Battling With The Autopilot

The airplane was manufactured in 1981. It had a 300 horsepower Continental TSIO-520-UB engine driving a constant speed prop. The pilot purchased it on September 9, 2015, and at the time of the accident, it had 5,310 hours. The pilot flew it for 37 of those hours. He held a private pilot certificate for single-engine and multi-engine airplanes. He was instrument-rated and held a third-class medical certificate. He had logged 1,541 flight hours as of October 30, 2015, with 1,374 in single-engine airplanes.

The airplane had an autopilot system installed. There was an autopilot trim switch assembly on the pilot's left control yoke grip. An autopilot disconnect/trim interrupt switch also was on that grip, and a control wheel steering (CWS) switch was on the yoke's right grip. Depressing the CWS switch with the autopilot engaged released the autopilot servos, allowing manual manipulation of the flight controls without the need to disengage and reengage the autopilot or reselect any modes of operation. When the autopilot was engaged, and the pitch attitude servo sensed control forces on the elevator control cables that continued for longer than 3 seconds, the autopilot computer activated the trim servo to trim out whatever forces were detected. The way the pilot could tell whether the autopilot system was making trim changes was to look at the manual trim wheel located in the lower part of the pilot's side instrument panel. If it was turning, the autopilot was trimming.

It was a moderately pleasant morning on November 20, 2015, at Orlando Executive Airport (KORL), with scattered clouds at 2,300 feet, visibility 10 miles, temperature 80°F, dew point 70°F, and the wind from 020 degrees at 8 knots. The flight was to be VFR to Gainesville Municipal Airport (KGLE) in Gainesville, Texas. At 11:14:40, the pilot contacted the Orlando tower controller advising he was holding short of Runway 7 and was ready to depart. "Bonanza foxtrot golf, Executive tower, turn left on course, cleared for takeoff," the controller replied.

The pilot responded, "Turn left on course, Runway 7, cleared for takeoff, and can you give us frequency (unintelligible) to talk to?" The controller told the pilot to remain on the current frequency "...for now, when switched departure will be, uh, one one niner point four." The pilot acknowledged at 11:15:03. At 11:18:47, the controller radioed, "November seven foxtrot golf remain outside bravo (Class B) airspace at or below one thousand five hundred and contact Orlando departure one one niner point four." The pilot radioed, "Alright, one one niner point four, what was the altitude restriction?" "At or below one thousand five hundred," the controller replied. At 11:18:59, the pilot responded, "At or below one thousand five hundred, I'm already at two thousand---I'm descending for seven, for seven foxtrot golf (unintelligible)."

At 11:19:09, the controller handling the Central Florida TRACON Satellite Radar Disney position called the tower controller on the interphone system and advised, "You need to tell foxtrot golf to descend immediately." The tower controller said, "Yeah, I'm starting him down." According to the transcript of radio communications, whatever instruction the controller radioed to the pilot at 11:19:32 was unintelligible, as was the pilot's reply. At 11:19:35, the tower controller handed off the flight to Orlando departure.

At 11:19:54, the pilot checked in with Orlando departure and advised he was at 1,800 feet MSL and was descending. The TRACON controller gave him the latest altimeter setting and asked if he had a request. The pilot said, "...I'm trying to pick up flight following on our VFR flight plan." The controller asked, "Okay, where to please." The pilot advised, "I'm going to KGLE, and when able we'd like to start our climb cause we've got, uh, an opening here." Presumably he was talking about an opening to climb between the clouds, which may have been more dense than the scattered clouds reported in the observation at KORL. The controller asked, "Okay, where are you going and how high do you want to climb, I'm not familiar with the depart...with the, uh, destination." The pilot replied, "Uh, we want to go to one thousand, excuse me, twelve thousand five hundred, and we're going to KGLE." The controller asked where KGLE was, and the pilot replied that it was Gainesville, Texas.

Two seconds later, the controller admonished the pilot: "Seven foxtrot, okay, you have not been given a clearance through the bravo, why are you climbing?" Then, the controller instructed the pilot to turn left to a heading of 270 degrees. There was no acknowledgement. It's speculation, but based on radio calls to come, it would seem logical that at this point the pilot was finding it progressively more difficult to control the airplane's attitude. It's likely that he was using more and more force to keep the yoke forward. Based on subsequent radio transmissions, I can imagine him at this point being consumed by pushing buttons and squeezing on the autopilot disconnect switch in increasingly desperate moves to stop the airplane from fighting him.

At 11:20:51, the controller radioed, "Seven foxtrot bravo, I need you to listen cause you're in my bravo without a clearance. Turn left heading two seven zero." The pilot replied, "Turning left two even zero; for some reason I could not get my (auto) pilot to disengage."

The controller radioed, "Thank you," and then told the pilot to squawk a transponder code of 1073. At 11:21:35, the pilot confirmed the code and added, "Listen, I think we need to put this thing on the ground. I don't know what's going on." The controller asked, "You want to return to Orlando Executive?" The pilot said, "Affirmative, can you help me get there?" The controller instantly responded, "Absolutely, seven foxtrot golf, maintain VFR, turn left heading two one zero and maintain one thousand six hundred, this will be vectors for Runway 7." At this point, we can surmise that the pilot likely realized he was going to need more help than the controller could provide. At 11:21:53, he transmitted, "Okay, listen, I, I have to use full force. Does anybody have any ideas what I can do to shut off this autopilot?"

The pilot's distraction due to the mental and physical strain he likely was experiencing led him to radio at 11:22:03, "And, uh, do you want to give me a heading again?" The controller repeated the 210 degrees heading, and the pilot of a Piper Saratoga radioed, "Pull your circuit breaker." The controller asked, "Seven foxtrot golf, are you able to descend?" The Bonanza pilot radioed, "I'm pushing as hard as I can on (the) yoke." The controller reassured the pilot not to be concerned with the airplane's altitude from ATC's standpoint and asked him to fly a heading of 180 in order to give him more maneuvering room and keep him further away from other traffic and Orlando International Airport's operations. The Bonanza pilot radioed, "Descending to one eight...I'm sorry, going to one eight." The Piper Saratoga pilot then called the TRACON controller, "...have your distress airplane pull the circuit breaker." The controller asked the Bonanza pilot, "Did you copy somebody, uh, suggested you pull the circuit breaker for the autopilot?"

At this point, the Bonanza pilot provided details of what was going on. "We pulled the circuit breaker, but it just, it's going up and then it's going down and it's going up and down and then, uh, we pulled the circuit breaker, but it just keeps porpoising up and down, and it's taking full forward to go down and full back to go up, to com, compensate." The Saratoga pilot suggested, "Power off the airplane and go nordo (no radio) for 30 seconds, power off the airplane and then come back up." The controller let the pilot know it was okay to go off frequency: "...you can do that and, uh, if you can make a right turn, a right turn so you're away from Orlando International until you get it situated." After there was no response, the controller repeated, "...if you still hear Orlando, if you can make a right turn, a right turn to stay away from Orlando International departures." At 11:24:24, the Bonanza pilot radioed, "Alright, make a right turn, coming to two one zero and, uh, we're powered way down."

The Saratoga pilot then got permission from the controller to talk directly to the Bonanza pilot and explained that he meant for the pilot to turn off the electrical master switch, not to power down the engine. The Bonanza pilot didn't quite understand, radioing, "Okay, um, and are you saying then you want me to turn the key to the off position?" The Saratoga pilot shot back, "No, don't turn the key to the off position, just turn your master switch, your electrical master switch, off." It took about 45 seconds of back and forth transmissions before the Bonanza pilot understood that the objective was for him to cut the electrical power and wait 30 seconds before bringing it back up.

At 11:26:14, the pilot radioed, "Alright, going off now and I'm getting some, uh, warning lights as well." The controller then radioed, "Seven foxtrot golf, before you go nordo, Executive tower has advised you are cleared to land Runway 7, seven foxtrot golf approved as requested, everything he asked you to do, you are cleared to land Runway 7 at Orlando Executive." At 11:26:27, the pilot responded, "Seven foxtrot golf, I'm sorry, I was (unintelligible) turn off." Two seconds later, the controller responded, "Seven foxtrot golf, roger, you are cleared to land Runway 7." At 11:26:58, the Bonanza pilot said, "Orlando, I'm (unintelligible)." That was the last transmission. At 11:27:11, the tower controller advised the approach controller on the interphone system, "Ah, he just rolled over, straight down, he's in the ground."

Witnesses did not notice any parts falling from the airplane during the descent. They said no smoke was trailing behind it. They said the airplane made a right bank, and then entered a vertical descent and crashed into the lake. The accident site was about 3.7 nautical miles south-southwest of the approach end of Runway 7.

When the wreckage was recovered from the water, investigators found that the pitch trim was in the full nose-up position. Investigators found this in the flight manual supplement for the autopilot: "...When the autopilot is engaged, manual application of a force to the pitch axis of the control wheel for a period of 3 seconds or more will result in the autotrim system operating in the direction to create a force opposite the pilot. The opposing mistrim force will continue to increase as long as the pilot applies a force to the control wheel and will ultimately overpower the autopilot. If the autopilot is disengaged under these conditions, the pilot may be required to exert forces in excess of 50 pounds to maintain the desired airplane attitude. The pilot will have to maintain this force while he manually retrims the airplane." How far in excess of 50 pounds? Investigators said the plane's manufacturer figured the force necessary to return the control column to neutral with full airplane nose-up trim at 120 knots calibrated airspeed is about 311 pounds.

The NTSB says the pilot likely felt pressure to get out of the Class B airspace and, without disarming the autopilot, pushed and held the control yoke forward to halt the airplane's climb. This action would have resulted in a reaction by the electric trim system running the trim in the opposite direction (airplane-nose-up) to relieve the pressure on the pitch servo. The investigation could not reach any conclusions about what could have happened to cause the airplane to porpoise up and down as described by the pilot, and did not explain what could have caused the pilot to have trouble putting back pressure on the control yoke, since full nose-up trim would be applying more back pressure than any pilot would want.

Testing of the recovered autopilot and electric trim components was complicated by their exposure to water and impact damage. Although the autopilot computer at first failed several tests, several months later during retesting the results were better and the system performance was determined to have been satisfactory.

The Safety Board determined that the probable cause of this accident was "the pilot's failure to maintain adequate airspeed while turning to join the final approach leg of the airport traffic pattern with full airplane-nose-up trim, which resulted in the airplane exceeding its critical angle of attack and experiencing an aerodynamic stall. Contributing to the accident were the pilot's misuse of the forward elevator flight control input with the autopilot engaged, which resulted in the full airplane-nose-up trim; his failure to recognize and correct the mis-trimmed airplane per the emergency procedures; and the excessive control forces required to maintain control of the airplane in the mis-trimmed condition, which resulted in pilot fatigue."

The investigators looked at more than 657 accidents, incidents and other occurrences involving Beech 36 series airplanes and found two involving the autopilot or pitch trim. In both, the trim was found in the full-nose-down position, and the pilot's failure to re-trim was cited in the probable cause. The accident reports did not attribute the extreme trim positions to system failures.

In this accident, the Safety Board's report says that although it tried, it could not determine why the pilot did not "promptly recognize and take corrective action regarding the mis-set trim..." Perhaps I can help with that: Had he ever been trained in how to recognize and deal with runaway trim? It's something an instructor who's done a lot of my training throws at me whenever I go under the hood. Experience the control forces building up while you're distracted with something else, and you'll eventually make looking at the trim indicator, cutting power to the electric trim and autopilot system, and manually rolling the trim wheel to relieve control pressure things that are second nature.

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, visit www.ntsbreporter.us or write to: NTSB Reporter, Subscription Dept., P.O. Box 831, White Plains, NY 10602-0831.