HazelNuts39, Chris Scott;
Originally Posted by HN39
Although I am used to defining FPA relative to the airflow, the mention of 'drift' should have alerted me to the fact that FPV on the PFD is relative to the ground.
Originally Posted by Chris Scott
My response to HN39 has belatedly reminded me that AoA CANNOT accurately be derived from a comparison of Pitch and FPA (flight-path angle): it is only strictly true in a stationary air mass.
Exactly right, HN39.
So, with reference to Chris Scott's remark, where are we?
We have two cases: One in which the crew sets the FPA and the aircraft follows it under autoflight/FMGC guidance, and one in which the airplane is descending "normally" under autoflight/FMGC guidance either in the "OpDes/FltLvl Chg" modes which are the Airbus and Boeing settings respectively, or a form of VNAV which is typically altitude or airspeed constrained (not following an FPA in other words) and referenced to the destination airport taking into consideration the entire flight planned path entered in the FMGC.
Just so everyone is aware, the use of FPA for descent from cruise is almost never used in routine operations. Some here who don't fly but who understand what's going on here may know why right away but if not its discussed below.
Discussion points numbered for reference only:
1. First, when an FPA is selected by the crew on the autoflight control panel, (I believe for all types and not just the A330), it is defined by and referenced to the inertial platform, (essentially, the aircraft) and is not referenced to the airmass or the aircraft groundspeed. Once we set, say "3.0" in the FCU (Flight Control Unit), the FPA is a physical descent path.
2. Now I understand that airmass characteristics such as temperature and humidity do affect barometrically-referenced data. It is my impression that this data would indeed be corrected for "local" deviations from standard, (understand the comment that the ADIRU's/ADC's cannot know what airmass characteristics are below the aircraft).
3. In other words, while I understand that a fine-grained aerodynamics analysis of the notions set out in para.1 would disagree, neither a pilot nor the autoflight system nor the instrumentation on the PFD/Standbys can expect as an outcome, "3-decimal accuracy" in flight, even though the guidance equipment will likely be sending such fine-grained data to the autoflight system; - it's just not possible with the airmass variations and aircraft inertia involved. But the "approximation" is, (we have to assume, because its all certified and we do it all the time), good for all practical work both enroute and during the approach/landing on, for example, the non-precision FPA approach described above.
4. So back to FPA and AoA, let's assume the crew has
selected a 3.0 FPA descent: As the wind component changes and as temperature (and humidity) changes during the descent minor adjustments are made to pitch to maintain the 3.0deg FPA - that means that the IVSI, (Inertial Vertical Speed) changes and of course the AoA may change slightly as well. In other words, the autoflight system will increase/decrease descent and in the latter case even add power to maintain the FPA set by the crew. You can now readily see why we don't use FPA for a routine descent. Both pitch and power are all over the place trying to maintain the theoretical path in space. In busy air, it's a mess and not very smooth for the passengers.
5. In the case where the FPA is set by the crew and the autoflight system flies it, the FPV symbol will be close to the setting because that's what the autoflight system is designed to do. When under VNAV or OpDes/FLCHG guidance, the FPV symbol will still indicate the actual FPA, whatever it is in the VNAV-guided descent.
6. The key here is, even as the descent is taking place the FPV symbol on the PFD indicates actual FPA for the aircraft in real time and so may be (guessing here guys) reliable as a practical working AoA.
7. I'm not sure if this got us anywhere in terms of deciding whether the algebraic difference between displayed pitch and displayed FPA by the FPV symbol is accurate in all cases, (the non-still-air case as described by Chris) but it may provide some grist for further thought. PJ2