Pipes & Hoses
Bending and flaring rigid pipe to the 37° aircraft standard; hose identification by lay-line and cure date; installation, clamping, bend radius and fire sleeves; inspection and pressure testing.
Pipes & hoses, summary notes
- Rigid pipes carry fluids between fixed points; flexible hoses connect points that move relative to each other or need vibration isolation.
- Rigid pipe ends are formed by FLARING, the aircraft standard is a 37° flare (AN/MS). The 45° flare is an automotive and refrigeration standard and must never be mixed with 37° fittings, because the seat angles do not match and the joint will leak.
- Pipes are bent to at least the minimum bend radius using a bending tool; a kinked, wrinkled or flattened bend restricts flow and concentrates stress. Wall thinning at the outside of the bend is checked against limits.
- Flexible hoses are identified by the LAY-LINE printed along the hose, which carries the part number and specification and doubles as a twist indicator, it must run straight when installed.
- Rubber hoses have a cure/manufacture date and a shelf and service life, so age is checked as well as condition.
- Installation: correct hose for the fluid and pressure, no twist, enough slack for flexing and for the length change that occurs under pressure, minimum bend radius respected, clamped at the specified intervals, kept clear of hot areas or fitted with a fire sleeve, and protected from chafing.
- Inspection: leaks, cuts, blistering, cover cracking, exposed reinforcement, corroded end fittings, security and routing, plus the life limit. Pressure-test after installation and check clearance through the full range of movement.
- ⚠ Exam trap: 37° is the aircraft flare. A twisted lay-line means the hose is under torsional strain and must be refitted.
- Aircraft flare angle
- 37° (AN/MS), 45° is automotive, never interchange
- Lay-line
- part number and specification, and a straight-line twist indicator
- Under pressure
- hoses shorten, always fit with slack
A hose is fitted with its lay-line spiralling around the hose. What is wrong and why does it matter?
The hose has been twisted during installation. Torsional strain in a pressurised hose greatly shortens its life and can unscrew the end fitting, so it must be released and refitted with the lay-line running straight.
Why is a flexible hose never installed taut between two fittings?
A hose shortens when pressurised and must also absorb relative movement between the components it joins. Fitted taut, it would pull on its end fittings under pressure and fatigue at the ferrule, so slack is specified.
Pipes & hoses concept map
Pipes & Hoses
Pipes & hoses quiz
Pipes & Hoses, quiz
1. A hydraulic system operates at 3000 psi. Taking 1 psi as 6.895 kPa, this is approximately:
20.7 MPa2.07 MPa207 MPa2. The standard aircraft flare angle is:
37°45°30°3. The lay-line on a flexible hose:
Carries the part number and shows whether the hose is twistedMarks the point at which the hose should be cut to length before the end fittings are made upIndicates the direction of flow, so the hose is fitted the correct way round in the system4. A flexible hose is installed with slack because it:
Shortens under pressure and must absorb movementLengthens with ageNeeds room for insulation5. Exceeding a hose's minimum bend radius causes:
Kinking and flow restrictionImproved flexibilityFaster cure6. Rubber hoses are checked for age using the:
Cure or manufacture datePart number onlyColour of the cover7. A fire sleeve is fitted to a hose to:
Protect it where it passes through a hot or fire zoneIncrease its pressure ratingPrevent twisting8. Exposed reinforcement braid on a hose means:
The hose must be replacedThe hose may continue in serviceOnly the cover needs repair9. A rigid pipe with a kinked bend must be:
Rejected or replacedStraightened and re-bentUsed if it still passes fluid10. After installing a hose the system should be:
Pressurised and checked for leaks and clearanceLeft for 24 hoursFlushed with solvent