Hi Kris!yeah I wasn't clear enough above in that only acceleration and possibly speed data from the IRU's is used (ie attitude, V/S, track, roll rate) should you lose ILS data. I don't think it would be wise for the AP to rely on its position data and try to navigate "blindly" towards the runway using only current position and ILS database position as the AP doesn't directly know how precise the actual position is!I believe the FMC can only tell to around 95% certainty how precise the position currently is (ANP).Regards,Mark

MArkAs I sais the NG does things better than the aged classic. The NG uses DME/DME fixing via 5 independant DME units which are completely separate from the pilot set Nav boxes. These continually update the FMC position even during an approach. That combines with the multiple GPS feed that is fitted to most NG's and map shift is far less pronounced. The -300 we fly has just 2 nav boxes and when set in AUTO are tuned by the FMC to give dme and VOR position fixing. This is fine while they are in AUTO but we need to switch to manual in order to get the ILS up. When we do this we remove the fixing and the FMC must rely completely on IRS positioning, we even get a helpful FMC message telling us IRS NAV ONLY. alas we don't even have GPS on our fleet (yet!!) so we really are back to IRS position only. when this happens it doesn't take long to end up with a large chunk of map shift particularly with old IRU's and after a long flight. The IRU's can be up to 5 miles out so the map really does look screwey! Hope that explains it a bitKris

>Hi Kris,>>"you should always back up and appraoch with raw data!">>I'm not sure I understand the meaning of "raw data"? Can you>explain further, please?>>Thanks,>FrankHe means you need to look at the course deviation indicator and glideslope bar, not just the flight director.

>Thanks, Leo.>>Follow up question:>Do the FD and VOR/ILS displays receive data from different>sources? I thought they were both controlled by the Flight>Control Computers, which rely primarily on GPS position>information, AFAIK?>>The Flight director is driven by the autopilot computers, and shows what the airplane attitude SHOULD be to achieve the goals of the Autoflight system.The CDI and glideslope indicators are driven by the radios - period. They show the aircraft's horizontal and vertical deviation from the localizer and glideslope center.The radios drive the indicators, and that's the "raw" data. The radios also drive the autoflight system, which drives the flight director.Hope this helps.

oh darnyou're right mark.The information I gave was mixed up.In the Airbus family, the IRS' provide alot of information during the final stages of the approach... I wouldn't call this unreliable at all...(information was presented to me during class about ILS by an A340 Flight Instructor(guy teaches on the sim, both the 300 and 600, in the 300 sim :-))BTW, accelerometers are used for quite alot of things lately.on the long 340-600 they have installed alot of additional accelerometers to counter some problems in stability e.g. the plane would be a bit wobbly due to it's lenght rendering it more 'flexy'... the accelerometers now provide information for the computers to intepret ...Cheers

"The NG uses DME/DME fixing via 5 independant DME units which are completely separate from the pilot set Nav boxes. "Debatable, Kris....There are still only two DME Interrogators on the NG as there is on the Classic, but each Interrogator has 5 (internal) channels. Normally, each FMC sends four DME autotuning signals to the VOR/ILS ("NAV") control panels. The manual (pilot set) and automatic tuning data is then sent from the NAV control panels to the DME Interrogators (One Manual plus four FMC tune signals = 5 signals per Interrogator).Only if the NAV panels fail will the FMC's send DME tuning data directly to the DME Interrogators.Cheers.Ian.

IanAs an engineer you know the ship better than I, I was led to believe that the FMC DME updating was separate to the NAV boxes we tune on the flight deck. The NAV page on the FMC shows 5 DME stations being interogated so I have to say I assumed that was how it was set up in the bowels of the E&E bay. WE have a couple of ex-Lufthansa aircraft with the NG type nav boxes and Update 10.5A FMC's so we do get a brief glipse into the future sometimes! All the best mateKris

Thank you, Tim!BR,Frank

Thanks for a very informative reply, Tim!Now I understand. :-)BR,Frank

>The NAV page on the FMC shows 5 DME stations being interogated so I have to say I assumed that was how it was set up in the bowels of the E&E bay.Hi, Kris.There are an awful lot of variations in avionics out there, so I can't say that I am 100% correct ;-)On some NG FMC pages, you might see a list of those five DME stations, but the header on those lists shows either "DME-L" or "DME-R". These represent the two DME Interrogators (Actually some NG's have 3 DME Interrogators, but I really don't want to go there!)Note that 99% of my info on the NG comes from manuals. I'd be lost without them. I need guys like you to tell me what really happens on the flight deck :(Cheers.Ian.