Ok, so I just did a lot of tests using the plane SITL.
First I did some flight plans with parallel lines every 2km on a 110 x 110 km square, the same surface than a single SRTM *.TIFF or *.HGT files, the goal was to get a *.DAT files ready to use on the SITL terrain folder. with a terrain spacing parameter of 30m I got files around 25MB, which is very close to the actual size of the SRTM download from Earthexplorer.
Then, I did a simple flight, 150km from takeoff and after 2 minutes airborne, I disconnected mission planner “GCS Failsafe off” and waited or connected Qgroundcontrol, without elevation data and internet.
In both case, after a couple of KM, the plane stopped receiving terrain data, meaning terrain-pending at maximum and terrain-loaded at 0, even if I have 24 000 square km of terrain data on the SD. So arduplane would use the last elevation as a reference point, if the terrain is raising, the plane would crash.
So it seems that Arduplane is never really reading what is on the SD card and how can I be sure terrain following RTL will even work at long distance?
If a dev could have a look I would be grateful.
Benjamin