Schreder Sailplane Annual Inspection Guidlines
by
Bob Kuykendall

In general, I believe that owner-assisted annuals are the best kind of condition inspection. The owner has a vested interest in both the safety and economy of the process, and often has more effective resources for information than the A&P. The owner is also often more familiar with the peculiarities of the aircraft, and has more time and patience to do it while causing the least amount of trauma to the aircraft.

I prefer to divide the annual condition inspection into the following five distinct phases:

1.   Opening up
2.   Cleaning up
3.   Inspection
4.   Lubrication
5.   Closing up

Itís hard if not impossible to put together a list of must-dos and inspection points for a homebuilt aircraft. The variation between individuals of a type means that a list that works for one example might miss critical points on another. In general, all you can do is look at the aircraft from the perspective of the question "What things are critical for safe operation?" and go from there.

Iíd be inclined to say that the most important element is structural integrity. So Iíd start by looking at all the critical structural parts for things that are cracked, broken, bent, or corroded. The second most important element is correct operation of primary (pitch, roll, yaw) control systems. So Iíd check the operation of the systems for ease of motion and freedom from any actual or potential binding.

Next, Iíd move on to make the same checks for secondary structural and control systems; things like flaps, landing gear, avionics, plumbing, and wiring.

The list below is my starting point for the must-check items on most HP/RS-series aircraft. It is based on my experience operating HP-11 and HP-18 ships, and on what Iíve read about HPs. Feel free to suggest additions or refinements:

1.   Opening up: Youíll remove all reasonable inspection ports, hatches, covers, and fairings. This means lots of screws running around loose, and lots of potential for damaging screws, screw holes, and covers. Use a separate snack-sized baggie for each cover, and mark the baggie with the location. Take your time, and pay special attention to the engagement between your tools and the fasteners. Every screw that you donít break, strip, or lose saves you about 20 minutes in the overall removal/replacement process.

For the typical HP, you will remove some permutation of the following:

         Remove the seat bottom, stick mechanism cover, and forward floor panel (if fitted and removable).

         Remove the floor panels (if fitted) from the right and left saddle wells (the wells outboard of the landing gear well).

         Remove the landing gear well top cover if fitted. If one is not fitted, you should probably make one to reduce drag. For the steel-framed ships, use a piece of thin Plexiglas and some refrigerator magnet tape.

         Remove the tailcone stinger (the fairing aft of the stabilizer attach plate).

         Remove the forward, center, and wing canopies.

         Remove the stabilizer fairings if they are removable. Many HP-11 have the fairings held on with sheet metal screws. Most if not all HP-18 and RS-15 have the fairings bonded to the aft fuselage. If the fairings are bonded on, youíll want to disengage the lower pin and fold the stabilizer to allow inspection access to the attachments. If the fairing is bonded on, and the surface is semi-permanently bolted or taper-pinned on, youíre sort of painted into a corner.

2.   Cleaning up: Use brushes, rags, and a vacuum cleaner to remove all dust, dirt, lint, twigs, grass, and other foreign material from the areas you opened up. Itís amazing how much of that stuff can accumulate over a season.  It all adds weight, and it all has potential for interfering with the aircraft systems.

In particular, youíll want to be on the lookout for mouse and bird droppings. They are corrosive, and indicate that there may be beasties and whatnot distributed into places that you canít see.

3.   Inspection: The inspection will basically consist of making sure that the aircraft conforms to acceptable practices and is of a condition for safe operation. Unfortunately, whatís acceptable can vary widely between mechanics and can even vary situationally between aircraft. Thatís one of the reasons that many mechanics wonít do homebuilt inspections - they donít have any solid guides to go by. In my experience, most A&Ps will inspect a homebuilt with an eye towards making sure that it meets up with what they know of AC43.13. I suppose that some may want to look at the plans as well, but Iíve never encountered such a situation.

Anyhow, for most HPs, these are some of the things I pay special attention to, in no particular order:

         In general, anything thatís supposed to move should be checked for smooth motion, with no binding, interference, or stickiness. Anything that should not move (like wings, structural components, and stabilizers) should be checked for security and stiffness. The wings and stabilizers, in particular, might have a bit of play, but excessive play should be noted and addressed. I like the wings to have no more than about ĺ" of fore-aft play and an inch or so of vertical play at the tip. As Iíve written before, the size and weight of the wings, and the relative limberness of all of the glider parts, makes it difficult to get a good assessment of wing play. For the stabilizers, I like to have no more than about 3/8" of vertical (rather, diagonal) play, and about 1/8" of fore-aft play at the tip.

         Carefully inspect the ruddervator drive angles where they go into the pockets on the ruddervator drivers (those things made with Ĺ" square steel tubing). The drivers are steel, and the drive angles are aluminum, so thereís potential there for galvanic corrosion. I like to keep mine painted and greased, but I still look closely at them at every opportunity.

         Inspect the cotter pins on the clevis pins that the ruddervator drivers pivot on. They are in a position that doesnít allow for much freedom of motion, and they tend to get beat up.

         For the flap drive torque tube, also check the cotter pins (if fitted). On the HP-18 in particular, the cotters get beat up like they do on the ruddervator driver clevises.

         In fact, make it a point to track down and inspect every cotter pin in every clevis pin on the aircraft. Dick used a lot of clevis pins in his designs, because it saves weight and parts, making things lighter and simpler.

Going off on a tangent, on occasion thereís been a bit of a philosophical debate about which direction clevis pins should be installed. Some people say you should always install them head-up so that gravity will tend to hold them in place in the unlikely event that the cotter pin is omitted or breaks. But in my HP-11, almost all of the clevis pins in the control system are installed head-down. The reason that I like it better that way is that it makes it easier to inspect the critical part of the connection. When you look at the head of a clevis pin, you usually canít tell whether or not thereís a cotter pin through the hole at the other end. All you can tell is that the head hasnít broken off. And, címon, has anybody ever seen a clevis pin with the head broken off? But when youíre looking at the tail of a clevis pin, you can tell in an instant whether itís cottered.

         Inspect the stabilizer attach pins or bolts for security and freedom from corrosion. For HPs with spring-loaded pins, make sure that the pin is cross-bolted per the safety bulletin. If the pin is cross-pinned, I strongly recommend that you replace the pin with an AN-3 bolt as described in this bulletin.

         Use a flashlight and inspection mirror to carefully inspect the aft bulkhead to which the ľ" tailplate is bolted. That hefty-looking plate is bolted to a fairly light formed bulkhead, and there have been reports of cracking where the flange and web of the bulkhead meet. Iíve only seen this in ships where an unsprung tailwheel was fitted. Thatís why I use the coil spring suspension on my HP-18, and the leaf spring on my HP-11.

Anyhow, get right in there and take a critical look at where the flange meets the web. If itís dusty or dirty, clean it off to get a good look. But if itís painted, you probably shouldnít go to the length of stripping or rubbing the paint off. That would expose the part to greater corrosion potential. Besides, if the part is painted and it cracks, the paint will most likely crack as well.

         Inspect the stabilizer fittings for looseness, corrosion, or trauma.

         Inspect the stabilizer and ruddervator exterior for corrosion or damage.

On any bonded HP, youíd want to take a close look at the ruddervator trailing edge for any signs of trauma, cracking, or delamination along the bond line. Youíd also want to make sure that there are at least two rivets in each trailing edge, one at each end of the bond line.

         While youíre at it, go to the wings and check the flap and aileron sections to the same criteria as the ruddervators. Be especially mindful of the inboard flap sections, since they carry the entire cumulative torsional loads of all of the flap sections on the wing. And again, check for the presence of the anti-peel rivets at each end of the trailing edge bond line.

         Check the rivet joints between the ruddervator tip plate and the ruddervator skin for cracks or trauma, especially on the underside near the hinge axis.

         Sight carefully up the aft fuselage to look for any sign of wrinkles, loose rivets, or trauma.

         At the wing carrythrough(s) on the fuselage, look carefully at all pin holes for signs of excessive trauma. They will always look at least a little bit beat up. However, there should be no signs of heavy gouging or notching in the holes or at their faces.

         On HP-18, use your inspection mirror and flashlight to check the aluminum stiffener plates installed on the bottom surface of the wing deck where AN4 bolts secure the five aluminum carrythroughs to the deck. Look for cracks or signs of overtightening. Udo has suggested replacing the plates with thicker aluminum or steel plates to better distribute lift loads. If you are so inclined, be sure to inspect the underlying fiberglass for trauma while the plates are removed.

         On HP-18 and RS-15, check for binding between the parts of the aileron and flap control system. On some ships, there may be an issue where the aileron bellcranks or rod ends contact the transverse flap torque tube before reaching the specified aileron travel. Look for scratches on the transverse tube that indicate interference.

         Carefully inspect all cable runs, and run a cloth over as much of the cable as you can to look for broken strands. Wherever there are any fairleads or rub points, carefully inspect the cable to make sure that no single filament is worn more than 50% of the way through. There are diagrams in AC 43.13 that show the characteristic "hourglass" shape of a strand that is worn too far.

Be especially critical of areas where phenolic material is used for a fairlead. Studies have shown that the grain of the phenolic material tends to become embedded with abrasive particles, which accelerates cable wear. In my HP-11, Iíve replaced all phenolic fairleads with blocks of Teflon or Nylon-6. It wears faster than the phenolic, but the expensive and hard-to-replace cables wear much more slowly.

         Carefully inspect all exposed push-pull tubes for excessive wear, kinks, scratches, gouges, or wrinkles. If your ship tends to honk or squeak on pitch or rudder inputs, consider removing and waxing the push-pull tubes inside the aft fuselage. However, if you do that, be very careful not to damage or disrupt any part of the control system while removing or replacing the tubes. Be careful to replace them on the correct sides, and with the correct end forward.

Getting off-topic, thereís always the possibility of an inspection actually making an aircraft less safe than it was before, by damaging or disrupting parts or systems that were happy and functional before the inspection intruded upon them. So you must seek balance between functional inspection and disruptive intrusion.

         Also check the aileron P-P tubes that run up the flap coves on the wing.

Make sure that the tube sections are securely joined at the rivet joints.  Check that the Teflon or nylon parts of the guides are secure and not cracked or broken.

         Carefully inspect all electrical wires, liquid or gas plumbing, and other secondary systems. For the most part, the criteria of the inspection should correspond with the criticality of the system. However, all such stuff must be inspected to make sure that it will not interfere with, damage, or intrude upon any of the primary structural or control systems. Wires and hoses should be confined to neat bundles that are well supported away from moving parts. Urine drains should be inspected for security, and to make sure that they donít leak or spray their corrosive payload into the aircraft. Copper oxygen plumbing should be free of kinks, and should be insulated from aluminum structural members to prevent chafing and galvanic reaction. Also, long runs of copper tubing should contain at least one loop of tubing to absorb physical expansion and contraction from temperature changes.

         Run the gear slowly through a complete extension and retraction cycle. Or better yet, get someone else to cycle the gear while you watch. There should be no binding or rubbing, and especially no potential for binding that prevents the wheel from extending. If there is excessive friction to the system, find out why and fix it.

         Check the landing gear door springs. If you have actual steel springs, check that they canít bind and block extension or retraction. If you have chunks of bungee cord, check that thereís still some bounce to them, and that theyíre not rotten or mouse-eaten.

         Check the wheel and tire for cracks, weathering, and excessive wear. For HPs with the band-on-tire brake, check that the tire isnít worn too far through where the band touches it. Spin the tire and eyeball it for flat spots, wobbles, or lack of balance. Glider tires rarely wear out from normal use. Far more often, they get flat spotted being parked on too long, cracked from sun exposure, or worn through from touching down with the brakes locked.

         Check the operation of the wheel brake. With the brake applied, you should not be able to turn the wheel by hand. All actuation cables, cable sheaths, or plumbing should be secure and in good condition. For disk brakes, inspect the thickness of the pads as well as you can without removing the caliper.

         Carefully inspect the restraining cable that keeps the oleo struts of the landing gear from exceeding its travel length, and replace it if you find any broken strands or unusual wear. There has been at least one incident where one of these has broken, caused the gear to come apart and self-retract.

Most worrisome is the possibility that parts of the unrestrained gear mechanism might get shoved through the forward bulkhead of the landing gear well. Thatís a very good reason to keep an eye on the condition of the restraining cable, and also a good reason to always wear a parachute for a little extra protection from flailing parts.

         Inspect the landing gear well for mouse intrusion potential. Any opening greater than about ľ" inch should be screened, shielded, or blocked to keep mice out. Mice will chew up seats, wires, and plumbing, can be a health hazard, can interfere with controls, and are generally corrosive. Do what you can to keep them out, but be careful that your mouse exclusion measures donít interfere with any primary or secondary structures or systems.

         For HP-18, make sure that the universal joint for the side stick P-P/torque tube is of the correct type as specified in this safety bulletin.

If the joint has a fixed rubber shield around its working parts, itís probably the correct one. If it is of the open type where you can see the yokes, spider, and pins, itís probably the suspect J-50 joint, and should be replaced.

         For HP-14, check the main wing fittings to make sure that they are not of the hermaphroditic Slingsby type as per this safety bulletin.

The Slingsby forgings are elegantly designed, nicely shaped, well machined, and almost certain to crack. I believe that all of the original Slingsby fittings have been removed from service, but you should check your ship at least at the first inspection. What you are looking for are big blocky fittings machined from aluminum bar stock, with three knuckles on one wing, and two on the other wing. Those are the good ones. The ones you donít want are elegantly curved, have three knuckles on all four fittings, and have steel bushings pressed into the main pin bores. Hereís a picture of a pair of the bad ones.

         For HP-18, carefully inspect the rod ends at the forward ends of the ruddervator P-P tubes. These are prone to abuse from slamming the stick to the full up or full down stops, and there have been one or two that popped open and shed the balls from the ball bearing unit. Fortunately, the design of the mechanism is such that the rod end will remain secure even if so damaged. For the retrofit center stick mechanism Iíve been selling for the HP-18, I have the installer replace the RE3M6-2N with a pair of spherical bearings that allow greater angles of misalignment. But the center stick also has good pitch input stops to protect the rod ends from excessive misaligmnent.

         While youíre at it, inspect all of the rod ends that you can get to; that should be every rod end in the ship except the ones buried in the wing at the inboard end of the aileron. For the most part, Dick used high-quality ball-bearing rod ends like the Fafnir RE3M6-2N. Iím pretty sure that Dick got a great deal on these through WWII surplus. But that means that now all of the rod ends in your aircraft are about 45 years old, and theyíve never been greased since they were manufactured. Fortunately, the original grease is extremely long-lasting stuff. But you should start to think about a program to remove, pressure-grease, and reinstall all of the rod ends. But again, thereís that issue about balance and doing no harm. Maybe keep it in mind for the annual next year.

         Check all of the rod end installations to make sure that there is adequate thread engagement between the rod end and whatever it is threaded into. I like to have things set up so that all of the rod ends have at least 6 threads of engagement, and preferably more like 8. When I first got my HP-18, some of the rod ends had as few as 3 threads of engagement, and that made me nervous (but not until I found out about it). Iíve heard that engineering studies of threaded joints show that most of the load is carried by the first thread and a half. That may be so, but I like to have a several-x safety factor for such joints.

Going off on a tangent, letís do the math for a typical rod end thread engagement. The shank of the RE3M6-2N has ĺ" of threads at 24 threads per inch. That means that there are 24 x .75 = 18 threads on the shank. Some of the threads will be hidden by the AN316-6 jam nut, which is about 3/16" thick - 3/16 x 24 = 4.5 threads to be exact; lets call it 5 threads. So when I am looking at an RE3M6-2N installation, I like to be able to count no more than 7 threads remaining on the shank - at least 6 threads are hidden by whatever the rod end is attached to, and 5 threads are hidden by the jam nut. Any more than 7 threads, and I get nervous. In some cases, Iíve been able to adjust the rod ends to get more engagement at one end, less engagement at the other, and no overall change in the eye-to-eye length of the push-pull tube. In other cases, Iíve made adapters or just fabricated longer P-P tubes.

         Also check all cable turnbuckles to make sure that they have adequate thread engagement. There should be no more than three threads exposed on the clevises or forks at either end of the barrel. Also check the turnbuckles for secure safety wire wrapping.

         For ships with canopy transparencies that are glued to aluminum frames, check that the transparency is secure. I guess that a few glue voids or spots of delamination would be OK. But anything over a couple of inches drastically increases the potential for the transparency to unzip and depart the aircraft. From reading Flight International, I learned that in the UK they call those BFO Incidents, for "Bits Falling Off."

Dick recommended using layers of contact cement or rubber cement for securing the transparencies. So far, Iíve had good luck using plain old Liquid Nails. Of note, Dick didnít recommend using the EA9430 epoxy that the wings are bonded with. Thereís no record that he recommended against it, but I think that the fact that he had that glue at hand and recommended something else is reason enough not to use it. I suspect that the thermal coefficient difference between aluminum and acrylic is such that a more pliable, less rigid adhesive is required.

Iíve heard that some HP operators have also installed nylon screws at the corners of the transparency and frame as a safety backup for the adhesive.

         For bonded wings, set the wings up on sawhorses, with one stand each at the root and tip of the wing panel. Tap around with a light plastic screwdriver handle or other non-marring tool to look for areas of delamination. Some delamination is okay, but itís really hard to say how much is too much. If there are areas of delamination, youíll want to map them on a sheet of graph paper so that you can monitor them for spread. Flip the wing over to check the bottom surface - just getting under it and looking up at it will show you a falsely optimistic picture of how tight the skins are.

         For riveted wings, check for loose rivets and other badness. In general, the riveted HPs are entirely conventional in their wing construction, and any mechanic familiar with Cherokees and Cessna 172s should be comfortable with them.

         Look carefully at the exposed parts of the wing spar for corrosion, cracks, scratches, or notches. In particular, look for places where the spar has gotten beat up from assembly or disassembly operations. The 7075-T6 material used in the wing spars is very strong stuff, but it does tend to be sensitive to notches that can become crack initiation sites. Minor scratches or defects in the middle of a face are usually inconsequential. But scratches or notches along corners and edges should be addressed.

         Check carefully for places where contact with trailer fittings might have trapped moisture against the wing parts, causing corrosion.

         Check the wing tips for pedestrian impalement potential. Sharp, jagged corners on skids or trailing edges should be dressed.

         Check all interior areas for signs of water entry. Anywhere that water can get in should be protected from the elements, and further, there should be a way for the water to get out. On my HP-11, I have drilled holes in the leading edge of the wings at the tip. During the soaring season, I have these taped over with aluminum foil tape. During the winter, I leave these open to let out any water that might leak into the trailer and get into the wing. When any substantial amount of water gets into a wing, and then freezes, it is bad news.

         In the cockpit, check to make sure that all the paperwork is in order. You should have some permutation of the ARROW documents:

Airworthiness certificate - No exceptions. You probably have a pink Special Airworthiness Certificate issued for the purpose of operating an amateur-built aircraft. It is somewhat a matter of pride to me to have a white Special Airworthiness Certificate for the HP-11, which means that the aircraft is enough of a veteran that the pink coloring has faded completely away.

Registration certificate - Again, no exceptions. The name on the certificate should be you or your corporate incarnation, and the address should be valid.

Radio station license - You only need one of these if youíre going to fly internationally. That probably means you donít need one.

Operating Limitations - For almost all homebuilt gliders, the Special Airworthiness Certificate is only valid when accompanied with a separate list of operating limitations issued by the original inspector. I hope you didnít lose that letter, or file it where it couldnít be found; youíre supposed to carry it in the glider. There should also be a list of operating speeds and limits at hand, although I think you can substitute appropriate markings on the airspeed indicator.

Weight and balance - There should be information at hand about the weight and balance situation of the aircraft. For the HP-11 and the HP-18, I carry diagrams of the datum and empty CG, and the pilot CG. I also carry placards that show the calculated minimum and maximum pilot weights.

         Check to make sure that there is an EXPERIMENTAL marking with 2" letters, and also a passenger warning. You can find the appropriate verbiage and placement info in 14 CFR 49 (what we mostly call the FARs). Few inspectors are really picky about the placement, but it should at least be close to conforming to the rules. If youíre lax on this one, they may go looking specifically for other stuff to bust you on.

         Inspect all of the pitot/static plumbing for security and freedom from foreign matter. You might find that bugs have built nests inside the tubing, but that they didnít completely block the tube. Clean out all such things that you find.

         Check the release mechanism for binding or stickiness. Check to make sure that, when deployed, it takes at least a couple pounds of pressure to pull the retainer back far enough to get the tow ring into it. It should be about as tight as it can be and still let the average lineperson engage the tow ring.

         If an oxygen system is fitted, check the oxygen cylinder for the date of its last inspection. I believe that most of the aluminum and steel cylinders common in glider service must be hydrostatically checked every five years.  That may sound exotic and expensive, but itís actually a cheap and common inspection, and any welding supply shop can have it done for you. However, note that such shops are rarely very good about returing to you the same cylinder that you submitted. So if you care at all about getting back the same cylinder you started with, be sure to paint your name and phone number on the cylinder. You might also consider painting the cylinder an odd blue or green color to distinguish it from all the others. Avoid red or orange colors, so that nobody mistakes it for an acetylene cylinder and blows themselves up. 

4.   Lubrication: Carefully lubricate all pivot points. Take care not to put more oil onto the pivots than capillary action will draw into the joint.  Excessive oil or grease will just attract and hold dust, and the dust will accelerate abrasion and wear. Here are some tips about lubrication:

         Donít use WD-40 as a lubricant; it is primarily a water-displacing protectant, and not a lubricant. Use light machine oil, or SAE 20 weight oil instead. I like using the Tri-Flow product that they sell for bicycles. It comes in both bottles like sewing machine oil, and in spray cans like WD-40.  Once the solvent and propellant evaporates away, the oil that remains has about the consistency of SAE 20 weight, about like youíd want.

         Dick has recommended against oiling piano hinges, so I usually donít.

         Keep oils and greases away from the parts of the oxygen system. A little oil on the outside of the copper plumbing wonít hurt anything. But if you get a little oil on the tubing, and then handle the tubing while disconnecting it, youíll get oil on your hands. And if you handle the parts of the joint while reconnecting it, youíll get oil from your hands inside the system. Then, when you apply 2000 psi to the system, that little bit of oil will oxidize rapidly (read: burn). The heat from that oxidation may weaken the system enough for it to rupture. That rupture will liberate great volumes of 100% oxygen into the interior of the glider. So, the best thing is to keep the parts and surfaces of all oxygen systems clean and free of oils. Use denatured alcohol to wipe off any oil or grease that you do get on oxygen system parts.

I think Iíve written elsewhere that I think that denatured alcohol is the most glider-friendly and glider-useful solvent, and that everyone should have a pint of it in their trailer.

         It is debatable whether oiling ball-bearing units in bellcrank bearings, pulleys, or rod ends does much good. All of these things are factory lubed with fine grease. Adding oil will often just thin the grease enough so that it runs out, leaving the bearing with less lubrication, and not more. The best thing to do with these items is to use a bearing re-greasing tool like Aircraft Spruce sells. Thatís often not practical, since youíd have to remove or disrupt lots of stuff to use the regreaser.

Generally, what I do is to check all bellcrank bearings, pulleys, and rod ends for motion and friction. Ones that are smooth and free, I just leave alone. Ones that are sticky and easy to get at, Iíll remove and regrease.  Ones that are sticky and hard to get at, Iíll try getting some oil into to see if I can free them up enough to go another year. If not, Iíll remove, regrease, and reinstall the bearing.

         For spherical (non-ball-bearing) type rod ends, I use just a drop or two of oil, just enough for capillary action to draw between the ball and the housing.

         Donít oil cables or get solvent on them, especially not near places where the cable bends to go over a pulley or through a fairlead. Cables are factory treated with a thick lubricant that protects them from internal friction between the strands. Any solvent or oil you apply will remove or thin the internal protectant, and also attract dust and other abrasives.  

5.   Closing up: Basically the reverse of the opening-up phase. The difference is that all of the screws and fasteners have been through one cycle, and that there is a distinct possibility that you might have lost one or two of them. Again, take your time and pay attention to tool engagement. Also, be careful not to over-torque small fasteners.

Well, thatís all that occurs to me at the moment. Again, feel free to suggest additions and refinements. bob@hpaircraft.com