Hi all,Just wondering if you could explain to me Map Integrity? I have found it on a couple of checklists. Is it something to do with checking the FMC and Map mode on the EHSI??ThanksBarge

Hmm never seen it but I imagine you're right. In most 757/767's there are no GPS'es so the FMC position is caculated from the 3 IRS'es and VOR/DME correction. Since the IRS'es build up errors over time (they only measure acceleration along the three axes and integrate that number into speed, position etc. so any error will get bigger over time, on average), after a while, you may notice a "map shift". This means that the location on your EHSI is noticably different from your actual position. Since VOR's can be drawn on your EHSI but are simply drawn by the FMC taking their database coordinates and the FMC position, they can still be in the wrong place on your EHSI picture (eg. you're right over a VOR and the EHSI picture shows you one mile east of the VOR).This is always a consideration when doing a tricky approach without an ILS, tricky meaning with high obstacles/terrain or other traffic (routes) in the vicinity. Especially since the approach and the largest FMC position error are always at the end of the flight.Iz

Iz/anyone with time to answer, Maybe you could clear something up for me...After doing some pond hops lately, I notice that -for example- when finishing the ocean crossing (IRS NAV ONLY), the moment when the first VOR comes in to radio range, the LNAV path shits a little on the EHSI and the aircraft turns to re-intercept. I have assumed (perhaps incorrectly) this was the Radio position updating/overiding the IRS-3 position when the VOR came back into range. The Radio position was "updating" the IRS to an actual position, therefore the small course correction. Maybe this is a "PIC" thing? Also, I do not see a "Map Shift" notification on the FMC when this occurs. I beleive I should see that message if there is -any- course correction/path/map change as a result of Radio updates, no? Maybe the Map Shift notification only occurs above a certain threshold/amount of a shift (i.e. more than 5 miles?).If the above is correct, then maybe I am missing something and need to re-read the IRS system manual....following on from this, would the VOR's not constantly update the IRS postion whenever there are VOR's in range resulting in near perfect HSI display/path/map 'integrity'? This would result in little or no error at the arrival airport...whereas the comments I see here indicate that any error would peak at the end of the route, i.e at landing....so I have to conclude I am definitely misunderstanding something when it comes to IRS and Radio postion updates. Sorry to babble, no doubt I have things muddled up. Please help.Rob.

To calculate where you are in the world the FMC looks at the three IRS inputs. It then "mixes" these three inputs to come up with an average position, which is where the FMC shows the aircraft on the map display. At the bottom of the FMC progress page it will display IRS (3). Should one of the IRS's fail, or its position differ from the other two by a certain amount, it will disregard the input from that IRS and the prog page will display IRS (L C), indicating it is no longer taking an input from the right IRS.Due to the innacuracy of IRS systems the FMC is will also be constantly trying to confirm its position relative to radio navaids. There are many restrictions on when it can do this, for example you must be within 40nm of the station for a VOR DME update, within 20nm for a LOC update, the position lines must cut at a certain angle for a VOR VOR update.When the FMC obtains a position fix from a radio aid, it is not updating the IRS position, it is simply adding the radio update to the "mix" of positions generated by the IRS's.When MrBoeing built the 767 IRS systems were new, and accuracy not as good as today, so he decided it was worthwhile notifying crew if they had not recieved a radio update in the last 15 mins, or by 15000ft on descent (IRS NAV ONLY). However these days, even after a few hours without a radio update, the map will generally be accurate and little if any map shifting will occur.A map shift occurs when you have been out of range of a radio update (such as over-water flights, or flights over certain parts of Asia where navaids dont work anyway), and the FMC has been relying on the IRS position only. If the IRS's have been "drifting" and their position is inaccurate, then the first radio update the FMC recieves may show that that aircraft is 1 or 2nm away from the IRS position. In this case the FMC quickly changes the aircrafts position based on the update, and the map will shift, showing the aircraft as not being on track. LNAV will simply turn to re-intercept the track. The is NO annunciation of a map-shift, other than prog1/2 showing what type of update you are getting (this may be on the Radio page of a pegasus FMC), the prog2/2 showing you are now left/right of track.There are some airports (such as Darwin and Perth) where you arrive over water, without having had an update in 2-3 hours, and have to do a VOR approach. This is where mapshifts become critical, as the only update you can recieve is a VOR DME from a single aid, which does not do much to improve the accuracy of the map. It is very disconcerting during a VOR approach, to have the MAP display showing you 1-2nm from the runway centreline while on approach.AT the end of each flight the FMC calculates how far out the IRS's were during the flight, and comes up with a IRS error, which we much record. If the error is greater than a certain limit the IRS must be considered U/S.

"Also, I do not see a "Map Shift" notification on the FMC when this occurs. I believe I should see that message if there is -any- course correction/path/map change as a result of Radio updates, no? "Perhaps an option, Rob(?).Can't say I've heard/read about it tho'.Just following up on what HPSOV has said....When flying in areas of good constant VOR/DME coverage, the FMC doesn't actually mix IRS and radio positions (not in the sense of "averaging"). It simply uses the radio position. However, in your case, this rule is probably being bent a bit: When Map Shift is being corrected after the aircraft has come into range of a VOR/DME station or DME/DME stations after a long overwater sector, the FMC has to figure out how to smoothly shift the aircraft from the IRS-only position to the radio-only position. I guess some sort of mixing/unmixing algorithm would be used in this case.When using radio-only nav, the IRS triple mix position is still being computed in the background, and the radio position and the IRS position are compared periodically inside the FMC. If the IRS (3) position and the radio position vary too much from each other, the FMC switches back to IRS only (it assumes the radio stations are faulty, rather than the 3 IRS's). In this respect, the IRS is given a higher status than radio navigation. For this to happen, however, I'd say that the discrepancy between the radio and the IRS position would have to be quite large (in the 10's of nautical miles). If it happened with small discrepancies, the aircraft would not resume radio navigation after a long overwater sector (small IRS drifts would be seen as radio station faults). IRS-only position is used on the ground (unless the aircraft is fitted with GPS). This is because the signal coming from the radio stations are not deemed reliable enough at ground level (too many buildings, etc, distorting/blocking the signals). The aircraft has to be at a certain height above ground level before navaids are used.Hope this helps.Cheers.Ian.

Fantastic answers gents, thank you! I finally understand the relationship between radio and irs nav properly, the haze has lifted, a dim light brightens in my head.The timing is good as I will be doing the YSSY-KSFO run on Friday night with the fly-in...a long over water flight with several hours of IRS only time. The route takes us over HNL, I will be looking for the affects of the VOR's as I pass overhead.Great stuff.Thanks!Rob.

"I finally understand the relationship between radio and irs nav properly, the haze has lifted, a dim light brightens in my head."Good to hear, Rob... Of course, we could always complicate it for you :-hahThere's "radio bias", polar navigational differences, etc.... :-)

Hmm Rob and you're a real pilot or a virtual one?

Hi Iz,Unfortunately I am only a virtual pilot, a dreamer, a wanna-be ;-)Thanks again for your answer, great stuff to know.Ian - no need to make it any more complex...I think I have it now ;-)Cheers,Rob.

hey Rob,glad to hear it's clearing up for you :)ANyway, are there any plans for you to become pilot? I've heard north-american airports sell cheap avgaz, so a PPL shouldn't be very expensive, compared to f.e. Antwerp, ?Normally, the VFR practical part of my ATPL(starting sept. 2003) should be in Tyler(Texas, for 12 weeks) because of 'financial' benefits, and meteorological benefits.I'll try and make the short NZ hop, but then it's of to bed... gliding lessons this weekend :-) if weather allows :-( Cheers

A PPL is out of my budget right now, with getting married later this year and trying to save to buy a house, my love of flying has to take a back seat for the forseeable future.If I am a really good boy this year, I might get myself a CH Yoke in a couple months ;-)Good luck with the gliding...something about getting into a plane without an engine that bothers me ;-)Rob.

"...something about getting into a plane without an engine that bothers me ":-lolhehehehehehehei had that thought aswell before they actually let me fly one of these beauties... i can assure u, the sound of only wind passing over the canopy and sometimes the instructor YELLING at me is the best I ever heard :)No it's just the fact i'm flying that's attracted me into it, and the fact it's 'the most cheap way', as i have a pending ATPL of about 100.000euros, i really gotta keep expenses small.U should try it though, if you get the chance, you'll love it, expecially the cable-launches are awe-frikkin-some, gives a glint of the feeling carier pilots must have, it's actally obliged to raise a hand symbol for the catapult to be launched :) (not exactly 0-300km/h in 3 seconds though....but still fun).And Good luck with the marriage over there :)Greetz from rainy antwerp