I assume you have the FS2004 SDK installed? If so, to attach an effect that displays all the time:
First create a small cube in GMAX that will be converted into an attachpoint. The effect will be attached there. Then select this object, and:
1. In GMAX, MaxScript/Run Script.
2. Browse to your GMAX/gamepacks/[FS2004 gamepack name]/scripts folder, if needed.
3. Choose the Attach Tool GMAX script and click Open.
4. Browse to your FS0024 Effects folder.
5. Click the Effect radio button.
6. Give it a name like WaterEffect1. Each must be different.
7. Click the Attach and Effect button.
The Effect Params box allows the effect to display at certain times, etc. See the SDK for details.
The SDK has a good explanation of this process in your FS2004 gamepack/Docs folder, AttachTool.doc.
I'm not sure this works for flyable aircraft models, but this is the process.
I am adding jet boat drive units to a seaplane to speed up water takeoff. I need the water jet to display for a second or two as the aircraft lifts off the water, then for the water jet column to appear to drop/shoot into the water below as the water jet drive runs out of water. The drive nozzles are also steerable, so a static effect just will not work. I wasted a week editing the stock Old Faithfull .fx into a very good water jet .fx
So, Plan B, create the water jet from animated parts and use VIR dummies to hide the displays at the correct time. Just a little eye candy.