Narrative:

We received this aircraft from another base; it was used for pilot training [that] night. I did an airworthiness check on it today and noted many differences/mods from our normal base aircraft. I also noted that there was a hole cut in the tail rotor gear box (trgb) cover to check the oil level of the trgb. This is something I have seen in other aircraft so I did not take particular notice of this. Additionally I noted that the hole was obviously there when the aircraft was painted due to the condition of the paint around the hole indicating it had been there since before the aircraft was painted. The check airman came in later this evening and noted the hole in the cover and asked if I had seen anything in the aircraft documents that would allow for the hole in the trgb cover. I was not able to find anything on short notice. The check airman made an entry in the aircraft log book as a 'suspected unapproved modification'. I removed the aircraft from service and ordered a replacement cover.during the conversation the check airmen stated that there were several aircraft that had the hole done and did not have any approval. [Company] has done these in the past and if [they are] aware of these as 'unapproved mods' it should be forwarded to all [company] bases that these are not authorized and if there is a hole in the trgb cover it must be replaced.it would be helpful if there was a way to document minor modifications; ie like 337's. So you could search for minor modifications to airframes. Over the years there have been many minor mods that only have log entry's or are buried in some work order that we do not have access to; these are done during maintenance; work orders and outside vendors. If we do not have ready access to a list of these we are unable to quickly or easily reference any minor modification; repair or inspection. Inspected a dent in the finlet on a 407; you can circle the dent with a marker and date & initial it. But in 6 months any marking of that on the ship will be gone. Then a FAA guy comes in and asks if this was check. All I can say is yes it was check a while back but there would only be a nondescript entry to find in a stack of old log books.

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Original NASA ASRS Text

Title: Several mechanics reported that a BHT-407 Helicopter aft tail rotor gear box fairing had an undocumented hole added to access the gear box oil level site gage.

Narrative: We received this aircraft from another base; it was used for pilot training [that] night. I did an airworthiness check on it today and noted many differences/mods from our normal base aircraft. I also noted that there was a hole cut in the Tail Rotor Gear Box (TRGB) cover to check the oil level of the TRGB. This is something I have seen in other aircraft so I did not take particular notice of this. Additionally I noted that the hole was obviously there when the aircraft was painted due to the condition of the paint around the hole indicating it had been there since before the aircraft was painted. The Check Airman came in later this evening and noted the hole in the cover and asked if I had seen anything in the aircraft documents that would allow for the hole in the TRGB cover. I was not able to find anything on short notice. The Check airman made an entry in the aircraft log book as a 'suspected unapproved Modification'. I removed the aircraft from service and ordered a replacement cover.During the conversation the Check Airmen stated that there were several aircraft that had the hole done and did not have any approval. [Company] has done these in the past and if [they are] aware of these as 'unapproved Mods' it should be forwarded to all [Company] bases that these are not authorized and if there is a hole in the TRGB cover it must be replaced.It would be helpful if there was a way to document minor modifications; IE like 337's. So you could search for minor modifications to airframes. Over the years there have been many minor mods that only have log entry's or are buried in some work order that we do not have access to; these are done during maintenance; work orders and outside vendors. If we do not have ready access to a list of these we are unable to quickly or easily reference any minor modification; repair or inspection. Inspected a dent in the finlet on a 407; you can circle the dent with a marker and date & initial it. But in 6 months any marking of that on the ship will be gone. Then a FAA guy comes in and asks if this was check. All I can say is yes it was check a while back but there would only be a nondescript entry to find in a stack of old log books.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.