I've seen a couple of questions related to contrails here in the last few weeks, so I thought I'd post something more thorough...When engines (regardless of type) combust fuel, the majority of the gas that is exhausted is carbon dioxide and water. That exhaust gas rapidly comes to an equilibrium with the surrounding air. That equilibrium, a complicated balance of temperature, vapor pressure, and concentration of water, determines not only if the water present in the exhaust will condense, but how long the condensate will persist. Thus, given the right atmospheric conditions, any engine which emits hot exhaust gas could produce a contrail as the gas is cooled and pressure reduced...None of these complexities are simulated in FS9's weather engine. That doesn't mean you can't have fairly realistic contrails emitted from ALL of your aircraft - AI and flyable, and here's how:First download (don't install anything yet, just download them) these two files from the AVSIM library: and[link:library.avsim.net/esearch.php?CatID=fs2004util&DLID=37019]Markus Brunner's contrail utilityJan's file is important for two reasons: 1.) it's got an additional contrail effect which we will use below, 2.) that contrail effect is superbly annotated to show how to modify the other contrail effects to produce contrails that are tailored to your liking.So, unzip it to a temporary folder. In that folder, there is an effects folder which contains a file called fx_A_I_Contrail.fx - copy that file into your FS9Effects folder. Jan has kindly provided modified aircraft.cfg files for the default planes, but you can delete them - markus' utility is going to make the modifications for us!Next, unzip Markus' utility to any directory you like. Run the AIContrails.exe file and you're presented with the following screen:http://cobweb.dartmouth.edu/~gridlock/SG-Contrails03.jpgHere I show my recommended settings for Jet and Rocket powered craft. Since jets and rockets burn the most fuel, their exhaust has the highest concentration of water which means (simplistically) that the exhaust should reach the point of condensation at a higher temperature than other types of engines. Also, these contrails are likely to take a little longer to form (due to the higher temperature of the exhaust it takes longer to reach equilibrium), but persist much longer. Thus, I have selected a formation temperature of -15 oC. Also, I've selected the fx_A_I_Contrail.fx file from Jan as this effect has the contrail start point offset a bit more from the aircraft than the others, and the persistance is 6 minutes (about 50 miles for a average jet). If you want only AI aircraft to produce contrails, uncheck the "Flyable Planes" box. Make sure your path to FS9 is correct and click start. The utility will parse all of your installed aircraft, and properly configure all of your jets and rockets.The real power of Markus' utility is this: You can select a different temperature and effect file for any type of plane.http://cobweb.dartmouth.edu/~gridlock/SG-Contrails02.jpgThis shot shows my recommended settings for Turboprops and turbine powered helos. These craft burn a little more fuel and piston engine aircraft, but no where near as much as jets. Thus, colder air is required for contrail formation because of the lower water content in their exhaust. I show here at setting of -25 oC. Also, I've set these planes to use the fx_contrail_l.fx file, which is formed close to the aircraft, and persists for about 30 seconds, and is a little smaller than Jan's effect.http://cobweb.dartmouth.edu/~gridlock/SG-Contrails01.jpgLast, since all combustion engines could produce condensing exhaust, here are my recommended settings for piston powered craft. A formation temperature of -35 oC will rarely produce contrails, and by using fx_contrail_s.fx those contrails will be small and short lived (just a few seconds) as -35oC air is usually pretty dry.The last thing I'd recommend it that you open Jan's fx_A_I_Contrail.fx file in notepad and examine the file along with Jan's comments. You'll quickly learn that each effect is composed of emitters and particles. By changing the "lifetime" parameter of the particle(s), the length of the contrails can be altered. For example, I find that the contrails produced by turboprops are a bit paltry at 30 seconds, so I've edited the default microsoft fx_contrail_l.fx effect file to produce contrails that last 90 seconds. Similarly, I found that the short contrails produced by piston craft were undetectable with the default 1.5 second lifetime, which I've increased to 20 seconds. By tweaking these parameters, you can alter existing contrail effects, or create new ones such that each type of aircraft has it's own distinct contrail.I hope that this sheds some light on how to impliment these great utilities to produce a more realistic flight environment.Thanks to Markus and Jan - great work!EDIT-->if anyone is really interested in contrail formation check out these websites:http://cimss.ssec.wisc.edu/wxwise/class/contrail.htmlhttp://www.aerospaceweb.org/question/atmosphere/q0111.shtmlhttp://www.wrh.noaa.gov/Flagstaff/science/contrail.htmBest,sg

Thanks for this informative, enlightening post. I had downloaded the 2 files you referenced yesterday, but it was very late at night, and after reading through Jan's read me, I decided to get some sleep and tackle it today. Found your post before having a go at it, and I'm glad I did! This made it incredible simple. Thanks again!

Scott,First that great AutoStar program and now this good stuff. You are really cutting into my flight time!Hope this helps,JimActiveSky Supporthttp://www.hifisim.com/images/as2004proudsupporter.jpg

Sorry Jim!:-smoochBest,sg

SG,Thank you. You did a great job with all, the explanation was really great.Mark.

I plan on updating this post with info on how to do the same thing - have different accurate contrails depending on engine type - using the recently released 3D contrail effects, however, I'm pretty deep in several other projects. As time allows, I'll update the post, but I think you can guess it involves renaming the 3D effects, and plugging those names into markus' utility as above...In the meantime, here's the new link for the NWS info on contrails:http://www.wrh.noaa.gov/fgz/science/contrail.php?wfo=fgzBest,sg