Corendon Dutch B738 near Athens on Jul 8th 2022, sluggish aircraft control
Last Update: August 8, 2024 / 20:56:25 GMT/Zulu time
Incident Facts
Date of incident
Jul 9, 2022
Classification
Incident
Airline
Corendon Dutch Airlines
Flight number
CD-512
Departure
Heraklion, Greece
Destination
Amsterdam, Netherlands
Aircraft Registration
PH-CDF
Aircraft Type
Boeing 737-800
ICAO Type Designator
B738
On Nov 17th 2022 Greece's Air Accident Investigation and Aviation Safety Board (AAIASB) reported the occurrence was rated a serious incident and is being investigated.
The occurrence aircraft remained on the ground in Athens for about 54 hours, then positioned to Amsterdam and returned to service another 10 hours after landing there.
On Jul 11th 2023 the Dutch Onderzoeksraad (DSB) reported in a brief interim report, they are investigating the occurrence. The DSB wrote:
The incident occurred on June 8, 2022 within the local air traffic control area of Heraklion International Airport Nikos Kazantzakis on the island of Crete, Greece. The aircraft had just taken off for the return flight to Amsterdam Airport Schiphol. The process update is given because the investigation takes longer than a year.
Immediately after leaving the runway, abnormal steering corrections were necessary to keep the aircraft level (wings level). It was unclear what caused the control problems. The flight crew decided to divert to Athens, Greece. The approach and landing were without any significant problems. An initial inspection revealed that an aileron cable had broken.
The study is currently in the analysis phase . The final report will be published in due course.
On Aug 8th 2024 the Onderzoelsraad released their final report concluding the probable causes of the serious incident were:
Due to a failure of the left aileron control cable, the aircraft got into a degraded state of safety, as it partially lost a primary flight control causing a continuous roll effect to the left.
The aircraft remained controllable as the flight crew managed the roll effect by applying rudder and aileron input. The aircraft flight control system design aided in keeping control over the aircraft attitude and flight path. It is to be noted however that the serious incident occurred during favourable weather conditions as the atmosphere was smooth.
For the system configuration on the PH-CDF, the flight control check performed by the flight crew (as part of the Before taxi checklist) is not effective in detecting a deteriorating or failed flight control cable between the feel and centering unit and the ailerons. A timely detection of a defect depends on an adequate maintenance process.
The failed aileron cable was according to material specifications and it was the original cable since aircraft delivery in January 2000. Detailed examination showed that both the failed and paired cable were affected by corrosion and wear and lacked sufficient lubrication. The absence of sufficient lubrication promotes wear and corrosion.
The Dutch Safety Board could not determine why the cables were not properly
lubricated. The maintenance documentation shows that the required inspections were performed and that the tasks were signed off as completed. However, it cannot be ruled out that both cables have not been treated and lubricated in accordance with applicable maintenance procedures.
In view of the Dutch Safety Board, it is important that - in particular for cables that have aged in cycles and flight hours – a ground engineer timely detect defects. In general, for ground engineers, the detection and determination of the extent of wear during a Detailed Visual Inspection (DVI) of the cables is a difficult task because of the small size of the cable inspected ‘on condition’.
Insufficient lubrication was the case for PH-CDF, but the overall effect of wear over lifetime of the aileron cables contributed to the failure as well. It could not be determined that aging (in terms of normal wear) of the aileron cable was a more dominant factor than the effect of lack of lubrication which promoted more than normal wear and corrosion.
The required interval limits for the DVI have been stretched from every 12 to 18 months in 1996 to 4,000 flight cycles or 24 months as applicable in June 2022. Given the effect of corrosion due to lack of lubrication as found on PH-CDF, it cannot be determined that the inspection interval of the DVI (4,000 flight cycles or 24 months) valid at the time of the event, was set too broadly.
Reviewing the last three decades with hundreds of millions of flights, there is no immediate reason to doubt the effectiveness of the current maintenance procedures. The Dutch Safety Board concludes that - despite the serious incidents with PH-CDF and TF-HHB – a safety recommendation is therefore considered not to be appropriate.
The DSB analysed:
The roll effect to the left upon lift-off and afterwards was caused by the non-functioning of the left aileron of the aircraft, which came and remained in an upward position. This was caused by a failure of the aileron cable for the left aileron down input in the wheel well. As revealed by Flight Data Recorder (FDR) data, the aileron cable failed when the crew performed the flight control check as part of the ‘Before taxi checklist’. The original aileron cable, installed since aircraft delivery on 15 January 2000, was still in place, meaning that the cable had accumulated 75,776 flight hours when it failed.
Laboratory analysis shows that the composition of the broken cable was according to the prescribed Boeing Material Specification (BMS) and Aircraft Maintenance Manual (AMM) specifications. However, both the failed and paired cable were affected by corrosion and significant abrasive wear, including squeezing, and showed a lack of sufficient lubrication.
The signed off maintenance documents - like task cards – only show that the Detailed Visual Inspection (DVI) had formally been accomplished. It does not describe – nor is this required - a detailed condition of the inspected cables. Furthermore, the Dutch Safety Board did not find peculiarities that could relate to the corrosion and nonlubricated condition of aileron control cables when it reviewed the maintenance and aircraft tech log documents. Therefore, the Dutch Safety Board decided not to continue an in-depth reconstruction of the DVI or other circumstances, as it would likely not reveal why the cables showed a lack of sufficient lubrication.
Referring to Section 2.7.2, as the DSB assessed the pulley itself (hence, the pulley was not sent to the laboratory for examination), the DSB needs to address the conclusion of the laboratory that the pulley near the cable fracture was not properly rotating.
Since the operator had the aircraft in service from May 2012, the assessed pulley had been installed at least more than 10 years. The DSB found that a normal contact pattern of the aileron cable was in accordance with the AMM. It is therefore assumed that the assessed pulley had no adverse effect on the cable other than that cable displacement over the pulley affected normal wear. The history of the pulley(s) and the effect on the condition of the aileron cable were not further investigated, as it would not explain the lack of lubrication of the aileron cable anyway.
Non-detection of cable failure prior to departure
Flight crew statements revealed that during the flight from Schiphol to Heraklion no flight control difficulties were experienced and flight data showed normal aileron behaviour of both ailerons. Data analysis revealed that the cable had failed prior to taxi, explaining the flight crew information that abnormal behaviour of the left aileron occurred after lift-off from Runway 27 at Heraklion.
The flight control check by the pilots prior to taxi particularly focused on the functioning of the flight control systems. Flight data reveals that the left aileron cable happened to fail during the aileron flight controls check. This check could not have detected the aileron cable discontinuity between the aileron feel and centering unit and the ailerons21, as there is no aileron position (deflection) feedback for the pilot, nor are the ailerons visible from the cockpit.
The Aircraft Technical Log (ATL) showed no entries that could be related to the cable failure.
Aircraft Registration Data
Incident Facts
Date of incident
Jul 9, 2022
Classification
Incident
Airline
Corendon Dutch Airlines
Flight number
CD-512
Departure
Heraklion, Greece
Destination
Amsterdam, Netherlands
Aircraft Registration
PH-CDF
Aircraft Type
Boeing 737-800
ICAO Type Designator
B738
This article is published under license from Avherald.com. © of text by Avherald.com.
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