RNP APCH in the Cold: Baro-VNAV, Temperature Limit and RAIM

Only the LNAV minima (MDA 1 460 ft) are available. The LNAV/VNAV DA of 1 240 ft must not be used.

Reason

• Baro-VNAV builds the vertical path from barometric altitude, assuming an ISA temperature profile.
• In air colder than ISA, the pressure levels are compressed towards the ground, so the altimeter over-reads and the aircraft is lower than the computed path says. Following the baro-VNAV path in the cold therefore places the aircraft below the designed geometric path, eroding the obstacle clearance that the procedure designer built in.
• The chart's -12 C limit is where that erosion reaches the design margin. At -20 C, the margin is gone, so the baro-VNAV approach is prohibited - not merely 'less accurate'.

What to fly instead

• The LNAV procedure: a non-precision, step-down approach to the MDA of 1 460 ft.
• Apply the published cold-temperature correction to the LNAV procedure altitudes, including the MDA, using heights above the altimeter-setting source.
• Brief the step-down fixes carefully: with no vertical guidance and a low temperature, the approach must be flown as a stabilised continuous descent with the corrected minima as the decision point.

The requirement. RNP APCH depends on on-board performance monitoring (RAIM/ABAS here, since no SBAS). The approach may only be flown if the monitoring is predicted to be available for the whole approach, from the initial approach fix through the missed approach.

The check

• Run a RAIM prediction for the actual time window: 1005 UTC at the IAF to about 1018 UTC or later at the missed approach point, plus the missed approach itself.
• The NOTAM covers 0900-1130 UTC, and the entire approach falls inside it. The prediction will almost certainly show no RAIM; the notice is not a general warning to be read past.
• Check that the prediction covers the whole window, not just the ETA.

The decision

• If RAIM is not available, the RNP APCH cannot be flown; there is no other approach at the destination and no nearby alternate within fuel.
• Options: request a delay from ATC so as to arrive after 1130 UTC (checking fuel and holding), or divert to an aerodrome where an approach is available - and the diversion decision must be taken early enough to keep the fuel.
• The weather being good enough for a visual approach does not change this: the crew cannot plan to 'have a look' when the cloud is broken at 2 000 ft in an area of high terrain.
• Do not continue simply because the aircraft is RNP-approved; approval, signal availability and prediction are three separate gates.

What changes

• LPV uses SBAS to generate a geometric vertical path from the satellite solution. The vertical guidance does not depend on barometric altimetry, so the baro-VNAV temperature restriction does not apply and the LPV minimum could be used at -20 C.
• That would restore a vertically guided approach and a lower decision altitude than the LNAV MDA, which matters with a 2 000 ft cloud base and high terrain.

What does not change

• The GNSS availability problem remains. The PRN 23 outage still has to be assessed, this time against SBAS availability and the LPV integrity service, not against ABAS RAIM. If SBAS is not available for the window, the LPV approach is equally unavailable.
• Barometric altimetry is still used for the go-around and for the aircraft's other altitude constraints, so the cold-temperature correction still applies wherever a barometric altitude is flown or compared - for example the missed-approach altitude.
• The crew still need the LPV capability in the operator's approval and in the aircraft's documentation; an RNP APCH approval alone does not cover LPV.

In short: SBAS removes the temperature limitation, but not the availability limitation, and the decision still turns on the prediction for the actual time window.