Summary of Responses to Proposed AD PAD/F28/93 and PAD/F100/97

Commenter 1

Comment: Cost to conduct each prefight detailed visual inspection is $20. Should read preflight

CASA Response: Agreed PAD changed to preflight as this is a typographical error.

 

Commenter 2

Comment:

Item 1. Refer to the latest issue of the Fokker All Operator Message AOF100.230#05.

Item 2. Refer to SBF100-32-175 to formally cover the accomplishment instructions for the Detailed Visual Inspection.

Item 3.  Strongly supported a compliance period of at least 12 months, and preferably 18 months as it is not possible if the reduced timeframe is imposed.

Item 4. Recommendation to specify the required inspection in the AD in such a way that the operators do have the flexibility to have the inspections at out-stations accomplished by flight crew members specifically trained and authorised for that purpose.

CASA Response:

Item 1. Agree to refer to latest Fokker All Operator Message AOF100.230#05. This version of the AOM specifies at a minimum the inspection is carried out every flight day.

Item 2. A detailed inspection is necessary whilst these units are in service as they have the potential to crack quickly, and such inspections are achievable at main bases. Those flights that are to a remote location do present an issue for compliance with this requirement. However, regulation 39.004 allows an alternative to be presented to CASA for the preflight DVI but it must be of equivalent safety.

Item 3. Agree to changed compliance time to 1 February 2024 for removal from service of the affected main landing gear pistons. 1 February 2024 aligns with the expected removal from service by EASA of the affected pistons. This would be just over 18 months since the investigation report was provided to CASA by EASA.  CASA acknowledges industry feedback that requiring piston removal from service within six months after the effective date of the AD, will not be logistically manageable. The number of spare units readily available at the moment is simply insufficient to replace the pistons that are presently still installed on the aircraft.

Item 4. CASA considers there are issues with pilots conducting the inspection, as pilots are not specifically trained in this level of detail in a walk around inspection. This level of inspection requires thoroughly cleaning the lower aft side of the Main Landing Gear Piston, in preparation prior to conducting the detailed inspection. It also requires using a strong flashlight and magnifying glass to perform the detailed visual inspection. In order for pilots to conduct the inspection would require a change to the expositions of Part 42 (refer Chapter 15 of MOS 42) and Part 145 (refer 145.A.37(f)) to cater for this level of inspection (refer 15.2.1 List item 6 of MOS 42).

 

Commenter 3

Comment:

Item 1. As the high risk time period for hydrogen embrittlement cracking has passed (see report of Dr Romeyn), the likelihood is extremely low to the point that the existing inspection process or heightened one should be sufficient and that there is no obvious basis for requiring replacement of all the components after 2,000 calendar hours have elapsed (or 5,000 calendar hours if a more conservative position is preferred).

Item 2. The pre-flight inspection is also unnecessary, and a detailed daily visual inspection is more appropriate.

Item 3. It is anticipated that CASA will, in considering the matters raised in this submission, canvas the views of other interested parties and subject matter experts. As the potential consequences of the proposed airworthiness directives are extremely serious, the submitter seeks the opportunity, as part of the process of procedural fairness, to address any further comments, submissions on technical questions, and in any event requests a meeting with CASA and other stakeholders at which the technical issues and competing views can be addressed.

CASA Response:

Item 1. Fokker Services (FS) and Collins have finalised their analysis however the two parties could not come to an agreed position. The variation between the two results from FS and Collins is the result of differing analysis, FS reports states that; as calendar time evolves that the induced surface stress imparted by the inappropriate overhaul procedure will relieve and the component will tend towards a normalised state. However, this would be subject to the piston not being subject to any stresses during that time. If in service, then this stress relief is not possible This would require recording appropriate information and the pistons being stored under the appropriate conditions. Whilst the Collins report PS3837-6-03 refers that they are unable to verify the outcome of the plating process applied to the 29 subject pistons and hence the eventual resultant state and that they should be removed from service within six months.  

EASA as the certifying authority recommendation to CASA was to put in place mandatory actions in the forms of mitigations (a detailed visual inspection is carried out before each flight) and corrective actions (removal of the units before 01/02/2024) as soon as possible (within 18 months after 1/8/22). 

EASA as the state of design has asked CASA to address the unsafe condition that has been introduced from the incorrect overhaul practices with the intention that the data in the investigation report being used. The investigation report concluded that removal from service of those pistons that had been overhauled was the only appropriate way to ensure that the unsafe condition was addressed. If CASA failed to take timely action to address this unsafe condition concern, then EASA, as the state of design, will initiate an EASA AD in line with their recommendation to CASA, mandating the removal from service to ensure the continuing airworthiness of the type design and that the units are not introduced into the worldwide supply chain.

The decision to remove all pistons from service was also recommended by Collins Aerospace, the manufacturer of the main landing gear pistons. EASA and the OEM had considered the metallurgical science of hydrogen embrittlement cracking as part of their assessment process and ultimately determined that the effected parts were to be removed from service within 18 months.

One submitter provided a metallurgical report which recommended that the incorrectly overhaul pistons could remain in service if they survived the initial period without failure. This leaves an unacceptable risk as the failure of these pistons may not be benign which could lead to accident involving a transport aircraft. EASA was provided with a copy of this report and reaffirmed their position that the only safe way to manage these incorrectly overhauled pistons was to remove them from service. Collins assessment concluded that the state of these pistons is unknown. Therefore, the only appropriate course of action is to remove these pistons from service.

The Aeronikl overhauled pistons should have been removed from service as mentioned in service bulletin AOF100.230#05 as of 1/3/22 as the nickel coating was twice as hard as the maximum hardness prescribed.

Following the PAD consultation CASA has verified with EASA requirements stipulated within the EASA Letter to CASA, noting the latitude on the removal from service period.

Item 2. A detailed inspection is necessary whilst these units are in service as they have the potential to crack quickly, and such inspections are achievable at main bases. Those flights that are to remote locations do present an issue for compliance with this requirement. However, regulation 39.004 allows and alternative to be presented to CASA for the preflight DVI but it must be of equivalent safety.  

Item 3. CASA has published PAD/F28/93 and PAD/F100/97 for industry consultation after consideration of the technical input from the State of Design National Aviation Authority, EASA, the type certificate holder Fokker Services and Collins Aerospace, the main landing gear component manufacturer and CASA’s investigation in relation to this cracking. Therefore, CASA has consulted widely, and this summary of responses consolidates all input including technical and costs.

 

Commenter 4

Comment:

Item 1. Vary Requirement a. Compliance from “prior to each flight” to “once per flight day” aligned with Fokker Service Bulletin SBF100-32-175.

Item 2. Vary Requirement b. Compliance from replacement “within 6 months” to replacement “within 12 months” of the effective date of the AD.

CASA Response:

Item 1 and 2.

A detailed inspection is necessary whilst these units are in service as they have the potential to crack quickly, and such inspections are achievable at main bases. Those flights that are to a remote locations do present an issue for compliance with this requirement. However, regulation 39.004 allows an alternative inspection schedule to be presented to CASA for the preflight DVI but it must be of equivalent safety. 

 

Conclusion

CASA has fixed simple typographical errors in the PAD.

PAD/F100/97 has been published as AD/F100/98.

CASA has referenced the latest All Operator Message AOF100.230#05 and has included the Service Bulletin SBF100-32-175.

CASA did not agree with allowing the affected pistons to remain in service after 2,000 calendar hours as both EASA and Collins Aerospace has recommended removing pistons from service. If CASA failed to act on this recommendation, it would be enforced by EASA at a later date.

CASA agreed to changing the compliance date from 6 months to the later date specified initially by EASA 01/02/2024 for removal from service.  This change was made on the basis of the consultation received from industry that 6 months was a unobtainable time frame for sourcing replacement components.

CASA did not agree with allowing pilots to carry out the detailed visual inspection within the subject airworthiness directives as it would require additional training of the pilots and the use of basic tools; However, regulation 39.004 allows an alternative to be presented to CASA (AMOC) which should meet the criteria below.

  • Training provided by Part 145 organisation for the specific maintenance service
  • Amendment of expositions for organisations under Part 42 and Part 145
  • Pilot would have to clean the surface, carry a strong flashlight and magnifying glass
Date:
Online version available at: https://www.casa.gov.au//summary-responses-proposed-ad-pad/f28/93-and-pad/f100/97
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