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)

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...:)

FD -Interesting compared explanation

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

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.

BOAC
You should have got out more, dear boy - Khartoum, Addis Ababa, and almost anywhere in West Africa all spring to mind.

FD
Surely 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 I tend to think of it as designed for raw data flight, not just NPA's.

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

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.Exactly 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. 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.

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://img196.imageshack.us/img196/794/radiopos.th.gif
http://img195.imageshack.us/img195/2962/bias.th.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)

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.