What’s the story on Sierra main landing gear doors? Have any owners lost a door in flight? Do the main gear door brackets crack? What is the best way to attach the gear doors?
Technical Editor:
The upper gear door brackets work fine on the Sierra, when properly fitted. You should just barely be able to tell that they have very slight clearance on the upper strut ‘arms’, so that they can move very slightly when needed. They are NOT supposed to be fitted so that they hold the door tightly against the strut, and are hard to ‘wiggle’ with the lower bolt removed. If set that tightly, they WILL crack and fail. Standard heavy washers are used to space the door attach points off of the bracket tabs, as needed for clearance. The idea is to space the doors so that they don’t pull up hard into the wing when retracted, and don’t stress the brackets. Since this is just about the only retract plane on the market in which you can extend the gear while ABOVE normal indicated max cruise speed (155 MPH versus normal indicated cruise of 140 MPH; these are IAS, not TAS), there is no reason for the brackets to be an issue, if they are properly set up. The strong gear enables you to extend the gear for drag and stability, if you are being upset by turbulence in bad weather.
There are two (some would say three) key reasons for door loss. The number one reason is the tire hitting the gear door because of old or wrong donuts. The dead giveaway for this is the presence of visible scrub marks on the inside of the door. This usually occurs on landing, and can crack and break both brackets and the center bolt. It goes unnoticed, and the door departs on the next (or some subsequent) flight.
If they don’t break outright, the brackets will get an incipient crack in the corners where they wrap around the strut. Normal stresses will propagate the crack until failure. But that happens slowly. If the owner normally cycles the gear at as low a speed as is practical, one side of either bracket can be totally separated, without loss of the door in flight. I have had to repair brackets, which had clearly been broken on one side for months if not years. That is the purpose of the clevis pin that connects the two ears of each bracket. If one ear cracks through, the clevis pin ensures that the bracket remains supported by the strut. One error I find is the presence of clevis pins that are much too long. The pin should be just long enough to reach through, with its washer and cotter pin installed, so that a broken bracket still gets well-aligned support from the strut.
The second/third cause is loss of the center bolt, or its head. I have seen them gone, and I have seen them broken. Both events have the same two causes:
– The door is incorrectly installed, with the bolt not bottomed out solidly in its hole; or
– The stress riser from the center hole drilled through the body, coupled with a angled load rather than pure tension, snaps off the head.
LOST BOLT: The factory bolts come with a Nylock insert in the threads, and a grease fitting in the head. People adjust the door fit (to the wing) by turning the bolt in or out, expecting the Nylock to keep it from coming all the way out. It comes out anyway.
BROKEN HEAD: When the bolt backs out far enough, or the tire hits the door, the bolt head is subject to a severe ‘prying’ action. If it doesn’t fail right away, it will the next time the gear is retracted. With the bottom bolt gone, which is the primary structural support (as opposed to positioning support) for the door, the top brackets will rip away, like tearing paper.
Beech is partly to blame for this. The can’t make their minds up on the best way to handle the bolt, and have gone through at least three iterations. Only the latest iteration is now available as parts, and it will not properly position the older doors. If used as outlined, it leaves the lower/outboard end of the door too far from the wing. Due to the thickness of the spacer, the door cannot be adjusted further in/up. The sole advantages of this system are the one-piece retaining flange and spacer, and the solidly-installed bolt. The spacer fits the door hole, and has an integral retaining flange on the outside. The very thin Teflon anti-chafe washer goes over the bushing, under the flange. The bushing goes through the door, and a rubber grommet is placed over the protruding bushing. Then the bushing is bolted down solidly to the knee pin.
Unfortunately the new bushing is too long for most existing doors; and the grommet holds the door out from the knee. This keeps the door from mating properly with the wing. There are several possible solutions. All begin with putting the heavy Teflon anti-chafe tape (FAA-approved, available from Spruce) on the surface of the strut knee, where the door is likely to touch it.
– Use a hacksaw to narrow the bushing, and refinish the surface. It is tough stainless steel, but it will cut fine with a good blade. You want the bushing to pull the door down until it is snugly against the tape. If you pull it any further, tightening the bolt will actually bow the door the wrong way (away from the wing). Then you can solidly install the bolt, with the bushing tight against the knee pin. If you find that the bushing is too tight/narrow, and is bowing the door, just use proper washers/spacers to bring it back out as needed. The washers should have the same OD as the pin and bushing, the same ID as the AN5 bolt, and go between the pin and bushing.
– The alternative is to use the original parts (spacer and retaining washer), but with the same technique. A spacer with the AN5 bore, the OD of the door hole, and slightly more than the thickness of the door, is placed in the door hole. The Teflon washer is placed over the slightly protruding outside. An AN970 washer (fender-style, but high-strength) goes against the spacer, the bolt installed, and the assembly bolted down tightly against the pin. This setup will usually require some small thickness of additional spacers or washers between the pin and door spacer, in order to get the door properly positioned against pin and wing. It isn’t quite as ‘neat’ as the later style one-piece flanged SS bushing, but it is more adjustable and works fine.
The final weak link is the grease fitting bolt. Beech originally used a standard AN5 bolt, and the knee pins had to be disassembled to be lubricated. When the greaseable pins came along, so did the hollow bolt with Nylock thread insert and drilled center (with the fitting in the head). This approach was taken with the expectation that a grommet behind the door would push the door out, while the bolt would pull the door down. The bolt would be left wherever the door fit right, and the Nylock would keep the bolt in its hole. Doesn’t work. I strongly recommend that the door be spaced as needed using solid spacers or properly-sized washers; and that the bolt be tightened down solidly against the stack. I further recommend that the grease-bolt be used only during the 100-hour lubrication. A standard, solid AN5 bolt, of the proper length, should be used to install the door for flight. This approach will end bolt loss and breakage. It further enables you to make your own lube bolt, if you don’t have one, without spending the fabulous sum that Beech wants for what used to be a ten-dollar bolt.
To reiterate:
Remove the rubber seal strips on the doors, and get the old adhesive off. Use a file to ‘draw-file’ the door edges where they abut the wing skin (when the gear is up). Touch-up with epoxy primer and paint. Apply a strip of Teflon anti-chafe tape to the wing skin, where the door edges might touch. Rig the gear and door so that the door edges lightly touch the tape.
When you rig the fore and aft top brackets, make sure that they are not ‘pre-loaded’ with stress on them; it will cause them to crack. I have a very few new brackets, if you need one or more. Some people line the bracket with a layer of inner-tube rubber or thin chafe strip, where they wrap around the gear leg. I have not experimented with that.
At the bottom retaining bolt, definitely use the solid AN5 bolt. Apply chafe tape to the knee, where the door contacts the knee when it is held flush by hand. Measure the distance from the face of the door to the end of the knee pin.
Use the flanged, one-piece, stainless-steel door retaining grip-bushing, P/N 169-110029-61… Use a good hacksaw to trim the bushing to the correct length. Finish off the sawed surface.
Install Mylar anti-chafe washer P/N 169-110029-63 on the bushing, and put the bushing through the gear door hole. Use an internal-serration lockwasher, and install the AN5 bolt. Make sure that the bolt grip length is such that the threads get full engagement into the knee pin, and torque the bolt. With the lockwasher, the solidly stacked-up parts, and the torqued AN5 bolt, the bolt will neither come loose nor break.
I avoid using Loc-Tite on the threads. If you use it, fragments are left behind when the bolt is removed, and the greasing bolt is installed. The fragments can then get pumped into the grease passages, where they can cause clogging. I used to use Loc-Tite with the hollow factory bolt, designed to remain in place for greasing; in part because the bolt could not be torqued down tight.
Some additional notes:
If you make the bushing too short, the door will be pulled in at the center when you torque the AN5 bolt. That will actually cause the edges to bow away from the wing, making a snug fit very difficult. If this happens, you can use wide-flange -5 washers to space out the bushing from the knee pin. I find that Grade-8 SAE washers work well. They are wider than AN960 washers, and are Cad-plated like AN washers; good quality steel.
If you make the bushing too long, the door will be too loose on the bushing. You want to get a slight compression of the door against the anti-chafe tape on the side of the gear knee.
A key point is to have a solid stack-up of the parts. No free play, no big rubber grommets.
If you are having trouble getting the gear doors close to the wing, without the seals, check the gear rigging. The up-stops have to be adjusted virtually all the way up. There is supposed to be a significant minimum air gap between the up-lock hook and the flat on the gear leg (it is specified in the Shop Manual). I often find the gear dragging on the hook, rather than well above it.