Martin B744 at Harare on Jul 8th 2019, dropped part of flaps
Last Update: February 24, 2022 / 16:38:29 GMT/Zulu time
The occurrence aircraft is still on the ground in Harare about 33 hours after landing.
The airline reported the aircraft lost part of a flap. The loose part was found just outside of Harare and has not caused any damage. An investigation has been opened.
Zimbabwe's Civil Aviation Authority reported the aircraft dropped a flap in Chitungwiza but landed safely at Harare Airport.
Residents of Chitungwiza found the part and handed it over to investigators.
On Dec 16th 2019 the Dutch Onderzoeksraad (DSB) reported the aircraft lost the right hand inboard Fowler flap, the crew were able to control the aircraft and continue for a safe landing. The Civil Aviation Authority of Zimbabwe is conducting the investigation, the DSB has joined the investigation.
On Dec 14th 2020 the DSB reported: "Zimbabwe, the state of occurrence has delegated the whole of the conducting of the investigation to the Netherlands, as the state of registry and state of the operator. The Dutch Safety Board has instituted an investigation into the circumstances of the occurrence."
On Feb 24th 2022 the DSB released their final report concluding the probable causes of the accident were:
The right wing inboard foreflap of PH-CKA failed and partly separated, because of a fatigue crack failure of the foreflap outboard fitting lug. The fatigue crack was caused by pitting corrosion. The pitting corrosion had formed because of moisture that had accumulated between the inside of the foreflap fitting lug and the outside of the fitting lug bearing for over a long period of time. The cause of PH-CKA’s inboard foreflap outboard fitting failure is similar to other investigated occurrences concerning inboard foreflap separations, that occurred with the Boeing 747 series aircraft in the past.
The owner of the aircraft’s planning of the required maintenance on the inboard foreflap outboard fitting was compliant with Airworthiness Directive 75-20-05. The owner had incorporated Service Bulletin 747-27-2366 (Rev 3), that recommends the visual inspection and lubrication of the foreflap fitting within an interval period of six months, in such a way that an interval period of seven to eight months ensued. The owner had justified this longer interval period by referring to its experience with foreflap fitting anomalies across its Boeing 747 fleet in the past. The European Union regulations concerning continuing airworthiness allow for such an adaptable incorporation of service bulletin requirements into aircraft maintenance programs.
About one year prior to the failure of PH-CKA’s right wing inboard foreflap outboard fitting lug, the foreflap outboard fitting lug bearing was replaced during a regular D-check because of migration of the bearing. At that time, it was likely that a progressed form of pitting corrosion was present on the inside of the fitting lug. This corrosion was not discovered during the replacement. Therefore, the Dutch Safety Board emphasises that investigating the cause of anomalies of aircraft parts according to maintenance manuals and by observing standard maintenance practices is vital to ensure system safety.
Boeing issued Alert Service Bulletin 747-57A2367 on 15 November 2019 to further improve the reliability and the safe operation of the inboard foreflap and related parts of the inboard trailing edge flap system. This alert service bulletin requires replacement of the inboard foreflap outboard fitting lug at an interval that is well before the expected time of failure. On 30 March 2021, the Federal Aviation Authority mandated with Airworthiness Directive 2021-02-15 the maintenance requirements from Alert Service Bulletin 747-57A2367. This measure as imposed by the airworthiness directive has been incorporated after a prolonged period with various maintenance measures to improve the Boeing 747 inboard trailing edge flap system. The Dutch Safety Board considers that the alert service bulletin requirements and the mandatory compliance as imposed by the airworthiness directive are adequate to prevent future inboard foreflap outboard fitting lug failures. Hence, the Dutch Safety Board does not issue recommendations.
The DSB analysed:
PH-CKA’s outboard foreflap fitting failure
The maintenance organisation’s metallurgic failure analysis of PH-CKA’s broken right wing inboard foreflap outboard fitting determined that a progressed form of pitting corrosion and fatigue fracture was the root cause of the foreflap fitting failure. The failure of the outboard fitting caused the foreflap to come lose, move upwards and backwards into the airstream, then break in half and separate from the aircraft. The separated part caused subsequent damage to the aircraft.
The analysis further stated that the pitting corrosion was caused by moisture that had been present between PH-CKA’s inboard foreflap outboard fitting lug inner surface and fitting lug bearing. For moisture to enter and pitting corrosion to form on the inner surface of the fitting lug, play between the fitting lug and the bearing must have been present. Once pitting corrosion has started, the inner surface of the fitting lug gradually loses material which in turn increases the play, causing more moisture to enter. Because the forming of pitting corrosion is a long term process, it is likely that the pitting corrosion on the inside of the fitting lug was already present before the replacement of the fitting lug bearing of PH-CKA in July 2018.
According to SB 747-27-2366, the play, loosening and migration of foreflap fitting bearings may be related to skewed operation of the foreflap in flight or poor lubrication of the fitting lug bearing. Skewed operation or poor lubrication were not identified as causal factors with the foreflap fitting failure of PH-CKA by the maintenance organisation’s failure analysis. Nevertheless, the manufacturer had indicated that because of the flap system’s complexity, it is difficult to determine the actual cause of a partial foreflap loss and that skewed operation may contribute to the initiation of a fatigue crack.14 The skewed operation causes high stress loads on the fitting lug and fitting lug bearing. Therefore, the Safety Board remarks that skewed operation of the flap system as a contributing factor to the failure of the foreflap fitting lug, should not be ruled out entirely.
Similar outboard foreflap fitting failures
PH-CKA’s failure mode of the right wing inboard foreflap fitting as described in Paragraph 3.1.1 shows a similar corrosion and fatigue failure mode of the foreflap fittings as they were found in the occurrences listed in Paragraph 2.6.1.
Noteworthy is that of these occurrences, the service life of the outboard foreflap fittings at the moment of failure, was approximately between eleven and sixteen years. The service life of PH-CKA’s fitting was sixteen years, showing correlation with the mentioned failure time period. Altogether, the foreflap separation occurrence with PH-CKA shows a similar causality with previous foreflap separations.
Owner’s maintenance on inboard foreflap fittings
The owner of the aircraft incorporated the maintenance tasks from AD 75-20-05 and SB 747-27-2366 (Rev 3) in its regular maintenance cycle/planning. The AD 75-20-05 required a lubrication task which had to be accomplished at an interval rate of up to 24 months. The owner performed this maintenance task at an average interval of approximately seven months In the period 2014 to 2019. The owner complied with the mandatory requirements from AD 75-20-05.
Furthermore, the owner’s engineering department integrated SB 747-27-2366 (Rev 3) maintenance tasks into the existing maintenance program on 1 August 2016. The maintenance task that required a detailed inspection and lubrication of the outboard foreflap fitting and bearing was scheduled to be performed with an interval not to exceed 3,400 flight hours. This interval of 3,400 hours led to an actual inspection interval period of seven to eight months.15 The SB recommended a maintenance interval not to exceed six months. With the interval period based on aircraft hours as determined by the owner, the recommended interval period of six months was exceeded by one to two months. The owner’s rationale to deviate from the SB recommendation was that their experience with twenty one foreflap fitting replacements across their Boeing 747 fleet in the past, justified a longer interval. The European Union regulations concerning continuing airworthiness allow operators to incorporate service bulletins based on their experience with their fleet.
One of the inspection tasks of SB 747-27-2366 (Rev 3) is to perform a detailed visual inspection of the exterior of the fitting lug and the fitting bearing as they are assembled. The SB does not instruct to inspect the inside of the fitting lug for corrosion. The fitting lug bearing in its position in the fitting lug prevents such a visual inspection for corrosion. As such, the maintenance tasks from SB 747-27-2366 could not detect nor prevent corrosion on the inside of the fitting lug.
The maintenance documentation indicates that outboard fitting lug bearing No 6 of the foreflap fitting was found migrated and replaced during a regular D-check16 inspection on 22 July 2018; this was one year prior the failure of the fitting lug. After this replacement, the bearing was inspected and lubricated according to the SB, five months later in December 2018. This SB inspection did not reveal any discrepancies on the fitting lug or bearing. The fitting lug failed seven months later in July 2019.
During the above mentioned D-check in July 2018, a general visual inspection of the right wing inboard trailing edge flap system revealed that the outboard fitting lug bearing No 6 was migrated. This finding led to the replacement of the bearing that was mentioned earlier. The maintenance worksheet that described the work performed, does not contain remarks about the condition of the inner surface of the fitting lug. The documentation neither shows that further investigation was performed in order to find the cause of the bearing migration nor, does it show that the foreflap fitting was within technical specifications. The non-routine maintenance worksheet raised by the maintenance repair overhaul (MRO) organization had the SOPM 20-50-03 annotated as the procedure to replace the bearing. The SOPM 20-50-03 provided the work steps for the removal and installation of bearings. Yet, instructions to visually inspect parts of the foreflap fitting and bearing for defects and if needed, to use additional inspection methods to further investigate anomalies were provided in CMM 57-52-31. It should be noted that maintenance has to be performed according to the instructions of the maintenance manual, such as an SOPM or CMM and by observing standard maintenance practices.
Measures taken to prevent foreflap fitting failures
On 15 November 2019, which is after the incident with PH-CKA, Boeing issued alert SB 747-57A2367 to further improve the reliability of the trailing edge flap system. This alert SB requires operators to periodically replace specific parts of the trailing edge flap system. For the inboard foreflap outboard fitting specifically, a replacement interval of every six years or 5,880 flight cycles is required. This replacement time interval is well before the eleven to sixteen years failure time period of the foreflap outboard fitting, as also mentioned in Paragraph 3.1.2. Therefore, the periodic replacement of the inboard foreflap outboard fitting as required by alert SB 747-57A2367 is deemed an effective preventive measure, according to the Dutch Safety Board.
The owner of the aircraft started to implement the requirements of Alert SB 747-57A2367 on 2 December 2019. The owner’s maintenance documentation17 showed that the maintenance program was adjusted to be adhere to the Alert SB 747-57A2367 as of 28 February 2021. On 30 March 2021, the FAA published AD 2021-02-15, that mandates all applicable actions from Alert SB 747-57A2367.
Aircraft Registration Data
This article is published under license from Avherald.com. © of text by Avherald.com.
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