Myanmar National Embraer 190 NLG Jam: Maintenance Error? We’re Not Convinced! (XY-AGQ, Mandalay)
On 12 May 2019 Myanmar National Airlines Embraer 190 XY-AGQ suffered damage to its forward fuselage during the landing in Mandalay, on a flight from Yangon, after the Nose Landing Gear (NLG) failed to extend.
The 89 occupants were unharmed. A maintenance error narrative was accepted without question by the technical press. We aren’t convinced! https://youtu.be/Jl5xXzj8n64
The Aircraft Accident Investigation Bureau of Myanmar (AAIBM) issued their safety investigation report on 16 June 2020 (after just 13 months). They explain that analysis of the Flight Data Recorder (FDR), and the weight-off-wheels switch signal in particular, showed that as the NLG retracted the shock strut had compressed enough to disengage a self-centring cam which keeps the nose wheels central as the NLG enters the bay. The nose wheels turned and became offset by about 20°. Any more that 8° offset means the wheels will jam when the NLG is lowered, which is what happened. On 23 previous flights the weight-off-wheels switch signal had also changed from ‘WOffW=True’ to ‘WOffW=False’ during the retraction. The investigators concluded that a similar event could have occurred on those flights.
The aircraft had previously undergone fault-finding to try to rectify a failure message relating to the weight-on-wheels system, first observed on 8 May 2019. The engineer involved stated that the aircraft had…
…encountered a Landing Gear WOW System fault just after take-off from Yangon and returned to Yangon and the Pilot-in-command entered a Landing Gear WOW System fault message into the Technical Logs book.
The engineer said that the maintenance action performed was cleaning the Nose Landing Gear sensors, connector and targets according to Fault Isolation Manual (FIM) (32-61-00-810-890-A00). After that he checked CMC [Central Mainteance Computer] of the aircraft, that there were no WOW SYS fault messages active.
The aircraft was put back into service that day but on 11 May 2020, the crew felt the NLG vibrating during take-off and gear retraction. They recorded in the Technical Log records that “Nose Wheel Tension is Stronger than Normal During Take off and Landing Rolling”. Maintenance was performed that night, with the engineer stating that the task performed…
…was the Nose Landing Gear shock Strut N2 Chamber Servicing, according to Aircraft Maintenance Manual (AMM) 12-12-09-600-804-A. He also stated this condition, was not included in FIM. Therefore, he decided on the appropriate maintenance action for this fault based on his experience. After that he checked the Dimension H of sliding tube of Nose Landing Gear based on the temperature 30ºC , it was within limit in the graph of (AMM) 12-12-09-600- 804-A.
The accident occurred on the next flight.
The AAIBM Conclusion
The investigators concluded:
Maintenance actions were not properly done as per the fault isolation manual in rectifying the intermittent Weight off Wheels System Fail fault and in addition, poor workmanship in performing the Nose Landing Gear Strut N2 servicing.
Our Observations and Warnings on WYLFIWYF and the FFP Fallacy
However, disappointingly, the investigators actually provided no evidence for their conclusions! They identify no omission or error by the first engineer. They don’t comment on the dilemma the second engineer faced, namely the absence from the FIM of the oddly phrased problem reported nor do the investigators assess the adequacy of the maintenance instructions in general. The investigators also do not identify why the N2 Chamber servicing was either incorrect or, indeed if correct, may have contributed to the jam on the 24th flight. There are also no details of the inspection of the NLG.
- Investigators training has been orientated to ‘error investigation’. Sometimes courses that are (re-)titled as ‘event investigation’ or ‘occurrence investigation’ are still primarily focused on interview of frontline staff and their behaviour with technical information simply being given by the instructor. Interview skills are important but just one of many skills investigators need.
- Investigations are reviewed / approved by panels or review groups who work to a judgemental process that is primarily orientated around classifying errors and behaviour, determining culpability (or ‘accountability’) and whether disciplinary action should be instigated.
- The motivation for investigations is simply to satisfy third parties (regulators, customers etc) and show the ‘system’ is OK.
In turn, such investigations can also fall for the FFP Fallacy too. We’ve also seem statistical studies that have misleadingly classified any procedural issue as FFP.
We have previously written:
- James Reason’s 12 Principles of Error Management
- Back to the Future: Error Management
- Airworthiness Matters: Next Generation Maintenance Human Factors
- Rockets Sleds, Steamships and Human Factors: Murphy’s Law or Holt’s Law?
- Safety Performance Listening and Learning – AEROSPACE March 2017
- B1900D Emergency Landing: Maintenance Standards & Practices
- B747 Landing Gear Failure Due to Omission of Rig Pin During Maintenance
- When Down Is Up: 747 Actuator Installation Incident
- Lost in Translation: Misrigged Main Landing Gear
- Maintenance Human Factors in Finnish F406 Landing Gear Collapse
- ERJ-190 Flying Control Rigging Error (UPDATED 25 June 2020 with Final Report – very different levels of detail and analytical rigour evident)
- NTSB Confirms United Airlines Maintenance Error After 12 Years
- Fire After O-Ring Nipped on Installation
- A Lufthansa MD-11F Nose Wheel Detached after Maintenance Error
- B767 Fire and Uncommanded Evacuation After Lockwire Omitted
- What Leaks in Vegas Stays in Vegas – A320 Hydraulic Failure
- A319 Double Cowling Loss and Fire – AAIB Report
- CFM56-7 HPC Titanium Fire Due to VSV Maintenance Assembly Error (United Boeing 737-924)
- Uncontained CF6-80 Failure: American B767-300 28 Oct 2016
- Inadequate Maintenance, An Engine Failure and Mishandling: Crash of a USAF WC-130H
- Crazy’ KC-10 Boom Loss: Informal Maintenance Shift Handovers and Skipped Tasks
- Hoist Assembly Errors: SAR Personnel Dropped Into Sea
- Contaminated Oxygen on ‘Air Force One’
- Meeting Your Waterloo: Competence Assessment and Remembering the Lessons of Past Accidents
- UPDATE 25 June 2020: Japanese Jetstar Boeing 787 GEnx-1B Engine Biocide Serious Incident
- UPDATE 28 June 2020: Maintenance Issues in Fire-Fighting S-61A Accident
- Also see our review of The Field Guide to Understanding Human Error by Sidney Dekker presented to the Royal Aeronautical Society (RAeS): The Field Guide to Understanding Human Error – A Review
The pilot and the flight test engineer were completing a production test flight of the helicopter. A review of a surveillance video showed that, as they were preparing to land, the lower section of the left side of the engine cowling folded back and struck all four main rotor blades, which resulted in substantial damage to the blades. According to the operator/manufacturer, postaccident examination of the helicopter revealed that none of the fasteners on the left side of the cowling remained attached and that three of the fasteners were missing. No evidence of any preaccident discrepancies with the fasteners, the cowling, or the design and conformity of the helicopter structure was found.
According to the manufacturer/operator, before the flight, the pilot reported to maintenance personnel that he had seen that the fasteners attaching the generator duct to the right side of the same cowling appeared long. Maintenance personnel then removed the one-piece engine cowling, and a mechanic then replaced the generator duct fasteners and reinstalled the cowling with the help of another mechanic. The mechanic said he specifically fastened each of the fasteners on the cowling himself and checked all the fasteners after the cowling was reinstalled. Once the cowling was reinstalled, a quality inspector visually inspected the engine cowling fasteners. He did not see any anomalies and signed off that the work had been completed. The flight test engineer performed another preflight inspection of the helicopter and did not note any issues with any of the cowling fasteners. It is likely that the mechanic did not properly install the cowling fasteners before the flight, which resulted in the cowling coming loose and striking the main rotor blades.
The NTSB determined the probable cause of this accident to be:
The in-flight failure of the lower left side of the engine cowling due to the mechanic’s improper installation of the fasteners, which resulted in the cowling impacting all four main rotor blades.
The specific maintenance error is however not identified.
Aerossurance’s experienced investigators can help coach your investigation team, so they don’t make the mistake WYLFIWYF and or fall for the FFP Fallacy.