IRS-Only Navigation in RNP 10 Oceanic Airspace
A wide-body is 5 hours into an 11-hour Pacific crossing at FL350. Both FMCs show 'GPS PRIMARY LOST', followed by 'NAV ACCURACY LOW'. The crew has no DME/DME or VOR coverage. The left and right IRS positions now differ by 4 NM, and the FMC position is 2.5 NM from the left IRS and 1.5 NM from the right. Both IRUs were aligned at the gate and have been operating normally. The aircraft is cleared in RNP 10 airspace. Fuel is adequate for destination plus 45 minutes. The nearest suitable diversion is 2 hours away. The crew has paper charts and a standby compass. The destination weather is forecast fine, but there are no radio navaids along the route. The autopilot is engaged in LNAV and the aircraft is on the cleared track.
1. Explain how the FMC is computing position and why LNAV may remain available.
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With GPS primary lost and no DME/DME or VOR coverage, the FMC is navigating in IRS-only mode. Each IRU provides a position, and the FMC blends the IRS positions, often with air data and a wind model, to produce a best-estimate position. The FMC can still provide LNAV because it has a position and a track, but the integrity is lower and the position error grows with time as the IRUs drift. The 'NAV ACCURACY LOW' message means the FMC's estimated position uncertainty has increased, and the crew must compare it with the required navigation performance. RNP 10 may still be met for a limited period, but the aircraft's AFM or QRH may specify a maximum IRS-only time. The crew should not assume that the loss of GPS primary means immediate loss of RNP 10; they must check the actual navigation performance and the applicable time limit.
2. Which IRU is suspect, and how can the crew confirm a drift rather than a normal IRS difference?
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A single 4 NM difference after 5 hours is not by itself proof of an IRU failure, because IRS drift is normal and can be of that order. The FMC position is a blend, so it will lie between the two IRUs and will be pulled toward the drifting one. The crew should look at the trend: record the three positions every 15–30 minutes and compare the rate of divergence. If the left IRU is moving away from the right at an abnormal rate, it is suspect. Cross-check the IRU's heading and attitude; a failed IRU may show a heading flag or attitude error. Compare with the last known GPS position and the planned track. When a navaid comes within range, update the FMC and see which IRU agrees with the fix. Do not deselect an IRU on a single snapshot; use the trend and corroborating evidence.
3. What operational action is appropriate once a faulty IRU is confirmed?
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Once the suspect IRU is identified, deselect it and continue with the remaining IRU. The aircraft then has a single IRS, so there is no redundancy for attitude, heading, or position. The crew should assess whether the remaining IRU can support RNP 10; if not, they must request a change to a non-RNP route or a lower altitude where navaids are available, or divert. The nearest suitable diversion is 2 hours away, and fuel is adequate for destination plus 45 minutes, so a diversion may be feasible but should be decided early. Notify ATC of the degraded navigation capability and request any assistance, such as radar vectors or a route with DME coverage. Monitor the remaining IRU closely; if it fails, the aircraft will lose primary attitude and heading, and the crew must use the standby instruments and consider a diversion to the nearest airport.