Airbus Flight Path Angle
 

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!

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.

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

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

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

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....

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

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.

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!

VDEV use
 

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

C_Star
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.

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?

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.

Airbus Flight Path Angle
 

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?

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?

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.

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.

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...

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

This may help..

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