http://www.aaiu.ie/upload/general/4719-0.pdf
Seems that lessons of previous accidents are either forgotten or ignored.
A few degrees off-track 1 mile from a way-point is negligible, 30 miles or more away is problematical.
Is one of the several factors that killed the people (including a CAA examiner) in the above accident.
Have a look at the safety recomendations to the CAA and ask how many have been followed through!!!!!
GPS deviation is
not angular it is linear.
VOR - 1 dot right = 1 degree off = 1nm at 60 and 0 at the VOR (subject to system errors).
GPS - 1 dot right = (perhaps) 1nm off at 60nm and 1nm off at 0nm and will parallel the track 1nm off
provided each dot represents 1nm (again subject to system errors).
That final provision depends on scale selected.
A pilot who uses say 2nm full scale deflection normally can believe that they are within 1nm of track when less than 1/2 scale deflection. However, if in error the scale has been set to 5nm full scale then they will be twice as far off track as they think all the way past the fix.
VFR GPS can not be used as sole source information for navigation.
Approved GPS instalations when in the enroute mode are (for the most part) RNP 5. That means +/-5nm which will keep the flight on the airway but not guarantee that the flight remains outside the zone that the track misses by 2nm. Working to such small tollerances requires PRNAV and to follow a coded PRNAV procedure.