Accident Probe: Pitch Trim Runaway

While automated flight systems generally are reliable, we need to be vigilant and prepared to disable them.

5

After the primary flight controls—elevator, rudder and ailerons on conventional airplanes—the most frequently used secondary control is pitch trim. That’s because a typical flight in any airplane involves a range of speeds, and pitch trim is used to minimize control forces throughout the operating envelope. Even if the pilot can supply that force, it’s fatiguing. Along the way, a popular feature is powered pitch trim, typically using electric-powered servos and installed as part of a multi-axis autopilot/flight director system.

Of course, a powered trim system brings with it additional failure modes, and its presence means the system is easier to misconfigure, adding to maintenance and inspection requirements, and the risk of catastrophic failure, including what some would call a worst-case scenario: the powered pitch trim system fails in a manner that commands the nose down and it can’t be disabled. This is not unlike what happened to two Boeing 737 MAX jetliners in late 2018 and early 2019, resulting in a worldwide grounding, although their failure mode was more complex than the system aboard typical personal airplanes. While the systems are designed to be capable of being overpowered by a pilot, that’s not always the possible. Disabling them and reverting to manual trim—where someone in the cockpit uses a large wheel, crank or lever to minimize control pressures—is the usual cure.

With that in mind, anyone familiarized with a specific airplane should know how to disable a powered pitch trim system. The methods vary depending on the failure mode itself, but when all else fails, removing power from the system is only half the battle. You still have to fly the airplane, and return the pitch trim control to a neutral setting.

Background

On June 27, 2019, at about 2233 Eastern time, a Beech E55 Baron was destroyed when it impacted a residence and terrain while attempting to land at the Fayetteville Regional Airport (FAY), Fayetteville, N.C. The solo private pilot (male, 65) and one person in the residence were fatally injured. A second person in the residence was seriously injured. Night visual conditions prevailed.

The flight was cleared for takeoff from FAY’s Runway 4 at 2227:36. Two minutes later, at 2229:27, the pilot reported a problem to ATC and stated he wanted to return. The controller cleared the flight to land on Runway 4 and, at 2230:52, the pilot stated, “It’s really wanting to pitch down bad for some reason.” No further communications were received from the pilot. At 2233:18, the controller initiated emergency procedures.

The accident site was about two miles southwest of the approach end of Runway 4. A witness reported seeing the airplane veer sharply, followed by a steep descent prior to impact. The debris path began with freshly cut treetops, descending at about a 35-degree angle and extending about 50 feet on a magnetic heading of 270 degrees to the back of a residence. Sections of the right wing, left horizontal stabilizer and right engine came to rest inside the residence; the main wreckage came to rest in the front yard, upright.

Investigation

Examination of the wreckage revealed one propeller’s blades exhibited S-bending, chordwise scratching and leading edge gouges, consistent with being under power. The other propeller was not located. The landing gear and flaps were retracted. Flight control continuity was confirmed from all control surfaces to the cockpit yoke. The fuel selectors were in the “on” position. No pre-impact anomalies were found with either engine.

Measurement of the aileron trim actuator corresponded to a full-down tab on the left aileron. Measurement of the rudder trim actuator corresponded to a five-degree nose-left trim. Measurement of both elevator trim actuators corresponded to 10-degree, full-tab-up, nose-down pitch trim.

A pilot-rated friend of the accident pilot flew with him three days before the accident. The attitude and heading reference system (AHRS) fail caution light illuminated but went out shortly after takeoff. It illuminated again during a test at the end of the day’s flying. The pilot stated he would have an avionics shop investigate.

On the day of the accident, the pilot checked on the status of the avionics shop’s work and was informed the repair had not been completed. The shop needed information from the EFIS manufacturer and noted the only issue would be that the autopilot would not engage. The pilot then took the airplane for the accident flight.

Testing and examination of the pitch trim servo revealed it did not operate within design parameters—the solenoid would not disengage when power was removed without having to manually spin or bump its clutch. Additionally, the electrical cable connection “resulted in intermittent motor operation.”

Testing revealed the pitch trim servo’s clutch tension required 45 lbs. of force when the required breakaway force setting should have been 13 ± 2 lbs. The fastener securing the clutch to the pitch trim servo had been overtightened, resulting in the breakaway force needed to overcome it being about 30 lbs. too high. The overtightened clutch would have greatly increased the force required to override the clutch of the pitch servo.

Probable Cause

The NTSB determined the probable cause(s) of this accident to be: “The pilot’s failure to disengage an uncommanded nose-down pitch input. Contributing to the accident was improper maintenance of the pitch trim servo, which would have precluded a physical override of the pitch servo.”

According to the NTSB, “Examination revealed that the cotter pin used to secure the castle nut retaining the [pitch trim servo] clutch appeared to be new, and the castle nut did not align with the factory orange torque strip setting applied at the time of servo completion and final testing. The orange torque strip on the nut and the orange torque strip on the tension washer were about ¾ of a turn off. At some point during previous maintenance, the castle nut securing the clutch on the servo was overtightened, which caused the breakaway force needed to be 30 lbs. over the specified limit. The overtightened clutch would have greatly increased the force required to override the clutch of the pitch servo.”


Disabling Autopilots And Electric Trim

According to the NTSB, emergency procedures documentation for the make/model of the accident aircraft listed the following methods for disabling the electric pitch trim system and the autopilot. These steps will be similar for other types and equipment:

  • Turning off the autopilot master switch;
  • Pulling the autopilot and trim circuit breakers;
  • Turning off the radio master switch;
  • Turning off the electrical master switch;
  • Pushing the go-around switch on the throttle grip; or
  • Pushing the TEST EACH FLT switch on the autopilot controller.

Aircraft Profile: Beechcraft E55 Baron

Image: Adrian Pingstone

OEM Engines: Continental IO-520-C

Empty Weight: 3291 lbs.

Maximum Gross Takeoff Weight: 5300 lbs.

Typical Cruise Speed: 195 KTAS

Standard Fuel Capacity: 100 gal.

Service Ceiling: 19,100 feet

Range: 934 NM

VS0: 73 KCAS


Jeb Burnside is the editor-in-chief of Aviation Safety magazine. He’s an airline transport pilot who owns a Beechcraft Debonair, plus the expensive half of an Aeronca 7CCM Champ.


This article originally appeared in the June 2021 issue of Aviation Safety magazine.

For more great content like this, subscribe to Aviation Safety!

5 COMMENTS

  1. The article seems to use the terms pitch servo and pitch trim servo interchangeably. Can you clarify which one? Or was it both? May be hard to do if the NTSB is the one failing to make the distinction clear. Your quote from their report says “Contributing to the accident was improper maintenance of the pitch trim servo, which would have precluded a physical override of the pitch servo.” Improper maintenance of the pitch trim servo does not affect the ability to override of the pitch servo.

      • No. They are not the same servo.

        I read the NTSB report. There is some confusion within the report between the two servos, but at the end it seems to be pretty clear that the PITCH servo is the one that had the slip clutch set too high. “Testing and examination of the pitch, roll, and trim servos at the manufacturer’s facility revealed that the roll servo and trim servo operated within design parameters; however, the pitch servo did not.” But, the trim tabs were at the stops, which indicates that the PITCH TRIM servo probably moved them there for some reason. The PITCH servo being set too high meant that overcoming the out-of-trim condition was harder to do than it otherwise would have been, if the AP was engaged.

        No clues in the report as to why the pitch trim tabs were at the stops, but that can happen easily enough by simply having the AP on while trying to hand fly, with all of the AP working as designed.

        I think a lesson to be learned is that trying to hand fly with the AP on can get you in a bad state in a hurry, when the trim starts trying to counteract what you’re doing. Can’t say for sure that’s what happened here, but the clues point that direction.

    • Pilots need to be organized, such as with reference cards and/or committing some procedures to memory.

      My airline knowledge includes Pacific Western Airlines having plasticized cards in a slot on top of the glareshield, and a quick-reference flip-over binder stowed somewhere (pages about half size of normal ‘letter’ paper).

      IIRC the fated Lion Air 737MAX Captain was digging through a quick-reference binder while having the F/O fly the airplane. But he did not properly inform the F/O of what he experienced before handover. Crew on previous flight had disabled stabilizer electric trim in response to what they diagnosed as runaway trim. Crew of the fated Ethiopian 737MAX flight were aware of existence of the MCAS system but got a bit confused and ran out of time (fast airplane due hot-high airport and not much altitude above ground).

      Outcomes of the 737MAX fiasco include recognition of crew workload and confusion, and benefit of even more training.

      Coulson Aviation trains pilots fighting forest fires to the point of ‘muscle memory’, it claims.

      Note in this case it sounds as though the detail procedure for the Baron was an alternative to over-riding the trim system, which was very difficult because of improper adjustment of clutch over-ride mechanism.