Map Shift
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Yes / No.
One of the problems involves de -s electing navaids - notably ILS which are NOTAMed as out of service.
If you go to the WAYPOINT / NAVAID page you will probabaly find that you can only DESELECT a single navaid.
Hence the FMC may, position update, on an invalid ILS/DME.
One of the problems involves de -s electing navaids - notably ILS which are NOTAMed as out of service.
If you go to the WAYPOINT / NAVAID page you will probabaly find that you can only DESELECT a single navaid.
Hence the FMC may, position update, on an invalid ILS/DME.
Join Date: Mar 2002
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It certainly can still happen. Some of our 757s do not have GPS updating and so rely very heavily on DME-DME, VOR-DME etc updating. Some of the briefs for our destinations make quite a point about continually monitoring for map accuracy as we descend into areas with high terrain but poor navaid coverage.
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The IRS is driving the basic nav solution .. but it gets the wanders and needs to be reset as it were .... hence the importance of GPS, DME/DME or whatever updates ... to wipe the error slate clean.
Per Ardua ad Astraeus
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Two g/a's in 'Classic' 737's due to EGPWS being 'triggered' by a run-away map position. GPS (NG) stops this, but it can still be a nuisance 'cos some SOPs require an automatic g/a at night even if fully visual.
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I can't help but keep wondering if "map shift" is a problem invented by someone who relied too much on following the magenta line without crosschecking other instruments...
In a "steam gauge" airplane with IRS but no FMS or Nav Display, "map shift" happens every time an update is made or lost, and the autopilot and/or flight director move the airplane half a mile left or right to match the latest solution. We all get used to the phenomenon, but also crosscheck the HSI and/or RMI when VOR or ILS is available. If you religiously crosscheck your Nav Display with raw VOR data, "map shift" should never catch you by surprise.
In a "steam gauge" airplane with IRS but no FMS or Nav Display, "map shift" happens every time an update is made or lost, and the autopilot and/or flight director move the airplane half a mile left or right to match the latest solution. We all get used to the phenomenon, but also crosscheck the HSI and/or RMI when VOR or ILS is available. If you religiously crosscheck your Nav Display with raw VOR data, "map shift" should never catch you by surprise.
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.. and therein lies the problem ...
some of us are of the view that one should never quite trust the whizz bang gear too much ... I can recall on the 733 without DME/DME after several hours ... having the map show the runway in quite a different position compared to where we knew it was from looking at it on final ... were a chap to cheat and try a letdown on the magenta line without anything updating the position data ... could be embarrassing.
some of us are of the view that one should never quite trust the whizz bang gear too much ... I can recall on the 733 without DME/DME after several hours ... having the map show the runway in quite a different position compared to where we knew it was from looking at it on final ... were a chap to cheat and try a letdown on the magenta line without anything updating the position data ... could be embarrassing.
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Flying across the Oggin or the Sahara you are a bit short of navaids, so it follows that you <have to> trust the map.
Once you have done it for a few hours at a time it is quite compelling. Having said that, going into somewhere like Moscow, map shift occurs quite often and can present a problem. Cross checking with navaids is always good - but if you have just cross checked with the navaid that has given the dodgy update, then you are still none the wiser!
How do you tell which of the navaids is dodgy, unless you cross check with three?
Once you have done it for a few hours at a time it is quite compelling. Having said that, going into somewhere like Moscow, map shift occurs quite often and can present a problem. Cross checking with navaids is always good - but if you have just cross checked with the navaid that has given the dodgy update, then you are still none the wiser!
How do you tell which of the navaids is dodgy, unless you cross check with three?
Per Ardua ad Astraeus
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"map shift" should never catch you by surprise."
Mach 3
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I can't help but keep wondering if "map shift" is a problem invented by someone who relied too much on following the magenta line without crosschecking other instruments...
I guess 90% of map-shift events on EFIS a/c in Northern/Southern Europe are never even noticed.
Its only on an IMC day (because otherwise you'd be looking out the window right?), with high terrain around on final approach, nav-aids with restricted coverage and your map display down at the highest res you'd ever even notice.
I've noticed it once in the last 3 years into AGP I believe.
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Map shifts can occur for a number or reasons such as incorrect ppos initialization of the IRS, spurious ground Navavid data, NavDB error, or excessive drift of the IRS (especially where navaid update not available). On modern aircraft, this problem is mostly illiminated if the aircraft is equipped with GPS and the FMS uses GPS as the primary position source (cross-checked against DME/VOR/IRS for reasonableness). The actual position blending can differ between aircraft types. The most important thing in all of this is that the EGPWS should ideally receive a position based on GPS which is by far the most accurate (since selective availablilty was turned off - thanks to Bill Clinton). In the event that the FMS position may become shifted from the actual position, it is essential that EGPWS has a GPS based position to ensure that the last line of defence against CFIT can provide adequate protection against FMS blended position errors.
Map shifts may also occur where there are discrepancies between the geographical positions in the database, and the actual locations on the ground - China is one area where this may occur. The published facility co-ordinates are not in the WGS-84 datum used by the FMC....
