I guess this is MRS
ILS Y 13L approach?
Some part of these differences are part of what PapaChuck mentioned, slant error distances.
The other issue is data accuracy, there are some tolerances built into these waypoints/intersections...
The problem here lies with ZEBRA... where exactly is ZEBRA?
Is ZEBRA calculated from MTG or from ML
? 1 degree at 15NM
is about 1/4NM (it's 0.26NM
), the location of ZEBRA is within a circle of 0.13NM
radius of R336.5 - R337.5 from MTG, and R312.5 - R313.5 from ML
Entry by LatLon also has it's "resolution error"... with the format of DD MM
.x DDD MM
.x, the resolution of the fix is 0.1 minutes... that is slightly above 0.07NM
for the Marseilles area.
So, if your airplane is RadNav over IRS, then your accuracy is as good as the RadNav itself... If your airplane has GPS Primary over IRS, then your accuracy can never be better thn 0.07NM
anyways when relying on the FMC.
Now, ZEBRA to CI1LY having a 0.2NM difference between the chart and FMC is "understandable", and U're using Jepp charts... the LIDO charts say it's 4NM (no decimals) because it does not go to 0.1NM increments until the final approach segments, likely because of the circular probability error of ZEBRA's location.
A further look into this, gives something that's more interesting... where exactly is the glideslope intercept? D10.7 ML
? or somewhere else? The glideslope is 3.00 degrees... 3492ft is intercepted at 10.98 NM
slant distance... and 10.97NM
ground distance... assuming a flat earth!
The slant distances would be correct for 3.00deg GS
, but the heights at D10.7 and D3
.9 at 3degswould put you below 3500 and 1270ft (albeit above the minimum segment altitude specified (1190)... those altitudes fits a 3.065deg glideslope instead...
Again, there are tolerances built in... To assume it is 100% precise, you'd go mad pretty quickly!
The question therefore is... did you go below the segment minimum altitudes? If yes, go and write a complaint about it and someone will get a calibration flight done.
If not, then, the differences are just "tolerances" and "data resolution errors".
Jepps tend to have too much info that'll get the precisions freaks sweating in a near minimum approach... I prefer the LIDO charts instead... the numbers are realistically more workable.