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alampl
3rd Jun 2010, 20:14
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.

EMIT
5th Jun 2010, 20:45
Hi,

Not being familiar with the exact equipment status of your aircraft, this is a best guess answer:

If you have a page where the ANP for IRS(3) is given, as a piece of information that is separate from your "actual" ANP with GPS or other updates (DME/DME or VOR/DME), then that IRS only ANP will just be determined by a standard algorithm base on "average IRS performance". Reason for this - when you have only IRS info, you have no "real" position to compare IRS position with, so you(r) FMS can only work with assumptions, which is the standard IRS deviation algorithm (make a best guess).

Neupielot
6th Jun 2010, 00:12
Any ideas how we get an ANP 20nm on IRS(3) after only 8 hours of operation, every time?


There shud be a limitation chart Drift vs Flight hrs somewhere in the aircraft ops manual. Is that still within limits? im not very experienced but 20nm sounds....very serious. So u can't do RNP 1 stuff i'm guessing.

NSEU
6th Jun 2010, 02:19
EMIT is correct. ANP is simply based on a standard value for the available sensors and their time in use. This is a probability value.

Your IRU's will tell you their error. It will be the IRU value you use when comparing it against the charts.

If the IRU's were showing a 20nm error, then I would check this against your charts. Our charts show that it is right on the edge of the limit where you would have to observe the IRUs on the next sector (20nm is not enough to reject it on the first flight).

If your aircraft is fitted with Enhanced GPWS, there should be a GPS card within the EGPWS computer.

Regards.
NSEU

rudderrudderrat
6th Jun 2010, 07:40
Hi,

Without any position updating, your IRS position will drift within a tolerance published in the manuals somewhere. On our TriStars the INS error was (3 + 3T) nms where T = (Flight Time + Taxi Time) in hours.

On the Airbus with IRS, we simply look at the residual ground speed error when parked, and do a rapid re-align if it's less than 5 kts or a full re-align if it's greater than 15 kts. (I don't have the info to hand for maximum allowable drift error.)

The FMC adds a position bias (calculated from DME/DME fix or GPS) to the IRS position to produce your MAP. Provided your GPS position was good - you would still be able to do RNP 1.

20 miles error in 8 hours would have been well within our old INS tolerance.

8che
6th Jun 2010, 09:29
You need to check the IRS monitor function. On the ground only you will see the maintance function on the CDU options page. (init ref should take you to it). Line select the maintance fucntion and then line select IRS monitor. This will show the IRS drift rates for the flight just flown. I believe the limit is now 2.5nm/hr.

This check is normally done by the crew after ETOP's flights.

There is definitely a fault if the EICAS is flaging up when you turn the GPS off. Are you sure your in a good area of radio updating when you turn it off ?

p.s rudderrat, the IRS's always drift with or without radio updating. The updating is for FMC position not IRS position. They are 2 seperate things.

alampl
6th Jun 2010, 12:26
Thx so far for your replayīs;

Our maintanace told me that if the GPS is inop, you are not allowed to fly with your NAV recievers in Manual tuning mode; your radio updates are inhibited.

They checked all IRSīs, and they very completly in limits, now they think only the representaion on the PROG 2/4 is incorrect. The issue is still under investigtion, but it caused a big wave through Boeing and our FMS Manufacturer.
After the upgrade to EGPWS there was also an "upgrade" of the Position Software, and now they think the bug happened there. Looks like we are flying around with this error in our System since there.

The Ground speedīs of the IRSīs was never greater than 4 kt, even with an IRS(3) navigational performance of 20 nm.

rudderrudderrat
6th Jun 2010, 12:36
Thanks 8che. That's what I meant - but I didn't explain it at all well.

Before FMS was available - we could update the INS displayed position (position hold & type in the co-ordinates). On shut down - we would "Flush the Fix" to display the INS uncorrected position, compare with the gate co-ordinates and log the Desired Track and Distance errors.

EMIT
7th Jun 2010, 20:21
file:///C:/DOCUME%7E1/Ed/LOCALS%7E1/Temp/moz-screenshot.pngHi,

Try to locate this Boeing document D926T0120, perhaps via the Boeing website.
Title is "RNP capability of FANS 1 FMCS equipped 757/767"

On Page 30 you will find a table, specifying the algorithm for IRS only ANP values. Please note that "flight time" is time since system switched to NAV mode.

Sorry, am unable to copy paste the table into this posting.