Why does the HSI have to be set to the front course heading for ILS approaches?

[Contacttower]

I should know the answer to this since I have an IR....

When using a VOR an HSI will always be a command instrument, yet when using a localiser, whether it be inbound on the back course, inbound on the front course or any other combo of course and direction it has to be set to the front course heading otherwise it will reverse sense...why are localisers and VORs different in this respect?

[421dog]

If you think they are, just run out and try a back course with a VOR without a backcourse switch.

The reason an HSI reads "correctly" on a back course when set to the localizer heading is that the "arrow" is actually pointing towards where the localiser will be when you cross it, whereas in the case of a VOR, you are "from" a station that doesn't have a "from" side, and you are going "to" it so the needle has no option but to sense backwards.

[galaxy flyer]

Technically, w/o engaging the autopilot, you don't! But have you ever tried to apply a crab angle when the CDI is 90 degrees off. You have, say, the CDI set at 090 on an ILS to runway 18. It will work fine except try making a 3 degree correction while looking at the CDI off 90 degrees.

This is not so for coupled approaches, the A/P needs to know the course.

GF

[eckhard]

why are localisers and VORs different in this respect?

I think the main difference is that VORs transmit 360 different 'signals', whereas a Localiser only transmits one, (or two if you count the back-course).

The VOR transmitted signal is phase-modulated to produce 360 discrete 'on course' signals.

The Localiser transmits two 'lobes', the intersection of which provides the 'on course' signal.

As Galaxyflyer says, you don't need to set the HSI at all in order to receive the correct localiser indications. It broadcasts its two lobes and it doesn't care what you have set. The VOR broadcasts its 360 different signals and your receiver simply 'knows' which one of these radials it is on. The HSI has to be set in order to make sense of this one known fact with respect to your desired course.

Hope this helps because I'm confused now!:confused:

[rudderrudderrat]

I thought it was because the VOR has a "TO / FROM" logic inbuilt. (This logic was displayed on early VOR receivers).

The ILS has no such logic so the front course has to be the reference, for correct sensible display.

[bookworm]

That you can track VOR in that way with an HSI is something of a double fluke!

If you fly towards a VOR with a conventional CDI with the OBS correctly set (magnetic track to the VOR), indications will be correct. If you fly towards a VOR with a conventional CDI with the OBS set 180 degrees off (magnetic track from the VOR), indications will be reversed.

Try the same with an HSI. In the latter case, the indications are still reversed. But the left and right indications are with respect to the direction of the course arrow. Because the course arrow is now pointing downwards, the entire readout system will be upside down. Thus the readout is reversed twice, and reads in the correct sense.

For a localizer, the direction of the course arrow is irrelevant. You can fly the ILS with anything set on the OBS of a conventional CDI. However, if you set the course arrow of an HSI to 180 degrees off the localizer, the course arrow is now pointing downwards and the readout system is reversed -- just once now -- so the instrument reads in reverse.

The autopilot, of course, doesn't care which direction the course arrow is pointing. So if you try to fly towards a VOR with the course arrow set 180 degrees off, the autopilot still sees reversed indications and fails to track the VOR.

[Checkboard]

Nice explaination, bookworm :ok:

That was the rational behind the design of the HSI - by spinning the CDI (Course Deviation Indicator) needle around a compass rose, and slaving the compass rose to the aircraft heading, the HSI became a "command" instrument - for VORs it always operates in the command sense. All you have to do is look at it after setting up your radial to understand your current position with respect to that radial.

It saved all the mental combobulations involved with a fixed CDI display.

As both VOR and ILS systems show your displacement from a line based on the phase difference between two radio signals, the same CDI has always been used for both systems. With an ILS signal, the "radial" is fixed, so the OBS isn't used by the system at all - it just detects the phase difference, and displays that on the needle. For the old fixed CDI display, it didn't matter what you put on the OBS when flying an ILS - as this bit isn't used at all by the ILS.

Then the HSI came in, and the CDI turns around with the aircraft's heading. Now when using the ILS you need to set up the needle so that it points "up and down" with respect to the panel for it to operate in the command sense, and the only way to do that (as it rotates on a slaved compass card) is to set the OBS to the track you are going to be flying when you are established on the ILS (i.e. the inbound track).

For ILS the two lobes, the "left" and "right" lobes, are set up to display correctly for front course (the most common) approaches. If you are flying an approach to the reciprocal runway you are pointing the aircraft 180º in the opposite direction for the approach - but the ground staff haven't run out, dug up the Localiser antennae and spun them around 180º for you! The "Left" and "Right" lobes are therefore 180º out of sync - so you need to turn the CDI "upside down" to correct that. (Or flip the backcourse switch on the old fixed CDI indicator to reverse the sense.) :8

[Contacttower]

Thanks bookworm, that is a very good explanation. :ok: