Bird On the Electric Bus
 

Senerio:

Airbus, Flying reference is Bird.
On Selected Approach to Track an Inbound VOR radial of 180 degrees (inbound track of 360 degrees)

Once ESTABLISH on the radial inbound. TRK selected will be 360 degrees. No further input is done after this.

Question:

If the wind CHANGE will the Bird Keep the same VOR radial track? or will it get blown off and parallel track the radial?

:confused:

It guides you to fly the selected track. It will not track the radial.

If you move away from the radial by handling or over compensating, you will parallel the radial.

In practise, it does the job very well and provided you cross check with the needle from time to time, you will be fine.

To track the radial, you would need an FMS2 box with course out/radial in function - then you can define the radial and track it in NAV.

But yes it will keep flying the selected track, with varying heading in case the wind changes.

Your tracking of the radial will be less influenced by any wind changes than the IRS drifts themselves.

In reality, it is quite common to see an IRS drift after a couple of hours:
Hence yor bird will "only" be + or - 1 or 2 degree accurate.

So when correctly tracking a radial or an ILS, it's normal to see you get slowly off track, even with no wind. A very small correction (+/- 1 or 2 degrees) on the FCU selected track should compensate that drift.

what's that ? is it the new theory of drift due to wind ?
you're mixing two completely different situations.
first deals with change of heading to maintain a desired track due to wind, second is to compensate for IRS drift by amending the desired track !!

to answer the initial question, yes if you're flying manually the bird will be blown off by the increasing wind and you'll have to amend heading to realign bird and vertical blue track line. not doing that will create a deviation from the track increasing with time.

Foff, before jumping on people, try to understand my message...

First, initial question was already answered by Tom...and yes you need to align the bird with the blue deviation bar to follow a track whatever the wind(!) as you nicely said.

I was just adding (not mixing :=) a case here where when established on a track, you can see yourself slowly blown off...due to IRS drift and not the wind. That's it.

So answer to the question is:

Once ESTABLISHED on an inbound radial 180 degrees, this is on Autopilot, the 360 track selected for the Bird will KEEP this inbound radial, EVEN if the wind drastically CHANGES? Correct me if I get all of you wrong. :8:cool:

Yes.

But depends on what you call drastically...Turbulent and gusty?

To be picky, the a/c will maintain the selected magnetic track, within the limits of a human-designed system. That may or may not result in maintaining the radial - the VOR system accuracy is ± 5˚, some radials bend, shortfuel's IRS drift remarks are valid.

- crikey! I will definitely stick to Boeing, then. :) I've never seen an 'IRS Drift' which will take me off an ILS in the few minutes I am tracking the LOC (not since the Harrier GR3's rubber band map system which used to leg it into East Germany at high speed with monotonous regularity)..... + of course, FMC position update negates IRS drift, surely?

PS What is 'bird'...?

BOAC, the bird is a visual representation of your flight path vector (FPV), laterally showing your track, and vertically showing your flight path angle (FPA).

We can either select Heading/VS mode OR Track/FPA. With Track/FPA we have a little visual representation of the aircraft showing your FPV that appears.

Generally we use Track/FPA mode if flying a NPA or flying visually. We also have a flight director associated with the bird, but it isn't very good to fly with manually. So, generally we fly an NPA with the bird on, the FD's on (known in this case as the flight path director) AND the autopilot on; or autopilot off, FDs off. e.g. you get on your inbound QDM, you are in track mode, so you are laughing all the way down. Regarding FPA, 0.3nm from the FAF select 3 degrees down, and it will fly down beautifully. So varying speed or wind doesn't really affect you with AP on, without AP on you'll have to turn the aircraft to keep the bird on the selected track. Don't get onto managed NPAs for now!

Quite handy also, if you put the top of the tail of the bird on the 0 degree line on the PFD, you will be going down at a flight path angle of around 3 degrees.

The track line in a Boeing is just as much influenced by IRS drift as the similar displays in an Airbus. The "bird" is the velocity vector icon that is displayed on the attitude indicator whenever the pilot changes from selecting heading to selecting tracks.

- I think you'll find that in the Boeing 'Track' is computed from the FMC position, ie normally DME/DME or GPS? In any case, you've either got an INCREDIBLY powerful VOR or LOC if you can get ANY noticeable IRS drift in the time span available (or a crappy IRS...................:)).

EDIT: ACTUALLY I'm not sure about Boeing's 'Track', but I have NEVER seen IRS drift of the magnitude you are suggesting on a 737!)

The 'Classic' 737 suffered from an 'out-of-date' FMC mag variation database which would give you 'drift' in still air while tracking a radial and 'offset' the localiser a degree or 3 plus change the runway QDM for you.

Since when you track an ILS with IRS or FMC?...


Anyway, bus skippers here got my point ;)...and the drift I am referring to is typically as "big" as 1 or 2 degrees.

This drift on the bus, while tracking with the bird, is enough to make you fly within one dot of your LOC deviation scale.
Some people would think they have a crappy handling, but it's not all their fault.

Corrective action:

-Identify the drift: established on the LOC and accurately tracking your runway track, if your localizer deviation index keeps having a tendancy to go left on your scale, you may have a drift of +1°
-Fly back on the localizer and track now your (RWY TRK - 1°)

For flights within 4 hours, 1° correction should be enough.

Not convinced?
On your next A/P flown ILS, check your Actual Track Symbol (green diamond) compared to the ILS course pointer on your PFD heading scale.
If there is no IRS drift, they should be perfectly aligned.
Most of the time, they're not. You've just noticed the drift.

Greetings BOAC..
In an attempt to introduce clarity I offer the following to everyone...

Each pilot's FPV (bird) displays the track calculated by the onside IRS, and is subject to IRS errors which Airbus say may be up to ± 2°. When maintaining a track in manual flight you may therefore need to make a small correction in order to fly the required track.

When the AP is engaged as per the original question the AP uses the onside FMGC to fly the selected track (in the FCU window). I cannot find an FCOM reference but I assume this will be the updated value. The FPV will still display the onside IRS track. The a/c will maintain the track precisely - I reckon any observed error in following the radial is more likely to be due to VOR system errors than anything else.

Summary: the FPV may have errors, the AP will be very accurate.

- ME - NEVER! The quote was yours, not mine! If you MUST use fancy gismos to fly an ILS......................do REAL pilots actually use this for an ILS?:eek:

TP - thank you for the sanity. I'll stick to doing it my way, I think:). Yes, countless people do forget the 'errors' of the VOR signal, although I have never seen anything like +/- 5 despite the caveat.

BOAC - You must really be bored to stick yourself into a very tiny, very type specific thread. Type that you obviously doesn't have.

I said something close to "you track your runway track with your FPV. This FPV is fed by drifted IRS".

As you began your first post with your own conclusion:
I suggest you stick yourself to it :)

Yeah, try it our way on Boeing...:confused: not possible, (i think) you don't have that function!


Now, if you are curious, you can always ask. But please, don't feel frustrated. Even the majority of airbus pilots never heard about what I am talking about.
They may have read it somewhere in the FCTM, but very few noticed it in real life.

Don't worry, "REAL pilots" as you say, disconnect A/P at 500 feet after a 10NM level off at 2000ft and full automated ILS approach :E

The others, not so "REAL pilots", sometimes use the bird to fly raw data ILS.



TP-

Do you have a reference for that?

Fctm 04.020 P5

...well, you cannot tell from this that each FPV is fed by onside ADIRS...that's my point. So where this info comes from.
Actually, I would be grateful if you have it. I don't wanna dig into AMM :yuk:


FCTM extract:
The FPV is computed from IRS data, therefore, it is affected by ADIRS errors.
An error may be indicated by a small track error, usually of up to ± 2°. This
can be easily determined during the approach.
The FPV is also computed from static pressure information. Therefore, the bird
must be considered as not reliable, if altitude information is not reliable.


EDIT: I got it: Vol 1.34.10 page 1 ADIRS schematic.
That's right, in normal ops, ADIRU1=>...=>FPV1, respectively ADIRU2...

Hmm - I wonder what that FPV selector does on the 737...........?