Crosscheck your QNH Setting

The modification mentioned in the presentation are incorporated in the FCOM revision published in Q4 2024.

The NAS STD4 does not address change in the management of QNH in the avionics. Flight crews should cross check QNH by all available means. Please refer to the conference presentation with regards to your own operation.

How accurate is the GPS altitude. Would a cross check of the GPS altitude against the altimeter be good practice.

Can we use GPS altitude to cross check your altitude?

The GPS altitude has its own accuracy, called VFOM (Vertical Figure Of Merit), and uses the WGS84 model. Both aspects could make the crosscheck between GPS altitude and Baro altitude difficult i.e. a difference could be noticed but not related to a wrong QNH setting. However, this can still bring awareness to the crew and encourage them to confirm the QNH with ATC.

The GPS altitude is available on GPS NAV page of the MCDU, available through PROG 3/3 page, but it is not available on the PFD.

How can we include QNH setting in FDM Analysis?

The analysis presented during the conference was done comparing the recorded QNH setting with the expected one. The expected QNH is calculated comparing the Baro altitude at landing with the runway elevation.

In current NAS and next NAS STD4, there is no such caution message.

The insertion of QNH in the FMS allows an anticipation of the delta Baro in the VNAV profile for the crossing of the altitude constraints, and to have the STD flashing at the appropriate location. Once the source switches from STD to QNH, the ICP setting becomes the main source for guidance on altitude.

If such ATC system requires the QNH value itself, aircraft need capability to downlink the QNH information. Today, the ATR does not have such capability.

Investigation are still in progress. Currently some in service aircraft are equipped with additional flight data recorders to support the troubleshooting.