airsp
19th Jan 2012, 17:32
Hi there,
Just wondering on the A320 or even other aircraft, which angle deviation correponds to one dot deflexion on the glide slope ?
and for the localiser ?
Cheers
Checkboard
19th Jan 2012, 18:23
http://www.pprune.org/engineers-technicians/420878-glideslope-dots.html
Without getting too technical, it depends on the glidepath angle. For the Glidepath, full scale deflection of the CDI - five dots or 2.5 dots depending on the type of instrument - is defined by the Displacement Sensitivity of the Glidepath. This is known by ILS tecchies as the 'Width'. It is dictated by ICAO Annex 10 as ±0.48 Theta, where Theta is the Glidepath angle (between 2° and 4°). For a 3° Glidepath angle, this works out for full scale deflection as 4.44° (full scale fly down) and 1.56° (full scale fly-up). However, linearity across this span is not guaranteed. It is reasonably linear to half scale but outside that all bets are off! For this reason, the engineers and flight inspectors measure at ±0.24 Theta. However, please bear in mind that despite what the CPL textbook diagrams show, a glidepath is not sharply defined. It is produced by the mixing of signals reflected off an imperfect surface (the ground in front of the glidepath) so if you imagine the quality of the reflection that you see in a creased and crumpled piece of aluminium foil that has been flattened out, you might get some idea of what the glideslope signal 'looks like' in reality.
http://www.pprune.org/tech-log/455385-full-scale-deflection-cdi.html
The LOC course sector width does depend on runway length, and you are correct that the nominal width is to have full deflection 105 m (350 ft) either side of the threshold*.
The width is, however, limited to six degrees. On shorter runways, this creates a problem which is solved by instead having full deflection equal to the same displacement at point B, which is 1050 m (3500 ft) from the threshold.
This is specified in ICAO Annex 10 Vol I §3.1.3.7.
The 2.5 degree figure often given is just another "close enough and won't confuse the public" figure.
Cheers,
Fred
*) Or the ILS reference datum, if we are to be strict about the definitions. That's where you'll be when crossing the threshold.
The bit about the glide path quoted above is incorrect, I'm afraid. Try this one (http://www.pprune.org/archive/index.php/t-359552.html) instead.
For a glide path, half scale deflection should correspond to 0.12 times the glide path angle.
Can't really agree on the poor shape of the GPs either. Sure, there can be plenty of structure but when within tolerances they provide good guidance - as they should.
That geezer rambling on about the LOC got it about right though, as far as I can see Bet it was more dumb luck than anything else... ;)