cianpars

The CAT of ILS depends on 2 things: RVR and Decision height.
RVR is the Runaway visual range, and it's how far you can see on horizontal plane (assuming that you are on the runaway it says how many feet you can see). The DH is the altitude when the pilot must see the runaway lights, else he must abort the landing. The categories are:
 
CAT I: DH>200 ft, RVR>2400 ft
CAT II: 100 ft< DH <200 ft, RVR>1200 ft
CAT III: in general DH<100 ft and RVR<700 ft, but there are different sub-classes ABC. CAT III C dosn't require and DH or RVR, an aircraft with CAT IIIc can landing with no visual, only using instruments.
 
A barometric altimeter translates the air pressure into height.  Assuming that the altimeter is set to real pressure an mean sea level (typically QNH), then your altimeter will read height above mean sea level.  However, a radio altimeter sends a radio signal to the ground and will measure your actual height above the ground at that point.
 
Note that if your altimeter is set to QFE then it will read the height above the airfield that the QFE is provided for.
 
IAN

http://www.saitek.com/uk/blog/?p=45http://www.saitek.com/uk/blog/?p=45

The first time this happened to me, I cured it by deleting the FSX.CFG file and let FSX build a new one.
 
The second time it happened, the first fix didn't work and I traced it to an issue with my default flight, so I created another.
 
I've also heard of this happening with certain videocard settings, so perhaps try defaulting these.
 
Good luck
 
IAN