***This has been fixed!***
A reliable 300+ mile flight I have been using is KPAM to KMLB across Florida. Pick IFR low altitude (high altitude is fine for the Airliners) underneath the plane selection on the World Map and it comes out to just over 300 miles. Takes 2-3 hours for most planes, less for the airliners and jet. Once you start your flight, you can bring up the AI Control menu and turn on Control Aircraft and the co-pilot will fly the whole thing from takeoff to landing while you do other things. If you also modify the weather to be night and raining, you can make progress on all 3 of these other achievements while working on this one.
There's also a bit of taxiing at the end of the flight which will make small progress toward
The 747 on this flight did struggle with the landing and landed in a neighborhood instead of the runway, but my tracker still went up and this achievement (which you will unlock along the way to this one) popped upon landing on some houses, so it still counts.
Additional note: On a couple flights I used the Sim Rate Increase feature to finish them faster and tracker did not go up after those flights. I cannot verify for 100% certain that the Sim Rate Increase function disables achievement progress, but I have stopped using it and the tracker has resumed incrementing after every flight.
There are 20 aircraft in the standard edition. I suggest copying down the list and marking them off as you complete them. You should also verify in your Pilot Profile that the achievement tracker went up by 5% after you complete each flight because there is no other indicator of progress to tell you which planes you have finished and which you still have to do. Update:
meochan913 suggests checking your flight log in your Pilot Profile to try to help figure out which plane you missed if you feel like you've done them all. Make sure all planes have a lengthy flight with a takeoff and landing registered. You can't see the actual distance but if it was only a 30 minute flight or something it's not going to have been 300 miles.Turboprop
Daher TBM 930
Textron Aviation Beechcraft King Air 350i
Textron Aviation Cessna 208 B Grand Caravan EXAirliner
Boeing 747-8 IntercontinentalJet
Textron Aviation Cessna Citation CJ4Propeller
Aviat Pitts Special S2S
Cub Crafter X Cub
Diamond Aircraft DA40NG
Diamond Aircraft DA62
Extra EXTRA 330LT
Flight Design Flight Design CTSL
JMB Aircraft VL-3
Robin DR400/100 Cadet
Textron Aviation Beechcraft Bonanza G36
Textron Aviation Cessna 152
Textron Aviation Cessna 172 Skyhawk (G1000)
Zlin Aviation Savage CubAdditional note:
if you have any of the Community Liveries installed, several people have reported them preventing progress on this achievement. So if you're sure you're meeting the requirements and the tracker isn't going up, uninstall those and try again.