So... I was flying the PMDG 737-800 from Atlanta to Orlando. As I was climbing through 25,000 ft, I noticed that the plane was not producing any contrails. I checked the temperature, and it was at about -40 degrees celsius, while the Total Air Temperature was at -8 degrees celsius. Keep in mind that I typically see contrails at this point. Just to make sure that it's not the PMDG, I hopped into the F-18 that comes with the simulator at the NASA Shuttle Landing Facility (KTTS). As it turned out, that didn't produce any contrails either, even as I went up through 35,000 feet. I do not understand why the contrails would all of a sudden stop appearing, when they were showing up when I flew a few days ago.

I am using Active Sky Next and simulating real-world conditions.

Special effects turned on?

Yes, the special effects detail is on high.

I like using this file for contrails. Maybe this will work if you don't already have it?

Avsim library:

longer_contrails_fsx_fs9.zip

Here is another one:

aicontrails.zip

Andrew Grey

Just to clarify: all my AI aircraft have contrails, but I don't for whatever the reason.

It has been my experience that contrails can disappear if the Top Down view distance is set too high.   Maybe this is peculiar to my situation, but as soon as I zoom down to begin seeing the aircraft, the contrails mysteriously appear.

Contrails appear by default above 29000 ft and below -39°C, unless max_contrail_temperature in the aircraft.cfg is set to a higher value (e.g. -30°C).

See the contrail related parameters here: https://msdn.microsoft.com/en-us/library/cc526949.aspx