As someone mentioned earlier, if you have an in-cockpit camera than you can use the equivalent of the clapper board to get sync. And if the prop is visible, you also get sync (unless recording from the aircraft intercom which is always totally silent until the squelch is broken, or PTT is pressed).
I think the Alps are covered in Go-Pros with suction cups attached… There are better ways… Somebody with a TB20 mounted one on the footstep, but on the GT you can’t because it retracts with the landing gear.
Contrast handling is a problem. These cheap cams just don’t have the dynamic range. Some of the clouds in mine are washed out as a result.
I have discovered a great way to sync sound and video: fly into a thunderstorm 
You get cracks on the sound track and you line them up with the flashes…
Just as an update to an old thread, I have a load of flying videos here which have the ATC sound track. All were done by recording the sound separately, currently on a Tascam DR-05 mp3 recorder, and simply starting the camera (currently a modified Sony X3000) at the same instant as the sound recorder, and then replacing the X3000 sound track (which is basically useless) with the DR-05 mp3.
I have not had any reason to play with the more advanced methods mentioned in this thread, and there is no need for e.g. jam sync. There might be if doing a long video (say 1-2hrs+) where the pilot’s face is visible and he/she is doing some narrative; then you must have accurate lip sync.
I have not seen this thread before but to add my 2 pence now. There are many ways to sync picture and sound and for many years and even today the old clapperboard and its various off shoots like mic taps and light flashes syncing to pops on the sound track have served the purpose.
Eye matching sound and picture has also long been a tool in the assistant editors arsenal.
But in this particular case if I am understanding the discussion correctly I would look for a much simpler method. Eg I would use the inbuilt time on the external video camera and record my sound on a cheap video camera with good sound recording technology (and there are many because many of the old video cameras don’t have the picture quality but do have perfectly good sound quality).Point the internal camera at a digital watch and calculate the offsets to use in editing. Without a time code generator you will of course get some drift but that can be sorted in the edit unless you have long pieces of lip sync dialogue where it becomes a little more difficult unless you have an edit package which can slow and increase frame rate within a scene.
I would use the inbuilt time on the external video camera
I am not sure whether any video format contains the date/time. MP4 AFAIK doesn’t; it starts where the file starts, and it just plays at the frame rate at which the playback device plays it (the desired frame rate is stored in the file). MP3 is the same. Timecode generators record the data on a separate track. That said, I have never played with timecode equipment.
It isn’t an issue where lip sync is not needed; quartz crystals are good enough to within a second, over some hours.
The challenge remains where the video and audio recording devices are separated, and are started and stopped separately; that is where the timecode stuff gets used. Sure; pointing the video camera at one’s watch solves that problem (to maybe 1 second) for the video, but what about the audio?
Most modern camera/recorders have a built in calender and clock. Sometimes you have to search for the information, but it is usually there.
If it is set to time of day it doesn’t matter whether you turn it on or off it will always be time of day.
The same is true of the watch, which is acting as a basic time code generator. As I said all you need to do is calculate the offset and sync it in the edit. Syncing the 2 clocks to time of day at the beginning of the day is the most difficult part and will likely leave you out of sync by up to a second but it’s easy to adjust in the edit.
The bigger problem is drift because the cameras, unless of the highest quality and linked to a time base correcter, will run at slightly fluctuating speeds. This is where an editor has to use his/her skills at resyncing to something in picture and readjusting the sound track.But as I wrote with no long pieces which need totally accurate sync this shouldn’t be a problem
After all it’s done in movies and commercials every day.
You can also fit a remote which will run and stop the camera on the ext of the plane to your smartphone or a simple clicker device. Unless you want to spend a bit of money you will only be able to turn the camera on or off. Focus, zoom, camera movement and lens cleaning need something a bit more sophisticated which usually costs a bit extra.
