High pressure	multiple wire braids	3 000 psi and above hydraulic systems
PTFE (Teflon)	PTFE inner liner with stainless braid	wide temperature range, compatible with most fluids incl. phosphate-ester; must not be sharply bent or straightened once set

• Inner tube material must match the fluid: Buna-N (petroleum fluids, fuel), neoprene (petroleum, not phosphate-ester), butyl (phosphate-ester, e.g. Skydrol - not petroleum products), PTFE (almost all).
• Hoses carry identification: a lay line (to check for twisting), size, manufacturer, cure/manufacture date (age limits for rubber).
• Size of a flexible hose is given by the inside diameter in sixteenths of an inch (dash numbers matching the tube OD they replace).

Key fact: Rigid tube size = outside diameter; flexible hose size relates to inside diameter. Install hoses without twist (check the lay line) and with slack (5-8 % of length) for movement and pressure contraction.

Identification of fluid lines

Lines are marked with tapes or decals (about 1 in wide) carrying a colour code, the function in words, a geometric symbol and, where relevant, a hazard marking and a flow-direction arrow. Commonly quoted examples: fuel red, hydraulic blue/yellow, lubrication yellow, breathing oxygen green, fire protection brown. Always confirm with the aircraft manual - colour schemes differ between standards. Aluminium alloy tubing may also carry a painted colour band identifying the alloy.