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An aircraft NOT equipped with temperature-compensating system is executing an RNP APCH to a Baro-VNAV minimum. The minimum temperature to execute the procedure (TMIN) is -8°C. The temperature at the airport is +10°C. The pilots have set the appropriate FMS settings based on the charts of the approach and the aircraft follows the FMS guidance. During the final approach segment, ATC instructs the pilots to go-around immediately as a result of insufficient separation from the surrounding terrain. What caused this outcome?

  • A

    Incorrect altimeter setting leading to the aircraft being lower than indicated.

  • B

    The pilots miscalculated the rate of descent and selected an inappropriate vertical speed.

  • C

    The temperature at the aerodrome is above the limiting temperature.

  • D

    The vertical path in the FMS was incorrectly coded as it is NOT shown to the pilots.

Learning Objective Explain why an RNP APCH to LNAV/VNAV minima based on Baro-VNAV may only be conducted when the aerodrome temperature is within a promulgated range if the barometric input is not automatically temperature-compensated.

LNAV/VNAV minima based on BARO-VNAV – the approach horizontal guidance is managed based on GNSS and the vertical profile is guided by barometric altimeter, as opposed to using GPS-SBAS altitude.
For aircraft using baro-VNAV without temperature compensation to conduct the approach, low temperature limits are reflected in the procedure design and identified along with any high temperature limits on the charted procedure.
Baro-VNAV approaches require certification of altimeters with adequate accuracy to enable completion of the approach. While executing this type of approaches, we should take the following into account:

  • It is crucial to set the correct QNH on the altimeter, to avoid offset altitudes during final approach. QNH is given by ATC, and updates are also given if it changes.
  • Temperature values affect 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. Consequently, obstacle clearance minima may be jeopardised if temperatures are below a certain limit. In this case, that limit is -8°C, and it is unlikely that this is what caused ATC to call for a go-around, as the OAT was 10°C.

Since temperature was not the cause of a low approach (it was higher than the minimum required), the pilot may have entered a wrong altimeter setting which led the aircraft being lower than indicated.

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