ILS question

[RedKnight]

Trying to get my head around this question but I don't seem to agree with the prescribed answer.

Qn: You are established on the localiser to the aerodrome on a heading of 220. If you adjust the dial of the OBS to read 110, what will happen to the position of the CDI needle?

Ans: Nothing

That intuitively does not make sense since I would expect a CDI deflection commensurate with the degree by which the current heading deviates from the intended/selected track on the OBS.

Thoughts please?

[BackPacker]

The OBS instrument is used for VOR and ILS tracking (and sometimes GPS as well).

The OBS ring is only used when you track a VOR, to establish the VOR inbound/outbound radial. For an ILS the inbound radial is fixed so the OBS ring doesn't do anything.

Likewise, the GS is only used when tracking an ILS, not when tracking a VOR.

[RedKnight]

Extremely helpful, thank you.

[peterh337]

The ILS localiser is not a VOR radial. It is just two fixed beams, hence the OBS setting is ignored.

Nevertheless it is taught as good practice to set the OBS to the runway inbound track, as a reminder.

However this is not the whole story. In some instruments, e.g. an EHSI (http://www.peter2000.co.uk/aviation/sn3500/p2-view.jpg), you do actually have to set the OBS (which on an HSI is called a course pointer) to the runway inbound. If you don't do that, and e.g. have it pointing in the opposite direction, or something vaguely like that, the instrument will assume you are flying a back course approach and will suppress the glideslope indication (because a BC approach is a nonprecision approach, without a glideslope).

[thing]

One of the radio checks I do before taxiing is to select the ILS frequency, get the ident, rotate the OBS and make sure that the needle doesn't move, so the answer is quite correct as the above have pointed out.

[AdamFrisch]

I just recently learned this myself during my instrument training in my aircraft. My old twin only has one VOR/ILS, so I have to switch frequency constantly on the same instrument to catch cross radials. It's a crazy archaic way to do it, but it will have to do for now until I install more fancy stuff. But on my first ILS approaches I set the track in the OBS, then switched and turned the OBS for my stepdown radials. My instructor just said don't bother setting the ILS track (but know it), just set the next cross radial, as the ILS will be the same whatever you put in. It was a great revelation and it reduced my workload from insanity, to just crackers..;)

[Captain Smithy]

That's interesting Peter. I didn't think that BC approaches were used anymore as the modern ILS Localisers have very directional antennae that don't allow a BC approach to be flown. I know that in the past it used to be used quite a bit in the States, I didn't think modern aircraft still had the capability to do BC however as it was obsolete.

Smithy

[Tinstaafl]

BC approaches are still fairly common in the US. Also, I can't think of an aeroplane that can receive the front course but can't receive the backcourse. Limiting backcourse reception would be a matter for the localiser antenna design to eliminate the backcourse signal.

[peterh337]

This

http://www.peter2000.co.uk/aviation/sn3500/bc.jpg

is what you get on a Sandel SN3500 (http://www.peter2000.co.uk/aviation/sn3500/) EHSI if you have the course pointer pointing backwards instead of forwards :)

A Back Course indication, and glideslope suppressed.

A normal KI525-type HSI, or a basic CDI, couldn't care less what the CP or OBS are set to. However, if you are driving an autopilot from it, then it may well matter because in NAV/APR modes the AP uses the CP as a "heading bug" and uses the deviation of the deviation bar to modify the heading. So that's another reason to set the bearing to the right value, though I am sure this will be system dependent.

Is there any way to emit a localiser without emitting a back course equally - using the standard type of LOC antenna array which runs across the width of the runway?

[BillieBob]

Is there any way to emit a localiser without emitting a back course equally....Yes....using the standard type of LOC antenna array which runs across the width of the runway?No.... (min 10 characters)

[FlyingStone]

I set the front course of the localizer on the CDI instrument anyway, even though the representation on the instrument remains the same whichever course you set. One of the advantages of this is that you can easier see the WCA (wind correction angle) during approach, since one would set heading bug to the actual heading that keeps the aircraft on the localizer - since this is the heading you will have to maintain in order to fly towards the localizer in case of missed approach (unless a procedures dictates immediate turn at MAPt). Peter also mentioned that some autopilots require the correct course to be set in order to fly the ILS accurately - Piper Autocontrol is one of them.

But the real advantage is that, if you have the luxury of standby frequency on the NAV receiver, you can preset the VOR if the runway has a VOR approach. In this case, if localizer goes offline, it only takes one push, turning the approach plate around for the new minima and off you go on the VOR approach (provided you're not very close to the runway). However, I usually have the VOR for the VOR approach tuned and set on the 2nd CDI so I can cross-check during the approach. And nothing happens so fast in an IFR flight you wouldn't have time to change the VOR frequency and set the appropriate radial - unless you're flying single-pilot jet I suppose :)