Looking at e.g. this STC for a Hartzell alternator it lists a large range of planes, many of which will be damn hard to even physically find, to check for whether the alternator will fit under the cowling.
Did they really do that?
I can see avionics AML STCs are much easier; you need to demonstrate that it won’t do any harm, but the installation is normally always doable dimensionally.
@pilot_dar may know.
I have done an EASA Minor Change for electronics with a long list of planes and EASA wanted us to do one test installation. For the other planes there was good reason to believe it would also work and it was approved. But it is down to the person doing the installation to do a full check on every installation. But that was for a generic non essential part. I assume if you do an autopilot STC things will be much more complicated.
Much has been claimed over the years about STEC having done their autopilot AML STCs using a process which is no longer available, where testing was not done on the actual airframes. I don’t know if this is actually possible, but there are many reports of their APs oscillating (in pitch usually) in some loading conditions on some types, which suggests it wasn’t tested all around the envelope on all the types. It appears that King APs were thus tested.
An AML STC for normal avionics is of course “safe”; you merely need to be sure that the box can be screwed in.
Many years ago I tried to install a K&N air filter on the TB20. It had an STC for various types. It didn’t fit and it turned out that the STC owner tested it only on a TB10. Some people did install them on the TB20, by hacking the filter container metalwork. I sent it back; the gain was at most some very small fraction of an inch of MP.
Peter wrote:
Much has been claimed over the years about STEC having done their autopilot AML STCs using a process which is no longer available, where testing was not done on the actual airframes
I would not be surprised. Seeing that nowadays many of our planes do NOT get the STC for newer autopilots as really every single type needs to be tested, it is hard to understand how they would have done it earlier, when most planes get the STC.
Peter wrote:
but there are many reports of their APs oscillating (in pitch usually) in some loading conditions on some types, which suggests it wasn’t tested all around the envelope on all the types. It appears that King APs were thus tested.
I don’t get that, but the Stec AP we use is less than optimal in roll if there is a little wind even. It doesn’t hold tracks very well and even less so ILS Localizers. In Pitch however we are quite happy so far.
It depends a lot of the type of STC and if you are capable to demonstrate a safe installation and the effects on other airframes without investigating them in detail. We do this a lot. On EASA side, there are many knowledgeable engineers working in certification and you mostly need to convince them on a technical level with hard facts that these installations will comply to the regulations.
