Rigging Checks and adjustments

Rigging Checks

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-- at 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 stabilizers without enough UP travel that caused the pilot to always land at 90 mph, 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.

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 mechanicsm 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 is subject to several possible faults. The cables can be incorrectly tensioned, the adjustment of the front cables to the control yoke and be miss-ajusted 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, and they 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.

<To be continued>