Hydraulic Systems Tubing
Lifelines to power and motion control
By Terry Karl and Mark Morrow
When a fully loaded B747-400 finally lands after 10 or more hours in flight, some 600,000 lbs. of aircraft and passengers travelling at approximately 170 miles per hour must come to a stop — safely. While we all know that hydraulic pumps, valves, brakes, and thrust reversers must work properly to accomplish this, we often don't think about the ubiquitous metal tubes that carry the hydraulic power through and to these subsystems. These "lifelines" of the hydraulic system must also be in optimum operating condition to meet these demands.
Most of today's attention in hydraulic system design and maintenance is paid to the critical, large subsystems and components, such as primary and secondary flight control actuators, engine-driven pumps, auxiliary pumps, electric motor pumps, and power transfer units. The emerging technologies surrounding the "all electric aircraft" and EHAs (electrohydrostatic actuators) command much more attention than simple hydraulic tubing. These technologies and accessories drive both the complexity and costs associated with aircraft hydraulic systems. As many mechanics are aware, however, even with all these advancements and the increased sophistication of hydraulic systems today, fluid leakage remains as one of the most aggravating causes for unscheduled maintenance actions. One of the leading causes of fluid leakage involves hard tubing and tube fittings. This article focuses on hydraulic tubing and various methods of maintenance and repair for these tubes.
Hydraulic power lifelines
Hydraulic tubing and flexible hose styles are, indeed, the lifelines of hydraulic power and motion control. These distribution systems carry high-pressure (typically 3,000 psi) hydraulic fluid power to all the various points in the aircraft where work needs to be done. Although not nearly as glamorous as the power and motion control accessories, nevertheless, problems associated with tubing and fittings cause some 30 to 60 percent of aircraft delays and cancellations. A typical commercial transport can contain up to several hundred yards of tubing and thousands of fittings and fitting connections. Hydraulic tube sizes can range from as small as a quarter inch up to one and a half inches. Material types used range from aluminum (typically 6061T6) to corrosion-resistant steel (CRES alloys such as 300 series and more robust materials such as 21-6-9) to exotic titanium material (such as 3Al-2.5V). Tubing wall thickness for a given tube size can vary also, since the effort to save weight in aircraft design is optimized. Therefore, the mechanic's dilemma in hydraulic tube repair can be magnified quite quickly.
Many factors can contribute to tube failures. The most commonly cited factors are chafing (either against structure or at clamping points), installation stress (created by runs of tubing going from fixed point to fixed point), and corrosion (due to environmental conditions). While thorough precautions are taken in the selection of materials, design, and installation by the airframe manufacturers, it is still possible that tube leaks can occur. To minimize these possibilities, maintenance of tubes and fittings should always be accomplished in accord with the airframe manufacturer's recommended practices as detailed in the airframe maintenance manual.
Tubing maintenance basics
Hydraulic system tubing maintenance can be proactive in the sense that routine inspections for evidence of weeping and leakage can be done visually or by wiping exposed surfaces with a clean, white rag to determine if fluid is present. Tube clamping points should be examined visually for evidence of looseness and possible chafing against the tubes they are clamping. Loose or damaged clamps or clamps whose protective elastomer padding is worn or damaged should be replaced. Tubes should be examined visually for evidence of scratches, nicks, dents, or any other malformations or defects that could cause failure points. Tubing should also be free from excessive accumulations of other fluids such as cleaning fluids and other foreign substances that might eventually lead to corrosive damage to the tube. Finally, tubing should not be in direct contact with airframe structure, other tubes and/or hose or other accessories that might lead to chafing.
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.
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.