Straighten Up & Fly Right

Flight Control Rigging

July 2014

This month’s ShopTalk will educate the reader on aircraft flight control rigging. We will show the reader how to evaluate their aircraft and verify the flight controls are properly rigged and how to correct most discrepancies. This ShopTalk is focused on Mooney airplanes, but much is applicable to Pipers. Both utilize engine mount offsets to compensate for engine torque and propeller wash and P‑factor. Cessnas utilize elevator and rudder offsets to balance these forces.

Technically and legally speaking, your plane was not certified by the FAA to be out-of-rig and there are no optional documents from any aircraft builder allowing you to fly that airplane in that condition. As we all know, during an annual, when your IA mechanic signs his or her name and certificate number in the airframe log book, that aircraft meets its type certificate data all the way back to when it was manufactured. This includes all supplemental data (STCs), and all optional approved items that may have been installed over the years (337s).

To be able to correctly do this job some special tools are required: airplane jacks, a digital level, Mooney rudder and aileron/flap travel boards, a straight three-foot bar to tape to the bottom of the control wheels and a ten-inch straight bar along with two C-clamps to clamp this bar across the pilot's rudder pedals and, of course, the correct maintenance manual for this aircraft. Please study the maintenance manual as the first step in this process. Some of the procedures mentioned in this articles are detailed in the manual. For instance, where do you measure that the airplane is level? By the way, most of this work must be done by an FAA certified airframe mechanic.

The Flight Test is a simple one but must be done in a specific order, one step at a time. This flight test must be done in an uncongested area, on a smooth day with equal fuel load in both wings. Any lateral imbalance in the fuel load or the cabin load will bias a roll towards the heavy side. If the plane is equipped with any Aerotrim trim tabs, before departure, visually verify the tabs are straight regardless of cockpit indicators. It requires straight and level cruise flight for 10 - 20 miles. The pilot will be concentrating on airplane operation so I strongly recommend utilizing a safety pilot to watch for traffic.

The Ball Test: In straight and level flight, select your usual cruise power setting and and stabilize the airspeed. Turn off the auto pilot and disable the Positive Control system (pull the button out of the yoke). With feet flat on the floor, hold the control wheel so the wings are level with the horizon. If there is rudder trim (not Aero trim) set it to the center of the rudder trim scale. In the turn & bank instrument, record the position of the ball with respect to the hash marks.

The Wing Level Test: Now, using the rudder to keep the ball centered (if not already centered), release the control wheel and begin counting seconds. A properly rigged Mooney will start a very slow bank in about 5-8 seconds. This bank should not start as soon as you release the controls! Record the direction, time counted and severity of roll. Repeat this wing level test a few times to get a trend.

While straight and level, ball centered, note the trailing edge alignment of the ailerons with respect to the flaps.

During landing, note the amount of elevator pitch trim and availability of elevator travel.

This completes The Flight Test.

Back at the hangar, review the flight test notes. If the ball was not centered during The Ball Test or the airplane's tendency is to roll positively to the same side during the wing level test then it is not properly rigged and it’s time to go up on jacks. Place the aircraft on jack stands with the nose wheel off the ground and free to move. Level the airplane for both axes.

Elevator pitch trim and travel limit was noted during the test flight. This is important but will not cause an out-of-rig condition that we focused on during our test flight. Elevator travel limits and pitch trim adjustments require an elevator/trim travel board. I rarely see these limits out of tolerance, but if the pitch limits are suspect, the trim/elevator travel need to be verified as correct (especially with the long nosed Mooney M20K,M,S,R,TN). Adjustment procedures are beyond the scope of this article.

Now that the airplane is up on jacks and level, the first item to look at is the turn and bank instrument. Is the ball centered between the hash marks? If not, then fix this first. If the airplane has an Electronic Flight Director, power it up and verify it is displaying level. A vacuum-driven horizon can be leveled if an external vacuum source is available. Never run the engine of an airplane while it is on jacks. Doing so could lead to an event that negates any efforts at rigging the flight controls, forever. If not able to calibrate a vacuum-driven horizon while on jacks, another leveling process will be addressed later in this article.

Using the ten-inch bar and C-clamps, the pilot’s rudder pedals are aligned with each other and clamped. Next, tightly tape the three-foot bar to the bottom of the control wheels so they cannot move. Remove the belly panel and locate, under the pilot's floor near the fuel strainer, the aileron/rudder interconnect springs. These springs should be stretched to the same length (typically 4.9 inches). If they are different lengths, adjust them to be equal.

Place the aileron/flap travel board across the wing at the correct wing station and verify that both ailerons are at the same degree of droop, 0 to 2 degrees. The ailerons will tend to rise during flight, so some droop is desirable. Adjust the ailerons to be equal and within tolerance.

Verify that the outboard flap stops are limiting the flaps and that the outboard flap angle is the same for each flap on our travel board scale. Adjust this before going any farther. For the controls surfaces (flaps, ailerons, elevator, rudder) there is one aeronautical engineering standard you must achieve: any control surface must be straight within 1 degree along the control for the plane to fly hands off correctly; no twist, no bends.

Now comes the part not in the maintenance manual. Using the digital protractor, measure all the outboard and inboard angles of the flaps and ailerons. Sight along the trailing edges to detect any bends and measure these also. Organize and record all measurements so they can be scrutinized from one wing tip to the other. Any flap or aileron that is twisted or otherwise bent more than 1 degree must be repaired. The flaps may be slightly lower than the ailerons as long as the flap angles (all four of them) are within1 degree of each other. If your Mooney flies slower than the book numbers it could be that full-up flaps are drooping and need to be adjusted up. I often observe, over the years, the inboard angle of the right flap is bent (twisted) down 2 to 4 degrees from people stepping on it.

When your Mooney was manufactured, all of its control surfaces were built in jigs that were recalibrated after ever so many manufacturing cycles. These surfaces have the same angle on the inboard side as the outboard side and all the way through the center, all within 1 degree. This is the structural standard that meets the FAA type certificate.

Another procedure not in the maintenance manual is to secure the elevators level using a clamp on the copilot's control shaft with the stabilizer trim in the takeoff position and measure the outboard and inboard angles on each elevator. As was done for the ailerons, verify that all four angles are within 1 degree of each other.

Check that the rudder is at 1 degree right on your rudder travel board. Due to normal play in the rudder linkages, set the rudder so it is no less than ½ degree offset when pushed by hand to the left and no more than 2 degrees when pushed by hand to the right. This will get the rudder as close to 1 degree right as possible. If the rudder needs adjustment (and most do), now is the time to set it correctly.

Unlike Cessnas with wing struts, Mooneys have no way to wash-in or wash-out the wing tips, so if the airplane wing has ever been pranged or re-skinned, it is imperative to verify that both wing tips have the same angle of attack at the end rib (not the fiberglass tip). Place your digital protractor at the same location from the leading edge on the outboard rib and verify that both wing tips are within 1 degree of each other. It is rare to see the wing end ribs out of rig, but a Mooney will not fly correctly if the ends of the wings are not within 1 degree of each other. Unfortunately, the only way to properly fix this is to re-skin the wing tip.

The engine is the next item to be checked. Start by examining the engine mounts and shims for integrity - no sag. Engine sag is more common with Lycoming than Continental engines in Mooneys. Vertical and lateral engine offset varies among the Mooney model line, so matching the engine position with a properly installed cowling is a good way to verify alignment. If the cowling has been modified (improperly rebuilt), this easy method won't work. Accurately realigning an engine from scratch involves engineering drawings and plumb bobs - beyond the scope of this article and most airframe mechanics.

Just how much 1 or 2 degrees of engine misalignment has on the overall rigging is probably negligible, but again, does the aircraft meet its type certificate?

If the airplane has any fixed or movable tabs, make sure they are straight before the next flight test. So, why is the service manual missing all these items mentioned in this article? When the manual was written it was based on a low-time airplane, not a decades old, twice bent airplane with 3,000 hours on it. Even a well kept, no incident airplane can have a bent right flap from people stepping on it over the last twenty or so years.

Remove all the clamps and locks and verify that the ailerons have the proper travel up and down. Verify that the aileron stops are hitting at the correct travel, reset them if necessary. Verify that the rudder has its full right and left travel and it is hitting the stops on the nose wheel truss before the stops in the tail cone. Adjust the stops as necessary to obtain the correct limits.

Close up everything (except for the instrument panel, see next paragraph) and go over one more time all the things that were loosened. Make sure everything is tight. This is a good opportunity to inspect the landing gear components and operation. Before lowering the plane off jacks, retract the gear and verify that all the gear doors are tight and none are hanging out in the slip stream. A sagging gear door will cause the plane to yaw. Once you are satisfied with the gear door fit, lower the gear and take the plane off jacks.

If the gyro horizon hasn't already been leveled, it will be done out on the ramp, with the engine running for vacuum pressure. Be sure to have the proper tools and have the plastic panel cover removed. Taxi to a spot on the ramp where the airplane is laterally level by observing the ball in the turn and bank instrument which has already been calibrated in an earlier step. Insure the vacuum pressure is adequate and steady during the adjustment.

The last step before The Flight Test is to put the airplane back to an airworthy condition (instrument panel cover, inspection covers, control locks, etc.)

Now it’s time for some fun. Verify that the fuel load is even and repeat The Flight Test. If rudder was needed to center the ball (The Ball Test), then the trailing edge of the rudder must be bent the opposite way the rudder was pushed. For instance, if right rudder was used to center the ball then bend the entire rudder trailing edge to the left, viewed from the back looking forward. This will push the rudder to the right in flight. The degree of bend should be small.

From The Wings Level Test, if the plane tends to roll in one direction, an aileron trailing edge needs to be adjusted. For instance, rolls to the left are countered by bending the left aileron trailing edge up. This will push the left aileron down thus favoring a right roll. Yes, if the opposite aileron's trailing edge is already bent up a bit, it may be straightened (bent down) for the same effect. Always bend the entire trailing edge. A very small bend goes a long way, so do not be heavy handed with this adjustment. It may seem like bending a ¼ inch strip will be ineffective, but along the entire length of the control surface, that trailing edge is a huge trim tab.

Obviously, these adjustments of flight surface trailing edges might not be the exact amount to get the airplane perfectly in trim. When your Mooney is rigged correctly it should fly like it is balancing on a ball. It will eventually roll one way or the other but never the same direction every time and never immediately after you release the controls. In flight, when checking the airplane rigging, follow The Flight Test steps - don't shortcut the process. Remember, balanced fuel and cabin load. Once the rigging is close, it is probably within the FARs for a pilot to make small trailing edge adjustments (once back on the ground).

As always if you have any questions about this article, feel free to call my aircraft repair shop at 307-789-6866 or e-mail me at shoptalk@knr-inc.com. Archived ShopTalk articles may be found on our web site, www.knr-inc.com. Until the next ShopTalk, enjoy flying your Mooney.