Ryanair B738 at Frankfurt on Jan 29th 2019, gear strut penetrates wing during retraction
Last Update: August 27, 2020 / 15:03:45 GMT/Zulu time
On Dec 2nd 2019 the US NTSB reported the Irish AAIU is investigating the occurrence, that occurred in Frankfurt (Germany). The NTSB joined the AAIU investigation under ICAO Annex 13 provisions.
On Feb 19th 2019 Boeing released following Multi Operator Message (MOM):
FROM: THE BOEING COMPANY
TO: Boeing Correspondence (MOM)
[MESSAGE NUMBER:MOM-MOM-19-0087-01B] Multi Operator Message
MESSAGE DATE: 19 Feb 2019 1253 US PACIFIC TIME / 19 Feb 2019 2053 GMT
This message is sent to all 737NG/MAX Customers, Regional Directors, Regional Managers and Boeing Field Service Bases.
SERVICE REQUEST ID: 3-4434607494
ACCOUNT: Boeing Correspondence (MOM)
DUE DATE: No Action Required
PRODUCT TYPE: Airplane
PRODUCT LINE: 737
SUBJECT: Information - Main Landing Gear (MLG) Hardware Penetrates Wing
/B/ 737 AIPC 32-32-93-01
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A 737-800 operator reported that their aircraft was climbing when it experienced a Main Landing Gear (MLG) retraction anomaly as indicated by the landing gear position indication and warning system. The flight crew evaluated the discrapancy and performed an air turn back. After 38 minutes from departure, the aircraft landed safely with no injuries reported. Damage was observed to the upper surface of the LH wing.
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The operator reported pin, part number 161A7301-1, was found resting on the lower surface of the wing, under the MLG actuator. It appears the pin (161A7301) migrated such that it was no longer engaged within the corresponding link. When the MLG actuator (273A2101) was commanded to retract the landing gear, the MLG actuator extended in a way that it's linkage penetrated the upper wing surface. The associated nut (BACN11N112CD), washer (161A7304), and split pin (BACP18BC04A14P) have not been located.
Please review Ref /A/ figure attached.
When the landing gear control lever is in the UP position, the hydraulic system pressurizes the MLG up lines through the selector valve to the MLG transfer cylinders and MLG downlock actuators. The transfer cylinders stroke. This delays full pressurization to the MLG actuators until the downlock actuators unlock the downlock mechanism. After the downlocks unlock, pressure increases in the head end of the MLG actuators and they extend to retract the landing gear. In the subject event, with the pin fully dislodged, there was no reaction point when the MLG actuator extended, thus the linkage traveled up and outward towards the wing upper surface.
The event aircraft delivered from Boeing in September 2018. Boeing has opened a quality investigation addressing the noted landing gear hardware.
AIPC 32-32-93-01 INFORMATION:
- Split Pin, BACP18BC04A14P, Fig 1 item 10
- Pin, 161A7301-1, Fig 1 item 15
- Washer, 161A7304-3, Fig 1 item 20
- Nut, BACN11N112CD, Fig 1 item 25
- Beam Assy Complete, 161A7100-X, Fig 1 item 95
- Link, 161A7114-2, Fig 1 item 160
Boeing recommends performing a visual check of the MLG retract actuator and linkage per AMM pageblock 32-11-00/601 (step 2.(F)(1.)) to ensure all hardware is properly secured; Pin intact, Nut fully installed, Split Pin installed and captures nut. In addition, Boeing recommends visually inspecting the MLG actuator attaching hardware as outlined in AMM pageblock 32-32-11/401.
These inspections are currently recommended for 737NG/MAX line number 7150 thru 7420. Boeing requests that operators report any negative findings.
In addition, Boeing recommends operators keep a maintenance record of this MOM recommended inspection for traceability purposes. In the event that further service action is issued, operators who have performed the visual check per this MOM can use their records to verify accomplishment.
On Aug 27th 2020 the Irish AAIU released their final synoptic report concluding the probable cause of the occurrence was:
Migration of the pivot pin from the walking beam hanger in the left main landing gear assembly.
- The pivot pin was not held securely in the correct position.
- Production line inspections did not identify that the castellated nut and cotter pin intended to hold the pivot pin securely in position, were either not installed or installed incorrectly.
The AAIU summarized the sequence of events:
At approximately 18.00 hrs on the 29 January 2019, the aircraft departed Frankfurt-Hahn Airport (EDFH). When established in the climb, the Commander called for ‘gear-up’. During gear retraction, the Flight Crew heard an unusual noise. This was followed by the aircraft rolling to the left, and the illumination of the left-hand landing gear red and green indication lights. The Flight Crew trimmed the aircraft to reduce the roll and attempted unsuccessfully to engage Autopilot Channel A. The Flight Crew then actioned and completed the ‘GEAR DISAGREE’ checklist in the aircraft Quick Reference Handbook (QRH). ATC directed the Flight Crew to continue to Flight Level (FL) 100 and turn right at their discretion. The Flight Crew made a further, successful attempt to engage the Autopilot Channel A. After reviewing the situation, the Flight Crew elected to return to EDFH, briefed the Cabin Crew and following ATC clearance, took up a holding pattern.
Whilst in the hold, the Flight Crew discussed the situation with ATC and requested that the Airport Rescue and Firefighting service be in attendance for their arrival, but they did not declare an emergency. On final approach, at seven nautical miles (nm) from EDFH, the landing gear was selected to down. All indications were normal, so the approach and landing proceeded as planned. No abnormal indications were observed during roll-out and taxi.
When the aircraft arrived at its assigned gate, the tower controller informed the Flight Crew that a small piece of metal from the aircraft had been found on the runway. Damage to the upper surface of the left wing of the aircraft was observed by the passengers and Crew.
The passengers and Crew disembarked the aircraft normally. The part found on the runway was subsequently confirmed to be part of the skin from the upper surface of the left wing.
The AAIU analysed:
Initial field enquiries carried out by the BFU, concluded that the probable cause of the failure was that the castellated nut and cotter pin used to secure the walking beam hanger to the aircraft structure had either been improperly installed or not installed at all (these parts were not found at the scene of the occurrence). This conclusion was supported by the evidence gathered by the Aircraft Manufacturer, who carried out a full review of the relevant production line processes, engineering drawings and associated data.
The fact that following the occurrence the pivot pin and tang washer were lost, meant that a full laboratory analysis of these components was not possible, and this may have provided further evidence. However, the Investigation is satisfied that the most probable cause has been identified: that during the manufacturing of the aircraft, the castellated nut and cotter pin were either not installed, or installed incorrectly, and that this was not identified on the production line. Production line records indicate two previous instances (in 2016 and 2017) where the installation of the castellated nut and cotter pin were missed. However, in these instances, subsequent production line inspections identified the error before the aircraft was released to service.
Following the occurrence, the Aircraft Manufacturer inspected the castellated nut and cotter pin installations on all of the aircraft of this type in their custody, i.e. aircraft that had not yet been delivered to customers. All of the castellated nuts and cotter pins were found to be installed correctly. In addition, no further issues were reported by affected Boeing 737 NG and Boeing 737 Max operators following an MOM which recommended that they carry out a visual check of the MLG actuator, linkage and actuator attaching hardware. Furthermore, no previous in-service occurrences had been reported.
The Investigation notes that the Aircraft Manufacturer’s own Investigation identified that although mechanics on the production line reported no issues with the installation of the castellated nut and cotter pin, they were not using the approved process, and due to access issues were applying torque to the head of the pivot pin, using a standard torque wrench adapter, rather than applying the torque to the castellated nut. The Aircraft Manufacturer’s engineering team subsequently reviewed and approved the method that was being used by the mechanics.
In summary, although this appears to be an uncommon occurrence, it does serve to remind aircraft manufacturers and maintenance organisations of the importance of inspection tasks during assembly and maintenance, and the damage that can be caused if these tasks are missed, or incorrectly executed.
This article is published under license from Avherald.com. © of text by Avherald.com.
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