Airbus Flight Path Angle

[killer.a]

Hello everyone,

I was trying to find out more info on the FPA and how it works, but couldn't find much useful information on the net or the FCOMs.

Specifically I want to know how the wind affects the FPA selected?

Is my assumption here correct?

If at 10 dme and at 3000' I select a FPA of 3 degrees. The aircraft will fly a constant 3 degree angle down to a specific point on the ground. In the case of a tailwind, this point on the ground will not move forward nor will the descent angle become shallow.

As the tailwind increases the GS increases and the ROD will automatically increase to keep this 3 degree angle?

If there were some updrafts that pushed the aircraft 30' above the 3 degree angle, every subsequent dme vs alt check, I would be 30' high. Therefore causing my 3 degree FPA to be parallel with the initial 3 degree FPA, causing the original specific spot on the ground to have moved forward.

Thanks for your time and thoughts. I would also appreciate it very much if anyone can provide any references that I am looking for.

Cheers!

[CaptainProp]

Yes, the aircraft adjusts the rate of descend required to maintain the selected FPA.

In the case of "updraft" you would have to adjust your FPA to get back on your profile. This is why, on an approach, we monitor distance vs altitude. To regain 30' in 1 NM you would need to correct .3 degrees.

[Lord Spandex Masher]

Erm, why wouldn't the aircraft maintain the selected FPA in an updraft? <rhetorical>

[CaptainProp]

I think he / she meant "if you end up getting high/low on profile"...

[Trackdiamond]

Also have to monitor the VDEV function to maintain the required FPA..

[CaptainProp]

The VDEV is only correct if the vertical profile is correctly programmed in the FMS and should be used with caution when flying selected (vertical) approaches with FPA.

The vertical profile from the approach plate is the profile you "have to" monitor, not the VDEV. Having said that, the VDEV gives you a good reference if available and indicating correctly....

[Trackdiamond]

Thanks for the added clarity.I was fixated on the approach plate FPA as reference.I concur.

[vilas]

Trackdiamond
If NAV ACC is high and approach plate parameters are confirnmed in the MCDU and not altered by the pilot you can do managed approach and Vdev will be correct, even if you do selected approach.

[C_Star]

Hi,

Expanding the OP's question - Does anyone know how FPA is actually calculated/derived by the ADIRU?

Is it based on:

a) AoA measurement - ie. PITCH-AoA=FPA (but then, it wouldn't take head/tailwind into the account), or
b) derived from GS and VS?,
c) a mix of above, or something else altogether?

Cheers!

[Trackdiamond]

Villas..correct.That is why yiu have to check NAV accuracy before commencing approach.

[TyroPicard]

C_Star
Does anyone know how FPA is actually calculated/derived by the ADIRU?
Yes, airbus know.
Pedant mode off... The IR portion of the ADIRU calculates the FPV... and as you know that uses attitude and accceleration info and does clever sums.

[OPEN DES]

Previously the Old QRH in the Unreliable Airspeed Procedure made reference to the FPV not to be used if remaining altitude (air data) was unreliable. This always made me believe that there was some sort of additional air data input to the FPV next to just IR.
Any thoughts?

[HazelNuts39]

Open Des,

Vertical speed is calculated as a 'blend' of inertial and air data. Inertial for the short-term variation and barometric for the longer term.

[OPEN DES]

Hi Hazelnuts,
Thanks for your reply. I am fully aware where the v/s is derived from.
The old QRH made a clear case of when remaining air data is affected: do not use the FPV. This suggests that the bird uses some sort of air data input as well. Any thoughts?

[C_Star]

I was told by one very knowledgeable Gentleman, that the main parameter used to derive the FPA is the AoA and was shown some drawings explaining how it worked. Seems plausible to me and actually quite clever - but I am looking for some official reference.

The only problem I have is that in this case the FPA would be referenced to still air - ie. headwind/tailwind component wouldn't affect it, only TAS and VS. Does anyone know if this is the case?

[TyroPicard]

C Star

Don't ever put your faith in knowledgeable Gentlemen... read the bleedin' FCOM for yourself... extract below. The difference between Pitch attitude and FPV on your PFD is a very good visual indication of AoA .. this works whether the FPV is level, climb or descent. Does not work the other way round, i.e. the IR cannot calculate FPV from the other two parameters.

"Each ADIRU is divided in two parts, either of which can work separately in case of failure in the other:
• The ADR part (Air Data Reference) which supplies barometric altitude, airspeed, mach, angle of attack, temperature and overspeed warnings.
• The IR part (Inertial Reference) which supplies attitude, flight path vector, track, heading, accelerations, angular rates, ground speed and aircraft position."

The IR part may well use info from the ADR part in it's calculations .. but as a practical pilot I couldn't give a rats about that... life's too short.

[HazelNuts39]

I suspect the ' very knowledgeable Gentleman' was not familiar with Airbus FBW airplanes. Shortly after joining this forum I made the error of assuming FPA relative to air, and it was quickly pointed out to me by another ' very knowledgeable Gentleman' that it is relative to earth.

[C_Star]

The "Very knowledgeable Gentleman"in question was actually an experienced Bus TRE - but that's actually irrelevant to the discussion.

Thanks Tyro for pointing out the FCOM reference - it's quite clear on what the FPV is derived from. Just wondered if there's more to that - as there often is with FCOM info...

[HazelNuts39]

The IR portion of the ADIRU calculates the FPVAnd the IR portion doesn't know the AoA - right? (Except on BUSS-equipped aircraft with the ADR's switched off).

[underfire]

This may help..

http://i44.tinypic.com/2my930w.jpg

[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!) :ok:

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 :

http://i474.photobucket.com/albums/rr101/Zab999/InertialAlt_Vzbi_zps2dc90482.jpg

[C_Star]

Ok, that explains it completely! :ok:

[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]

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?