Hang Glider Inspection

So, now your wings have seen some good airtime. What would be a routine maintenance program for this aircraft?

This article is intended as a general idea of maintenance check items, not as a complete, one-size-fits-all comprehensive guide. Your HG dealer, HG mechanic, and owners of your particular glider may also have good input for you. Collect all the wisdom and experience that you can, for this work. This article is just a starting place. Going solo on this job, with limited airframe maintenance experience, is not recommended.

Feel free to print this out.

At least before purchase, and once a year, or after any particularly noisy incident with your glider, you should perform a sail-off airframe inspection for your glider. Enlist the help of qualified HG technicians, for as many times as it takes to be comfortable with this process (such as the first, second, or third inspection cycles). Always, it will never hurt to have a second set of eyes on the job, working with you.

Obtain a copy of the current Owner’s Manual for your glider (most manufacturers have this document available for download, on-line). Check for any updates, recalls, or notices from the manufacturer, while you are there. Print out the manual, and put it into a three-ring binder or folder. For the first full inspection, have a digital camera and a way to save and display large pictures, such as a tablet or laptop computer, while you are working. A tiny display screen will not do the job, here. If the camera does not link to the computer directly, a USB memory-card reader can connect the camera's memory card to the computer as an external drive, usually for not much money.

Take lots of good pictures of the construction details, before you disassemble anything. Label tubes and plates clearly with words and arrows (using tape labels and a marker pen) in each picture, so you will know how the picture was taken, and which way it is to the Nose, Wingtip, Hang-straps, et c.) and whether the picture was taken from the top, or from the bottom of the glider. Print out these fine detailed pictures, use a three-hole punch, and add them to your printed Owner's Manual where needed. I recommend the use of plastic page protectors, in your printed three-ring manual.

To begin, set up the glider completely, without the ribs, but with the washout tubes (or sprogs) assembled. Go to each wingtip, and use these tubes to try to rotate the leading edges, causing the trailing edge of the sail to lift and sink. Any excessive looseness in this twisting test indicates wear at the bolt holes, or in the break-down assembly point. The washout or dive recovery of the glider wing can be reduced, with worn bolt-holes. A good airframe mechanic can install new sleeving there, to restore “new-tube” bolt hole tolerances, or else the badly-worn tubes should be replaced.

Perform a good preflight inspection, and make written notes of anything that you find questionable. Address these issues during the inspection process, and seek advice from your dealer or HG airframe mechanic, if needed.

A sail removal operation is a great time to think about replacing key bolts that may have high stress jobs. Most people think about a "heart" bolt or leading edge to crossbar bolt, but bolts often overlooked are the ones at the base of the control bar (if applicable). These can undergo major stress from less than perfect landings and are cheap to replace.

All used Nyloc nuts must be replaced with new Nyloc nuts, unless they can be secured with a safety ring or safety pin. Nyloc nuts become non-locking, after one use. Plan ahead, and have more new Nyloc nuts on hand than you will need, or simply replace all Nylocs with all-metal AN-grade locking nuts. Aircraft supply houses (linked below) will have AN-grade Nyloc nuts and all-metal locking nuts for sale. These places have telephone assistance, if you get lost in all the options.

As any part of the glider is being disassembled, check with drawings in the Owner’s Manual and/or make sketches of how things are put together (the order of assembly). Some Owner's Manuals will have pictures inverted (showing work being done on an inverted airframe); make written notes in the printed manual, where you find such “inverted” views. Before dis-assembly begins, use the marker pen (never use pencils or ballpoint pens on aluminum) to mark plates and tubes where cables and fittings are installed, so nothing gets bolted to the wrong hole when re-assembled.

Most double-surfaced glider airframes can be extracted from the sail through the bottom-surface zipper. The details of this process will vary with different gliders, so I recommend having experienced people for this work. Most single-surfaced gliders can be inspected by removing and replacing one wing tube at a time. If the sail conceals the keel, preventing a good inspection, then remove the keel separately, also. The center bolt of a glider takes the most abuse, normally. Check the tubing carefully, at that location.

On each tube, you are looking for cracks, dents, ripples, scratches, bending, and any deformations (tubes should be round, not oval in cross-section). Cracks may be found, radiating from bolt holes. This is an impending tube failure, and it must be repaired (by sleeving) or the tube must be replaced, before flight. Leading edge tubes may show some slight bending, but it should be equal, between the left and right leading edges. If one leading edge is bent more than the other one (and we are talking about slight bending, only, as being acceptable), then consult your dealer. Some slightly bent tubing can be un-bent, and some can not, but if in doubt, replace the bent tubing.

Airframe connection plates may be showing wear, and possibly gouging by hardware. This damage is usually caused by over-tightening the related bolts, and a lack of paste wax (which is both metal protection, and a dry lubricant). Minor wear may be acceptable, but if 10% of the metal is chewed away, you need a factory-new connection plate. Cracking may also be found in the metal, radiating from the bolt holes. No amount of visible cracking is acceptable, because metal cracking goes beyond what you can see. Worn and cracked connecting plates must be replaced. If you have any doubts, replace the connecting plate. Hey, we do this for *fun!* It's no fun in turbulent air, if you are woried about the airframe.

You can do a fair tubing inspection, when the glider is just set up on the control bar, before the wings are fully spread. You should both look and feel, along the full length of every tube. Your hands can find a dent which your eyes CAN and WILL miss, visually. Even with single-surfaced gliders, you can get your hands (if not your eyes) on a large percentage of the tubing in your glider, before the sail is tensioned.

We do a sail-off inspection because sailcloth can hide corrosion, wear spots, or other more serious airframe damage. Trying to find a dent by feel through sailcloth is perhaps better than nothing, but I do not consider this to be as reliable as hands on a bare tube, even inside the sail. If your glider has a "break-down" shipping option, you can remove this half of each leading edge, for a better inspection. Pay particular attention to the condition of the tubing, anywhere it exits a sleeve. A "kink" or ripple (or any deformation) in the tube there is extremely serious: it is immediate cause for repair or replacement of the tube, BEFORE the glider is flown again.

Bolts can be inspected for wear, rust, or other damage. The cadmium (gold-looking) coating on AN bolts prevents corrosion. Once it is gone, rust may attack. Any bend in a bolt is the most likely place for rust to get inside a bolt (inter-granular corrosion). Do not tolerate a bent bolt, then; replace it. Inspect bolts for bending by rolling them down a plate mirror, with the bolt head just beyond the edge of the mirror. Sight along the plane of the mirror, trying to look under the bolt as it rolls; the mirror will “double” the bend of the bolt, visually. Practice this “eyeball” skill with old household bolts rolling on the mirror. The mirror will let you find and replace even slightly bent bolts.

All tubing and hardware can be waxed to prevent corrosion, using carnauba automotive paste wax. Apply the wax, let it dry, and wipe any excess haze from the airframe tubing and connecting plates. Do not buff. Leave the wax haze on the bolts. The wax is a “dry lubricant” as well as protection from corrosion. Zippers in sails or harnesses can be lubricated with a few swipes of the hard clear wax used by makers of fine furniture and gun-stocks (do not use soft, oily candle wax). Before anybody asks, I do not know how to get liquid lubricant stains out of sailcloth.

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AN bolt science: AN bolts decoded

Inspect the sail stitching, along all seams. With one hand above the sail and one hand below, rub the stitches between clean, flat hands, using a circular motion, moving along each seam as you rub. Good stitching will not be affected. Worn or broken stitches will pop up, indicating a need for some new stitching there. You can do this sewing by hand, with a common sewing needle and UV-proof upholstery thread. You can not break upholstery thread with your bare hands; it would cut skin, before you could break it bare-handed. Most fabric shops will carry upholstery thread.

You can sew the new thread right into the existing stitching-holes, made by the factory. Start where the sail stitching is still good, and sew past the bad stitches, maybe by the width of a hand on each end of the job. For your first pass in this repair process, your stitching should look like this:

/ / / / / / / / /

Your second pass in sewing should look like this:

\ \ \ \ \ \ \ \ \

. . . so that when you are done with the second pass, the completed repair job there will look just like the original stitching, like this:

/\/\/\/\/\/\/\/\/

You can cut and sear the ends of the new thread with a hot knife, or small soldering iron. Get the iron hot, plugged in while in the next room, then bring it to the job, plug it in again, and cut the loose threads with it. Take the hot tool back into the next room, when finished, to cool. Never have a hot tool anywhere around your sail, more than absolutely necessary. Consider a hot tool as a disaster, just looking for a place to happen, when HG sails are involved.

Cable replacement will be about the most common maintenance item on any hang glider, except maybe for battle damage. See the “preflight” cable checks, below, for details about cables. Old cables can pull most of the load of new cables, in static pull testing. What old cables lose with age is the ability to withstand shock-loads. After about four years, depending on use, abuse, and environment, the shock-load capability of old cable goes downhill fast. If you fly a lot, or fly in big air, you may wish to replace the bottom cables after two years, and the top cables maybe half that often. Any cables more than six years old should not be trusted for flight; they should be kept as the patterns needed for new cables, though.

Any new cables obtained for your glider should exactly match these old cables in length. Check new cables against old cables for length by putting one long bolt through each end of the old cable. Put the new cable ends on these same bolts, and pull the pair of cables tight. Any slack in either cable then means the new cable is not the correct length. Sidewires are not very critical in length, but nose-wires and tail-wires can cause the keel to curve in flight, if they are not correct in length.

Curving the keel can cause a constant turn (which the pilot must correct continuously), or the glider may turn easily to one side, and resist turning in the opposite direction. The same thing can happen if the downtubes are not equal in length, left and right, at the bolt-holes. With the downtubes removed from the glider, put one long bolt through the bolt holes of both downtubes, and hold the tubes together and parallel. Then, you should be able to check visually at the other ends of the downtubes, to see if the downtube lengths are equal. For some gliders, the physical length of the downtubes will be critical. For some gliders, the distances between the bolt-holes must be equal, for each tube.

If you plan to make your own new cables, the most important tool needed is a “Go-Gauge.” To my knowledge, only one new cable has ever failed, which was caused by the wrong parts being assembled with the wrong tools, and a TOTAL failure to own and operate a Go-Gauge, to check the finished work. You will need one heavy headless nail for the common end of all cables, and a second headless nail (at the correct distance from the first nail) for each cable that you plan to make. My “cable fixture” is now part of my garage. Label each “second” nail as the glider-type and wire-name it will produce, such as “Falcon 195 sidewire.” If you can fit your old cables having zero-slack on your cable fixture, you are good to go. Simply make up the new cables, with zero slack on your cable fixture. Always make the longest cables first. Then, if there is a mistake, you can cut off a bad end, and make a shorter cable from the longer "bad" cable later.

I recommend using two NICO sleeves at each end of each cable, separated by two inches (5cm) of cable. There are several reasons for this practice, which is done by most HG manufacturers. In brief, a single-NICO cable will almost always fail at the NICO, when pulled to destruction. A double-NICO cable will usually fail somewhere in mid-span, when pulled to destruction. No single NICO has ever slipped, to my knowledge, if it was the correct NICO for the cable, crimped with a proper swaging tool, and checked with a Go-Gauge.

The major mail-order aircraft shops

Aircraft Spruce and Specialties

Wicks Aircraft Supply

can sell you a cheap nuts-and-bolts NICO swaging tool, but these gadgets are a lot of hassle. A bolt-cutter type of swaging tool is much easier to deal with. You can find good quality swaging tools (used by the telephone company) on eBay sometimes. Any NICO swaging tool is acceptable, though, if you check the final assembly with a Go-Gauge. The aircraft shops will have a Go-Gauge and commercial-grade swaging tools for sale, also. Do not make ANY aircraft cables, unless you have a Go-Gauge on hand.

Checking NICO sleeves

If a bolt-cutter type of swaging tool does not produce the correct amount of crimping action, as shown by the Go-Gauge, it will have some cam-type or screw-type adjustments available, to set the clearances needed in the jaws. A poorly crimped NICO sleeve is not acceptable. Make adjustments to the tool and test with scrap cable, until the Go-Gauge passes the resulting work.

A bolt-cutter type of swaging tool does not need to be expensive. As of September, 2011, from Home Depot (in the USA):

Home Depot, USA

A bit about HG assembly and preflight inspections:

I recommend that this hands-on check (or as much as practical) of your tubing should be a part of your preflight inspection, which would be done during the usual assembly process. It can also be done during the tear-down process, if you suspect that any damage may have been done during a noisy landing.

I also recommend two separate checklists: one for the glider assembly, and one for the preflight inspection. Getting it together properly and looking for equipment problems are two separate tasks. I do not recommend that you try to combine them. The tubing check is most practical during the assembly process, but the preflight inspection should be an entirely separate operation, otherwise. These checklists will vary from one glider to the next, so consult with your dealer, and past and present owners of your glider model for the best information, there.

In the most general terms:

You should be inspecting your tubing for any dents, bending, or rippling near sleeves. Any pattern of scratches in the aluminum will only get worse, so determine what causes these scratches, and pad or protect the aluminum from any further damage there. This damage is usually gradual, and may happen when the glider is transported on an inadequate or unpadded vehicle rack. Glider parts may be chafing when in the cover-bag, also, indicating a need for separate pads there, to protect the tubing in transit.

Inspect bolts for security, meaning that all hardware locking systems are in force. Locking (metal or Nyloc) nuts MUST have a minimum of two bolt threads showing beyond the nut, because AN-grade hardware is tapered near the ends of the threads, and the locking system of the nut does not grip fully on these tapered threads. I am very aware that some HG manufacturers do not seem to know this. In those cases, a longer bolt will be required. Bolts may also have nuts secured by safety pins or safety rings. Especially in hard-to-reach places, you should prefer safety rings over safety pins, for non-locking nuts. Safety rings spring CLOSED; safety pins spring OPEN, if jostled. NEVER over-tighten any HG bolt; if you crush (ovalize) the tubing, it becomes weaker at the bolt location. If you remove a Nyloc nut for any reason, it must be discarded and replaced with a new nut, unless you can secure the nut with a locking ring or safety pin through the bolt. You can not restore the lost “locking power” in a used Nyloc nut. All-metal locking nuts may be re-used.

Control bar tubing should be straight. Do not straighten a slightly bent downtube more than once. Do not straighten a badly bent downtube at all; replace it. Any control bar fittings and hardware should be undamaged. There should be a safety cable installed in any basetube, where this is possible. The bolts that assemble the basetube usually hold the ends of this safety cable, inside the basetube. A safety cable is vital, if you are using wheels. A hard landing on wheels can compromise a basetube, to an unknown level of reliability. A safety cable in the basetube then is cheap life insurance.

The sail should show no sun-fading, or missing stitches along any seam. The sail area near the hang straps will carry the most loading. Monitor this area of the sail more carefully, as a matter of course.

Ribs and washout struts will have various ways to be secured in the sail; check each one. Check that the sail is secured properly at each wingtip, and at the rear of the keel.

Cable coatings should be clear, so you can see corrosion if it happens on the cable. Damaged cable coatings will often indicate possible damage to the cable inside, at that spot. Any broken strands in aircraft cables are bad news. A single broken strand almost always indicates more broken strands in that location, maybe where they can not be found. Replace any such frayed cable, before the glider is flown again.

Cables should not be kinked. Any kink (especially near the cable ends) is cause for cable replacement. Do not tension the glider, unless all cables are pulling straight, not twisted in any way. Any cable which tries to become kinked repeatedly has a twist in it, lengthwise. Unbolt the cable at one end, and allow the cable to untwist into a relaxed state. Then install the cable as before, with no twist involved. Any kinked cable must be replaced with a new cable. There is no possible inspection that can tell you if a kinked cable is safe to fly, or not. I recommend that you assume the worst, if a cable gets kinked. New cable is cheaper than the results of limitless optimism.

With the glider fully assembled, check the operation of all moving parts in the glider. Washout struts should pivot as intended. VG systems should operate smoothly, and through the full range of travel. Tie a knot in the VG cord, where the stop-cables prevent the crossbars from going too far forward. This knot will then stop at the VG cleat, and act as a back-up to the stop-cables. This knot may become a nuisance, if your glider does not fold up with the knot in place. I would still choose to have some form of back-up system on the VG stop-cables. It does not need to be a knot in the VG cord, but make it trustworthy.

Sight along each leading edge, and become familiar with the appearance of each wing. Note the twist visible in each wing, and insure that the twist is equal in the left wing, and in the right wing. Check that luff-lines are not twisted, or hung up on any part of the glider. Inspect the primary and secondary hang straps for wear, sun-fade, or contamination by water or chemicals.

Inspect the harness for any loose or missing stitches, untimely wear on ropes or straps, or sun-fade. Check the locking mechanism of the carabiner. Check the parachute pins which normally keep the parachute container closed.

Individual gliders may have other issues to check in the preflight inspection, besides these more universal concerns.

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