Bird On the Electric Bus

[YankeeGolf]

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:

[javelin]

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.

[tom775257]

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

[shortfuel]

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.

[foff]

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


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.

[shortfuel]

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.

[YankeeGolf]

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:

[shortfuel]

Yes.

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

[TyroPicard]

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.

[BOAC]

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'...?

[tom775257]

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.

[Max Angle]

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.

[BOAC]

The track line in a Boeing is just as much influenced by IRS drift as the similar displays in an Airbus. - 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.

[shortfuel]

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?

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.

[TyroPicard]

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.

[BOAC]

Since when you track an ILS with IRS or FMC?- 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:

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. (Call for a trainer:)) A very small correction (+/- 1 or 2 degrees) on the FCU selected track should compensate that drift.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.

[shortfuel]

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

ME - NEVER! The quote was yours
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:
crikey! I will definitely stick to Boeing, then.
I suggest you stick yourself to it :)

I'll stick to doing it my way, I think
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.

do REAL pilots actually use this for an ILS?
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-

Each pilot's FPV (bird) displays the track calculated by the onside IRS

Do you have a reference for that?

[TyroPicard]

Fctm 04.020 P5

[shortfuel]

...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...

[BOAC]

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

[FlightDetent]

BOAC, it's the subtle differences that when not thoroughly understood make the debate heated sometimes. Unlike 737 Classic (never flown NG) AB has pilot selectable TRACK function that can be used in lieu of normal HDG lateral mode.

By definition the TRACK reference mode will adjust for drift in changing wind conditions - within the technical limits of a man-made design. This actually answers the original question.

Coupled to TRACK mode is Flight Path Angle mode that does pretty much the same thing in vertical plane. So you have a choice to use either HDG-V/S or TRK-FPA. In TRK-FPA reference a small trajectory symbol is superimposed over PFD called flight-path-vector, commonly referred to as "bird".

Anybody could fly ILS beam using TRK-FPA (or HDG-V/S) if there were a reason to do it except of that being a grossly silly idea. OTOH, TRK-FPA (or bird) is designed for NPAs - for NDB APCH once on final track you select TRK to appropriate figure, at FAF FPA do required value (i.e -3,3 deg) and irrespective of wind and GS changes everything pretty much takes care of it self.

The subtle difference from 737 (suppose) is the fact that AB designers decided not to introduce VOR-beam (radio based) tracking function to FD. Instead of VOR/LOC pushbutton AB only has a LOC pb. So to track a VOR radial the procedure is to activate "bird" and once established inbound set INB CRS and in theory no further adjustments are required.

In real life small adjustments within 1-2 deg are required to keep the VOR raw data centred. Possible reasons:
- radial is scalloping so changes needed to follow the imperfection
- radial is orthodromic (curved track) by definition whereas TRK flies straight ahead
- once displaced (by gust that was not perfectly compensated by the guidance and AP due to AC inertia) from the ideal centre, TRK would continue parallel to radial unless corrected
- momentary drift of the commanding IRS

The IRSs drift exactly the same as on Boeing, probably owing to the fact that they are identical piece of equipment. Residual GS when stationary on ground is typically 1-3 kt. If such drift is 90 deg to desired track to be flown it indeed needs to be compensated conventionally. Of course this doesn't affect the NAV accuracy / FMS position as that is derived from MIX 3 IRS data that is constantly updated by DME/DME and then pinpointed through dual GPS with RAIM. Again, I would be surprised if Boeing system were any different.

Hope this removes some of your fears :ok:

Yours,
FD (the un-real)

[BOAC]

Thanks FD - a clear and helpful explanation. It was the 'silly idea' of flying an ILS in FPV that was really confusing me! Didn't realise you poor deprived folk could not F Dir a VOR radial (or presumably couple?).

You will no doubt recall from your 737 days that the HDG/TRK selection was only Pin selectable by avionics, but that NPAs were a 'doddle' and flown using the track line as a 'sheepdog' on the Nav display to 'corral' the NDB pointer, which removed any mag variation or IRS errors.

As an aside, the WORST 737 IRS error I ever had was 122kts on stand caused by an incorrect 'part number' IRS unit.

Fearing less now...:)

[shortfuel]

FD -Interesting compared explanation

if there were a reason to do it except of that being a grossly silly idea

Can you develop this? What and why is so silly to fly an ILS with the bird?

[CONF iture]

Also disagree on that one FD :
The Bird is a great tool to fly an ILS too, even highly recommended in degraded level of equipment such as ELECtrical EMERgency CONFIGuration.

[TyroPicard]

BOAC
Yes, countless people do forget the 'errors' of the VOR signal, although I have never seen anything like +/- 5 despite the caveat.You should have got out more, dear boy - Khartoum, Addis Ababa, and almost anywhere in West Africa all spring to mind.

FD
momentary drift of the commanding IRSSurely if the AP is engaged the guidance bit of the FMGC is providing steering commands - not the IR? And will it make any difference to the accuracy?

and OTOH, TRK-FPA (or bird) is designed for NPAsI tend to think of it as designed for raw data flight, not just NPA's.

[BOAC]

Khartoum, Addis Ababa,- nice offer, TP, but I'll say no to those. I think +/- 5 was not quite the story there?

[FlightDetent]

In attempt to explain I have mixed together modes of guidance (NAV(LNAV), HDG/TRK, LOC; DES(VPTH), OPDES(LCH), VS/FPA, GS) and reference (attitude or flight path vector). But first for a few mistakes I spotted afterwards.- radial is orthodromic (curved track) by definition whereas TRK flies straight aheadExactly the other way around. Radial (or any other signal) is orthodromic i.e. straight line, and so is track. Constant heading would be curved line that requires changes to follow signal over significant distance.flown using the track line ... which removed any mag variation or IRS errors. On a second thought do not think the IRS part is a technical fact. We want to fly a certain direction which is displayed by track indication on ND (same on Bus) but the target shown is not actually where the aircraft is going, but where the IRS think it is going and affected by momentary drift. This is witnessed by residual GS, or when in LOC mode by the fact that track indicator does not always really overlay the radio signal mark, or that the track (and heading) readout differs between CM1 and CM2 instruments. Flying track reference by eye (Classics&AB) or chosen regime FPV (AB) does not exempt us from errors of IRS drift, however miniscule.

shorfuel / CFi: What I meant to say was that normal OPS ILS is flown in Flt Dir mode LOC-GS and not HDG-V/S (TRK-FPA). I included this due to to somewhat confused discussion in earlier posts. Of course all fellow Borg know that hand flown ILS with F Dir guidance is best done with crosshair LOC-GS and raw data approach should be aided with flight path vector on PFD. (Hey, what kind of practice is that?:ooh:) Abnormals are a different cup of tea.

NAV accuracy / FMS position as that is derived from MIX 3 IRS data that is constantly updated by DME/DME and then pinpointed through dual GPS with RAIM. Possibly confusing. Mix 3 IRS position is compared with DME/DME fix pinpointed by dual GPS. The DME/GPS fix then becomes the FMS position. Constantly updated error vector between mix IRS and DME/GPS pos (bias) is remembered. Should you lose radnav updating and gps then bias is applied as a correcting vector to mix IRS reference in order to calculate the perceived aircraft (FMS) position.

http://img194.imageshack.us/img194/2111/mixirs.th.gif (http://img194.imageshack.us/my.php?image=mixirs.gif)
http://img196.imageshack.us/img196/794/radiopos.th.gif (http://img196.imageshack.us/my.php?image=radiopos.gif)
http://img195.imageshack.us/img195/2962/bias.th.gif (http://img195.imageshack.us/my.php?image=bias.gif)

Tyro: I believe that track is a primary output from IRS to ND/PFD heading tape. Hence the drifting of IRS affect the readout directly as witnessed on ILS approach and by comparison between onside and offside instruments.

Food for thought, thanks gentlemen.

FD (the un-real)

[shortfuel]

FD - now I see there was a misunderstanding ;) As you said flying an ILS with TRK and FPA guidance modes active on FMA (i.e A/P ON) would be a stupid idea (or HDG-V/S btw), but CONFiture and I were referring to flying an ILS with the bird (FPV) without A/P.

Your posts are interesting.

As for the momentary drift of IRS: I think the drift is anything but momentary.