Using Airbus FLS in High Temperatures
Did a FLS approach the other day in temperatures well above ISA and it left us quite high on the approach. This left me wondering about the FLS performance with regard to vertical guidance in high temperatures.
The FCOM clearly states that FLS only compensates for temperatures below ISA but nothing is mentioned with regard about the opposite case.
In the traditional managed-selected case for a non-precision approach a pilot that correctly flies the approach using the distance-altitude crosschecks will remain high on the approach if temperatures are above ISA, but the difference between indicated and geometric altitude would diminish as he descends. E.g. for an airport at sea level with ISA+25 and a platform height of 3000ft the plane would start the approach at 3300ft (+300ft) but by 700ft would be at 770ft (+70ft). This would be easy to correct.
I was then wondering if this principle also applies to an FLS approach. As far as I can tell FLS creates a 'Beam' from the anchor point using the glide angle of the approach that is coded in the FMS. The aircraft then intercepts this point from the correct distance. For a 3 degree slope the interception at a platform of 3000ft should be at 9.4 DME from the threshold. The aircraft then flies a Pseudo-G/S on a computed beam. The real question here is how exactly does the FLS calculate the beam and its own performance on it? And how is this effected in high temperatures?
If the aircraft simply calculates the beam using distance vs. altitude and then cross checks the aircraft's position on it then I see that there should be no difference in how the aircraft flies as compared to the managed-selected method. However, when we flew the approach we were distinctly higher vs. the PAPIs (and yes the PAPIs and the FLS Beam were an identical 3 degrees) than I would have expected on a managed-selected approach. In fact by the time we had reestablished on the PAPIs the F-G/S was almost at full scale deflection.
Thanks