From here – this is regarding the FAA process
Doing a From 337 for the alteration wouldn’t be a big deal. The hard part is to find the substantiated data for the FAA. If no one has ever done it before, getting the right FAA inspector to sign off on it will take time to get the data he wants. But once you have a baseline of approvals, the future ones will be a lot easier
Interesting, NeumannJ. That sounds like the Field Approval process. Could one apply to an FSDO for the signoff as it is “obvious” there is not likely to be a structural issue in the lower cowling, or would one need a DER 8110 design package to support it? In my case the cowling is fibreglass but in others it will be metal. Installing an off the shelf inspection door (which has all the right metalwork) would probably help.
As regards the EASA process, I wonder if somebody has done something similar?
Most people I deal with would say “yeah, just do it, who cares about it anyway”. There are so many aircraft and variants out there, if somebody asks, my answer would be “that was a little known factory option at the time, a few aircraft have it”.
This may be a consideration, for an off-the-books mod. I know from personal experience that an FAA DAR would spot such a mod in 5 seconds, and would expect a European CAA inspector to do likewise. Then you have to buy a new cowling, which for a TB20 may not exist until a crashed specimen (but with an undamaged lower cowling) appears… It could be a whole lot of fun given that the aircraft has been deregistered by then 
This is not a mod which could be removed.
Peter wrote:
I know from personal experience that an FAA DAR would spot such a mod in 5 seconds, and would expect a European CAA inspector to do likewise
Really? First of all you never deal with a DAR or even less so an EASA inspector and if they found it, they would say “yes, that’s a sensible thing and for the record, I did not see it”.
Peter wrote:
hen you have to buy a new cowling
There are approved repair methods for fiberglass and sheet metal. I would never do such improvements without a strategy how to revert them. Not so much because of the fear of being grounded but because I have to reveal everything when I sell the aircraft and the buyer might not be comfortable. However, this one would be a no-brainer, it might even be possible within CS-STAN.
Show me any aircraft older than 10 years and I will give you a list of at least 5 illegal modifications within a few minutes.
If I were to see this mod on an aircraft and had to get fix it, it could easily be “repaired”. Especially composite cowlings.
But this has me thinking. With today’s composite technology, splash molds of the cowlings could be made fairly easily. And with that you can make a door and back-up frame. You can take a perfectly good aircraft, makes your molds off of its cowling and there you go. Off to the races. A little fabrication work and you could make the panel flush or just have it protrude over the skin.
These are the sort of pre-made covers
I imagine they must be available with a variety of curvatures, or none.
