Heading Shows Dashes After a Display Swap
After a display unit is replaced in a two-IRS business jet, the new display shows valid pitch and roll but heading as dashes. The other display, fed from the same ARINC 429 output of IRS 1, shows valid heading. The technician concludes the IRS is failing and plans to swap it. Each IRS drives both displays through separate 429 ports. The replacement display is a serviceable unit borrowed from another aircraft, and its configuration strapping and software part number were not verified before installation. At the display connector, the 429 bus is a twisted shielded pair with pins designated A and B.
1. What do the symptoms prove, and what do they rule out?
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Proven good: IRS 1 is transmitting heading, the shared bus wiring is sound, and the label is present in the aircraft's data architecture - because the other display reads it.
Ruled out (or nearly so): a failed IRS. Replacing it would be a shotgun repair that leaves the real fault in place.
Most likely: the receiver side. Heading-only loss with good attitude is consistent with:
- the display being configured to listen to IRS 2 as its heading source, with IRS 2 off or failed - dashes for one parameter while others are valid is the signature of a source-selection problem;
- an SDI / source-destination mismatch, so the display discards the heading word as not addressed to it;
- a receiver software part number whose label table does not match the aircraft configuration.
A swapped A/B leg is possible but usually kills the whole port, not one label - keep it on the list, but low priority.
2. List the specific checks you would make, in order, and what each one tells you.
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- Compare configuration with the removed unit. Same source selection, same SDI strapping, same software part number. This is the cheapest check and the most likely fix.
- Read the display's configuration report from the maintenance pages - confirm which IRS is the active heading source and that IRS 2 is not selected.
- Verify the software part number against the aircraft's approved configuration, not just against a generic serviceable-unit list.
- Check pin A/B at the connector against the wiring diagram: A leg to A leg, B to B. Note that A429 is polarity sensitive; a swap usually produces no data at all on that port, so a pass here further supports a configuration cause.
- Only after the above passes, substitute a known-good display or move the unit to the other position to see whether the fault follows the unit.
Do not disturb the IRS or its wiring while the other display is receiving valid data from it.
3. After the fix, what verification and documentation is required before return to service?
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- Perform the functional check called out in the aircraft maintenance manual for the affected system, verifying heading on both displays, in both IRS modes, and after a normal alignment cycle.
- Confirm the configuration report shows the intended part numbers and source selections after the change, not just that a load completed successfully. A load-complete message only proves bits moved.
- Record the part numbers removed and installed, the configuration settings, and the reason for the change in the aircraft records.
- If the borrowed unit's software was changed to match this aircraft, note that it may no longer be interchangeable with the aircraft it came from - flag it so the other aircraft's records stay accurate.