IRS Positon 757
Hallo;
On one of my last flightīs with B 757 we discoverd on the PROG Page 2/4 that the IRS (3), the tripple mix IRS POS had an actual navigational performance of 20 nm. So we checked the groundspeeds of the IRSīs; they only had a GS of 1-2 kt. And there BRG/DIS was only 0.5nm. There was no way that we could have an ANP of 20.
First we thought we have a problem with the IRSīs on this particular plane, but we found out that every airplane of our fleet showed the same ANP of the tripple mix POS.
At the parking pos. it shows 0.0 but after 15min taxi it goes up to 1.5nm, after 3 hours flight it gets stuck at about 4nm, after 6 hours it holds at 10nm, and so on, until it reaches 20nm; it stays there. We tried that on all planeīs of our fleet.
So we selected the GPS OFF, went on Manual on the VOR/DME Recivers, and than we became EICAS Unable RNP. That was normal system behaviour.
The plane is certified to fly with no GPS, and imagine you have no radio updates available; we would end up with an ANP of 20nm.
Any ideas how we get an ANP 20nm on IRS(3) after only 8 hours of operation, every time?
thx for infos
A
Ps.; I think the GPWS always takes the IRS (3) POS, not sure.