Rigging Checks and adjustments

Introduction

Mechanical Difficulty:

This is a job which an owner/pilot can do without any additional supervision or sign off, because you are only CHECKING the control travel, not making adjustments to the primary flight controls.

Checking the rigging on a Grumman is a very simple job if you have the factory templates shown in the Maintenance Manual. They are available for rent from FletchAir on a weekly basis.

Many rigging checks can be made without the factory templates, with slightly more effort. Each year at the AYA Convention, the rigging templates and the Rigging Check volunteers are available to help owners check for discrepancies in their rigging of their planes.

The goal of rigging checks (and adjustments as needed) is to bring the plane into compliance with its Type Certificate Data Sheet specs, and to improve the handling and performance of the plane. In the hundreds of planes checked at the AYA conventions since 1998-- the first Cody Convention when I spearheaded the effort to start the rigging checks, I have yet to see ONE PLANE that had correct rigging on all surfaces. All that I have personally checked had at least 2 measurements out of spec. Too little or too much control displacement, but always at least 2 out of spec!

Significant performance changes can be obtained by removing the errors and getting the plane correctly rigged. At conventions its been sort of a tradition to help correct the plane that "wins" the worst rigged plane award. There have been elevators without enough UP travel that caused the pilot to always have to land at 90 mph in order to be able to flare, planes with bent outboard aileron bearing brackets that had made it impossible to fly "hands off" since the plane was purchased by its current owner, and an amazing array of other rigging errors that induced all sorts of strange handling characteristics that the owners just put up with. Then there were those rigged so poorly that they had significant performance deficits. Some people have spent many thousands of dollars for "speed mods", and found more performance in FREE rigging adjustments than in all their combined speed mod improvements.

Rigging Adjustments

Mechanical Difficulty:

Any adjustments to the flight controls requires appropriate certificates and a log book sign off. You will need an Airframe mechanic to supervise and sign off the following work.

After rigging checks determine that there are control travels outside the specified limits, the next step is to attempt to correct the errors. An experienced A&P should be able to use the factory templates, the maintenance manual, and perhaps this page of instructions to correctly rig any Grumman.

Elevator and Rudder

The adjustments are under the plastic tail cone. They consist of 4 stop bolts with jam nuts on them to prevent unwanted movement of the stops. Loosen the jam nut on the stop in question (7/16" offset box end wrench is the best tool I've found), and then turn the stop bolt IN for more travel and OUT for less. Re-check with template or protractor to confirm the adjustment, and when correct, re-snug the jam nut. Most of the Type Certificate Data Sheet specs give a range of travel that is acceptable. Adjusting to the maximum travel will give the most control authority, and may provide the last bit of control that you need some day.

Tip: The lower rudder bearing is frequently worn or extruded from cold flow as a result of the constant tension of the rudder cables. To test for this, slowly push the rudder to the stop, such that the control horn JUST touches the stop. Now look closely at the lower bearing and support bracket, and gently push the rudder more firmly against the stop. You will almost certainly see some movement of the rudder torque tube in the bearing. If so, then the rudder will move beyond the initial limits set by the stop as the pivot point moves and changes the geometry of the hinge and stop. A new bearing insert will tighten the clearance up significantly.

Elevator Trim

The trim tab can not be "adjusted" for total travel, but it can be adjusted to center the travel such that the UP and DN limits are correct. The most common cause of elevator trim miss-rigging is lack of maintenance. The trim wheel in the console between the front seats drives the indicator, and it also drives a pair of bevel gears that convert the motion of the wheel into rotation of the trim drive, a long aluminum tube just above the floor in the tail cone. Failure to clean the old dried grease and grit from the gears, from the bearings that support the trim drive tube, and from the threads and actuator nut in the plastic tail cone can lead to excess torque required to move the trim. This can cause the bevel gears to slip, and cause the trim position indicator to no longer be sync'd with the actual position of the trim. The fix is to clean all the trim drive parts and re-lubricate them, and then set the indicator to match the elevator trim by disengaging the bevel gears and re-meshing them. Its also important to check the backlash in the gears and adjust the depth of their engagement, so that the trim won't easily return to its mis-adjusted state.

Flaps

The first step is to inspect for the correct AMOUNT of total travel. If the flap motor and drive assembly is moving the flaps too much or too little travel, then its probably a limit switch problem, either adjustment or failure. If the correct amount of travel in total degrees of movement is correct, then the flap drive mechanism can be adjusted via the Heim joints on each side to place the flap in the correct position at the UP stop. The Heim joints, one pair for each side, are located below and just outboard of the back seat cushion. The seat bottom must be flipped over and the canvas cover removed to be able to see the adjustments. Begin by loosening the jam nut on the threaded area between the joints. Then remove the bolt in the top or bottom Heim joint and allow the joint to disconnect from the flap arms. Rotate the Heim joints relative to one another. Closer together raises the flaps and further apart extends them. Insert the bolt to hold the joint to the flap drive arm and check position of the flap. Repeat until the flap is positioned correctly per the template or protractor. Secure the joint by installing the nut removed earlier and properly snug the jam nut on the threads between the joints. Repeat for the opposite side.

Tip Many planes have excessive play in the flaps. This seems especially prevalent on the AA-5 series, which have a more complex flap drive and more places to allow wear and increase the play. This would also be a good time to verify that the aileron torque tube is accurately centered within the flap torque tube. This keeps the pins on the special flap drive bolts from possibly wearing a groove in the aileron torque tube.

Ailerons

Rigging the ailerons are subject to several possible faults. The cables can be incorrectly tensioned, the adjustment of the front cables to the control yoke can be mis-adjusted relative to the aft "turn around" cable that connects the 2 cables from the yokes, the aileron outboard bracket can be bent or damaged, and the outboard aileron bracket can be positioned incorrectly on the aft spar. Any of these can cause large errors in the angular position of the ailerons, and also synchronization problems side to side.

Cable tensioning requires a cable tensiometer, which ought to have its calibration checked, if you are unsure of its testing and certification status. The basic operation is to clamp the tensiometer onto the cable in question and read the tension on the scale appropriate for the diameter of the cable. Adjustments are made by removing the safety wire or locking clip that holds the turnbuckle such that it can not let the cables unscrew from its ends. The first thing one needs to know about turnbuckles is that they have RH threads on one end, and LH threads on the other, such that if one holds the cable ends, and rotate the turnbuckle, you will either loosen or tighten the cable tension. It is important that you not turn one cable to change the tension.

The safety clips or the safety wire are NOT to be reused, so be sure to have some on hand before you start. Additionally you will need to be familiar with the correct way to safety wire a turnbuckle, you might be surprised at the number of A&Ps that can't do it right without looking at the book. The advantage of the MS21256 style safety clips is that they are quick and easy to install correctly. (Note that both the -1 and the longer -2 size clips are needed on Grummans, depending on which cable system is being worked on.} They clips can be installed one-handed in a tight spot where trying to wrap safety wire would be very difficult. I've seen a mix of turnbuckles installed on the fleet. Some have the groove in the threads for the safety clips, and some don't, so safety wire may be the only option in some cases.

Repairs When Adjustments Won't Correct the Rigging

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Sometimes there is damage to the plane, or in rarer cases, planes that left the factory without the means to achieve the correct control travel. In these cases, more extreme measures are needed. The good news is that the approved data to make these repairs has already been published in most every case.

Tip

Because these procedures involve work that is very Grumman specific, and in most all cases new holes being drilled, these may be jobs that should involve one of the more experienced Grumman mechanics and some scheduling and travel, rather than giving the local Cessna dealer the job.

Drooping Aileron Counterweight

Incorrect drilling by the factory, or in the case of a new aileron being installed in the field, incorrect drilling by the installer. Unique to the AA-5x series. Grumman published a factory repair. You can tell that your plane has this problem if you hold each aileron centered by putting your thumb and finger on the trailing edge of the wing tip and holding the aileron such that it has equal thickness above and below the wing tip, and then reaching under the wing tip to feel if the aileron counterweight is 1/4"-3/8" inside the wing tip. If its flush with the lower surface, then its a little out of position, and if its actually below the lower surface, then its far enough out of position that its affecting the up and down limits of travel, and should be re-drilled per the factory instructions.

Aileron throw incorrect

See Drooping aileron counterweight as one possible cause. Another is a bent or mis-positioned outboard aileron bearing bracket. The proper repair for a bent aileron outboard bearing bracket is a NEW BRACKET from FletchAir. To determine if the bracket is bent, remove it and check on a flat surface to see if the center line of the bearing hole is the same, no matter which side you view it from. Check the bracket with a square to see that it does not lean to either side. To detect a mis-positioned outboard bearing bracket, measure the centerline of the bearing bracket from both surfaces of the wing (after confirming the bracket is straight, per the above instructions), and if not, examine the aft spar and the mounting holes for evidence that the bracket was drilled off center top to bottom. To repair a mis-positioned outboard bearing bracket, a doubler of at least the thickness of the aft spar is fabricated to fill the space on the front side of the aft spar (note proper radius to match the stamped curve in the rib), and it is double-flush riveted, such that the new holes to be drilled in the aft spar will not interfere with the rivets. The bearing bracket is clamped into place and the new holes are drilled through the spar and the doubler. Slightly longer AN3 bolts will be needed to account for the spar doubler.

If the travel is reduced in both directions, it might be the case that the aileron stops were installed on the wrong side of the bracket by a mechanic following the Maintenance Manual.  The manual has an incorrect drawing, and the stop should be installed this way.

Split Elevator

When each half of the elevator is at a different angle. Check for this by aligning one elevator tip with the horizontal stabilizer, and then measuring the opposite tip to see if its mis-aligned with the stabilizer. Even slight differences in the zero position of the two tips can induce a rolling moment that must be corrected by adjusting the aileron trim tabs. This causes drag from the elevators and from the ailerons, and can add up to several knots of speed lost on the top end. The factory designed repair is the align the tips and ream the bolt hole(s) with a "G" letter reamer and install an NAS 6604 "X" oversize bolt to fill the larger hole. This mates the elevator half to the control horn and can cause fit up issues should the elevator ever need to be replaced. Because the oversize bolt is only ~0.015" oversize, there is only a small amount of adjustment in elevator alignment available. It is suggested that one side be re-drilled, moving the elevator halves as close to aligned as the bolt diameter increase will allow, and then, if needed, the other side be drilled to center it to the first side, thus doubling the total error that can be corrected. After reaming the holes it will be necessary check and adjust the elevator stops for proper travel.

Excess Play in Flaps

There are a number of causes, all wear related. The difficulty is in locating the real cause.

It can be the bearings on the flap drive tubes, the bracket that holds the flap drive tube on the inside of the honeycomb, the flap motor bracket cracking under the motor where it can not be seen unless the motor is removed, the bearings at the ends of the jack shaft that transfers the movement to the other side, or in the links that connect the jack shaft to the arms on the flap drive tubes. A little play in 3 or 4 locations can be several degrees by the time its all added up at the flap trailing edge. If your flaps have play, its probably a good plan to start on the list above and check/repair each item until you have eliminated the play.

Split Flaps

Although probably not showing up as a rigging problem right now, ANY history of split flap extension requires careful investigation, because the problem did not "heal" itself, and lies dormant waiting for a bad moment to re-appear. The flaps are driven by an aluminum arm that clamps the flap drive tube and is pinned by a pair of special bolts. The tip of the bolt has the threads removed, such that a round pin is created, and this pin fits into a hole in the flap drive tube. Improper assembly causes the flap drive tube to not be properly aligned when the pinning bolts are tightened, and this causes the bolt to deform the tube rather than to lock it in place. This leaves only the friction of the clamp to hold the flap in alignment, and sets the stage for split flaps. The repair is to remove the flap drive tube and either do some blacksmithing to raise the dimpled metal and restore the holes to flush with the rest of the tube, or to swap sides with the tubes and use the other set of factory drilled holes to install the pinning bolts correctly.