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 8th May 2010, 18:09 #1 (permalink) Thread Starter   Join Date: Apr 2009 Location: worldwide Posts: 40 A320 accuracy check 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° ?
 8th May 2010, 18:20 #2 (permalink) Join Date: Mar 2006 Location: Choroni, sometimes Posts: 1,975 Don't understand. More than one ore three miles off is off. It's that simple.
 8th May 2010, 23:41 #3 (permalink) Join Date: Jul 2007 Location: *** Posts: 341 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. Nic
 9th May 2010, 19:43 #4 (permalink) Thread Starter   Join Date: Apr 2009 Location: worldwide Posts: 40 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)
 9th May 2010, 20:51 #5 (permalink) Join Date: Aug 2007 Location: UK Age: 26 Posts: 53 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!
9th May 2010, 21:11   #6 (permalink)

Join Date: May 2007
Location: Italy
Age: 29
Posts: 206
Quote:
 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).

Hope that helps,
fredgrav

10th May 2010, 02:06   #7 (permalink)

Join Date: Apr 2010
Location: in the flight deck
Posts: 91
Quote:
 what if the bearing difference is 5, 20 or 40° ?
Quote:
 2. BEARING: Check that the needle (raw data) overlies the corresponding blue navaid symbol (FM computed).
hmm.....meaning no difference allowed?

 10th May 2010, 06:22 #8 (permalink) Thread Starter   Join Date: Apr 2009 Location: worldwide Posts: 40 thtat's the point !
10th May 2010, 06:36   #9 (permalink)

Join Date: May 2007
Location: Italy
Age: 29
Posts: 206
Quote:
 hmm.....meaning no difference allowed?
You got it !

 10th May 2010, 09:46 #10 (permalink) Join Date: Oct 2009 Location: UK Posts: 1,272 Hi foff, 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 10:48. Reason: maths example
 10th May 2010, 11:44 #11 (permalink) Join Date: Mar 2002 Location: Euroland Posts: 2,816 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.
10th May 2010, 12:42   #12 (permalink)

Join Date: May 2007
Location: Italy
Age: 29
Posts: 206
Quote:
 by my simple trig that's 3 degs at 60 nm
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 ?

Thanks,
fredgrav

 10th May 2010, 13:23 #13 (permalink) Join Date: Oct 2009 Location: UK Posts: 1,272 Hi fredgrav, Our company makes it "simples". The primary aids we use for the approach are the ones published on the Jepp. If we have GPS Primary & accuracy is High - then we may use the MAP overlay as an aid to SA.
 11th May 2010, 11:20 #14 (permalink) Join Date: Oct 2009 Location: UK Posts: 1,272 Hi Wally, If you are not NAVigating to a waypoint because you have selected a HeaDinG - then it can't apply any constraints because it may never get there. Last edited by rudderrudderrat; 11th May 2010 at 11:22. Reason: spelling
 11th May 2010, 12:39 #15 (permalink) Thread Starter   Join Date: Apr 2009 Location: worldwide Posts: 40 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