Back Beam ILS

[JOBBA]

Can anyone tell me that when using a back beam ILS, do you put the inbound QDM in the course window or the reciprical. Many thanks in advance

[Jetdriver]

Actually it does matter.

The course selected should be the one for the full ILS that is the approach that has the localiser and Glideslope.

Selecting back course reverses the signal sensing.

[Nick Figaretto]

That's not totally right, is it? It will depend on what kind of instruments your aircraft is equipped with.

If you have an "old style" ILS instrument, you have to steer opposite of the Course Indicator, and what inbound course you set in the window doesn't matter.

However, if you have a HSI (Horizontal Situation Indicator - a yellow CDI with a moving middle section, placed on your gyro compass) you set the CDI to the opposite of your inbound course when flying a backbeam LLZ. That way you don't have to steer opposite of your indications.

I didn't think there were any full ILS'es around using back beam localizers, since the frequency of the GP is "slaved" to the LLZ frequency. You would get the GP information of the opposite runway when you use a back beam localizer, which is really not much helpful. Actually pilots are recommended to ask ATC to turn off the GP signal of the opposite runway when flying a back beam LLZ approach to avoid the confusing GP signals on the instrument.

If there are full "back beam ILS'es" around, there would have to be two separate GP antenna systems using the same frequency, which can be switched on and off depending on runway in use.

I've just never heared of that. Let me know if there are any.

Nick.

[Spoonbill]

In the UK the use of the localiser back beam as an instrument approach is not permitted.
Anyone who uses this procedure in IMC down to minima is treading on extremely dodgy ground. Attempting to use the glide path as well is just plain stupid.
NICK, I don't know about the rest of Europe, but in the UK, ATC are not permitted to alter, switch on/off any instrument approach aid, this has to be done by the telecomunications engineers. This is because the equipment has to be checked that it is radiating correctly prior to being released into service.

[Dan Winterland]

Back beam procedures now only exist in Canada (I think) and are particularly prevalent on the East Seabord airfields where the predominant wind is Westerly, but if there is an Easterly off the ocean the weather is far worse and you need the lower minima.

The reason you set the reverse course is that the ILS does not transmit a radial like a VOR, but a fly left or right signal. If you are flying down the back beam, you would receive the fly right signal when right of the CL, i.e the wrong way. By turning the HSI course to the inbound QDM, you are effectively reversing the indication to make life easier.

Some instruments (like the 747-400 standy horizon which has ILS indications built in) have a sense reverse switch to provide the correct indications.

[tincancowboy]

Nick is right.

As i am a canuck and because we use them all the time. In mountains where an ILS won't fit, or if it is not cost effective, Candians use a localizer backcourse (backside of an ILS)aka. back beam or a plain old localizer . I believe that I have used them in the states too. This is just a more precise non precision approach to allow lower minima and easier stepdowns (usually with a DME). Though, it is a heavy work load and not much fun.
Using a CDI (window washer), you set the front course and the needle reverse senses.
Using an HSI you set the front course and it acts like the lateral information on an ILS.
On some EFIS the display shows BC indicating a backcourse and inhibits glideslope coupling. When using a CDI or HSI, the glide slope will flop around but not give correct information. We simply ignore it. ATC does not turn off anything. You can also use a backcourse for a missed approach. Some auto pilots wil actually couple to it. Hope this helps.

[ 10 September 2001: Message edited by: tincancowboy ]

[Nick Figaretto]

I believe there used to be a backbeam LLZ approach in ENHF (Hammmerfest, Norway) but not anymore. So now I don't think there are any LLZ back beam approaches in Europe.

As I have understood, to certify a LLZ for backbeam use, it has to have a separate antenna installation, but the same transmitter is used. At least that was what they used in ENHF, according to a navaid equipment engineer of the Norwegian CAA that I spoke with some years ago.

Not totally sure if that applies in Canada/US, though.

Nick.

[spekesoftly]

Hi Spoonbill

I don't wish to sidetrack the thread too much, but with respect, I disagree with your assertion that UK ATCOs are not permitted to operate ILS switching. I suggest that it may be policy at some NATS ATC units for the Tels engineers to exclusively operate the ILS selection, but this is not the case at all ILS equiped Airports. A number of Airports are H24, but do not necessarily have Tels engineers on site H24. In the event of a Runway change, ATC at such Airports have the authority, and the facility within the VCR, to select the ILS for the appropriate Runway in use. The functionality of the ILS can also be monitored from duplicate displays easily visible to the Controllers.

[ 12 September 2001: Message edited by: spekesoftly ]

[jbc2001]

In fact there are two main methods of selecting Back Course on jets. On Boeings the pilot sets the normal (front) course on the Flight Control Panel and the Flight Director compares this with the aircraft's current heading - if it's more than 90 degrees you get Back Course. On other systems, such as most Collins APS or FCC, there is a "BC" button that must be selected by the crew as part of the approach procedure.
BTW, there's at least one other country that uses BC mode, the government airfield at Beijing apparently utilizes the back course from the International airport.