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SATURDAY, MAY 18, 2019

737 Pitch Trim Incidents

REVISED TO INCLUDE OTHER EVENTS => 23 JUNE 2019Pitch trim is an abstract term to represent the ability to reduce column forces by moving thestabilizer 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 actuatorsmotors. Starting with the NG, the Boeing 737 relies on only one electric trim actuator, with manualwheel trim as the backup. There is no documented 737 accident as a result of stabilizer/pitch trimmalfunction or failure (prior to JT610 and ET302).

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

Ethiopian ET302 encountered high opposing forces due to the mistrim; it has been assumed thatthe ET302 actuator was not jammed, and there has been no concern raised that the ET302 clutchwould 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 withsome 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 asan 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 ofreliability.

737 STAB TRIM occurrences are not considered a serious incidents therefore don’tusually appear in NTSB reports, rather they are reported within airlines SMS andreporting systems to NAA. Most International States don’t provide public access tosuch information.

As a rule of thumb I find the number Canadian accidents per type to be around onetwelfth 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 10-16per annum of which at least one might be a stabilizer jam.

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Brian notes that my search was incomplete, that the events would not be revealed in the NTSBreports 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 byincluding them. There are three groups:

1. CADORS reports2. 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 atleast one per year, with overall stab trim non-functional about one per month. There are enoughexamples to assume at least on jam per year.

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

On October 10, 2001, at 1627 hours Pacific daylight time, Alaska Airlines flight 497, aBoeing 737-700, N615AS, experienced a jammed horizontal stabilizer trim actuatorduring the takeoff initial climb from the John Wayne Airport-Orange County, SantaAna, California. The flight was destined for Seattle, Washington; however, the flightcrew diverted to, and landed at, Los Angeles International Airport, Los Angeles,

CONCLUSION

NASA ACCIDENT DATA BASE

Alaska Flight 497

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 atboth the pilot and co-pilot controls and the manual trim control was jammed andimmovable.

To emphasize the point, the flight crew observed that arriving maintenance personnel had nomore success.

According to the pilot, maintenance personnel who met the airplane on arrival couldnot move the stabilizer control either and believed the actuator gearbox wasjammed.

The flight crew were insistent that the manual trim was not usable. That the simulator forces weremuch less than what they encountered.

In a subsequent company interview, on November 13, 2001, the pilot reiterated thatthe 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 pilotsaid he had trained for the jammed stabilizer emergency in the simulator; however, inthe incident at hand he had applied "a lot more force" than was required in thesimulator 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 wasseized. Further examination of the actuator motor revealed that the motor wasmechanically seized.

The actuator was replaced. Nothing else was replaced.

The actuator motor was replaced with a serviceable unit and the aircraft was ferriedto Seattle for inspection. No other faults were found in the pitch trim system and theairplane 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, inorder to manually trim the stabilizer, it would have been necessary for the flight crewto have exerted sufficient force on the trim wheel to cause the motor clutch to slip inaddition 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 pitchtrim. Having two actuators was the original source of the two 737 cutout switches, elec main andautopilot. This 737-300 autopilot actuator failed when the circuit breaker "popped". The autopilotran out of elevator authority. The pilot disconnected the autopilot and flew the airplane manuallywithout further incident.

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

While the pilot complained of some difficulty controlling the airplane manually (electric pitch trimwas 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.

From Brian Lowe:

A word search of CADORS (2000-2019) for “737 STAB out of TRIM” identifies 14occurrences where the 737 stabilizer motor malfunctioned, and in two of theses themanual trim also jammed. (Some of the occurrences involved stab trim switches onlyworking 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 trimstab to null elevator position), it favors the failure of the autopilot trim function.

Jet2 G-ELCA

CADORS REPORTS

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

The 737NG uses one actuator with two control interfaces, one for manual electric and the otherfor 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 circuitnon-functioning. However with two actuators, the mechanical components have moreindependence.

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

Two of the incidents involved some difficulty/jam in moving the stabilizer manually, the rest makeno mention of any problem.

Here is a QRH excerpt.

Initial information from T.S.B. Initial Notification (#A01O0254): The Air Canada Boeing 737 aircraftdeparted 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 nosedown using the control wheel switch. The autopilot was also found to have limited trimauthority. Nose up trim was normal and full manual trim was available. The flight crew elected toreturn to Toronto (LBPIA) and the aircraft landed below maximum landing weight without further

September 9, 2001 Incident at near Toronto

incident. Maintenance replaced the stab trim main electric actuator. Trim was functionallychecked normal. 59 S.O.B.

-->Failure of the stabilizer trim actuator/Partial non-functioning. Manual wheel trim used withoutincident.

Shortly after take-off, the Boeing 737-281 aircraft [C-FTWJ] was climbing through 1300 feet ASLwhen 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 engineforward and aft fuel pumps low pressure lights were illuminated. There were no other associatedwarning lights or master caution lights. The pilots noticed that the electric stab trim, number 2communication radios and the number 2 navigation system were unserviceable. The captaininformed ATC of the situation, leveled off the aircraft at 4000 feet, and advised that they werereturning to Abbotsford. Subsequently, the first officer found the TR2, TR3 and gasper fan circuitbreakers had popped. The aircraft landed in Abbotsford without further incident. Companymaintenance inspected the aircraft electrical system and determined that the number twogenerator had failed. The operator will file a Service Difficulty Report with Transport Canada.

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

The WestJet Boeing 737-700, C-FWSF, was on a scheduled flight from Toronto to Calgary. Whilelevel at flight level 390, the autopilot disengaged along with an associated warning horn. Themaster caution flight controls light illuminated and a speed trim failure was displayed on theoverhead 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 todestination using the manual trim system. The aircraft landed with a Vref additive of 15 knots foran uneventful landing at Calgary. Company maintenance personnel tested the speed trim systemand changed out a stabilizer trim motor. The aircraft has subsequently been returned to servicewithout further problems.

September 6, 2003 Incident at ABBOTSFORD BC (CYXX)

February 25, 2004 Incident at Between Calgary and Cancun, MexicoWESTJET FLIGHT 1382, A BOEING 737-700, REGISTRATION C-GUWS, WAS ENROUTE FROM CALGARY TO CANCUN, MEXICO. APPROXIMATELY TWO HOURS INTOTHE FLIGHT, WHEN THE AIRCRAFT WAS OVER THE CENTRAL CONTINENTALUNITED STATES, THE ""STAB OUT OF TRIM"" LIGHT ILLUMINATED. THEASSOCIATED NON-NORMAL CHECKLIST WAS COMPLETED; HOWEVER,THE ELECTRIC TRIM REMAINED INOPERATIVE AND TRIM CHANGES HAD TO BEACCOMPLISHED MANUALLY. MAINTENANCE CONTROL WAS CONSULTED ANDCIRCUIT BREAKERS 6B AND 610 WERE PULLED AND RESET. THE ""STAB OUT OFTRIM"" LIGHT CONTINUED TO ILLUMINATE INTERMITTENTLY, AND THE PILOTSCONTINUED TO MANUALLY INPUT THE REQUIRED TRIM CHANGES.THE STABILIZER EVENTUALLY BECAME COMPLETELY JAMMED AND THE ""JAMMEDSTABILIZER NON-NORMAL CHECKLIST"" WAS COMPLETED. THE STABILIZERREMAINED JAMMED FOLLOWING COMPLETION OF THE CHECKLIST, AT WHICHTIME MAINTENANCE CONTROL REQUESTED THAT CIRCUIT BREAKERS 6B AND 610BE PULLED AND LEFT OUT. MANUAL CONTROL OF THE STABILIZER WASRESTORED AND A NON-NORMAL LANDING WAS COMPLETED, WITHOUT FURTHERINCIDENT, IN CANCUN. WESTJET IS WORKING WITH BOEING TO IDENTIFY THEPROBLEM. CHECKLIST PROCEDURES ARE ALSO BEING REVIEWED.UPDATE MAINTENANCE AND MANUFACTURING REPORTED THAT SDR#20040309004 HAS BEEN ISSUED THIS MORNING. 'OPEN STATUS' AIRCRAFT #009REPORTED A JAMMED STABILIZER IN CRUISE ENROUTE FROMCANCUN TO YYC ON FEB.25/04. THE CREW WAS ABLE TO FLY THE AIRCRAFTUNDER MANUAL CONTROL AND LANDED SAFELY UNDER ABNORMAL LANDINGCONFIGURATION. MAINTENANCE REPLACED THE STAB TRIM MOTOR AND THEAIRCRAFT WAS RETURNED TO SERVICE WITH NO FURTHER INCIDENTS TOREPORT. WESTJET MAINTENANCE/FLIGHT OPS ARE WORKING WITH BOEING TOFURTHER INVESTIGATE. WILL UPDATE THE SDR AS MORE INFORMATIONBECOMES AVAILABLE.UPDATE MAINTENANCE AND MANUFACTURING ADVISED THAT THE OPERATORREPLACED THE STAB TRIM MOTOR IN ACCORDANCE WITH MM 27-41-71. THEAIRCRAFT WAS RETURNED TO SERVICE WITH NO FURTHER INCIDENTS TOREPORT. BOEING HAS BEEN NOTIFIED AND A RESPONSE IS EXPECTED ONFRIDAY. UPDATES WILL FOLLOW AS SOON AS THERE IS MORE INFORMATION. ANSDR WILL BE FILED WITH UPDATE UPON RECEIPT OF BOEINGRECOMMENDATIONS.

-->STAB TRIM ACTUATOR FAILURE THAT LEFT THE STABILIZER JAMMED (MANUALWHEEL CONTROL) FOR A PORTION OF THE FLIGHT. NOT CLEAR IF IT WASPOSSIBLY A CLUTCH BREAKOUT OR ELEVATOR OPPOSING FORCE ISSUE, BUT BYTHE ACTION TAKEN SOUNDS MORE LIKE CLUTCH BREAKOUT PROBLEMMARCH 27, 2004 INCIDENT AT BRANDON MUNI MB (CYBR)

UPDATE Maintenance and Manufacturing advised that the company reported that the crew ofaircraft tail #218 NG, reported; ""autopilot disengaged-master caution flight control illuminated-speed trim fail light illuminated-no electric trim-no autopilot."" Following in-flighttroubleshooting/non normal checklists, the aircraft landed in YYC without incident. Maintenancereplaced electric stabilizer motor IAW MM 27-41-71 and tested system IAW MM22-11-81 and MM27-41-71. No faults were found. The aircraft was returned to service with no further problems toreport. This unit, p/n: 6355C0001-01 is a new EATON ""C"" model which the vendor hascompletely overhauled because of problems with their ""B"" model. This was the first such stabtrim problem with the new ""C"" model and is currently being investigated by EATON, BOEINGand 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 aPCB 'meltdown', the root cause of the PCB meltdown is believed to be a K3 relay failure. Theoverall EATON STM 'C' model reliability is still under investigation by EATON and Boeing. Thereare 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.

The Westjet Airlines Limited Boeing 737-7CT aircraft (C-FWAI, operating as WJA656) was on ascheduled IFR flight from Edmonton International Airport (CYEG) to Toronto (LBPIA) (CYYZ). Theflight crew reported a stabilizer trim problem to the North Controller. ARFF staff were called to beat 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 disengagedand the aircraft was levelled at FL390. After disengaging the autopilot, the flight crew noted thatthe 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 flightcrew 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 andthe aircraft was returned to service.

Supplemental information received from Westjet Airlines Limited [2005/10/31]: The Safety Officeradvised 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 whichtime 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-normallanding was completed in Toronto. The flight crew declared a non-critical emergency andreceived 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 thestabilizer trim motor test IAW AMM 27-41-71. All tests were passed and the aircraft was returnedto service.

-->A relay failed and left the electric trim non-functioning. It appears the flight continued withoutdifficulty using manual wheel trim.

WJA 5018, a B737-700, departed Kelowna for Puerto Vallarta and shortly after takeoff, the flightcrew advised ATC of a stabilizer issue and requested to divert to Calgary. They also requestedtheCFR equipment be on standby for their landing. The aircraft landed safely at YYC at 0413z, TSBEdmonton is looking into the occurrence.UPDATE TSB reported that the WestJet Boeing 737-700, registration C-FZWS, was operating asflight WJA 5018 from Kelowna, B.C. (CYLW) to Puerto Vallarta, Mexico (MMPR). During the climbfrom CYLW, the crew received a "Stab Out of Trim" annunciation and stabilizer trim would notrespond to electric trim inputs. After consultation with company maintenance, WJA 5018 divertedto Calgary, AB (CYYC) where the aircraft landed with ARFF on standby. Company maintenancereplaced 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.

WJA468, Boeing 737-500, enroute from Toronto (CYYZ) to Halifax (CYHZ) inbound for Runway05 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.

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

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

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

UPDATE TSB: A06A0001: WJA468, a Westjet Boeing 737-76N, had departed Toronto en route toHalifax when the "Stab Out Of Trim" caution light illuminated. The crew attempted to operate theelectric trim without success. They completed the "Stabilizer Trim Inoperative" non-normalchecklist, which instructed the crew to use a maximum flap setting of 15 degrees. ApproachingHalifax, the crew informed control tower personnel that the minimum airspeed of the aircraftwould be 150 knots and that they did not require assistance. ARFF were on standby on theterminal apron when the aircraft landed safely without further incident. Maintenance inspectionrevealed that a flap limit switch had malfunctioned. The switch was replaced and the aircraft hasbeen returned to service.

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

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-trimlight illuminated again a short time later. At that point neither the electric trim nor the manualtrim would respond. The checklist was consulted and the aircraft diverted to the maintenancebase in Edmonton. No emergency was declared and ARFF assistance was not requested for thelanding.The electric stab trim, manual stab trim and autopilot stab trim systems were tested inaccordance with the Boeing 737 maintenance manual and no discrepancies were identified. Atest flight was performed during which the stab trim functioned normally. As a precaution theelectric and autopilot trim cut-out switches were replaced, the manual trim system was cycledthrough the full range of travel, and a trim cable inspection was carried out. No faults were foundand the aircraft was returned to service.

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

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

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 lightilluminated. ATC was informed and a go-around was commenced. The crew declared anemergency, checklists were actioned and vectors supplied for an ILS approach to 08L. Theaircraft 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 andwas vectored for a second approach. Aircraft landed 08L without further incident. Six aircraft wereheld outside the terminal airspace.

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

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

UPDATE TSB reported that WestJet flight 517, a Boeing 737-700, registration C-FBWJ, waslevelling at a cruising altitude of FL 400, en route Montreal to Calgary, when the pitch trim wheelstarted 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 controldifficulties were present and although the aircraft equipment no longer met the minimum RVSMairspace requirements (no autopilot), the flight remained at FL 400 as per ATC instructions andcontinued to Calgary. An emergency was not declared; however, ARFF was requested to standby for the arrival. Full manual trim was available and the flight landed in Calgary without furtherincident. 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

February 25, 2008 Incident at YELLOWKNIFE NT (CYZF)

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

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

stabilizer. It is not associated with using manual electric trim. Both Autopilot and Manual Electrictrim share a common actuator. It was not clear if the crew used manual electric trim.

Update: Follow-up information received from Airworthiness ? National Maintenance Operations[2014/08/20]: A Civil Aviation Safety Inspector (CASI) looked into this occurrence. The CASIcontacted the company and was advised that the flight crew reported experiencing aircraft controlissues (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 inairspeed. On further investigation, the flight crew found that the MAIN ELEC and AUTOPILOTstab trim cut-out switches on the centre pedestal were not in the normal position (causingStabilizer 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 normalposition. Company Maintenance Control advised the flight crew to return the aircraft to thedeparture 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 notconfirm any active faults and latched speed trim failure was reset. There has been no recurrence.Thecompany managing this occurrence through their safety management system (SMS) process andconducted 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 ofMach Trim system) and the flight crew did not notice this condition prior to departure. Theinvestigation resulted in a Corrective Action Plan to address the deficiencies. The CASI wassatisfied with the company?s handling of this occurrence, their corrective action plan and advisedthat 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 flightfrom 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 theSTAB TRIM was not responding with autopilot selected. As the aircraft increased airspeed, itbegan to pitch nose down. The Captain took control, manually trimmed the aircraft and declared aPAN PAN emergency and returned to Dublin International Airport. The landing was uneventfulwith ARFF standing by. Maintenance staff were called on board and found both of the STABTRIM 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 normalposition but this was not noted when setting trim for takeoff or in climbout. Nothing wrong, otherthan switch position.

TSB Report #A15F0009: Sunwing Airlines flight SWG587, a Boeing 737-8HX (C-FLSW), was enroute from Vancouver to Cancun when the "Stab Out of Trim" warning light illuminated. The crewactioned the relevant checklists and consulted with company maintenance before declaringa 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 andSWG586) between Vancouver and Cancun. In both cases, the crews declared PAN PAN andcontinued to destination. After the first occurrence, maintenance inspected and tested the systembut did not detect any faults so the aircraft was returned to service. Following the secondoccurrence, 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/n6355C0001-01) was replaced, the system tested, and the aircraft was returned to service.Following the final occurrence (flight SWG587), maintenance replaced the STM and the FlightControl Computer (FCC) before returning the aircraft to service. During pre-flight checks for thenext flight, the crew reported that the stabilizer electric trim wouldn't work when Nose Up wascommanded from the Captain's or First Officer's control column switches, though Nose Down trimwas 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 systemand determined that the TE flap limit switch (S245) was at fault. The switch was replaced and theaircraft was released to service. There were no further occurrences reported on the followingflight.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 stabtrim actuator and FCC would include most of the circuitry. The flap limit switch (S245) reference isuncertain, but it can be the reference that stops manual electric trim from going below about 4units with flaps up, and has no bearing on autopilot control.

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

January 18, 2015 Incident at Cancún International Airport

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) toToronto/Lester B. Pearson Int'l, ON (CYYZ) had previously advised that they were experiencingmechanical issues to air traffic control (ATC) enroute to CYYZ. Upon reaching 25 nautical milesnorth 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 withoutfurther incident.UPDATE from Airworthiness: A Civil Aviation Safety Inspector (CASI) looked into thisoccurrence. Stabilizer Electric trim failed after departure. Crew coordinated with OperationalControl Centre (OCC) and Air Traffic Control (ATC) and it was determined that the flight could becontinued to CYYZ with manual trim. The technical condition of the aircraft did not allow the useof 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 wasdeclared. Maintenance Action: STAB trim motor was replaced. No further occurrence reported.TSB#A18O0127: C-FTDW, a Boeing 737-800 aircraft operated by Sunwing Airlines, wasconducting 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 whenusing the electric trim switches. The flight crew completed the QRH and continued the flight usingthe manual trim. In coordination with the operator?s Operations Control Center (OCC) and ATC, itwas determined that the aircraft could continue the flight to CYYZ at a lower altitude. During thedescent 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 thelanding could be performed on minimum fuel, however ATC informed the flight crew that theywere number 6 for the approach. A PAN PAN was declared, and the aircraft was cleared for theILS approach to Runway 15L where the aircraft landed without further incident with ARFFstanding by.

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

At 2122Z, a WestJet Boeing 737-8 (C-GZSG/WJA1245) from Tampa, FL (KTPA) toToronto/Lester B. Pearson International, ON (CYYZ) declared PAN PAN and requested priorityhandling. WJA1245 reported they had a STAB out of TRIM snag and were requesting priorityhandling for Runway 15L at CYYZ. WJA1245 touched down at 2147Z. WJA1245 was able to taxito 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 flightand 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 duringthe 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’smaintenance replaced the Stabilizer Trim Motor, and the aircraft was returned to service after asuccessful test of the system.

--> Stab Out of Trim is reference to autopilot unable to zero elevator command by trimmingstabilizer. It is not associated with using manual electric trim. Both Autopilot and Manual Electrictrim share a common actuator. The checklist should allows for manual electric trim rather thanmanual 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/PierreElliot Trudeau, QC (CYUL) to Punta Cana, Dominican Republic (MDPC) requested to return toCYUL due to a technical problem. No emergency declared. SWG426 landed without incident. Noimpact 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

December 1, 2018 Incident at In the vicinity of: TORONTO / LESTER B. PEARSON INTL ON

(CYYZ) (Declared emergency/priority

December 20, 2018 Incident at 15NM MONTRÉAL / PIERRE ELLIOTT TRUDEAU INTL QC (CYUL)

FAA SDR

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 inputclassification/categorisation and are therefore difficult to capture specific occurrenceswithout running a wide trawl and eyeballing each entry. Again like CADORS reportingSDR 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 TIPOne runaway due to brake failure.One runaway from Speed Trim - replaced FCC and AOA vane.

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

--> Relay fused caused uncommanded motion.

2000-07-10 Stabilizer Runaway 737-200

2001-01-26 Stabilizer Runaway 737-500

2002-12-17 Stabilizer Runaway 737-800

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

-->At moderately high altitude, nose-down runaway (autopilot off). Pilot countered by aft columnmotion and column cutout switch along with nose-up manual electric command stopped runawayin 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 tobe within the actuator, as it was replaced and no further notice.

2002-08-15 Stabilizer Runaway 737-300

2003-08-07 Stabilizer Runaway 737-400

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

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

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

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

2006-09-11 Stabilizer Runaway 737-500

2009-06-23 Stabilizer Runaway 737-800

2011-01-10 Stabilizer Runaway 737-400

2011-05-20 Stabilizer Runaway 737-800

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

--> Relay fused cause nose-up stabilizer runaway. Presumably cutout switch was used withoutfurther issue.

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

one event common actuator failed/non-functionalone event runaway - unknown failure modeone event actuator failure leads to jammed stabilizer - landing OK

2017-12-28 Stabilizer Runaway 737-700

2019-04-05 Stabilizer Runaway 737-800

AVIATION HERALD (EVENTS IDENTIFIED BY BRIAN LOWE)

one event stabilizer jam due to deice fluid freezing

By Simon Hradecky, created Wednesday, May 2nd 2012 21:50Z, last updated Wednesday, May2nd 2012 21:50ZAn American Boeing 737-800, registration N811NN performing flight AA-451 from ChicagoO'Hare,IL to San Francisco,CA (USA), was climbing out of Chicago with the autopilot engagedwhen the crew received a speed trim and a stabilizer out of trim indication. The autopilot wasdisconnected, 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 safelanding about 50 minutes later.

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

http://flightaware.com/live/flight/AAL451/history/20120502/1520Z/KORD/KSFO

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

By Simon Hradecky, created Wednesday, Nov 27th 2013 17:29Z, last updated Wednesday, Nov27th 2013 17:32ZNorways AIBN released a preliminary report reporting that the aircraft was on an ILS approach toKittila's runway 34 at about 3250 feet with autopilot and autothrust engaged, flaps set to 5degrees, gear up, when the stabilizer trim actuated for about 12 seconds trimming the nose upresulting in a climb and loss of airspeed, as result autothrust systems applied full engine thrustwhich in turn created more pitch up movements and a more rapidly decreasing airspeed. Whenthe pitch went through 20 degrees nose up both pilots began to apply full forward pressure on thecontrol columns applying a combined force of 207lbs on the yoke, the nose however continued torise 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. Atthat point the stall warning and stick shaker activated for about 4 seconds. The crewsubsequently managed to regain control of the aircraft.

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

Preliminary findings of the investigation showed that both primary and secondary input arm of theright hand Power Control Unit (PCU) driving the aircraft's elevator and stabilizer trim had beenblocked with the autopilot unintentionally elevating the nose of the aircraft. Both PCUs wereremoved 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 analysisrevealed traces of dried up de-icing fluid. Tests of both PCUs revealed normal function with noanomaly.

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 October2013 Boeing changed the aircraft maintenance manual to introduce new de-icing proceduresrequiring operators now to apply de-icing fluid at an angle from the front and not from the sideand requiring the stab trim to be at the takeoff position during de-icing.

--> Deicing fluid jammed the stabilizer trim.

By Simon Hradecky, created Wednesday, Jun 5th 2013 22:21Z, last updated Wednesday, Jun 5th2013 22:21ZA United Boeing 737-900, registration N37422 performing flight UA-1451 from Phoenix,AZ toWashington Dulles,DC (USA), was climbing out of Phoenix, when the crew stopped the climb atFL230 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 currentlyestimated to reach Washington with a delay of 6 hours.

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

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

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

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

http://flightaware.com/live/flight/UAL1451/history/20130605/1530Z/KPHX/KIAD

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

By Simon Hradecky, created Friday, Jul 26th 2013 23:22Z, last updated Friday, Jul 26th 201323:22ZAn American Airlines Boeing 737-800, registration N825NN performing flight AA-2088 fromMiami,FL to Tampa,FL (USA), was climbing out of Miami when the crew received a temporarystab trim fail indication, then noticed the engine power was not in agreement with the set climbpower, autothrottles were slow in response. Both autopilots could not be engaged. The aircraftwas about half way into the flight when the crew decided to return to Miami due to the problemsand weather in Tampa. The aircraft landed safely back on Miami's runway 26R about 40 minutesafter departure.

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

http://flightaware.com/live/flight/AAL2088/history/20130726/1800Z/KMIA/KTPA

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

By Simon Hradecky, created Wednesday, Nov 13th 2013 22:45Z, last updated Wednesday, Nov13th 2013 22:45ZA United Boeing 737-800, registration N37255 performing flight UA-1275 from Cleveland,OH toDenver,CO (USA), was climbing out of Cleveland when the crew stopped the climb at FL190 dueto stab trim problems and decided to return to Cleveland for a safe landing about 40 minutes afterdeparture.

A passenger reported that after the return to Cleveland passengers were asked to board areplacement 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 wasready to depart, the aircraft subsequently made it to Denver without further incident.

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

http://flightaware.com/live/flight/UAL1275/history/20131108/1343Z/KCLE/KDEN

-->no idea

By Simon Hradecky, created Sunday, Jul 23rd 2017 19:04Z, last updated Sunday, Jul 23rd 201719:05ZAn Alaska Airlines Boeing 737-800, registration N513AS performing flight AS-617 from LosAngeles,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 systemswasn't working. The aircraft levelled off at 4000 feet, subsequently climbed to 5000 and 7000 feetdue to ATC sector boundaries while working checklists and preparing the landing back. Theaircraft landed safely on Los Angeles' runway 25L about 45 minutes after departure and taxied tothe apron advising no further assistance was needed.

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

http://flightaware.com/live/flight/ASA617/history/20170722/2055Z/KLAX/KSEA

--> no idea

By Simon Hradecky, created Sunday, Aug 27th 2017 19:38Z, last updated Sunday, Aug 27th2017 19:47ZAn American Airlines Boeing 737-800, registration N914NN performing flight AA-2253 fromBoston,MA to Chicago O'Hare,IL (USA) with 161 people on board, was climbing out of Boston'srunway 22R when the crew requested to level off at 9000 feet due to an issue. When departure

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

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

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

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

queried "what's the issue", the crew replied with only one word: "Trim!" The crew subsequentlyadvised they were still working the checklists, no assistance was yet needed. About 6 minuteslater the crew formally declared emergency and requested to return to Boston, they needed anice long runway and a long established final, they had a lot of fuel (about 4-5 hours). The crewdecided they needed to burn off fuel for about 40 minutes requesting different altitudes forholding, then the aircraft positioned for an approach to Boston and landed safely on Boston'srunway 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 andresumed service.

http://flightaware.com/live/flight/AAL2253/history/20170822/1435Z/KBOS/KORD

--> no idea

By Simon Hradecky, created Monday, Apr 30th 2018 22:18Z, last updated Thursday, May 3rd2018 10:30ZA Ryanair Boeing 737-800, registration EI-DYN performing flight FR-7476 from Pisa (Italy) toTenerife South,CI (Spain), had been enroute at FL370 and was nearing the top of descent whenthe 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 Tenerifedecided 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 adelay 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 motorwas found defective also disabling manual trim (jamming the trim wheel). The pilot of an aircraftlanding into Tenerife South reported a positive windshear, while pilots landing into Las Palmasreported strong winds but no windshear prompting the diversion to Las Palmas. The crewconfigured for a flaps 15 landing in line with QRH procedures, requested a long approach andmade a safe landing.

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

By Simon Hradecky, created Monday, May 27th 2019 21:29Z, last updated Monday, May 27th2019 21:29ZA UTAir Boeing 737-500, registration VP-BXQ performing flight UT-759 from Surgut (Russia) toKhujand (Tajikistan), was climbing out of Surgut's runway 25 when the automatic trim activatedshortly after takeoff resulting in a trim runaway. The aircraft levelled off at 750 meters (about 2500feet) for about 60 minutes, then climbed to 6000 feet. After burning off sufficient fuel the aircraftpositioned 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 thespontaneous continuous activation of the trim shortly after takeoff.

-->not sure what caused the runaway.

The entire series on 737 MCAS

What Happened on ET302?AoA Vane must have Failed, the Boeing Fix is In, Senate Grills FAAHow Did MCAS Get Here and What Hurdles Remain?Taking the Next Steps while Awaiting on the Preliminary Report from ET302Ethiopian ET302 similarities to Lion Air JT610What have we learned this week?Comparing Ethiopian ET302 to Lion Air JT043, JT610Angle of Attack Vane Failure Modes

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

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

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Peter Lemme

peter @ satcom.guruFollow me on twitter: @Satcom_GuruCopyright 2019 satcom.guru All Rights Reserved

Peter Lemme has been a leader in avionics engineering for 38 years. He offers independentconsulting services largely focused on avionics and L, Ku, and Ka band satellite communicationsto aircraft. Peter chaired the SAE-ITC AEEC Ku/Ka-band satcom subcommittee for more than tenyears, developing ARINC 791 and 792 characteristics, and continues as a member. Hecontributes to the Network Infrastructure and Interfaces (NIS) subcommittee developing ProjectPaper 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) forACARS, satellite communications, DFDAU, DFDR, ACMS and printers. Peter was lead engineerfor Thrust Management System (757, 767, 747-400), also supervisor for satellite communicationsfor 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 perspectivesfrom both operating and designing airplanes. Hundreds of hours of flight test analysis andthousands of hours in simulators have given him an appreciation for the many aspects that driveaviation; whether tandem complexity, policy, human, or technical; and the difficulties andchallenges to achieving success.

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