Just thought I'd chip in with what I've been taught (mostly on my FI(A) course);
PEC = Position Error Correction.
The altimeter does not read accurately due to the fact that local pressure fluctuations around the airframe will cause errors in its measurement. It is impossible to position a static port to reduce this error to zero.
I belive PEC for a light twin is unlikely to be greater than 50'. This is why 50' is added to minima during instrument approaches.
Obviously the radio altimeter on the autobuses means that there is no PEC to apply.
Pressure Error = Putting the wrong subscale setting into the altimeter.
Error in altimeter reading due temperature deviation from IAS = Density Error.
Standing by to be corrected by someone who actually knows what they are talking about!
EK