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Old 11-30-2012, 09:12 PM   #1
quarkie
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Default Incorrect position data after in-flight GPS/INS re-aligning

After in-flight engine failure and emergency restart re-aligning NAV systems in CDU results in incorrect position reported by all three GPS, INS and Blended mode sources. Position from this sources becomes shifted by certain offset in all 3 dimensions (you can see negative altitude value on one of the screenshot below). This heavily affects navigation data on IFCC. Ground realigning and complete shutdown - start-up sequences doesn't fix the issue, position is still shifted.

On this screenshot you can see on the TAD that aircraft is close (withing 1-2 nm, also confirmed by visual) to WP1 which has GG 376 460 grid (same on CDU WP page of this waypoint), while POS page on CDU reports my grid as GG 405 284, as well as it reports negative altitude of -631 feet. On IFCC you can see reported distance to WP1 as 9.7 nm.
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Issue is easily reproducible
Steps to reproduce:
1. Attain high altitude
2. Fly inverted until engines stop due to fuel system failure.
3. Shutdown engines by pressing RAlt+End and RCtrl+End.
4. Start APU, restore gen power, start engines.
5. On CDU choose page NAV -> page ALIGN and issue INFLIGHT aligning.
6. Wait until T = 5.5.0.8, switch to NAV mode, switch to EGI mode.
7. Try to validate your position data on POS page in CDU.

Track included, recorded with 1.2.2 DCS World A-10C module
Attached Files
File Type: trk ifcc_bug.trk (466.6 KB, 28 views)

Last edited by quarkie; 03-01-2013 at 09:01 PM. Reason: track version
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Old 11-30-2012, 10:20 PM   #2
msalama
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So the generator power is completely lost at some point? Maybe that's the explanation right there
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Old 11-30-2012, 10:29 PM   #3
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Yes. This is the cause, but not the explanation. After in-flight re-alignment aircraft should receive its correct position. Otherwise, - what is the purpose of this feature then?

EDIT: And as I mentioned, the issue remains after landing, shutdown and cold start with ground aligning.

Last edited by quarkie; 11-30-2012 at 10:39 PM.
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Old 03-01-2013, 08:45 PM   #4
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The issue is still present in DCS World: A-10C 1.2.3.9871

In-flight navigation systems realignment after loss of power results in corrupted position calculated by CDU. Affects navigation data on the HUD.

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On screenshot:
Real data: TAD WP1: DK 519 754, barometric altitude: 830 ft
Corrupted data: CDU POS page: DK 595 596, altitude: -2094 ft (negative altitude)
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