FMS Position Errors

[Oggy]

Hi Folks, can anyone help explain this to me?

I recently had a situation on a 733 with FMS U10.5. After Takeoff (Hdg090) the Flight directors tried to take me off the SID, by turning right when the Sid was straight ahead, so after the normal wtf heading select made and SID flown manually.

Upon further investigation The Aircraft position was found to be 9w of our position. So it must have been finger trouble putting in the position by myself me thinks.

The thing that i can't understand, is having verified the position by stand, airfield details on the route section and watching ourselves taxi to the hold while on Map mode. The next time I looked after LNAV had been pressed we were heading East on the SID and the Aircraft wanted to turn south west.

So if the aircraft thought we were west of the first point on the SID why would it try to get us to fly South west?

As I say I'm putting my hands up to finger trouble but I can't understand why this most likely of scenarios would give the symptons I experienced.

I standby for your suggestions and ridicule. :)

[QNH1013]

Does sound strange Oggy. Even with the 9W initially error, wouldn't you get a Runway position update once you press TOGA on takeoff? Then it should all look correct on the Map display.

[Oggy]

I agree QNH, if anything the symptons we got are the opposite to what I would have expected, ie no runway until after pressing TOGA. I can't see a logical explanation yet, neither has anyone I've discussed it with.
It was nice to fly and be able to navigate without the comfort blanket for a while:)

[Icebreaker]

We've had several map shifts on takeoff in our company too....same thing as you described. Subsequently out comes an OEB from the manufacturer and they usually say it's a software problem which will be corrected in the future.
It's possible that the runway threshold update stores an incorrect lat/long from the database and there's your mapshift.

[Dehavillanddriver]

What was the magenta line doing?

I got the impression from the first post that the flight directors commanded the aeroplane away from the magenta line depicting the SID - is this right?

If so, a map shift wouldn't cause that.

Also if using U10.5 you probably have got GPS input into the FMC which negates the effect of inputting the wrong coordinates.

[Cornish Jack]

Any idea what the discrepancy in position was in NMs?
If it is more than (I think) 40-ish NMs the TOGA update won't take properly and the IR inputs won't update your position quickly enough to be useful.
You are in good (not to say, exalted) company. One large carrier did this, and, since the only remedial action to regain proper map function was to land and re-initialise the inertials, they continued on raw data!! - Luckily they had a heavy crew, 'cos it was a loooooooong sector! Another example from the long-haul world was down to the company operating from HR and LGW - yes, you've guessed it - and the distance is in the problem area.
Murphy does it again:\

[Oggy]

I can't remember seeing the magenta line until much later when diagnosing the problem in the cruise. The FMS position was about 9W of us, which i guess is in the order of 270 miles at UK latitudes (Dumped the theory after passing the exams:O )
and there was no radio position, the system was looking for beacons on the south west coast of Ireland.

As far as the runway coordinates it worked fine the next time with the same aircraft, so it must have been yours truly. I'm just confused as to why if it thought we were somewhere near Shannon why did it try to turn me towards the south west, when the initial points of the sid were in London.

The system has no GPS input on our aircraft, which leads me to a seperate thought and question. Does EGPWS use a GPS position to provide information and if so is it possible to feed this input to FMS? It's purely a theoretical question, not worried about the legislation, cost etc, it's just an urban myth running around the fleet at the moment.:O

[The Real Slim Shady]

Oggy

I believe that the internal GPS on the EGPWS only provides position information to that system; i.e there is no feed to the FMS.

The position error you initially had would not be updated on TOGA activation as the the FMS position is a composite position based on IRS / RHO-RHO updating / GPS (if fitted) and FMS position. Hence the error would be reduced on TOGA activation - new postion combined with erroneous position averaged out- but not removed.

[QNH1013]

Thanks Slim for the Tech info. Helps me understand what I am fiddling with a bit more too.

[techstar]

EGPWS is available with or without an internal GPS card. If you have an EGPWS installed with an internal GPS card, then there are 2 types available. An 8 channel GPS, which is used purely for providing a position source to the EGPWS, or a 24 channel GPS which is certified to en-route Navigation standards and which can also be used as a position source for the FMS. In fact, some B737s are using this solution today for the FMS as it provides a very cost effective alternative than installing Multi-Mode Receivers on the aircraft.

[Gary Lager]

Cornish Jack - I wonder if I've read the same incident reports as you mention...there was an incident where a crew misaligned their IRS before departure (finger trouble) and after leaving VHF Navaid coverage their FMS position diverged from the actual sufficient to cause a potential loss of separation (can't remember if it actually resulted in TCAS mid-atlantic or not). Certainlyerrors of the order of around 70nm involved.

Never never never enter MPNS airspace if you suspect that your IRS has not been aligned correctly or your FMS/IRS position information is invalid.

There are guidance notes in UK CAA AIC 33/2000 (see UK Aeronautical Information Service (http://www.ais.org.uk)) for loss of B-RNAV capability enroute - so that might be worth a thought if you suspect that FMS position is having trouble.

[Cornish Jack]

Gary
No, t'was going t'other way!
These things happen often enough - Murphy is alive and well!!:(

[The Real Slim Shady]

Experienced a couple of map shifts in the Far East due to a lack of DME updating even following a good alignment.

First one was a 4200nm jump, we never actually worked out WTHH, second was smaller, the order of a couple of hundred miles but still significant.

Fortunately, heading, speed and time until we came into range of a VOR / DME worked. Mid-Atlantic or Pacific though, different kettle of fish!!