Checklist item-"map integrity"?
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Disgusted of Tunbridge
Disgusted of Tunbridge
Join Date: Jan 2005
Location: Hampshire, UK
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You have to check that the map picture on your map display is showing your correct position before descending. If the IRS positions have wandered during the flight, sure the map display is showing your position in relation to radio aids, but how do you know you are actually there? So you tune one of those radio aids in, and check the bearing on your RMI, and the DME reading, and make sure that the position on the map display corresponds with the actual radio bearing/distance from the VOR. Then you know your map is showing your accurate position.
It happens that errors can cause map shift. I have had it twice, and it is very disorienting. You suddenly can't trust what your major position display is telling you, and great care has to be taken. Now with GPS inputs to the map display, it's always bang on the ball.....but how do you really know that?
It happens that errors can cause map shift. I have had it twice, and it is very disorienting. You suddenly can't trust what your major position display is telling you, and great care has to be taken. Now with GPS inputs to the map display, it's always bang on the ball.....but how do you really know that?
Join Date: Apr 2005
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If the country you are operating in is part of the WGS84, then the GPS co-ordinates have been verified as accurate. If the country/airport is not part of the WGS84, then the GPS update should be inhibited and the ND treated with caution because of mapshift possibility.
Join Date: Mar 2003
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On a 737-300 video that I own the "map integrity" was checked by entering a VOR in the NAV RAD page of the FMC and then cross checked by the map and RMI compass readings.
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Before GPS updating was common, this was a common problem. En route, the FMS position would be derived from the INS which wcould and would wander. Your map oictire would wander accordingly. You would first be aware of it when the FMS took a more accurate fix off a navaid - usually the ILS when you intercepted on approach whne you could find yourself 'shifted' by up to three miles in a second. This 'map shift' could cause problems so the 'Map Integrity' check was there to warn you of it in advance.
Map shift is largely a thing of the past with GPS updating. The 747-400 I used to fly still had it in the checklist as it could still shift a little. The A320 I fly now doesn't and I have never seen a shift.
Map shift is largely a thing of the past with GPS updating. The 747-400 I used to fly still had it in the checklist as it could still shift a little. The A320 I fly now doesn't and I have never seen a shift.
