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PB on Q1433

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  • PB on Q1433

    I am disapointed by the explanation of this question... The result to be given is a pressure altitude, and the explanation gave a result regarding to QNH...


    According to the chart the minimum obstruction clearance altitude (MOCA) is 8500 ft. The meteorological data gives an outside air temperature of -20°C at FL 85. The QNH, given by a met. station at an elevation of 4000ft, is 1003 hPa. What is the minimum pressure altitude which should be flown according to the given MOCA?

    this one only requires a temperature correction ?. MOCA 8500ft is QNH ?

    Temperature correction ?. 4% per 10° deviation from ISA

    ISA at FL85 = -2° C, so temperature condition is ?ISA-18 °C?

    Correction 7.2% from 8500ft to DATUM (4000ft).

    Datum in this instance is our met station at 4000ft

    7.2% x 4500 = 324ft

    Is it plus or minus 324ft ? high to low careful go ? temp is low ?. +324ft

    Answer 8824ft.



    Is there something i did not understand?

  • #2
    Re: PB on Q1433

    Not a good question - the explanation tries to fit the answer (which seems to be wrong) ... Just need to remember this one ...

    QNH/1013 correction = 10mb x 30 = 300 ft (+)

    Temp correction: ISA at FL85 = -2°C, so temp condition = ISA -18°C ... temp correction = 4ft x 18 x 4.5 = 324ft (+)

    Minimum Press Alt = 8500 + 300 + 324 = 9124 ft

    Answer not available ... I think because they have referenced Pressure Altitude to 1003 ... Closest answer 8800ft

    Tom

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    • #3
      Re: PB on Q1433

      Yes, I agree with you, I get the same result...

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      • #4
        Hi Tom,

        Sorry if I'm asking dumb question but what's the rationale behind simple comparison of 1003hPa @4000ft to 1013hPa? I would assume that as 1013hPa is at MSL and pressure drops with height by 1hPa per 27ft (say 30ft), so we need to calculate standard pressure at 4000ft, which is 1013-4000ft/30ft=879hPa and then calculate the difference between 879 and prevailing 1003 at that height?

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        • #5
          They are asking for pressure altitude, not the FL, so correction QNH- ISA 1013 not needed here, as we would be flying with the QNH on the altimeter. The temperature correction gives about 300 ft, so 8500 + 300 = 8800. That's my opinion.
          Maciej- QNH is the SL pressure, not the pressure at the 4000 ft. QFE would give you the pressure at the airfield elevation, which is 4000 ft above sea level.
          Rgds,
          Marek

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