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 ahead
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.
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?
) 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.
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)