Saturday, May 18, 2019

737 Pitch Trim Incidents

Pitch trim is an abstract term to represent the ability to reduce column forces by moving the stabilizer and elevator. This post will dwell on stabilizer trim reported difficulties.

All airplanes provide at least two means to trim the stabilizer, mostly using two seperate actuators motors. Starting with the NG, the Boeing 737 relies on only one electric trim actuator, with manual wheel trim as the backup. There is no documented 737 accident as a result of stabilizer/pitch trim malfunction or failure (prior to JT610 and ET302).

With sparse reporting to draw from, it can be surmised that a stabilizer runaway or failure occurs about once a month, with a jam about once a year (world-wide).  Just plugging this into a spreadsheet yields the failure rate for runaway, loss of function and jam.  I am just assuming about 5,000 737 during the time frame for the failures accounted for, and looking ahead with a larger fleet size.

Ethiopian ET302 encountered high opposing forces due to the mistrim; it has been assumed that the ET302 actuator was not jammed, and there has been no concern raised that the ET302 clutch would have oppressively opposed trimming manually, on top of the aero loads.
The original post only looked at events in the NASA data base. Brian Lowe contacted me with some research he had conducted, to which he shared with me and include below.

From Brian Lowe:
I found your articles on the 737 max extremely factual and educational, however as an air safety researcher,  I must take issue with your article 737 Pitch Trim Incidents (May 18, 2019) where you cite lack of NTSB 737 STAB TRIM reports as evidence of reliability. 
737 STAB TRIM occurrences are not considered a serious incidents therefore don’t usually appear in NTSB reports, rather they are reported within airlines SMS and reporting systems to NAA. Most International States don’t provide public access to such information.  
As a rule of thumb I find the number Canadian accidents per type to be around one twelfth of that of the US, which in turn represents 50% of all world wide activity. Therefore a projected estimate of STAB Trim occurrences worldwide would be 10-16 per annum of which at least one might be a stabilizer jam.
Brian notes that my search was incomplete, that the events would not be revealed in the NTSB reports unless something worse happened. On this point, we can all agree that is a good thing, that there are not more serious events! 

I used Brian's suggestion for the failure occurrences to calculate a failure rate (see table, above).

As Brian was kind enough to share his findings with me, I feel it important to complete the post by including them. There are three groups:
  1. CADORS reports
  2. FAA SDR database
  3. Selected events he culled from Aviation Herald


737 stabilizer trim has been very reliable. There are enough examples of runaway to assume at least one per year, with overall stab trim non-functional about one per month.  There are enough examples to assume at least on jam per year.

NASA Accident Data Base

The 737 pitch trim system has been extremely reliable when looking at the US NASA accident database. I searched for accidents and incidents with stabilizer, trim, runaway and came up with numerous collisions on the ground (with A319, A321, 767, 777, A320, MD81, L1011, several 737 Vs 737, and even one with a Cessna 401). There were a couple of elevator malfunctions. A number of structural damage due to foreign objects or otherwise.  But only two that involved trim itself, and neither a runaway.

Alaska Flight 497

On October 10, 2001, at 1627 hours Pacific daylight time, Alaska Airlines flight 497, a Boeing 737-700, N615AS, experienced a jammed horizontal stabilizer trim actuator during the takeoff initial climb from the John Wayne Airport-Orange County, Santa Ana, California. The flight was destined for Seattle, Washington; however, the flight crew diverted to, and landed at, Los Angeles International Airport, Los Angeles, California, at 1658.
Stepping through the incident, the flight crew went to manual trim.
The crew disengaged the autopilot and found the electric trim control inoperative at both the pilot and co-pilot controls and the manual trim control was jammed and immovable. 
To emphasize the point, the flight crew observed that arriving maintenance personnel had no more success.
According to the pilot, maintenance personnel who met the airplane on arrival could not move the stabilizer control either and believed the actuator gearbox was jammed. 
The flight crew were insistent that the manual trim was not usable. That the simulator forces were much less than what they encountered.
In a subsequent company interview, on November 13, 2001, the pilot reiterated that the trim manual control had been jammed and immobile; neither he, the first officer (co-pilot) or the mechanics who met the flight, were able to force it to move. The pilot said he had trained for the jammed stabilizer emergency in the simulator; however, in the incident at hand he had applied "a lot more force" than was required in the simulator and was still unable to move the manual control.
The actuator failed, as was expected.
Post flight examination revealed the horizontal stabilizer trim actuator motor was seized. Further examination of the actuator motor revealed that the motor was mechanically seized. 
The actuator was replaced. Nothing else was replaced.
The actuator motor was replaced with a serviceable unit and the aircraft was ferried to Seattle for inspection. No other faults were found in the pitch trim system and the airplane was returned to service. 
The resolution was that the flight crew needed to use more force to break out the clutch.
According to a representative of Boeing Aircraft Company, with the motor seized, in order to manually trim the stabilizer, it would have been necessary for the flight crew to have exerted sufficient force on the trim wheel to cause the motor clutch to slip in addition to the force necessary to overcome normal system friction.


The 737 originally had two actuators: one for manual pitch trim and the other for autopilot pitch trim. Having two actuators was the original source of the two 737 cutout switches, elec main and autopilot. This 737-300 autopilot actuator failed when the circuit breaker "popped". The autopilot ran out of elevator authority. The pilot disconnected the autopilot and flew the airplane manually without further incident.

The source of the circuit breaker trip was never identified. There were some minor exceedances in the equipment, but nothing that would explain any overcurrent.

While the pilot complained of some difficulty controlling the airplane manually (electric pitch trim was operable and was not noted for any issue), there was no technical fault found to explain it. The investigation concluded that the captain was over-controlling.

CADORS Reports

From Brian Lowe:
A word search of CADORS (2000-2019) for “737 STAB out of TRIM” identifies 14 occurrences where the 737 stabilizer motor malfunctioned, and in two of theses the manual trim also jammed. (Some of the occurrences involved stab trim switches only working in one direction.) 
I have offered a comment for the CADORS reports (-->) after each narrative provided by Brian. As Brian searched for the "Stab out of Trim" message (which is asserting the autopilot cannot trim stab to null elevator position), it favors the failure of the autopilot trim function.

The "737s up thru classic" used one actuator for manual electric trim and another actuator for autopilot trim.

The 737NG uses one actuator with two control interfaces, one for manual electric and the other for autopilot.

With two actuators, the loss of autopilot actuator does not render manual electric trim non-functioning. With one actuator, the failure of one control circuit may not render the other circuit non-functioning. However with two actuators, the mechanical components have more independence.

The manual wheel trim relies on a clutch to take control of the stab trim jackscrew by isolating the electric actuator(s). The break-out of the clutch can be difficult (as noted earlier) and in addition to any aero loads or elevator hinge loads.

Two of the incidents involved some difficulty/jam in moving the stabilizer manually, the rest make no mention of any problem.

Here is a QRH excerpt.

September 9, 2001 Incident at near Toronto

Initial information from T.S.B. Initial Notification (#A01O0254): The Air Canada Boeing 737 aircraft departed Toronto (LBPIA) (CYYZ), ON and was destined to New Orleans Airport, LA (U.S.A.) (KMSY). During the en-route climb, the flight crew noted that the aircraft would not trim nose down using the control wheel switch. The autopilot was also found to have limited trim authority. Nose up trim was normal and full manual trim was available. The flight crew elected to return to Toronto (LBPIA) and the aircraft landed below maximum landing weight without further incident. Maintenance replaced the stab trim main electric actuator. Trim was functionally checked normal. 59 S.O.B.

-->Failure of the stabilizer trim actuator/Partial non-functioning. Manual wheel trim used without incident.

September 6, 2003 Incident at ABBOTSFORD BC (CYXX)

Shortly after take-off, the Boeing 737-281 aircraft [C-FTWJ] was climbing through 1300 feet ASL when the pilots heard some circuit breakers pop behind the first officer's seat. At the same time, the captain noticed that the number one engine forward fuel pump and the number 2 engine forward and aft fuel pumps low pressure lights were illuminated. There were no other associated warning lights or master caution lights. The pilots noticed that the electric stab trim, number 2 communication radios and the number 2 navigation system were unserviceable. The captain informed ATC of the situation, leveled off the aircraft at 4000 feet, and advised that they were returning to Abbotsford. Subsequently, the first officer found the TR2, TR3 and gasper fan circuit breakers had popped. The aircraft landed in Abbotsford without further incident. Company maintenance inspected the aircraft electrical system and determined that the number two generator had failed. The operator will file a Service Difficulty Report with Transport Canada.

--> electric power fault cause loss of electric stab trim function.

February 25, 2004 Incident at Between Calgary and Cancun, Mexico

WestJet flight 1382, a Boeing 737-700, registration C-GUWS, was en route from Calgary to Cancun, Mexico. Approximately two hours into the flight, when the aircraft was over the central continental United States, the ""Stab Out of Trim"" light illuminated. The associated non-normal checklist was completed; however, the electric trim remained inoperative and trim changes had to be accomplished manually. Maintenance control was consulted and circuit breakers 6B and 610 were pulled and reset. The ""Stab Out of Trim"" light continued to illuminate intermittently, and the pilots continued to manually input the required trim changes. The stabilizer eventually became completely jammed and the ""Jammed Stabilizer Non-Normal Checklist"" was completed. The stabilizer remained jammed following completion of the checklist, at which time Maintenance Control requested that circuit breakers 6B and 610 be pulled and left out. Manual control of the stabilizer was restored and a non-normal landing was completed, without further incident, in Cancun. WestJet is working with Boeing to identify the problem. Checklist procedures are also being reviewed.
UPDATE Maintenance and Manufacturing reported that SDR #20040309004 has been issued this morning. 'OPEN STATUS' Aircraft #009 reported a jammed stabilizer in cruise enroute from
Cancun to YYC on Feb.25/04. The crew was able to fly the aircraft under manual control and landed safely under abnormal landing configuration. Maintenance replaced the stab trim motor and the aircraft was returned to service with no further incidents to report. Westjet Maintenance/Flight Ops are working with Boeing to further investigate. Will update the SDR as more information becomes available.
UPDATE Maintenance and Manufacturing advised that the operator replaced the stab trim motor in accordance with MM 27-41-71. The aircraft was returned to service with no further incidents to report. Boeing has been notified and a response is expected on Friday. Updates will follow as soon as there is more information. An SDR will be filed with update upon receipt of Boeing recommendations.

-->Stab trim actuator failure that left the stabilizer jammed (manual wheel control) for a portion of the flight. Not clear if it was possibly a clutch breakout or elevator opposing force issue, but by the action taken sounds more like clutch breakout problem

March 27, 2004 Incident at BRANDON MUNI MB (CYBR)

The WestJet Boeing 737-700, C-FWSF, was on a scheduled flight from Toronto to Calgary. While level at flight level 390, the autopilot disengaged along with an associated warning horn. The master caution flight controls light illuminated and a speed trim failure was displayed on the overhead panel. The non-normal checklist for speed trim failure was completed. Using ACARS, the crew discussed the problem with company maintenance and the flight continued to destination using the manual trim system. The aircraft landed with a Vref additive of 15 knots for an uneventful landing at Calgary. Company maintenance personnel tested the speed trim system and changed out a stabilizer trim motor. The aircraft has subsequently been returned to service without further problems.
UPDATE Maintenance and Manufacturing advised that the company reported that the crew of aircraft tail #218 NG, reported; ""autopilot disengaged-master caution flight control illuminated- speed trim fail light illuminated-no electric trim-no autopilot."" Following in-flight troubleshooting/non normal checklists, the aircraft landed in YYC without incident. Maintenance replaced electric stabilizer motor IAW MM 27-41-71 and tested system IAW MM22-11-81 and MM 27-41-71. No faults were found. The aircraft was returned to service with no further problems to report. This unit, p/n: 6355C0001-01 is a new EATON ""C"" model which the vendor has completely overhauled because of problems with their ""B"" model. This was the first such stab trim problem with the new ""C"" model and is currently being investigated by EATON, BOEING and the company. Company Reliability is currently waiting for final strip report from EATON. UPDATE Maintenance and Manufacturing reported that the actual motor failure was caused by a PCB 'meltdown', the root cause of the PCB meltdown is believed to be a K3 relay failure. The overall EATON STM 'C' model reliability is still under investigation by EATON and Boeing. There are no modifications identified at this time to the company's knowledge.

--> Failure of the stab trim actuator, which is named. Use of manual wheel trim without incident.

October 29, 2005 Incident at en-route from Edmonton to Toronto

The Westjet Airlines Limited Boeing 737-7CT aircraft (C-FWAI, operating as WJA656) was on a scheduled IFR flight from Edmonton International Airport (CYEG) to Toronto (LBPIA) (CYYZ). The flight crew reported a stabilizer trim problem to the North Controller. ARFF staff were called to be at the ready. The aircraft landed on runway 05 at 0352Z without incident.
Supplemental information from T.S.B. Initial Notification (#A05O0245): The Westjet Boeing 737 (flight WJA656, registration C-FWAI) aircraft flight crew observed the 'Stab Out of Trim' caution light illuminate as the aircraft was climbing out of FL380. The autopilot was disengaged and the aircraft was levelled at FL390. After disengaging the autopilot, the flight crew noted that the electric trim was not functioning. The 'Stab Out of Trim' and the 'Stab Trim Inoperative' Non-Normal Checklists were completed. The flight continued to Toronto (LBPIA), Ontario and the flight crew declared a non-critical emergency with ATS. The aircraft landed without further incident. Maintenance replaced the R384 relay for troubleshooting purposes on the Stab/Trim system and the aircraft was returned to service.

Supplemental information received from Westjet Airlines Limited [2005/10/31]: The Safety Officer advised that the flight crew had reported that while climbing out of FL380, the 'Stab Out of Trim' caution light illuminated. The autopilot was disengaged and the aircraft leveled at FL390, at which time the flight crew noted that the electric trim was not working. The 'Stab Out of Trim' and the 'Stab Trim Inoperative' Non-Normal Checklists were completed. An uneventful flap 15 non-normal landing was completed in Toronto. The flight crew declared a non-critical emergency and received vectors for a long final in order to keep the speed and trim changes to a minimum. Maintenance staff then performed trouble-shooting and replaced a relay before carrying out the stabilizer trim motor test IAW AMM 27-41-71. All tests were passed and the aircraft was returned to service.

-->A relay failed and left the electric trim non-functioning. It appears the flight continued without difficulty using manual wheel trim.

December 25, 2005 Incident at CALGARY INTL AB (CYYC)

WJA 5018, a B737-700, departed Kelowna for Puerto Vallarta and shortly after takeoff, the flight crew advised ATC of a stabilizer issue and requested to divert to Calgary. They also requested the
CFR equipment be on standby for their landing. The aircraft landed safely at YYC at 0413z, TSB Edmonton is looking into the occurrence.
UPDATE TSB reported that the WestJet Boeing 737-700, registration C-FZWS, was operating as flight WJA 5018 from Kelowna, B.C. (CYLW) to Puerto Vallarta, Mexico (MMPR). During the climb from CYLW, the crew received a "Stab Out of Trim" annunciation and stabilizer trim would not respond to electric trim inputs. After consultation with company maintenance, WJA 5018 diverted to Calgary, AB (CYYC) where the aircraft landed with ARFF on standby. Company maintenance replaced the electric stab trim motor and the aircraft was returned to service.

UPDATE Maintenance and Manufacturing reported that the stabilizer trim motor, p/n 6355B001-03, was replaced. No defect recurrence to this date. The part is at the vendor's on a repair order.

-->Stab trim actuator failed/non-functioning. Presumably the manual trim was used without issue.

January 3, 2006 Incident at HALIFAX / STANFIELD INTL NS (CYHZ)

WJA468, Boeing 737-500, enroute from Toronto (CYYZ) to Halifax (CYHZ) inbound for Runway 05 advised Terminal sector they did not want to go below 150 knots. No assistance required. Aircraft landed at 1447Z. ADM advised. ERS was on standby on the apron. TSB Case Closed. UPDATE TSB: A06A0001: WJA468, a Westjet Boeing 737-76N, had departed Toronto en route to Halifax when the "Stab Out Of Trim" caution light illuminated. The crew attempted to operate the electric trim without success. They completed the "Stabilizer Trim Inoperative" non-normal checklist, which instructed the crew to use a maximum flap setting of 15 degrees. Approaching Halifax, the crew informed control tower personnel that the minimum airspeed of the aircraft would be 150 knots and that they did not require assistance. ARFF were on standby on the terminal apron when the aircraft landed safely without further incident. Maintenance inspection revealed that a flap limit switch had malfunctioned. The switch was replaced and the aircraft has been returned to service.

-->Loss of stabilizer function. Assumingly the manual wheel trim was functional.

February 25, 2008 Incident at YELLOWKNIFE NT (CYZF)

UPDATE TSB reported that the Canadian North Boeing 737-200 operating as flight MPE 1722, was en route Thunder Bay to Yellowknife when the out-of-trim warning light illuminated. The electric trim was used to re-trim the aircraft and the autopilot was re-engaged. The out-of-trim light illuminated again a short time later. At that point neither the electric trim nor the manual trim would respond. The checklist was consulted and the aircraft diverted to the maintenance base in Edmonton. No emergency was declared and ARFF assistance was not requested for the landing.
The electric stab trim, manual stab trim and autopilot stab trim systems were tested in accordance with the Boeing 737 maintenance manual and no discrepancies were identified. A test flight was performed during which the stab trim functioned normally. As a precaution the electric and autopilot trim cut-out switches were replaced, the manual trim system was cycled through the full range of travel, and a trim cable inspection was carried out. No faults were found and the aircraft was returned to service.

MPE 1722, a B737-200 operated by Canadian North, was on a flight from Yellowknife to Thunder Bay when the crew advised Edmonton ACC that the aircraft had a stabilizer out of trim with occasional jamming. The crew diverted the flight to the company maintenance base in Edmonton. No emergency was declared and no ARFF assistance was requested.

-->A curious report, with speculative repairs done but no fault found. The "noise" and comments about jamming are alarming.

January 6, 2011 Incident at VANCOUVER INTL BC (CYVR)

UPDATE/ Add info from TSB: A11P0007: A Westjet, Boeing 737-7CT, C-GWSP, flight WJA453, was on approach to Vancouver runway 08L. At 2000 ft ASL the STAB OUT OF TRIM light illuminated. ATC was informed and a go-around was commenced. The crew declared an emergency, checklists were actioned and vectors supplied for an ILS approach to 08L. The aircraft landed without further incident.

Westjet Boeing 737 7CT flight WJA453, IFR Winnipeg to Vancouver on approach to 08L, executed an overshoot due equipment issue. Flight subsequently declared an emergency and was vectored for a second approach. Aircraft landed 08L without further incident. Six aircraft were held outside the terminal airspace.

--> Stab Out of Trim is reference to autopilot unable to zero elevator command by trimming stabilizer. It is not associated with using manual electric trim. Both Autopilot and Manual Electric trim share a common actuator.  It was not clear if the crew used manual electric trim.

October 28, 2011 Incident at CALGARY INTL AB (CYYC)

The crew of WestJet flight 517, a Boeing 737-700 en route from Montreal to Calgary, reported flap problems when 260 NM northeast of Calgary and requested that the airport fire fighters be on standby. No emergency was declared and the aircraft landed without incident at 1533z with Airport Rescue and Firefighting on standby.

UPDATE TSB reported that WestJet flight 517, a Boeing 737-700, registration C-FBWJ, was levelling at a cruising altitude of FL 400, en route Montreal to Calgary, when the pitch trim wheel started to make an unusual noise. The flight crew then observed a "Stab Out of Trim" indication. The appropriate checklist was completed and ATC was advised of the situation. No control difficulties were present and although the aircraft equipment no longer met the minimum RVSM airspace requirements (no autopilot), the flight remained at FL 400 as per ATC instructions and continued to Calgary. An emergency was not declared; however, ARFF was requested to stand by for the arrival. Full manual trim was available and the flight landed in Calgary without further incident. The TSB will be updated when the reason for the malfunction is fully determined.

--> Stab Out of Trim is reference to autopilot unable to zero elevator command by trimming stabilizer. It is not associated with using manual electric trim. Both Autopilot and Manual Electric trim share a common actuator.  It was not clear if the crew used manual electric trim.

September 7, 2012 Incident at Dublin International Airport (EIDW)

Update: Follow-up information received from Airworthiness ? National Maintenance Operations [2014/08/20]: A Civil Aviation Safety Inspector (CASI) looked into this occurrence. The CASI contacted the company and was advised that the flight crew reported experiencing aircraft control issues (after take-off) and were alerted by the illumination of the Master Caution, Flight Controls, Speed Trim light. The aircraft experienced a pitch down attitude accompanied by an increase in airspeed. On further investigation, the flight crew found that the MAIN ELEC and AUTOPILOT stab trim cut-out switches on the centre pedestal were not in the normal position (causing Stabilizer Electrical Trim issues). Level flight was achieved using the manual stabilizer trim wheel. The aircraft control returned to normal once the two cut-out switches were selected to the normal position. Company Maintenance Control advised the flight crew to return the aircraft to the departure airport. On arrival, the flight was cancelled and troubleshooting was carried out. SPEED TRIM (a) was latched on recall. Subsequent tests of the speed/stab trim system could not confirm any active faults and latched speed trim failure was reset. There has been no recurrence. The
company managing this occurrence through their safety management system (SMS) process and conducted a detailed investigation which identified Human Factors as contributing to this event. The cut-out switches were not returned to the normal position following maintenance (testing of Mach Trim system) and the flight crew did not notice this condition prior to departure. The investigation resulted in a Corrective Action Plan to address the deficiencies. The CASI was satisfied with the company?s handling of this occurrence, their corrective action plan and advised that no further Transport Canada follow-up was required at this time.
Initial information received from T.S.B. Daily Notification [#A12F0106]: The Sunwing Airlines Inc. Boeing 737-8K5 aircraft (C-FLZR, operating as flight SWG1540) had departed on an IFR flight from Dublin International Airport, Fingal (Ireland) (EIDW) to Burgas International Airport (Bulgaria) (LBBG). The aircraft was climbing through 3,000 feet ASL when the flight crew noticed that the STAB TRIM was not responding with autopilot selected. As the aircraft increased airspeed, it began to pitch nose down. The Captain took control, manually trimmed the aircraft and declared a PAN PAN emergency and returned to Dublin International Airport. The landing was uneventful with ARFF standing by. Maintenance staff were called on board and found both of the STAB TRIM switches in the off position. The operator has initiated their own investigation into this event.

--> Bizarre, but the crew did not notice the stab trim cutout switches were not in their normal position but this was not noted when setting trim for takeoff or in climbout. Nothing wrong, other than switch position.

January 18, 2015 Incident at Cancún International Airport

TSB Report #A15F0009: Sunwing Airlines flight SWG587, a Boeing 737-8HX (C-FLSW), was en route from Vancouver to Cancun when the "Stab Out of Trim" warning light illuminated. The crew actioned the relevant checklists and consulted with company maintenance before declaring a MAYDAY with ATC. The flight continued to Cancun and landed without further incident. The "Stab Out of Trim" light had also illuminated on the two previous flights (SWG585 and SWG586) between Vancouver and Cancun. In both cases, the crews declared PAN PAN and continued to destination. After the first occurrence, maintenance inspected and tested the system but did not detect any faults so the aircraft was returned to service. Following the second occurrence, maintenance conducted a BITE test of the DFCS (Digital Flight Control System) which identified the Stabilizer Trim Motor (STM) as being at fault. The STM (Eaton, p/n 6355C0001-01) was replaced, the system tested, and the aircraft was returned to service. Following the final occurrence (flight SWG587), maintenance replaced the STM and the Flight Control Computer (FCC) before returning the aircraft to service. During pre-flight checks for the next flight, the crew reported that the stabilizer electric trim wouldn't work when Nose Up was commanded from the Captain's or First Officer's control column switches, though Nose Down trim was reported to work normally (the system was reported to have operated normally on the pre-flight checks done prior to the two previous occurrences). Maintenance troubleshot the system and determined that the TE flap limit switch (S245) was at fault. The switch was replaced and the aircraft was released to service. There were no further occurrences reported on the following flight.
UPDATE: Sunwing Airlines flight number changed from SWG587 to SWG597.

--> Hard to say. Stab out of Trim comes from autopilot not being able to trim. Replacing the stab trim actuator and FCC would include most of the circuitry. The flap limit switch (S245) reference is uncertain, but it can be the reference that stops manual electric trim from going below about 4 units with flaps up, and has no bearing on autopilot control.

September 1, 2018 Incident at TORONTO / LESTER B. PEARSON INTL ON (CYYZ)

A Sunwing Airlines Inc. Boeing 737-808 (C-FTDQ/SWG802) from Vancouver Int'l, BC (CYVR) to Toronto/Lester B. Pearson Int'l, ON (CYYZ) had previously advised that they were experiencing mechanical issues to air traffic control (ATC) enroute to CYYZ. Upon reaching 25 nautical miles north of CYYZ (1950Z), SWG802 declared an emergency (PAN PAN) with ATC due to low fuel, requesting priority landing Runway 15L. SWG802 landed at 1954Z and taxied to gate without further incident.
UPDATE from Airworthiness: A Civil Aviation Safety Inspector (CASI) looked into this occurrence. Stabilizer Electric trim failed after departure. Crew coordinated with Operational Control Centre (OCC) and Air Traffic Control (ATC) and it was determined that the flight could be continued to CYYZ with manual trim. The technical condition of the aircraft did not allow the use of autopilot. This resulted in not being able to enter Reduced Vertical Separation Minimum (RVSM) airspace. Flying at the lower altitude increased fuel consumption. A PAN PAN was declared. Maintenance Action: STAB trim motor was replaced. No further occurrence reported. TSB#A18O0127: C-FTDW, a Boeing 737-800 aircraft operated by Sunwing Airlines, was conducting flight SWG802 from Vancouver Intl, BC (CYVR) to Toronto/Lester B. Pearson Intl, ON (CYYZ). After the departure from CYVR, the flight crew were unable to trim the aircraft when using the electric trim switches. The flight crew completed the QRH and continued the flight using the manual trim. In coordination with the operator?s Operations Control Center (OCC) and ATC, it was determined that the aircraft could continue the flight to CYYZ at a lower altitude. During the descent into CYYZ, the flight crew was informed by ATC that the planned Runway 15L was no
 December 1, 2018 Incident at In the vicinity of: TORONTO / LESTER B. PEARSON INTL ON
 (CYYZ) (Declared emergency/priority)
longer available, and that Runway 23 was now in use. Fuel calculations determined that the landing could be performed on minimum fuel, however ATC informed the flight crew that they were number 6 for the approach. A PAN PAN was declared, and the aircraft was cleared for the ILS approach to Runway 15L where the aircraft landed without further incident with ARFF standing by.

-->Manual Electric Trim failed/non-functioning. Reverted to manual trim. Was denied RVSM airspace, put fuel consumption up and required priority landing to ensure safe minimums.

December 1, 2018 Incident at In the vicinity of: TORONTO / LESTER B. PEARSON INTL ON
 (CYYZ) (Declared emergency/priority

At 2122Z, a WestJet Boeing 737-8 (C-GZSG/WJA1245) from Tampa, FL (KTPA) to Toronto/Lester B. Pearson International, ON (CYYZ) declared PAN PAN and requested priority handling. WJA1245 reported they had a STAB out of TRIM snag and were requesting priority handling for Runway 15L at CYYZ. WJA1245 touched down at 2147Z. WJA1245 was able to taxi to the gate. There were no issues with the priority handling. No operational impact.
UPDATE from Airworthiness: A Civil Aviation Safety Inspector (CASI) looked into this occurrence. Stab Out of Trim light just after "Flaps Up" on departure. Ran check list. Reverted to manual flight and trim.
Update TSB Report #A18O0172: C-GZSG, a Boeing 737 MAX 8 aircraft operated by WestJet, was conducting flight WJA1245 from Tampa Intl (KTPA), FL to Toronto/Lester B. Pearson Intl (CYYZ), ON. During the flight, the flight crew received a STAB OUT OF TRIM annunciator light. QRH procedures were executed, and the flight crew continued to CYYZ. Passing FL210 during the arrival sequence, the flight crew declared a PAN PAN as they entered Toronto Centre (CZYZ) airspace. The aircraft continued for an uneventful landing with ARFF standing by. The operator’s maintenance replaced the Stabilizer Trim Motor, and the aircraft was returned to service after a successful test of the system.

--> Stab Out of Trim is reference to autopilot unable to zero elevator command by trimming stabilizer. It is not associated with using manual electric trim. Both Autopilot and Manual Electric trim share a common actuator. The checklist should allows for manual electric trim rather than manual wheel trim if only the autopilot circuit was not functioning, and no runaway.


At departure from CYUL, a Sunwing Airlines Boeing 737-800 (SWG426) from Montreal/Pierre Elliot Trudeau, QC (CYUL) to Punta Cana, Dominican Republic (MDPC) requested to return to CYUL due to a technical problem. No emergency declared. SWG426 landed without incident. No impact on operations. 

-->Stabilizer out of trim indiation, autopilot actuator failed/non-functional - replaced.


From Brian Lowe:
A few of the Canadian reports mentioned submission of an SDR so I ran a few queries through the FAAs SDR database for 737 occurrences in years 2000 to 2018.
Stab out of Trim flagged in around 50 cases with about 5  mentioning complete jam’s.
Despite best intentions these databases suffer from poor data input classification/categorisation and are therefore difficult to capture specific occurrences without running a wide trawl and eyeballing each entry. Again like CADORS reporting SDR databases are only a small snapshot of the occurrences actually taking place. The real data is in the airlines databases.
Ran queries for Runaway’s and Uncommanded trim and found 11. 
One momentary "runaway" that was a fault of the manual electric trim actuator.
Three true runaways were from a fused relay.
One runaway from what was most likely the autopilot actuator path, but no parts were replaced.
Two runaways from the common actuator.
One runaway from the common actuator that was corrected by resetting it per Boeing TIP
One runaway due to brake failure.
One runaway from Speed Trim - replaced FCC and AOA vane.

2000-07-10 Stabilizer Runaway 737-200

-->Stabilizer runaway from what appears to be brake failure in the actuator. Manual Wheel trim was functional. Replaced both actuators.

2001-01-26 Stabilizer Runaway 737-500

--> Relay fused caused uncommanded motion.

2002-12-17 Stabilizer Runaway 737-800

--> Runaway on short final. Used cutout switch. Actuator then was completely non-functioning. Replaced actuator.

2002-08-15 Stabilizer Runaway 737-300

-->At moderately high altitude, nose-down runaway (autopilot off). Pilot countered by aft column motion and column cutout switch along with nose-up manual electric command stopped runaway in about two seconds. Text book. What was strange was it appears the crew did not select cutout, but found that the runway did not repeat? Not clear for sure. In any case, the runaway appears to be within the actuator, as it was replaced and no further notice.

2003-08-07 Stabilizer Runaway 737-400 

--> Relay fused in manual electric nose-up command. Crew countered with nose down electric command and presumably used cutout switch (no mention). 

2006-09-11 Stabilizer Runaway 737-500

-->too little data to make any assessment, but apparently no fault found.

2009-06-23 Stabilizer Runaway 737-800

-->No detail on the nature of the uncommanded motions, but noting that the FCC and AoA sensor were replaced. This would imply a failure in the Speed Trim circuitry.

2011-01-10 Stabilizer Runaway 737-400

-->On first glance, it looks like autopilot actuator runaway from speed trim command. No report of action taken, but assume that the cutout switch was used. Corrective action did not involve replacing any component.

2011-05-20 Stabilizer Runaway 737-800

-->Autopilot Speed Trim runaway.  Interfered with Manual Electric trim. Resorted to Manual Wheel trim, without any further issue. Boeing has a maintenance tip regarding a reset of logic in the actuator, there was no part replaced.

2017-12-28 Stabilizer Runaway 737-700

--> Relay fused cause nose-up stabilizer runaway. Presumably cutout switch was used without further issue.

2019-04-05 Stabilizer Runaway 737-800

-->No data on what triggered the nose-up runaway, but the fault was in the actuator.

Aviation Herald (events identified by Brian Lowe)

one event common actuator failed/non-functional
one event runaway - unknown failure mode
one event actuator failure leads to jammed stabilizer - landing OK
one event stabilizer jam due to deice fluid freezing

American B738 near Chicago on May 2nd 2012, trim failure

By Simon Hradecky, created Wednesday, May 2nd 2012 21:50Z, last updated Wednesday, May 2nd 2012 21:50Z
An American Boeing 737-800, registration N811NN performing flight AA-451 from Chicago O'Hare,IL to San Francisco,CA (USA), was climbing out of Chicago with the autopilot engaged when the crew received a speed trim and a stabilizer out of trim indication. The autopilot was disconnected, the crew found the electrical trim had failed but manual trim remained available. The crew levelled off at FL280 and after working the checklists returned to Chicago for a safe landing about 50 minutes later.

A replacement Boeing 737-800 registration N915AN is estimated to reach San Francisco with a delay of 3:15 hours.

--> Likely actuator failure/non-functional. No reported issues using manual trim.

Norwegian B738 at Kittila on Dec 26th 2012, unintentional steep climb on ILS approach

Norways AIBN released a preliminary report reporting that the aircraft was on an ILS approach to Kittila's runway 34 at about 3250 feet with autopilot and autothrust engaged, flaps set to 5 degrees, gear up, when the stabilizer trim actuated for about 12 seconds trimming the nose up resulting in a climb and loss of airspeed, as result autothrust systems applied full engine thrust which in turn created more pitch up movements and a more rapidly decreasing airspeed. When the pitch went through 20 degrees nose up both pilots began to apply full forward pressure on the control columns applying a combined force of 207lbs on the yoke, the nose however continued to rise and reached a maximum of 38.5 degrees nose up with the speed decaying to 118 KTAS (specifically true airspeed referenced by AIBN) before the aircraft started to pitch down again. At that point the stall warning and stick shaker activated for about 4 seconds. The crew subsequently managed to regain control of the aircraft. 

The AIBN annotated that the stall speed of 1G at the given configuration of the aircraft was 121 KIAS, the aircraft however did not stall because the wing load was slightly less than 1G. During the initial upset there has been no attempt to disconnect the autopilot, autothrust or actuate the stabilizer trim nose down. The AIBN said: "One or more of these measures would have improved the situation. Also, to the knowledge of The Accident Investigation Board Norway (AIBN), the stick force applied should have made the autopilot switch off automatically."

Preliminary findings of the investigation showed that both primary and secondary input arm of the right hand Power Control Unit (PCU) driving the aircraft's elevator and stabilizer trim had been blocked with the autopilot unintentionally elevating the nose of the aircraft. Both PCUs were removed from the aircraft, CT scans suggested the presence of small particles of foreign objects, when the PCUs were later opened at the labs no foreign objects were found. Laboratory analysis revealed traces of dried up de-icing fluid. Tests of both PCUs revealed normal function with no anomaly.

The AIBN revealed that current procedures require the stab trim set fully forward during de-icing, tests however showed the ingress of fluid would be reduced at a mid range position. In October 2013 Boeing changed the aircraft maintenance manual to introduce new de-icing procedures requiring operators now to apply de-icing fluid at an angle from the front and not from the side and requiring the stab trim to be at the takeoff position during de-icing.

--> Deicing fluid jammed the stabilizer trim.

United B739 near Phoenix on Jun 5th 2013, trim system failure

A United Boeing 737-900, registration N37422 performing flight UA-1451 from Phoenix,AZ to Washington Dulles,DC (USA), was climbing out of Phoenix, when the crew stopped the climb at FL230 and returned to Phoenix for a safe landing about 55 minutes after departure.

The aircraft was able to depart again after about 5.5 hours on the ground and is currently estimated to reach Washington with a delay of 6 hours.

A passenger reported the captain announced the failure of the trim system and associated failure of autopilot as cause for the return.

--> no idea what specifically was failed or how it failed.

American B738 near Miami on Jul 26th 2013, flight control problems

An American Airlines Boeing 737-800, registration N825NN performing flight AA-2088 from Miami,FL to Tampa,FL (USA), was climbing out of Miami when the crew received a temporary stab trim fail indication, then noticed the engine power was not in agreement with the set climb power, autothrottles were slow in response. Both autopilots could not be engaged. The aircraft was about half way into the flight when the crew decided to return to Miami due to the problems and weather in Tampa. The aircraft landed safely back on Miami's runway 26R about 40 minutes after departure.

A replacement Boeing 737-800 registration N926NN reached Tampa with a delay of 3 hours.

-->gremlins! seemingly unrelated events compound any simple analysis.

United B738 near Cleveland on Nov 8th 2013, stab trim problems

By Simon Hradecky, created Wednesday, Nov 13th 2013 22:45Z, last updated Wednesday, Nov 13th 2013 22:45Z
A United Boeing 737-800, registration N37255 performing flight UA-1275 from Cleveland,OH to Denver,CO (USA), was climbing out of Cleveland when the crew stopped the climb at FL190 due to stab trim problems and decided to return to Cleveland for a safe landing about 40 minutes after departure.

A passenger reported that after the return to Cleveland passengers were asked to board a replacement aircraft, the replacement aircraft however had some issue and could not depart. About 2-3 hours later the airline announced that the incident aircraft had been repaired and was ready to depart, the aircraft subsequently made it to Denver without further incident.

The incident aircraft reached Denver with a delay of 3.5 hours.

-->no idea

Alaska B738 at Los Angeles on Jul 22nd 2017, trim problem

By Simon Hradecky, created Sunday, Jul 23rd 2017 19:04Z, last updated Sunday, Jul 23rd 2017 19:05Z
An Alaska Airlines Boeing 737-800, registration N513AS performing flight AS-617 from Los Angeles,CA to Seattle,WA (USA) with 159 people on board, was climbing out of Los Angeles' runway 24L when the crew requested to level off at 4000 feet reporting one of their trim systems wasn't working. The aircraft levelled off at 4000 feet, subsequently climbed to 5000 and 7000 feet due to ATC sector boundaries while working checklists and preparing the landing back. The aircraft landed safely on Los Angeles' runway 25L about 45 minutes after departure and taxied to the apron advising no further assistance was needed.

The occurrence aircraft remained on the ground for about 8.5 hours then departed again for flight AS-617 and reached Seattle with a delay of 10 hours.

--> no idea

American B738 at Boston on Aug 22nd 2017, "trim!"

An American Airlines Boeing 737-800, registration N914NN performing flight AA-2253 from Boston,MA to Chicago O'Hare,IL (USA) with 161 people on board, was climbing out of Boston's runway 22R when the crew requested to level off at 9000 feet due to an issue. When departure queried "what's the issue", the crew replied with only one word: "Trim!" The crew subsequently advised they were still working the checklists, no assistance was yet needed. About 6 minutes later the crew formally declared emergency and requested to return to Boston, they needed a nice long runway and a long established final, they had a lot of fuel (about 4-5 hours). The crew decided they needed to burn off fuel for about 40 minutes requesting different altitudes for holding, then the aircraft positioned for an approach to Boston and landed safely on Boston's runway 22L about 70 minutes after departure. 

The flight was cancelled.

The occurrence aircraft remained on the ground for 44 hours, then performed a test flight and resumed service.

--> no idea

Ryanair B738 near Tenerife on Apr 26th 2018, blocked stabilizer trim

A Ryanair Boeing 737-800, registration EI-DYN performing flight FR-7476 from Pisa (Italy) to Tenerife South,CI (Spain), had been enroute at FL370 and was nearing the top of descent when the crew received a "STAB OUT OF TRIM" indication. The crew disconnected the autopilot, worked the related checklists and considering the weather with forecast windshear at Tenerife decided to divert to Las Palmas,CI (Spain) where the aircraft landed safely.

A replacement Boeing 737-800 registration EI-FZX delivered the passengers to Tenerife with a delay of 2.5 hours.

The stabilizer trim was found blocked. The aircraft remained on the ground for about 14 hours, then returned to service.

On May 3rd 2018 The Aviation Herald received information that the stabilizer trim actuator motor was found defective also disabling manual trim (jamming the trim wheel). The pilot of an aircraft landing into Tenerife South reported a positive windshear, while pilots landing into Las Palmas reported strong winds but no windshear prompting the diversion to Las Palmas. The crew configured for a flaps 15 landing in line with QRH procedures, requested a long approach and made a safe landing. 

-->Stab out of Trim. This led to a jammed stabilizer. 

UTAir B735 at Surgut on May 25th 2019, trim runaway

A UTAir Boeing 737-500, registration VP-BXQ performing flight UT-759 from Surgut (Russia) to Khujand (Tajikistan), was climbing out of Surgut's runway 25 when the automatic trim activated shortly after takeoff resulting in a trim runaway. The aircraft levelled off at 750 meters (about 2500 feet) for about 60 minutes, then climbed to 6000 feet. After burning off sufficient fuel the aircraft positioned for a landing on Surgut's runway 25 about 2:45 hours after departure.

The airline reported a problem with the trim channel prompted the return to Surgut.

Rosaviatsia confirmed the crew decided to burn off fuel in the holding area of Surgut due to the spontaneous continuous activation of the trim shortly after takeoff.

-->not sure what caused the runaway.

Peter Lemme

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Peter Lemme has been a leader in avionics engineering for 38 years. He offers independent consulting services largely focused on avionics and L, Ku, and Ka band satellite communications to aircraft. Peter chaired the SAE-ITC AEEC Ku/Ka-band satcom subcommittee for more than ten years, developing ARINC 791 and 792 characteristics, and continues as a member. He contributes to the Network Infrastructure and Interfaces (NIS) subcommittee developing Project Paper 848, standard for Media Independent Secure Offboard Network.

Peter was Boeing avionics supervisor for 767 and 747-400 data link recording, data link reporting, and satellite communications. He was an FAA designated engineering representative (DER) for ACARS, satellite communications, DFDAU, DFDR, ACMS and printers. Peter was lead engineer for Thrust Management System (757, 767, 747-400), also supervisor for satellite communications for 777, and was manager of terminal-area projects (GLS, MLS, enhanced vision).

An instrument-rated private pilot, single engine land and sea, Peter has enjoyed perspectives from both operating and designing airplanes. Hundreds of hours of flight test analysis and thousands of hours in simulators have given him an appreciation for the many aspects that drive aviation; whether tandem complexity, policy, human, or technical; and the difficulties and challenges to achieving success.

1 comment:

  1. I think the manual trim wheel is a mechanical linear actuator. The electric trim is an electro-mechanical linear actuator. The type of linear actuator on the 737 is a ball screw, not a jack screw, or so I've been told in the Seattle Times comment section.