Landing Gear Doors

Here you will find help and clarification to properly adjust the main landing gear doors. As an example see the photo below of the foul-fitting MLG door. One would assume this airplane passed its last annual, and part of that annual is a gear swing, right? Then why does this condition still exist? The “help and clarification” article has factory drawings showing correct brake hose routing which helps avoid this condition.

For the threaded tool mentioned in the “help and clarification” article below, order this part number 8419K33 from McMaster.com

Here are a few thoughts to consider if you are having a problem with the main gear doors retracting and/or fitting properly on your Comanche. Read section 6-39 inserted below from the Single Comanche Service Manual; see section 7-30 in the TCSM. There can be multiple issues to deal with while restoring the proper fit and adjustment of these doors.

1. Ascertain that there is an all around even clearance between the door and wing panel when the door is in the
   retracted position.
2. Adjust by loosening the door hinge screws in the wing panel repositioning the door and retightening the screws.
3. The gear door actuating rods should be adjusted so that a nine pound weight suspended at the center line of the inboard edge of the door in the retracted position will create a 0.125 of an inch deflection from the wing contour at the inboard edge of the door.

Per “c” above; read the 6-39 note bellow.

“Ascertain that there is an all around even clearance between the door and wing panel when the door is in the retracted position”. This image is a good illustration of a poor job of fitting the door after a repair.

To correct “a & b” above I would like to pass on 2 additional tips.

Firstly and important to consider, there should be minimum looseness in the gear door hinges. The Piper original and replacement hinges use a pin diameter of 0.116″ in a 0.125″ hinge bore; that makes for a naturally loose condition. Here’s an inexpensive remedy to tighten up the looseness without the laborious operation of replacing the hinges. Webco Aircraft has already accommodated this 1st tip in their PMA hinges.

Remove the AN4 bolt from the upper end of the gear door retraction rod. With the airplane on jacks and the tail tied to an adequate weight, retract the landing gear and remove the offending door. The door removal is done with the gear retracted because that way the strut is away from the area where you need access with your hand and tools to remove the hardware. Now with the door removed and on a suitable work area, drive the old 0.116″ diameter pin out of the hinge. Mount the hinge half in a vise, make sure it is straight, and then drill through the pin bore using a long #30 drill bit with  some lubricant; repeat for the door portion of the hinge. Procure a 1/8″ [0.125″] diameter stainless steel welding rod and cut a piece exactly 4″ long; deburr and chamfer the ends and now you have a new hinge pin which will greatly reduce that loose feeling. Apply a little grease or oil to the pin and assemble the hinge. Close both ends slightly to retain the pin; don’t get aggressive with this, just a little will do.

Next temporarily reinstall the door using the hardware in the center position hole only. Note – use #10 washer head screws [AN525-10R8] which have adequate shank length to capture the total thickness of the rib, hinge, and the skin. Keep in mind that you will need to use 2 #10 thick washers at each position under the lock nut; otherwise the nut may bottom against the shank portion of the screw before clamping the assembly [this is a very common condition from the factory]. Don’t use fully-threaded screws or bolts here.

If you want to avoid the time hassle of procuring these pieces, buy my kit for $30. I know it’s only worth a few bucks, but I have the time involved in procuring the components, making the hinge pins, and packaging; you may have a problem buying just one SS welding rod. This kit has what you need to perform the gear door hinge improvement process for both doors. All nicely packaged in a plastic tube so the hinge pins and the long #30 drill won’t get bent during shipping.

Position the door in the opening with a little forward bias – see note below. Using a fine-line marker, make a mark on the hinge through the holes in the wing skin where the hardware was removed; again remove the door. Using a 1/8” carbide burr and a high-speed rotary tool, or a rat-tailed file, enlarge the fore and aft holes in the hinge to the marked positions. Please don’t drill those holes larger; you may end up with a mess.

Here’s the note – understand that as the door is closing the natural tendency for the door is to be pulled aft due to the geometry of the actuating linkage and the force imposed by the [specified] 9-lb load mentioned in 6-39, C. Keep that note in mind when you adjust the door position, i.e. use a forward bias. Trick number 1 helps with this also.

If you have a bent retraction bracket, now is the time to rectify that condition.

This section A-A above is typical from Piper’s assembly drawings. Check the torque on the hardware where the long bolt attaches the rod end bearing to the strut. If that bolt, either AN8 or AN9, is allowed to rotate from inadequate torque, then the gear door retraction bracket likewise can move during the retraction operation, thereby disallowing proper gear door adjustment. Also look for a bent bracket (see image above) which is quite common after 45 year’s service. Here are the torque values for the long bolts: AN8 = 480 – 690 in-lbs dry; AN9 = 800 – 1000 in-lbs dry.

Note concerning 6-39, C on page 1. The 0.125” deflection means exactly that; you must have some deflection or the closure load on the door is excessive. Excessive and you may have a bent bracket, and stress the gear door near the hinge causing a crack, which is why you typically see that ugly patch repair at that location – again see the photo of the poor fit job after a repair above.

If you plan on doing a bunch of these gear door adjustments, here is a tool suggestion. Purchase an Easy-Adjust Threaded Connecting Rod, 4″ Overall Length, with 1/4″-28 Threaded Female Ends. This rod is aluminum, hex shaped, and has a left hand and a right hand threaded end. Also purchase male threaded rod end bearings and the jamb nuts, one each LH and RH. Use this tool in place of the retraction rod for the adjustment mentioned in 6-39, C. Mimic the length for each retraction rod and install the retraction rod with the correct length preset. You’ll spend about $60 for materials on this tool. See the tool photo below.

Another problem seen in this main gear door area is a damaged, broken, or missing door stop bracket; see the photos and the discussion below.

The main landing gear door stop is a small plate which I have named the “main gear door stop plate”; it is riveted to the wing at the rear of the gear door opening. The purpose is to help align the retracted gear door flush with the lower wing skin. The thin gear door stem bumper P/N 453-887 [in the twins it’s P/N 453-641] is also located here; without that bumper you will chafe the aluminum. And please don’t drill a larger hole to accommodate whatever bumper you can find at the hardware store. Let me know if you need the bumpers; I have both the thick and thin styles. These bumpers are included in my landing gear wiring replacement kits, and the landing gear push/pull cables [conduits] installation kits.

The most common cause of damage to this plate is from improper routing of the brake hose. Possibly when the brake hose is removed and not properly routed and secured when reinstalled, it can foul that plate upon gear retraction. See the damaged plate in the photo below and the drawing insets on pages 6 and 7 which show brake hose routing.

Another less common condition is improper orientation of the AN4 bolt at the gear fork where the strut oleo tube fits. If that bolt head is not aft, the lower torque link will touch the bolt. Then the oleo will not fully extend and the pivot between the torque links will be too far aft and consequently contact the plate and bend it upon retraction. This can be assembled incorrectly after tube replacement or other service/repair.

And yet another cause; retracting the gear with a deflated/compressed oleo.

This plate is not to be found by me in the parts catalogs [I have wasted way too much time looking, but it has to be there!], and one would assume it would be like so many other Comanche parts, NLA. So I had a fabricating shop make a set of forming dies to duplicate the plate including the joggle. I felt a doubler was a good addition; they’re $30 each which I know is too much. Remember I had to pay for the forming dies and forming the parts. Since this is an un-approved part from me, it might be a good idea if you did the “ownerproduced” route; or just fabricate your own – that’s called a disclaimer in legal parlance.

Plate, main gear door stop with thin rubber stem bumper.

Plate, main gear door stop repaired showing added doubler.

Fabricated main gear door stop plate with doubler; call this ownerproduced. The 3/16″ hole is for the rubber stem bumper. $30

Right side brake line location from typical Piper assembly drawing. Note this routing applies to the single-fork gear which has the brake caliper on the inboard side of the wheel.

Left side brake line location from typical Piper assembly drawing. Note this routing applies to the single-fork gear which has the brake caliper on the inboard side of the wheel.

Nose gear door problem-solver; read this below even if you don’t know you have a problem with those doors.

And FWIW, whoever installed the landing gear wiring harness kit in this 400 clearly didn’t bother to read and follow the instructions manual. This harness is routed incorrectly and is not secured with the cushioned clamps and hardware that I supply with all the kits. Another example of how men just cannot follow instructions, although I try really hard to understand what she really means!

Here is the correct routing and cushioned clamps locations for the nose gear harness on a 400.