Note: We are not exactly sure as to the wording of the question, and our initial feedback did not even mention being on approach. From context, we know that the examiner is asking about approach temperature correction, and therefore most likely about an RNP APCH to LNAV/VNAV minima using baro-VNAV (as per the learning objectives). Please send any further exam feedback to [email protected] or put them in the comments, thank you.
RNP APCH to LNAV/VNAV minima based on BARO-VNAV – the approach horizontal guidance is managed based on GNSS and the vertical profile is guided by barometric altitude, as opposed to using GPS-SBAS altitude.
These 3D approaches conducted in the vertical plane based on barometric reference require certification of altimeters with adequate accuracy to enable completion of the approach. Barometric descents are prone to effects of the atmospheric parameters affecting altimetry:
- It is crucial to have the correct QNH altimeter setting to avoid offset altitudes during final approach.
- Temperature effects also influence terrain clearance on final approach segment. Altimeters are calibrated in accordance with the international standard atmosphere (ISA). In case of low temperature, the pressure layers in the atmosphere move closer together resulting in a lower true altitude than indicated. This means that the approach path flown by an aircraft would be shallower than expected, with a lower glide path angle. Consequently, obstacle clearance minima may be jeopardised if temperatures are below a certain limit. The majority of RNP approaches will have an OAT minimum temperature limit, and the approach plate will state ‘uncompensated BARO-VNAV not authorised below X°C’.
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