I would like to preface the following comments by saying that many of the issues I raise also occur on MANY other aircraft types, but I have specifically examined the MRH 90 repair manuals and so the comment is made here.
I have looked at the MRH 90 repair manuals provided to the RAAF and to be absolutely frank, they are pathetic. I can not believe that in the 21st century there are so many fundamental deficiencies that have been known for decades to produce very poor structural outcomes and yet they are here again in the approved SRM.
Even a cursory assessment show that they still use a scuff-sand and solvent clean process for surface preparation with blind rivets into sandwich structure, and this was abandoned by RAAF for F-111 before 1992 because it always disbonded and caused corrosion in the skin and/or core as well as disbonding. This has been a known deficiency since Noah tried to repair the Ark. So why do we find it in a repair manual for a modern aircraft? It was scrapped on F-111 before 1980 and upgraded in 1992. Since 1992, the repair failure rate on F-111 was reduced from 43% to less than 0.07% by adopting improved repair methods.
Then there is the use a heater blanket at 100C (212F) on composite or sandwich structure without first undertaking a drying process. That has an incredibly high probability of either blowing the core from the face sheets or blowing the core cells apart or delaminating the composite skins if ANY of the structure exceeds 100C (212F). Consider this when you read the next paragraph.
Next is the use of a single heater blanket to heat the structure for repair irrespective of the presence of heat sinks under the blanket. This is combined with the fact that only ONE thermocouple is used. There are only two possible outcomes depending on the location of the thermocouple; either the adhesive is under-cured over the heat sink if the thermocouple is located over thin structure away from the heat sink, or the structure adjacent to the heat sink will be over-heated while the heater tries to supply sufficient heat for the heat sink as dictated by the thermocouple. Overheating will result in delamination of laminated composites and/or skin-to-core disbonding or blown core.
The correct procedures are outlined in
http://www.tc.faa.gov/its/worldpac/techrpt/artn0657.pdf where the MULTIPLE heaters are configured to match heat sinks and MULTIPLE sensors are used to ensure each heated zone remains below a temperature which would cause damage, while other sensors provide assurance of adhesive cure. That publication also provides guidance on evaluating surface preparation procedures and I can assure readers that scuff-sand and solvent clean would never in a million years meet those standards. I doubt that any of the approved MRH 90 repair method would meet these requirements.
The correct procedures were also contained in a RAAF publication DEF (AUST) 9005 which was an engineering standard on composite and adhesive bonded repairs. Recently RAAF downgraded the status of that standard so that OEM approved repairs such as these would no longer have to meet the requirements which delivered the significant reductions in repair failure rates outlined above.
I would be happy to discuss these issues with the OEM or any user, including RAAF rotary wing personnel.
Kindest regards
Blakmax