Detail with helicopter fuselage on the repair line in a factory
What works on a helicopter, is vulnerable to fatigue and overload and is most often overlooked? Nope, not the A&P (but good guess). It’s the tubes and flexible hoses (or lines) that transfer operational fluids throughout the helicopter systems.
When lines fail, results can be disastrous. Consider the June 2014 hard landing that left a U.S. air tour helicopter substantially damaged after it lost power shortly after takeoff, causing the pilot to autorotate and land in a field. The NTSB concluded a pneumatic fuel-control line fracture caused the power loss and accident, likely caused by “the repeated loosening and re-tightening of the fitting during required maintenance and inspection.”
Fatigue and overload are often the direct result of installation practices contrary to manufacturer’s operations and maintenance manual instructions. There continue to be far too many preventable accidents attributable to improper alignment, clamping and tightening of engine hard lines at installation.
Correcting this malady begins with respect of the lines and the job they perform. More often than not, removed lines are tossed into a box on the bench. After a perfunctory inspection, they are reinstalled, too often pushed, pulled, tweaked and clamped, and the B-nuts tightened — or overtightened.
So I figure a review of line removal, inspection and installation would be apropos. First, have the ops and maintenance manual close by for reference. Review manufacturers’ bulletins for recent releases. Prepare a clean spot on a bench for removed lines.
Before removing a line, note its routing and position in relation to others and its terminus fittings, as well as the exact position and condition of stabilizing clamps. Look for obvious signs of lines contacting or “rubbing” each other. For liquid lines, inspect the B-nut attach-to-fitting area for evidence of leakage.
When you loosen the B-nut, notice whether the line is under tension and jumps from the fitting. If so, determine why. Is the fitting misaligned? Is the line bent? Fix the cause before reinstalling.
During removal, note which lines have over-torqued B-nuts. Their flared ends and fittings will require closer inspection, as will the attaching cone ends. Remove all support clamps for cleaning and inspection. Note moisture in the area under the clamps, where surface corrosion is possible.
Be sure to place caps/covers on now “open” fitting cones after the line is removed. When placing removed lines on the bench, position them in order of which they were removed, with the first placed at the back of the bench. Then, as you clean and inspect them, you will have an accurate visual inventory of all the lines and the fittings to which they are attached. This makes replacement easier.
Clean all lines with approved solvent. Dry them with clean compressed air inside and out. Completely remove all the original “slippage marks” from the B-nut and the corresponding fittings.
Inspect each line, particularly areas under clamps. Inspect for twists, kinks, chafe marks and corrosion. Replace any line that does not meet the criteria in the maintenance manual. Also, check the line for the correct part number and replace it if one is not visible. Inspect flared ends for splitting or cracking (particularly those that were over-torqued) and corresponding fittings for wear and thread condition. Replace any support clamps whose cushion material is worn, soaked by fluids or missing.
Reject any hard line that has anti-chafe wrapping as it will hold moisture and cause hidden corrosion. Correct line routing precludes use of the wrapping.
When installing lines, do so on the fittings of components to be connected. Lines are designed to traverse the space between components with no torsion or contact with other lines. In their free state, each end should fit squarely on the fittings, and the B-nuts should be completely threaded with no resistance. If necessary, adjust fittings slightly to accommodate. Do not bend or tweak the lines to fit. Replace the line with a new one if it does not fit.
Insure vibration-dampening support clamps are properly located on the lines and that there is no contact with other lines. Pull the B-nuts on the lines tight using a torque-measuring wrench following the ops manual torque limits. As the lines are tightened, place a slippage mark on the B-nut (referencing the manual).
So what’s my line? That providing a safe helicopter means there are no “rudimentary or routine” jobs when it comes to maintenance!