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FMC Distance Mystery  
User currently offlinesmartt1982 From United Kingdom, joined Nov 2007, 225 posts, RR: 0
Posted (4 years 9 months 19 hours ago) and read 2998 times:


I have been told that you get slight differences in the magnetic track on the FMC compared to what you get on the charts does sometimes due to the FMC using its own magnetic variation calculation (sounds right?) But recently once or twice I have come across a difference in distances on an arrival procedure for no obvious reason and the track did not appear to bypass or cut the points on the Nav display which would change the distances to the chart.

On the chart the distance from Zebra to CI1LY is 4.3 and then to F113L is 6.8
On the FMC the distance from Zebra to CI1LY is 4.1 and then to F113L is 6.7.

As I said, on the map display there was no bypassing or over shooting of points, so what has caused the difference?

Any info?

2 replies: All unread, jump to last
User currently offlinePapaChuck From United States of America, joined Aug 2010, 136 posts, RR: 0
Reply 1, posted (4 years 9 months 16 hours ago) and read 2942 times:

The first thing that occurred to me was a possible slant range error due to DME from the localizer compared to ground track distance. It looks like the chart bases distance on DME, so it would appear that the chart is compensating for the slant distance error, hence the slightly longer distance between ZEBRA and CI1LY (4.3nm compared to 4.1nm). A distance of 4.1 DME at 3500' would be slightly further along you course as opposed to 4.1 based on a GPS or RNAV track, if that makes any sense. If that is indeed the case, your FMC would be calculating the actual distance over the ground, hence the shorter distance.

Between CI1LY and FI13L, the actual distance over the ground may be 6.7nm, but the actual distance traveled downhill while on the glide slope (based on the DME reading) may be 6.8nm.

This is pretty much a shot in the dark on my part. Someone with more knowledge please feel to correct me on this one.


In-trail spacing is a team effort.
User currently offlinemandala499 From Indonesia, joined Aug 2001, 7561 posts, RR: 76
Reply 2, posted (4 years 9 months 15 hours ago) and read 2926 times:

I guess this is MRS/LFML 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.


When losing situational awareness, pray Cumulus Granitus isn't nearby !
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