Flight Control And Trim Anomalies

A friend's Cherokee had just undergone its annual inspection for 2015. I had gone to the airport where the maintenance shop is located and had conducted a more-extensive-than-usual preflight before getting in for the trip back to my home airport. Everything was looking good. Engine start was exceptionally prompt and everything else came alive just as it should. Taxi out to the hold line was uneventful, and the needles on the engine instruments were exactly where they were supposed to be, with the oil temperature just starting to creep into the green. The magneto check was fine, the flaps worked, the strobes blinked away, the controls were smooth and free, and the radios sounded good. When I tried to check the trim for free movement and set it for takeoff, the trim wheel would barely move in either direction. Without hesitation, I radioed ground control that I needed to taxi back to the maintenance shop. Once there, I advised the somewhat alarmed shop owner that I wasn't going to fly with a trim system that wasn't working as intended. It took about a half-hour for a mechanic to open things up and make an adjustment that restored the trim system to the way it had been working before the annual inspection.

In its Safety Alert, the NTSB says pilots should "become familiar with the normal directional movement of the flight controls and trim surfaces of the aircraft." The Board says if you're very familiar with "normal," it's easier to recognize "abnormal."

After maintenance, pilots should "check systems more thoroughly than the normal preflight checklist implies. For example, if a preflight checklist states, 'Trim---Set Takeoff,' verify not only the trim setting, but also proper direction travel." The NTSB says pilots should be prepared to abort a takeoff if something seems wrong, avoid interruptions and distractions during preflight, and "if you suspect that there is a problem with a flight control or trim system, ask qualified maintenance personnel to inspect the aircraft."

The Safety Alert contained examples of accidents related to the issues it raised.

Piper PA-22-135
A tailwheel Piper PA-22-135 struck the runway immediately after takeoff from Penn Valley Airport (SEG), Selinsgrove, Pa. Both occupants were seriously injured. Visual meteorological conditions prevailed for the personal Part 91 local flight.

The private pilot told investigators he completed the before-takeoff checklist, but couldn't see the elevator from the pilot seat during the control check. He taxied the airplane onto the runway, applied full throttle and, when the airplane began to gain airspeed, pushed the yoke forward to bring up the tail and then eased it back. After the airplane lifted off, he pushed the yoke forward again, but the airplane continued to climb at a "low airspeed." The pilot decreased the engine power, the nose of the airplane lowered, and the pilot pulled back on the yoke to stop the descent. The airplane responded by descending at a higher rate. When the pilot applied nose-up trim, the airplane struck the runway.

A witness said after the main gear rose off the runway, the airplane climbed, then the nose dropped, and it descended in a slight left turn to runway impact.

In an interview, the mechanic who performed the last annual inspection decided to replace the elevator cables since they were frayed. The mechanic routed the new cables in with the part tags attached and a second mechanic confirmed the cable installation. Then, they performed a flight control check utilizing the control column to confirm the correct installation of the elevator cables. One of the mechanics noticed that a bolt was too short, removed one cable, replaced the bolt with a longer bolt and reattached the elevator cable. When asked if they verified the cable rigging, both mechanics stated that they confirmed the correct routing from the elevator cables to the control yoke. In addition, they stated there was no maintenance manual for the airplane and they used the illustrated parts catalog in order to install the cables.

Examination of the airplane by an FAA Inspector revealed that the elevator control cables moved opposite the direction that was commanded. When the elevator control cable was operated that corresponded to the yoke being pushed forward, the elevator control surface moved in the upward direction, which would position the airplane in a nose-up attitude. When the elevator control cable was operated that corresponded to the yoke being pulled aft, the elevator control surface moved downward to position the airplane in a nose-down attitude.

The NTSB determined that the probable cause of this accident was the reverse rigging of the elevator cables by maintenance personnel and their subsequent failure to verify that the rig­ging was correct during post maintenance checks, and the pilot's inadequate pre-­flight check.

Cessna T182
Just after taking off, when about 900 feet AGL and just starting the crosswind departure turn, a Cessna T182 experi­enced a flight control issue. The pilot executed an emergency landing to the St. Louis Downtown Airport (CPS), Cahokia, Ill. The commercial pilot and the pilot-certificated passenger weren't injured, and the airplane wasn't damaged. This was a post-maintenance check flight in visual meteorological conditions.

The pilot told investigators that, when in the crosswind turn after takeoff, the airplane "began to pitch steeply toward the ground." The airplane lost about 200 to 300 feet in altitude, and the pilot pulled back hard on the yoke to keep the nose level. The pilot confirmed that the autopilot wasn't engaged and asked the passenger to help him hold the yoke, which required "extreme back pressure." The pilot estimated that it took about 150 pounds of force to keep the airplane level. The passenger made the radio calls and declared an emergency while the pilot maneuvered the airplane to land on runway 12R, since it was the longest and widest runway available.

Although the pilot slowly added nose-up elevator trim in an effort to relieve the nose-down pressure, it had no effect. The manual elevator trim wheel indicated that the trim was in the full nose-up trim position. The passenger checked the circuit breakers for any circuit breaker that could account for the problem, but there was none. He looked through the airplane's rear window for damage, but could see none. As the airplane turned onto base leg, it still had an extremely strong nose-down tendency. The pilot and passenger were concerned that they wouldn't be able to flare the airplane, and it would contact the runway in an abnormal nose-down attitude.

The pilot remembered that work had been done on the elevator trim system and thought that there might be some kind of control-reversal problem. While still on base, the pilot added nose-down, instead of nose-up, trim using the electric trim on the control yoke. The nose-down control forces decreased, and he realized that there was a control reversal. A normal approach and landing followed.

Examination of the elevator trim system revealed that when the trim wheel was moved to the full down position, it resulted in the elevator trim tab moving to the down position, opposite of the position to which it should have moved.

The airplane had come out of a 100-hour/annual inspection and maintenance during which the elevator trim actuator had been replaced. The mechanic who performed the maintenance reported that he moved the elevator trim tab control wheel to the full tab-down position and "rigged to 15-degrees tab down." He then moved the elevator trim tab control wheel to the full tab-up position and adjusted the tab to "24-degrees tab up from neutral." He verified the up and down angle several times. A second A&P mechanic inspected the trim tab actuator installation before the inspection panels were closed by visually checking for foreign objects, cable safeties, cable runs and smooth movement of the cables.

The Cessna Model 182 Maintenance Manual (Rev. 18) provided the instructions for removing and installing the elevator trim tab actuator. It said, in part: "NOTE: Nose down trim corresponds to the tab UP position."

The NTSB determined that the probable cause of this accident was the mechanic's improper installation of the elevator trim actuator, which resulted in reversed elevator trim control, and his subsequent failure to detect the misrigging of the elevator cables during the post maintenance inspection.

PA-12
Although it had not yet adopted a probable cause, the Safety Board also mentioned an accident involving a Piper PA-12 that entered a nose-down descent and crashed shortly after takeoff from Merrill Field Airport, Anchorage, Alaska. The private pilot was killed.

Investigators learned that the air­plane had been undergoing extensive maintenance for about five years, and the accident flight was believed to be its first in a very long time. A pilot who was standing on the ramp saw the airplane climb steeper and steeper after takeoff, as if the pilot had no ability to make it level off.

Investigators established elevator con­trol cable continuity from the control horn to the rear control stick. Pulling the rear control stick aft to command airplane nose-up resulted in cable movement corresponding with a nose-down deflec­tion of the elevator. The mechanic assisting investigators confirmed that the elevator control cables were attached to the oppo­site locations on the elevator control horn, resulting in a reversal of elevator control inputs.

Peter Katz is editor and publisher of NTSB Reporter, an independent monthly update on aircraft accident investigations and other news concerning the National Transportation Safety Board. To subscribe, write to: NTSB Reporter, Subscription Dept., P.O. Box 831, White Plains, N.Y. 10602-0831.