I was wondering if anyone knew why when I taxi, no matter the aircraft, it always speeds up to around 25-30 knot GS and I always taxi at idle because of this. In an aircraft like the FBW A20N, I start by adding just a hair of power when taxiing from the gate and then immediately bring it back to idle, and after 15 seconds or so, it has sped up to 25-30 knots on ground speed. I was wondering if there was a fix to this. I use the Thrustmaster Airbus yoke and quadrant. 

I believe most aircraft model a low gross weight will result in what you describe, where a full load will require some thrust to keep going along. But, I've notice some real wonky ground handling with some/all aircraft where it's like taxiing on melted asphalt.

So, not an exact model by no means.

Long story sort: load up the aircraft to near max takeoff weight and try that.

 

Take a look at this description from an A320 pilot. The A320NEO with Leap 1A-26 engines has a lot of thrust at idle and causes this. The normal procedure is to let the airplane accelerate to 25-30 knots (when on a straight taxiway) and then apply the brakes to bring it back to around 10 knots. Repeat as necessary.

The real A320 when taxying in (especially at a landing weight when coming in from the runway to the stand) can usually cheerfully coast along on the idle thrust of one engine alone. Some airlines do this, shutting down one engine after landing, some keep both engines running. Usually when the things come on stand and are rolling up to the centreline marker, they will get to within maybe a foot or so of the stop point and then halt, but then they will have to have a bit of thrust applied just for half a second or so to get them rolling again and then immediately stomp on the brakes to stop using the safedock guidance onto the exact stop point.

So it's quite realistic to have your simulated aeroplane roll along on just idle thrust alone once it's had a bit of power applied to get over the inertia of being stopped. This is reminiscent of when you see those 'world's strongest man' contests where some bloke will pull an airliner along wearing a harness; the hard part for them is to get over the inertia the aeroplane has which is keeping it still, but once it is rolling, it's comparatively easy for them to keep it going.

It's worth bearing in mind too that lots of airport ramps and taxiways are not completely flat and level and it only takes a miniscule incline for an airliner to commence rolling. I actually had this happen last week; I pulled the lever on the nose gear of a B737-8 MAX forward so I could insert the steering bypass pin to fit the towbar, and just doing this (i.e. releasing the hydraulics to the steering) was enough to free the thing up so it started rolling backwards on the stand until the left main pressed up against a chock and stopped it. This was because the crew had released the parking brake after taxying on stand and getting the chocks in place; they do that to stop the brakes from seizing when they are cooling down. I had to use the tug to pull the thing forward off the chock so it could be removed before I could push the thing out, and when I did that and commenced pushing it out onto the taxiway, the tug was easily moving the thing off stand on just idle engine power. Airliners roll really easily once they are moving.

I remember this very topic been done to the death a few months after FBW started developing the A320NX. A number complained about the taxi speed of the aircraft, only to be corrected by RW A320 pilots that infact the case.