Flexible hoses are used in aircraft fluid systems to connect moving parts. There are three main types of hoses based on pressure rating: low, medium, and high pressure. Hose construction involves an inner tube, reinforcement layers, and outer cover. Common materials for each component are described. Proper hose installation requires slack, avoiding twists or sharp bends, and use of supports and clamps. Hose identification and maintenance procedures are also outlined.

1. A. ISMAEILI
AIRCRAFT FLUID LINES
AND FITTINGS

2. Flexible Hose
Fluid Lines

3. Flexible Hose Fluid Lines
Flexible hose is used in aircraft fluid systems to connect moving parts with
stationary parts in locations subject to vibration or where a great amount of
flexibility is needed. It can also serve as a connector in metal tubing systems.
Hose assemblies are heavier than aluminum –alloy or CRES tubing and
deteriorate more rapidly.

4. Hose Materials and Construction
Pure rubber is never used in the construction of flexible fluid lines. To
meet the requirements of strength, durability, and workability, among
other factors, synthetics are used in place of pure rubber.

5. Hose Materials and Construction
The are Two Basic Type of Hose used in aircraft and related
equipment.
✓ Synthetic Rubber
✓ Polytetrafluoroethylene (Teflon™ ) (PTFE)

6. Hose Materials and Construction
Synthetic Rubber Hose
Has a seamless synthetic rubber inner tube covered with layers of cotton braid
and wire braid and an outer layer of rubber-impregnated cotton braid. This type
of hose is suitable for use in fuel, oil, coolant, and hydraulic systems.
The hose is provided in low-, medium-, and high-pressure types.

7. Common Synthetic Materials used in the manufacture of flexible hose.
✓ Buna-N
✓ Neoprene
✓ Butyl
✓ Ethylene Propylene Diene
Hose Materials and Construction

8. Common Synthetic Materials
Buna-N - is a synthetic rubber compound that has excellent resistance to
petroleum products. Do not confuse with Buna-S. Do not use for phosphate ester
base hydraulic fluid (Skydrol™).
Neoprene - is a synthetic rubber compound that has an acetylene base. Its
resistance to petroleum products is not as good as Buna-N, but it has better
abrasive resistance. Do not use for phosphate ester base hydraulic fluid(Skydrol™).
Hose Materials and Construction

9. Common Synthetic Materials
Butyl - is a synthetic rubber compound made from petroleum raw materials. It is
an excellent material to use with phosphate ester base hydraulic fluid (Skydrol™).
Do not use with petroleum products.
Ethylene Propylene Diene Monomer (EPDM) - outstanding heat, ozone and
weather resistance, used in seals such as doors, windows, trunk and hood seals.
Hose Materials and Construction

10. Hose Materials and Construction
Polytetrafluoroethylene (Teflon™ )
Polytetrafluoroethylene resin, broad operating range (-65 ℉ to +450 ℉), shelf life
is limitless, covered with stainless steel wire braided over the tube for strength
and protection. Not affected by any known petroleum products, synthetic base
oils, alcohols and coolants, Broad usage.

11. Hose Materials and Construction
Polytetrafluoroethylene (Teflon™ )
✓ Material that is used for the inner liner of hose.
✓ Carries fluids at temperatures up to 450°F.
✓ Chemically inert, and it maintains its strength at these high operating
temperatures.
✓ Unaffected by any fuel, petroleum, or synthetic-base lubricants, phosphate-
ester base hydraulic fluid, alcohol, coolants, or solvents commonly used in
aircraft.

12. Hose Materials and Construction
Polytetrafluoroethylene (Teflon™ )
✓ Strong and highly resistant to vibration and fatigue.
✓ Takes permanent set after is has been in service.
✓ Remembers its shape/curves and wants to remain in the shape.
✓ Store Teflon™ hoses in the shape used in the aircraft

13. A support wire should be used to
prevent a preformed tube or one that
has taken a permanent set from being
inadvertently straightened when it is
removed from the aircraft.

14. Flexible hoses used for
brake systems have
sometimes a stainless-
steel wire braid installed
over the hose to protect
the hose from damage.
Hose Materials and Construction

15. Hose Materials and Construction
Three Types of Hose Ratings:
The types of hose are normally classified by the amount of pressure they are
designed to withstand under normal operating conditions:
✓ Low Pressure - below 250 psi. Fabric braid reinforcement.
✓ Medium Pressure - up to 3,000 psi. One wire braid reinforcement.
Smaller sizes carry up to 3,000 psi. Larger sizes carry pressure up to
1,500 psi.
✓ High Pressure - all sizes up to 3,000 psi operating pressures.

16. Flexible Fluid Lines Sizing
✓ The size of flexible hose is approximately its inside diameter in 1⁄16-inch
increments.
✓ The size refers to the outside diameter of a rigid tube that has the equivalent
flow characteristics.
✓ For example, a -8 hose has flow characteristics equivalent to a piece of -8, or
1⁄2 inch (8⁄16) rigid tubing.
✓ The inside diameter of the hose is not exactly 1⁄2 inch; it is slightly smaller to
allow for the tube wall thickness.

18. Flexible Fluid Lines Sizing

19. Lay Lines
Lay lines run along the length of a hose. They are yellow, red or white stripes,
incorporating MIL-SPEC numbers and various other manufacturers’ information.
Besides identifying the hose, it serves to indicate whether the assembly is
twisted when installed.

21. Hose Materials and Construction
Rubber hoses consists of three basic parts:
✓ Inner Tube
✓ Reinforcement
✓ Cover

22. ▪ Inner Layer - carries the fluid and therefore must be compatible (chemically) with the fluid
being transported and have the minimum amount of porosity. The four main compounds used
to construct inner liners are Neoprene (for petroleum-based fluids) Buna-N (better suited for
petroleum-based fluids) Butyl (phosphate ester-base hydraulic fluid (Skydrol)) Teflon (compatible
with almost every fluid carried).
▪ Reinforcement Layers - cover the inner liner and determine the hose’s strength.Common
materials used for reinforcement layers are cotton, rayon, polyester fabric, carbon-steel wire,
stainless steel wire braid. Diligent design of reinforcement layers can minimise the dimensional
changes of hoses under pressure.
▪ Protective Outer Cover - is usually made of rubber-impregnated fabric or stainless-steel braid.
It is put over the reinforcement to protect from physical damage or heat.
Hose Materials and Construction

23. The cure date is provided for age control. Its is
indicated by quarter of the year and year. The
year is divided into four quarters.
▪ 1st quarter – January, February, March
▪ 2nd quarter – April, May, Jube
▪ 3rd, quarter – July, August, September
▪ 4th quarter – October, November, December
Hose Materials and Construction

24. Hose Identification
Hose Materials and Construction

25. Most air or vacuum hoses and some aircraft instrument lines are not required to
carry high pressures. Therefore, low pressure rubber hose is typically used with
these types of installations. These hoses have a seamless inner tube and a
reinforcement made of a single layer of cotton braid. An outer cover of ribbed or
smooth rubber is used to protect the reinforcement from physical abrasion.
Low-Pressure Hose
MIL-H-5593

26. ▪ Low-pressure hose conforming to MIL-H-5593
specifications is made up of a synthetic rubber
inner liner, a cotton braid, and a synthetic rubber
outer cover.
▪ It is approved for a maximum pressure of 300 psi
and is primarily used for instrument installations.
▪ Low-pressure hose is identified by a broken
yellow lay line.
▪ The letters LP, and the manufacturer’s code
number and date marking.
Low-Pressure Hose

27. Medium-pressure hose is used with fluid pressures up to 3,000 psi. However, its maximum operating
pressure varies with its diameter. For example, smaller sizes carry pressure up to 3,000 psi while larger
sizes are often restricted to lower pressures.
Medium-pressure hose has a seamless inner liner with one layer of cotton braid and one layer of
stainless-steel reinforcement. A braid of rough oil-resistant rubber-impregnated cotton is usually used
as an outer cover. If the hose is used with a petroleum-based fluid, its inner liner is made of synthetic
rubber and its outer braid is gray-black. However, if the hose is used with Skydrol or any phosphate-
ester based hydraulic fluid, the inner liner is made of synthetic Butyl rubber and the outer braid is
colored green with SKYDROL written on it.
Medium-Pressure Hose
MIL-H-8794

28. Medium-Pressure Hose
▪ Medium-pressure hose conforming to MIL-H-8794
specifications has a seamless synthetic rubber inner liner, a
synthetic rubber-impregnated cotton braid reinforcement,
and a steel wire braid reinforcement. All of this is encased in a
rough synthetic rubber-impregnated cotton braid.
▪ Medium-pressure hose is suitable for carrying fluids under
pressures of up to 1,500 psi.

29. All high-pressure hose has a maximum operating pressure of at least 3,000 psi and uses a
synthetic rubber liner to carry petroleum products. This inner liner is wrapped with two or more
steel braids as reinforcement. To help distinguish high-pressure hose from medium-pressure hose,
the entire hose has a smooth outer cover. Most high-pressure hose is black with a yellow lay line.
However, a hose designed to carry Skydrol has a Butyl rubber inner liner and a green outer cover
with a white lay line.
High-Pressure Hose
MIL-H-8788

30. ▪ High-pressure hose, conforming to MIL-H-8788
specifications has a seamless synthetic rubber inner
tube and either two or three carbon steel wire braid
reinforcements. This hose is covered with a smooth
synthetic rubber cover and is suitable for operating
with pressures between 1,500 and 3,000 psi.
High-Pressure Hose

31. Teflon Hose

34. Swaged Fittings
▪ Hoses using swaged end fittings are
assembled on special machinery that is
typically not found in the shop. These
fittings cannot be removed and reused.
▪ Ordered by correct length from the
manufacturer and ordinarily cannot be
assembled by the mechanic.
▪ Can be also ordered properly equipped
hose assembly shop.
Flexible Hose End Fittings

35. Swaged Fittings

36. Detachable Fittings
▪ May be detached and reused if they are
not damaged; otherwise, new fittings must
be used.
▪ Save cost of replacing complete assembly.
▪ Reusable fittings consist of:
▪ Socket
▪ Nipple
▪ Nut
Flexible Hose End Fittings

39. Slack
▪ Hose assemblies must not be installed in a manner that causes a mechanical
load on the hose.
▪ Provide slack or bend in the hose line from 5 to 8 percent of its total length to
provide for changes in length that occurs when pressure is applied.
▪ Flexible hose contracts in length and expands in diameter when pressurized.
Protect all flexible hoses from excessive temperatures, either by locating the
lines so they are not affected or by installing shrouds around them.
Installation of Flexible Hose Assemblies

40. Flexible hose should be 5% to 8% longer than the distance between the fittings to prevent
straining the fittings.

41. Flex
▪ When hose assemblies are subject to
considerable vibration or flexing,
sufficient slack must be left between
rigid fittings.
▪ Install the hose so that flexure does not
occur at the end fittings. The hose must
remain straight for at least two hose
diameters from the end fittings. Avoid
clamp locations that restrict or prevent
hose flexure.
Installation of Flexible Hose Assemblies

42. Twisting
▪ Hoses must be installed without twisting to avoid possible rupture of the hose or loosening of
the attaching nuts. Use of swivel connections at one or both ends relieve twist stresses. Twisting
of the hose can be determined from the identification stripe running along its length. This stripe
should not spiral around the hose.
Installation of Flexible Hose Assemblies

44. Bending
▪ To avoid sharp bends in the hose assembly, use elbow fittings, hose with elbow-
type end fittings, or the appropriate bend radii. Bends that are too sharp reduce
the bursting pressure of flexible hose considerably below its rated value.
Installation of Flexible Hose Assemblies

45. Elbow fittings should
be used to keep
flexible hose from
having to bend at a
sharp angle.

46. Clearance
▪ The hose assembly must clear all other lines, equipment, and adjacent structure
under every operating condition.
▪ Flexible hose should be installed so that it is subject to a minimum of flexing
during operation. Although hose must be supported at least every 24 inches,
closer supports are desirable.
▪ Flexible hose must never be stretched tightly between two fittings. If clamps do
not seal at specified tightening, examine hose connections and replace parts as
necessary. The above is for initial installation and should not be used for loose
clamps.
Installation of Flexible Hose Assemblies

47. For retightening loose hose clamps in
service, proceed as follows:
Non-self-sealing Hose—if the clamp screw
cannot be tightened with the fingers, do
not disturb unless leakage is evident. If
leakage is present, tighten one fourth turn.
Self-sealing Hose—if looser than finger-
tight, tighten to finger-tight and add one-
fourth turn.
Installation of Flexible Hose Assemblies
Hose clamp tightening

48. Installation of
Flexible Hose
Assemblies
Hose Clamps
▪ To ensure proper sealing of hose connections and to
prevent breaking hose clamps or damaging the hose,
follow the hose clamp tightening instructions
carefully.
▪ These wrenches are available in calibrations of 15
and 25 in-lb limits. In the absence of torque-limiting
wrenches, follow the finger-tightplus-turns method.
▪ Use good judgment when tightening hose clamps by
this method. Since hose connections are subject to
“cold flow” or a setting process, a follow-up
tightening check should be made for several days
after installation.

49. Torque Wrench for Hose Clamps

50. Installation of Flexible Hose Assemblies
▪ Support Clamps are used to secure the various
lines to the airframe or powerplant assemblies.
▪ The most commonly used clamps are the rubber-
cushioned and plain.
▪ Rubber-cushioned Clamp is used to secure
lines subject to vibration; the cushioning
prevents chafing of the tubing.
▪ Plain Clamp is used to secure lines in areas
not subject to vibration.

51. Installation of Flexible
Hose Assemblies
Teflon™-cushioned Clamp is used in areas where the deteriorating
effect of Skydrol™, hydraulic fluid, or fuel is expected. However, because
it is less resilient, it does not provide as good a vibration-damping effect
as other cushion materials.

52. Shelf Life
▪ The total storage/shelf life of both types of hoses
shall be limited to 10 years from the cure date of
manufacture.
▪ During storage, periodic inspection should be
carried out once a year for signs of deterioration,
weather cracks, signs of corrosion on end fittings
etc. and hose pressure tested to 1-1/2 times the
working pressure every two years.
▪ Before installation on aircraft pressure test should
be carried out at 1-1/2 times the working pressure.

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