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QFF versus QNH

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  • QFF versus QNH

    I am just not getting these calculations at all. Can anyone help with the best way of visualising and solving querstions such as:-

    The QFF at an airfield in California located 69 metres below sea level is 1030 hPa. The air temperature is 10?C lower than a standard atmosphere. What is the QNH ?

    A. More than 1030hPA
    B. 1030hPA
    C. Less than 1030 hPA
    D. Not possible to give an answer

    I went with less than 1030 hPA but to be honest I dont really understand how I should go about the answer!!

    Can anyone help?



  • #2
    Re: QFF versus QNH

    I just remember it as follows:

    If the temp = ISA conditions
    QNH = QFF

    If above MSL
    If the temp > ISA then QNH > QFF
    If the temp < ISA then QNH < QFF

    If below MSL then this is reversed:
    If the temp > ISA then QNH < QFF
    If the temp < ISA then QNH > QFF




    • #3
      Re: QFF versus QNH

      Thanks Merritt.

      I would still like to understand what I am missing though.

      Any ideas anyone?



      • #4
        Re: QFF versus QNH

        Could one of the instructors maybe help with this? I think if I could understand how the calculation from QFE is carried out and how the calculation from QFE to QFF is carried out it may help. I try drawing diagrams to visualise QNH/QFE issues etc but I just get lost.

        I seem to have a real blind spot with this and it needs sorting before I can confidently move on.



        • #5
          Re: QFF versus QNH

          Hi dunny,

          I am writing this off line, so it may be overtaken by someone quicker off the mark.

          QFF and QFE problems are all linked to the fact that in colder denser air pressure changes more rapidly as you go up or down.

          Imagine you are on an airfield at 1000ft elevation and at 1000mb and you want to calculate the pressure at msl. In round figures for ISA conditions, at 30ft/mb you would make it 33.3mb higher at 1033.3mb. This is your QNH.

          If the air is colder than ISA you get more rapid pressure changes, and we?ll use 20ft/mb for convenience. Now, using ambient temperature and this ?real? lapse rate we calculate the pressure at msl and it comes out at 50mb higher at 1050mb This is your QFF.

          Now QFF is our best guess of the actual pressure at msl, but QNH is a lower pressure and zero on the altimeter with QNH set will leave you well above msl. So, in cold conditions QFF is a higher pressure setting than QNH but the QNH pressure level is at a higher altitude. As an aside, this means you can safely fly down to zero indicated altitude with QNH set and not hit the sea.

          If you look at this upside down, from an airfield below msl it means that now QFF is at a lower pressure than QNH. QNH set will give you a negative height and as you climb to zero on QNH you will still be below msl

          The General Rule is that in below ISA temperatures you indicated altitude on QNH will always be nearer to your station level than your true height

          This works for obstacle clearance as well. If you are climbing over a mountain in cold conditions your indicated altitude will put you nearer to your station level, which is lower than your true height

          This has been about cold air> If the conditions given are for temperatures above ISA then reverse the logic!



          • #6
            Re: QFF versus QNH

            Im guessing that the reason for the confusion is because the QFF reference changes dependent upon the pressure level whereas QNH does not. Remember that QNH is a sea level reference based on ISA conditions. QFF is a sea level reference based upon actual conditions. The relationship between QFF and QNH therefore changes depending upon the pressure and temperature.

            The only thing you need to remember really is the above MSL case in warmer than ISA conditions:

            If temp > ISA QNH > QFF

            you then know that colder than ISA is

            If temp < ISA QNH < QFF (note the < arrows point in the same direction)

            the below sea level case is then opposite.

            Thats how I make sense of it at least!



            • #7
              Re: QFF versus QNH

              Cheers guys. It makes a little more sense but I need to work more to cement it.



              • #8
                Re: QFF versus QNH


                In ISA conditions QNH = QFF; the setting which gives airfield elevation at touchdown (QNH) = the atospheric pressure at sea level (QFF). If it is warmer than ISA the theoretical column of air between the airfield and sea level would expand and the QNH would occur below sea level which means that QNH would be greater than QFF. Similarly, if it is colder the ISA the column of air would contract and the QNH would be less than QFF.