Airbus Flight Path Angle

[A33Zab]

Airbus used calculation for FPA = arc tan(Vzbi/GS)

Vzbi = baro-inertial Vertical Speed
(inertial for accuracy in dynamic maneuvers & baro for stability in time)
GS = Groundspeed

Vzbi is computed from Vacc (Vertical acceleration from IR), Hi (Inertial altitude from IR) and Hb (Baro altitude from ADR).


After upgrade to L4.2 standard the FPA is available with BUSS activated as it uses IR only for calculation.


FPAair = θ - (AoAc * cos(μ)) - (SSAc * sin(μ))

θ = pitch angle
μ = roll angle
AoAc = corrected Angle of Attack
SSAc = corrected Side Slip Angle

[C_Star]

Thanks A33Zab, excellent reply (as usual!)

Just to confirm I got it right:
- old ADIRU standard uses VS/GS to calculate FPA
- new ADIRU standard uses AoA and pitch/roll only
correct?


Two follow up questions, if you don't mind
- does it mean that the new ADIRU references the FPA to the air, or is it somehow corrected for wind?
- What's inertial altitude? Is that integrated Vzbi?

[A33Zab]

Hi C*,

Just to confirm I got it right:
- old ADIRU standard uses VS/GS to calculate FPA
- new ADIRU standard uses AoA and pitch/roll only
correct?

No,

Both uses Vzbi/GS to calculate FPA in normal operations.
If no Air Data is available (all ADR OFF/FAIL/FROZEN) the FPA is also not available.

In the L4.2 standard however the FPV returns as hybrid FPA when ALL ADRs are selected OFF (this action results in BUSS activation).
Honeywell calls data hybrid if GPS is involved, so I assume the hybrid FPA is based on IR and GPS data.

- does it mean that the new ADIRU references the FPA to the air, or is it somehow corrected for wind?

FPAair is used to calculate wind.

- What's inertial altitude? Is that integrated Vzbi?

See diagram, inertial altitude = hBI :

[C_Star]

Ok, that explains it completely!

[Take it Easy]

Hi A33Zab,

Very interesting... I couldn't find any reference anywhere... can you please give us a reference? That would be great!

Thanks a lot

[Minorite invisible]

Originally Posted by A33Zab

Airbus used calculation for FPA = arc tan(Vzbi/GS)

Vzbi = baro-inertial Vertical Speed
(inertial for accuracy in dynamic maneuvers & baro for stability in time)
GS = Groundspeed

Vzbi is computed from Vacc (Vertical acceleration from IR), Hi (Inertial altitude from IR) and Hb (Baro altitude from ADR).

Just read this with great interest.

So the Airbus makes a trigonometric calculation to computes its FPA based on an IR based angle zero, the horizon.

If the Horizon is IR based, and the other variables all IR based, with just a small component of ADR based barometric Pressure Altitude to give the IR based vertical acceleration some stability in time (exactly like the Airbus vertical speed indicator), do you think that the PFA must be corrected when being used in cold temperature operations ?

In practical terms, if one wants to do a 3 degree approach but has to cross the FAF at an indicated altitude higher than the published altitude to compensate for temperature, the apparent flight path angle may be 3.2 degrees based on indicated temperature but the real angle based on true altitude remains the same at 3 degrees.

Considering that the Airbus is using trigonometry to compute an angle based mostly on IR data, should the FPA selected by the pilot be the real angle (3 degrees) or the apparent angle (3.2 degrees)

[Goldenrivett]

should the FPA selected by the pilot be the real angle (3 degrees) or the apparent angle (3.2 degrees)

Since

Vzbi = baro-inertial Vertical Speed
(inertial for accuracy in dynamic maneuvers & baro for stability in time)
GS = Groundspeed

Vzbi is computed from Vacc (Vertical acceleration from IR), Hi (Inertial altitude from IR) and Hb (Baro altitude from ADR).

then you should use the apparent 3.2 degrees due to the baro input.

[frankice]

Really not clear how Vzbi is computed, why is Baro altitude taken into account? Why is the inertial altitude taken into account, shouldn't the vertical acceleration from the IR suffice?