RAIM loss and a cold-temperature baro-VNAV limit

RAIM (Receiver Autonomous Integrity Monitoring) uses the redundant measurements from the satellites in view to check, inside the receiver, that the position solution is consistent. If one satellite is transmitting corrupt data, RAIM detects it and, with fault detection and exclusion, removes it; if it cannot, it raises an alert within the time-to-alarm. Without RAIM you have no integrity assurance - the receiver may output a position that is wrong without warning, which is precisely the failure mode an instrument approach cannot tolerate.

Because there is no SBAS here, the RNP APCH depends on RAIM being available for the approach and for the terminal area around it. A prediction of "RAIM not available" for ETA -10 to ETA +25 means:

• The approach cannot be legally planned or flown in that window, regardless of satellite count or receiver health.
• At the planning stage the correct action is to select a destination or alternate with a non-GNSS approach, or to re-time the flight, or to nominate the ILS airport as the destination.

Five satellites showing on the FMS is not the test - satellite geometry, not the number of satellites, is what determines RAIM availability. The prediction is a planning gate, and it was a gate the flight plan failed.

What it means: the GNSS receiver can no longer verify its own solution, so the FMS has reverted to dead reckoning - position derived from the last good fix plus heading, air data and wind. Position error grows with time and with any wind error, silently and without an alert. The aircraft is now a much less capable navigation platform in IMC.

Actions:

1. Tell ATC: loss of GNSS integrity and degraded navigation capability; request vectors or radar monitoring.
2. Cross-check with conventional aids - VOR/DME, NDB, radar - and keep a continuous plot so you know where you are without the FMS.
3. Do not commence the RNP APCH; it is not authorised without RAIM, and the pre-flight prediction already confirmed that.
4. Review fuel for the ILS 20 minutes away: at, say, 420 kg/h that is about 140 kg, which is normally well within reserves. Confirm it before accepting.

Accept the ILS. It is a ground-based, monitored approach at an airport with 700 ft and 3 km, which is comfortably above ILS minima; it converts an integrity problem into a normal approach. The alternative - pressing on to a GNSS-only airport with DR navigation, a temperature-limited baro-VNAV procedure and no vertical guidance - stacks three problems on top of each other. If fuel or weather later make the diversion marginal, declare it early rather than late.

Cold-temperature altimetry: in air colder than ISA, the altimeter over-reads - indicated altitude is higher than the true altitude above the aerodrome. The aircraft is therefore lower than the altimeter suggests, and the error grows with height above the reporting station and with the temperature deviation. On a day at -22°C, seven degrees below the chart limit, that error is significant at procedure altitudes.

Why baro-VNAV is affected: baro-VNAV builds the vertical path from barometric pressure, not from a geometric (SBAS) path. Flying a fixed pressure-derived path in cold air puts the aircraft below the nominal geometric path, and the obstacle clearance built into the design is eroded. That is exactly why the procedure carries an aerodrome temperature limit and, where published, a cold-temperature correction table.

Interaction with the LNAV MDA: the MDA of 720 ft is a barometric altitude, so the same over-reading applies. Even if the temperature limit did not bar the procedure, a cold-temperature correction would have to be added to the MDA before descending to it. But here the chart limit of -15°C is a hard authorisation boundary: at -22°C the procedure is not authorised at all, and no amount of correction makes it legal. The correct answer is that the approach is unavailable for two independent reasons - RAIM and temperature - and the diversion is the only compliant option.