Half Scale Deflection

[FlapsFive]

Hello!

I had a search for this but I couldn't find it.

I am currently in the process of writing some software and need a quick pointer...

On an ILS approach, how many degrees off is Half Scale Deflection (for both Horizontal and Vertical, if different)? Is it the same for all standard ILS approaches? (talking major airports... Paris, Heathrow, New York etc.)

Thanks very much! :ok:

FF

[Office Pest]

Well looking at my old books on a standard HSI full scale for the localiser is 2.5 degrees and full scale on the glideslope is 0.7 degrees. So just half that. On the Airbus that I fly the amount of deflection shown is slightly different (please don't ask me why!)

Best Regards

Office Pest.

[FlapsFive]

Perfect! Thanks very much :)

FF

[ft]

Glide path
For a glide path, half scale deflection should correspond to 0.12 times the glide path angle.

Localiser
The displacement sensitivity should nominally give a full course sector 210 meters wide at the reference datum (threshold), or for CAT I localisers serving short runways 210 meters at ILS point B, which is a point 1050 m from the threshold. The course sector can go to maximum six degrees. The scale deflection should be largely linear to the offset from the centerline within the course sector.

In other words, the course sector, sensitivity of the localiser and thus the number of degrees of offset from the centerline which corresponds to half scale deflection depends on the length of the runway served. There are also no guarantees on actually reading half deflection at half offset, as localisers have been known to be only largely linear, as opposed to linear.

Reference: ICAO Annex X.

Nothing is ever simple... :)

Cheers,
/Fred

[FlapsFive]

Ah well, gives me an idea of what to plot!

Thanks very much for your help!

FF

[bfisk]

To add to the confusion; a lot of EFIS equipped airplanes change the sensitivity of the indicator (as opposed to the receiver) once established on the ILS. That is to say; half-scale on capture may be further from c/l than half-scale when established.

[ft]

To add to the confusion; a lot of EFIS equipped airplanes change the sensitivity of the indicator (as opposed to the receiver) once established on the ILS. That is to say; half-scale on capture may be further from c/l than half-scale when established.

That's an interesting feature I have never heard of before. Got a reference? What types? What would control the indicator sensitivity?

[FlapsFive]

This whole thing is only to try and demonstrate to members of the public how good/bad they are at flying an approach in a mock up of a sim on MS Flight Sim; as long as I have some reasonable approximations of the IR requirements for an ILS approach to plot on the graph I am not too concerend!

Thanks again!

FF

[bfisk]

ft:

That's what I can recall from my ATPL manuals, anyway. I looked it up now:

"The normal localizer deviation scale is 1 degree per dot. When the course deviation is approxemately 5/8 degrees deviation (5/8 dot) and VOR/LOC is enganged, the scale automatically expands to indicate 1/2 degree deviation per dot. The scale remains expanded until after landing rollout or on a go-around with radio altitude greater than 200 ft"

Nordian: "Instrumentation: Aircraft General Knowledge", Nordian, 2005. ISBN 82-8107-042-0


Perhaps someone with more actual EFIS experience than myself can add to this.

[ft]

Thanks, bfisk.

As my skepticism towards generalised course literature is by now rather deep, is anyone with access to an AOM able to confirm?

I'd also like to know how said change in presentation is conveyed to the pilot. What feedback does the system give when the sensitivity is changed?

Regards,
Fred

[Denti]

From the boeing 737 fcom (generic one):

The localizer pointer and scale appear at the bottom of the attitude indication when a valid signal is received. When the course deviation is slightly more than one–half dot, the localizer mode is engaged and track is within 5 degrees of the MCP selected course, the scale automatically expands.

This does mean however if you do a raw data approach or an IAN other than LOC/Backcourse that the scale does not expand since the LOC mode has to be engaged. The expansion of the scale is supposedly for CAT II/III operation.

[Wodrick]

Mechanical ADIs such as the Collins FD108 or 109 in a bygone era had an "expanded localiser" scale at the bottom of the instrument, usually hidden behind a shutter and exposed at LOC capture.