B737 3/4/500 FMS Nav accuracy
 

I'm interested in other operators' experiences with FMS Nav accuracy on B737 3/4/500 (Twin IRS + DME/DME radio update) aircraft. Particularly with the sort of 'errors' that you are seeing when doing a 'raw-data' fix check against FMS position.

Assuming that you are operating in a 'DME rich' area, how does the FMS navigation perform i.e. what are typical displacements of FMS position from radio position (n mls)?

What are typical displacements of FMS position from the IRS positions (n mls) and what would you consider excessive?

If heading into an area where DME support is limited or nil, do you have any SOP to select the most accurate IRS for FMS use to try to avoid spurious EGPWS alerts when the position drifts towards a possibly inaccurate IRS position following loss of 'radio' position?

Lastly, does anyone have any links to sites explaining how FMS nav operates as it 'downgrades'? (I'm aware of the Honeywell site and Chris Brady's)

BTTT - any takers please?

When using the Fix page vs. raw data to check the position-

Regularly seen errors involving the radial from the gound VOR to the aircraft as seen by the FMC due to the aircraft variation database being in error. Can usually predict this one happening if you compare the A/C true heading (from the IRS control panel in HDG/Sts) with its magnetic heading pre flight. Compare this with charted variation. Normally on affected A/C you will see a difference of 2-3deg.

Hardly ever seen errors with DME side of things- 90% come out at within .3nm

Thanks Sniff - don't know if you know, but VOR radials are only 'correct' to +/- 5 degrees (from memory!). That was the 'original' specification in the '40's for VORs.

Agree with the DME bit - probably nearer 98% +/- 0.1 ml?

In my previous existence the 737 FMC we used was the version from the stone age so it was quite often the case that the old bird would throw a wobbler on longish sectors and you would see big Map shifts so we always had to back up using conventional NAV ie Manual/Auto.

In my new shiny B733 we have a up-to-date version of the FMC and with the Required Nav Performance function it is obvious when the Radio or IRS updates are slow cos they get near the limits. However, even when operating down in the Canaries and to Funchal the worst I have ever seen is a displacement of 8NM which isnt bad considering the FMC is relying on IRS input only for nearly 2 hours !

BA consider the RNP figure to be good enough to constitute a NAV Fix check nowadays so must be reliable I suppose !