Pressure error correction....correction

[Dick Whittingham]

Both QNH and QFF start off as the observed, actual, pressure at the station. QNH is then calculated (the met term is "reduced")down to a theoretical sea level on the asumption that ISA temps apply. The easiest way to do this is to set your altimeter to station elevation and read the subscale. This is QNH. Thus, regardless of temp or pressure at the station, with the current QNH set the reading on your altimeter at touchdown remains the correct station elevation.

QFF is different. QFF is station pressure reduced to a theoretical sea level using station elevation and the single recorded station temperature. In other words it assumes isothermal conditions from the station down.

In ISA conditions QNH should get your altimeter reading zero at msl. In cold conditions (ISA minus) your altimeter will read zero before you reach msl on the way down and vice versa. QFF, as the calculation changes to take account of station temperature, will be reasonably accurately at msl most of the time. For this reason QFF is plotted on msl forecast charts.

Note that when climbing above the station cold = danger but in desending below the station cold = safer

Dick

PS. Yes temp error calculations should take into account the temperature profile in the entire column of air, but in practice we take the ISA deviation at the indicated height and apply Genghis's rule of thumb