Hi, I just looked into that for you. I started a random flight at LSZH at a random time and could see two heavy aircraft. I also checked the size of the parking positions in the developers mode, and the ones at this gate are all big enough for big airliners, so there should not be a parking position related issue.
I suspect the issue relates to the FS Traffic flight plan. The current FS Traffic LSZH flight plan does not have lots of big airliners in it as it's a 2022 flight plan which was affected by Corona. It means if you spawn at the wrong time of the day, you'll never see big airliners as they just won't be there at that time. It may help to set the timer in the FS Traffic Control Center Options menu to 180 instead of 90. This should show lots more parked traffic at all airports.
For the A340, this one does not exist in FS Traffic yet, but you can get it from AIG for free. Here's a quick instruction, happy to elaborate. https://flightsim.to/file/55822/fs-traffic-rush-hour-schedules-for-eddf-and-kjfk
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