📖 The lesson
Rigid pipes and tubing
- Materials: aluminium alloys — 1100-H14 or 3003-H14 (low pressure: instrument, vent lines), 5052-O (medium pressure, readily flared), 2024-T3 and 6061-T6 (hydraulic and fuel) — corrosion-resistant steel (high-pressure hydraulics, fire zones, landing gear) and titanium (3Al-2.5V, light high-pressure lines). Copper is used for oxygen and some instrument lines.
- Rigid tube size is the outside diameter, in sixteenths of an inch (a −6 tube is 3/8 in OD), plus the wall thickness in thousandths. Replacement tubes must match material, OD and wall.
- Cutting: a tube cutter (or fine hacksaw); square the end, then deburr inside and out.
- Bending: a hand or production bender, not below the minimum radius. Flattening in the bend must not reduce the diameter to below about 75 % of the original OD. No kinks or wrinkles.
- Beading forms a raised ring near the end of a low-pressure tube so a hose and clamp can grip it.
Fittings and unions
- AN flared fittings use a 37° flare, a sleeve and a B-nut. Aluminium fittings are anodised blue and steel fittings black; Aircraft flaring tools produce a 35–37° flare; an automotive flaring tool (45°) must never be used. A faulty flare cannot be cured by extra torque.
- Single flares suit most tubing; double flares are used on soft aluminium tubing of 3/8 in OD and smaller, where they are stronger and resist cracking. Flares must be concentric, smooth and free of cracks.
- MS flareless fittings use a sleeve pre-set (swaged) onto the tube; they suit high-pressure (3,000 psi) systems. Do not over-tighten: after finger-tight, a defined further turn (typically 1/6 to 1/3 turn) is applied.
- Permanent / swaged fittings (e.g. Permaswage, Deutsch) join or repair tubes; unions join two tubes, and bulkhead fittings pass through structure.
- Never pull a misaligned tube into position with the B-nut. Torque fittings to the table value; do not over-tighten, and use thread lubricant only as specified (none on oxygen lines).
Flexible hoses
- Low-pressure hose (below about 250 psi) has a single fabric braid. Medium-pressure hose (to about 1,500–3,000 psi depending on size) has one wire braid; high-pressure hose (3,000 psi and above) has several wire braids.
- Inner tube materials: Buna-N (petroleum fluids), neoprene (petroleum, not phosphate ester), butyl (phosphate-ester fluids such as Skydrol — not petroleum), and PTFE (Teflon), which is compatible with almost all fluids and a wide temperature range but must not be bent in reverse once formed.
- Hose markings: a lay line along the length (shows twisting), the size, the manufacturer and the date of cure (quarter/year).
- Install with 5–8 % slack, no twist, bend radius at least the minimum, and clamps about every 24 in. Fit fire sleeves in fire zones.
- Identification: lines carry colour bands with words and symbols (e.g. fuel — red, hydraulic — blue and yellow, oxygen — green, fire protection — brown) near each fitting and at bulkheads.
- Inspection: leaks, chafing, kinks, bulges, cracks, stiffness or blistering of hose covers, loose clamps; dents in rigid tube not deeper than about 20 % of OD outside the heel of a bend.
Springs
- A spring stores energy by elastic deflection. Its rate (stiffness) is the load per unit deflection: k = F ÷ x. Springs in series give a lower rate, springs in parallel a higher one.
- Materials: high-carbon steel (music wire, oil-tempered), alloy steels (chrome-vanadium, chrome-silicon) for higher stress and temperature, stainless steel and Inconel for corrosion or heat, and beryllium copper or phosphor bronze for electrical conductivity and non-magnetic use.
- Characteristics: free length, solid length, wire diameter, number of coils, rate, and maximum working stress. Springs are often shot-peened for fatigue life and plated or painted against corrosion.
- Applications: valve springs and relief valves (compression), control return and door springs (tension, torsion), spring-steel landing gear (leaf/flat), and disc springs in clamp-up joints.
- Inspection: free length (permanent set), load at a specified length, cracks, corrosion pits, wear and correct seating of ends. Damaged springs are replaced, not straightened.