Hi, it is said in the FCOM that the check is valid provided the distance difference is less than 3 miles (1 mile for approach). no mention of difference in bearing ? what if the bearing difference is 5, 20 or 40° ?
As a DME/DME is the preferred way to fix the position by the FMS (no GPS), only DME values might be mentioned. The second best conventional fix is Bearing/DME, that is what you are referring to. It's been a while since I have been on the 320, and I don't have access to a maual, but we just tuned a VOR and compared the Bearingpointer and the DME to the FMS map. Distance accurate and pointer pointing at the tuned station - accuracy check ok.
let's take an example then. FMGC says VOR/DME ABC is located bearing 150°/20 nm but raw data are 165°/21nm. according to FCOM a check is valid if distance difference is less than 3 nm but doesn't say anything about difference in bearing. the above test is then valid or not ? (15° of bearing diff & 1 nm of distance diff)
Not sure if its correct, but you could calculate the distance between the two points (raw data and FMGC); in this case about 5.44nm which is more than 3... then again, that might be looking at it in an overly mathematical way!
FMGC says VOR/DME ABC is located bearing 150°/20 nm but raw data are 165°/21nm. according to FCOM a check is valid if distance difference is less than 3 nm but doesn't say anything about difference in bearing. the above test is then valid or not ? (15° of bearing diff & 1 nm of distance diff)
The accuracy check is done both on distance and bearing: 1. DISTANCE: compare NAVAID distance with FM distance on ND or check with FM PROG page: error shall be less than or equal to 3 NM (1 NM on approach). 2. BEARING: Check that the needle (raw data) overlies the corresponding blue navaid symbol (FM computed).
You have to be practical about this - say you are looking for a position accuracy of better than 3 nms: If the DME distance and FMGC computed distance were the same, then the bearing could be off by the equivalent of 3 nms - by my simple trig that's 3 degs at 60 nm, or about 9 degs if 20 miles away.
edit e.g. If you had 2 VORs - both 20 miles away, A is due North from you and B is due West.
FMGC position check gives: A = 22.9 miles and due North of you, B = 20.0 miles and 278 degs from you.
I'd say both confirm I'm within 3 nms of my FMGC position.
Last edited by rudderrudderrat; 10th May 2010 at 11:48.
Reason: maths example
Speaking in general terms, when required to conform FMC position accuracy eg before MNPS I use 2 or 3 DME stations.
Tune and ident DME station. Ask FMS how far to DME station and compare the result.
Repeat for another station (or 2) preferably one left of track and the other right of track to get a good spread.
Provided that the FMC and DME distances are not more than the allowed distance appart each time then everything is OK.
I only use VOR/DME if there is no other DME. The reason is that the VOR has a +/- bearing accuracy which at distance is far wortse than the DME accuracy.
Having said that to check the accuracy using VOR/DME I would go with "rudderrudderrat". provided that the indicated position (VOR/DME fix) is less than the maximum distance from the FMC indicated position, the FMC indicated position is accurate enough.
It should be possible to check distance from FMC posiiton to IRS position and GPS position etc so that when one is no longer in range of DME/DME position fixing one can expect that the GPS / IRS positions will do the job.
Hi rudder, a small doubt's arising: should I be checking the NAV accuracy throughout the approach (eg. GPS primary lost), will I have enough time to apply the 60-to-1 rule in determining whether the distance between the NAV/FM bearing is within the max limit or not ? Being the probability for a discrepancy in bearing only not very usual and should discr. be unacceptable, at that point, I'd take FM position as unaccurate thus continuing raw data ... right ?
by design you must have a managed lateral mode to have a managed vertical mode. that's why when you go heading, initial CLB or DES, both managed modes, can't be maintained. then reversion with a triple click to selected (=not managed) mode, ie OPDES or OPCLB. coming back to NAV doesn't imply coming back to vertical managed mode previously engaged. you must repush to engage again the managed vertical mode