Re: FMS VNAV and altimeter setting.
MP, PM thanks,
My interest comes from considering that the so called ‘much safer’ VNAV profile approach may not be as error proof as an ILS if the QNH is mis-set.
Because the ILS GS is ground based, the glidepath is always correctly positioned above the terrain. So an altitude/range check at GS capture/FAF or at the marker, should detect an incorrect QNH setting; the aircraft will have not intercepted the GS at FAF or it will be at the wrong altitude at the marker.
A ‘baro based’ VNAV approach has no such protection due to the descent profile being defined with the aircraft QNH setting, which if incorrect could result in a hazardously low flight path. Furthermore, the procedure altitude/range checkpoints will indicate that the aircraft is on the profile, but it will not be.
Many VNAV approaches are at airports without radar, and I believe warning systems such as MSAW are also based on mode ‘C’ which is similarly affected by an error in QNH setting. Could an ATM Ppruner confirm this?
Using the Rad Alt will help, but this is only really effective at lower altitudes, i.e. on an NPA the Rad Alt should always be greater than 1000 ft before the IAF, 500ft before the FAF and greater than 250ft before MDA; I assume that VNAV profile approaches are similar, is this correct?
Specific SOPs/checks/instrument scans are required to detect a QNH error, e.g. there is little value in checking that the QNH settings agree if the initial value is wrong. The procedure should require that both crew members independently obtain and set the QNH before crosschecking. It is important to avoid the practice of one pilot obtaining the value from ATIS and telling the other what it is. This weakness is also subject to controller error in providing an incorrect value. Perhaps the best gross error check is to use the preflight destination landing forecast.
Therefore, although advanced technology has improved safety by reducing the risks of NPAs, it might, without additional crew vigilance, increase the risks of a VNAV approach in comparison with an ILS.