Hydraulic Systems Tubing

Hydraulic Systems Tubing Lifelines to power and motion contro l By Terry Karl and Mark Morrow April 2000 When a fully loaded B747-400 finally lands after 10 or more hours in flight, some 600,000 lbs. of aircraft and passengers...


In some cases, common aircraft cleaning solutions may also have long-term damaging effects on tubing life. If there are any doubts as to whether any of the cleaning solutions might contribute to decreased tubing life, consult with the airframe manufacturer's technical product support personnel.

Also, while it may seem self-evident, always check the tube fitting, or ÔB' nut connections to ensure they are properly torqued. Consult the airframe technical manual for various torque levels given different tube and fitting materials.

When inspection does reveal any of these problems, replacing the entire tube assembly or sections thereof may be required. Several repair or replacement strategies may be employed, depending on the circumstances and what is permissible according to the airframe manufacturer's maintenance manual.

Repair and/or replacement strategies
Following are repair or replacement alternatives, depending upon the aircraft's situation, the resources available at the time, the mechanic's skill and training level, and the urgency of bringing the aircraft back into flight-worthy condition:

Minor repair of the tube assembly and If the tube is scratched or nicked not deeper than 10 percent of the tube wall thickness, typically such damage may be burnished away using hand tools. However, if the scratch or nick is on the outside radius of a bend, then the tube should be repaired or replaced as outlined below. Tubes with severe die marks, splits, cracks, evidence of torsional twist, or dents should also be replaced.

Replace the entire tube assembly and Obtain a complete new tube assembly from stock. Make sure the new tube assembly can be installed without pre-stressing the end fitting connections. Too much pre-stress to the end fitting connections may significantly affect tube life. Use Illustrated Parts List part numbers, and follow the appropriate sections of the airframe manufacturer's maintenance manual.

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Install a temporary hose assembly and Many manufacturers permit the use of a temporary repair that consists of a hose assembly fabricated from field assembly or reusable hose fittings and bulk hose. Such hose assemblies are typically limited for use until the aircraft reaches the next possible maintenance facility that is capable of performing a permanent repair. Care should be taken to ensure the hose assembly meets the pressure requirements of the system it is being used with and that there are no fluid compatibility problems. In the case of hydraulic systems using either phosphate ester (typically known as Skydrol™ or Hy-Jet) or MIL-H-5606 petroleum-based products, Teflon™-lined, wired-reinforced hose is preferred. Hose assemblies should be clamped in the locations where the hard tube was clamped, and care should be taken that all other installation practices for hose are followed.

Major repair of the tube assembly and Many, if not all, airframe manufacturers permit repair of tube assemblies either on the aircraft or in the maintenance shop. Damaged sections of the assembly may be cut out and replaced in the form of a splice (see Figure 1.). Damaged end fittings may be cut off and replaced, but care should be taken that the repaired assembly fits the installation and that no pre-stress is imparted to the repaired tube due to misalignment or differences in length caused by the repair. After the defective tube assembly is removed from the aircraft, there are five basic steps to follow:

1. Cut the defective area out of the assembly
2. Deburr the cut ends
3. Form the replacement tube section
4. Install the new tube section and/or fittings
5. Inspect the new tube assembly

Always ensure that the repair tube material is of the correct material type, size, and wall thickness in accordance with the manufacturer's instructions.

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