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Chris Wright

rnav(gnss) approach difficulty...

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g'day from oz...searched the forums for earlier info/posts on this and drawn a blank.on some of the rnav approaches avaliable for oz aerodromes, the b737 (6,7,8 and 9 models) fmc legs page will not permit a spd to be entered that is less than 150kias at the FAF. this may be (often is!) quite a bit in excess of the ideal Vref+5 spd you'd want to enter for the FAF.this happens inconsistently with rnav procedures at different aerodromes. an example is adelaide - the rnav procedure for ypad works perfectly for rwy 05 (any of the three transitions) but for rwy 23, it is not possible to enter a spd less than 150kias at the FAF (Vref+5 of < 150kias can be entered for the MAP). the most up-to-date set of oz procedure files (including the ypad example) is available at merv archer's site http://www.austscene.com/ (merv keeps 'em up to date with the air services oz airacs - other procedures files (eg. richard s's navdata) can be (are!) often quite out of date for destinations oz.it's not a case of the procedure file having invalid code in it either as far as i can tell - the syntax for the approaches causing problems are fine when compared to those that work perfectly (i've written a couple of rnav approaches and they work fine - ie. i have enough knowledge of the syntax to recognise that the procedures are spec'd correctly).it's also not my particular set up - there's a group of guys in the same va and we all consistently have problems with the same set approaches at the same locations.is this a known problem? is there a way around it?have i provided enough background info?cheerschristophelloperthutc+8

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The speeds on the LEGS page are adjusted after you do the INIT REF (737); regardless, I recommend do not recommend flying any approach in VNAV PTH mode but instead use MCP SPEED and LNAV with V/S control. I don't believe the VNAV PTH modelling is good enough for approaches, especially LPV RNAV decent paths.You're correct in that the procedures are probably not at fault. I think procedures should have speed commands that are related to an ATC constraints, with few other speeds to accomodate many types of aircraft.

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