Flying a Backcourse (B747-400)
 

Folks, I'm trying to understand the procedure for flying a backcourse departure in a 747-400 and wondering if it's possible to simulate the ND/PFD backcourse presentation on the ground (at the gate).

I have read in my engineering training notes that the backcourse logic in the EIU's reverses the LOC signal and removes the G/S pointer and scale. On previous occasions, at the gate, I have entered an ILS course on the Nav Rad page which was greater than 90deg to the airplane heading/track and this resulted in the the G/S pointer being removed, but not the scale (I'm not entirely sure that this phenomenon is backcourse related).

Questions:

1) Are most loc antennae capable of producing signals on the reverse side (whether reliable or not is a different question).

2) During backcourse flying procedures, what crs is entered on the Nav Rad page (the normal front crs for that frequency?)?

3) Do you think it would be possible to generate backcourse conditions at the gate when all the gates at my local airport are on the front side of the loc antennae. i.e. how do the EIU's know that you are on the front or backside of the antennae? (I assume this logic works without FMC ILS database knowledge or IRU/GPS position). Is aircraft heading/track factored into the equation or does, perhaps, the lateral movement of the aircraft and subsequent increase/decrease in sensed depth of modulation cause the backcourse indications to appear :8

Thanks for any insight.

Regards.
NSEU

Hi,

1) Yes
2) The normal front crs for that frequency

3) "Do you think it would be possible to generate backcourse conditions at the gate when all the gates at my local airport are on the front side of the loc antennae?" Ans = No

"i.e. how do the EIU's know that you are on the front or backside of the antennae?" Ans = They don't know where you are in relation to the antenna.

If you take off from a runway with an ILS tuned and displayed, the sense of the LOC is still correct after you have flown over the LOC transmitter and are now on the other side. If you then turned around to fly the back course in bound to the reciprocal runway, the LOC sense will still appear to be correct.

e.g. Consider an ILS front course onto Rwy 27 and you are a bit South of the center line. The LOC deviation bar will suggest you to turn Right a bit towards the North. Now consider heading East towards the reciprocal runway, flying the back course and you are a bit South of the center line. The LOC deviation bar is still suggesting you fly a bit to the North, but because your ILS display is "inverted" (still referenced to 270 but heading 090) the displayed sense is to turn Left now.

Thanks! I assume that this is because the instrumentation automatically reverses the indications?

But what has changed, other than aircraft track (in relation to selected front course), to tell the aircraft instrumentation to swap loc sense?.

Here's a diagram (Not sure where I got it from, so apologies to the person who created it)

http://www.iinet.net.au/[email protected]

If the airplane is parked at "C" (still in the reception area), with the ILS front course entered, how does the aircraft know it isn't at "B"? Won't the instrumentation change the loc sense?

Thanks,
Cheers.
NSEU

P.S. The arrows in the diagram show non-automatic switching of sense

Hi NSEU,

Assume North at the top of diagram. With the front course of the ILS (North) selected,

At posn A (normal presentation), centre line is to the Right of 360 degs heading (i.e. to the East of you)

At posns B and C, because your Rose ILS display now has 180 degs at the top and the LOC is still displaced to the East of the 360 degs reference, it will appear that you need to turn to the Left (i.e. towards East).

Only use heading select, don't try and use LOC Capture else it will turn the wrong way.

Thanks again, Rudderrat.
I had some time to experiment on a 744 today.

In the following picture, the aircraft is parked at the gate, parallel with runway (07/25).

http://www.iinet.net.au/~b744er@ozem.../DSC01510a.JPG

The aircraft heading is 062. The ILS front course is 242. Which way do I fly to get to the runway (for a backcourse approach)? :)

Thanks,
Cheers.
NSEU

Hi NSEU,

I can tell from the effort you've gone through to pose this last question - that something's wrong.

Your Expanded Approach Mode shows you should turn to the Right to get to the Back Beam centre line - however I'm confused by the Expanded Localiizer Indication on the PFD. I wasn't expecting that to show turn right also - I thought that worked in the wrong sense.

I've re-checked my manuals and you have set up the Approch Mode correctly - so I'm confused also.

Below is for the approach. For a departure reverse it all. But my god why would this ever be interesting?

Input frontcourse = positive sensing. (so respond like you normally would).

Input the backcourse = reverse sensing.

During a backcourse you can’t arm localizer. So heading select/track select (very nice mode).

Make is easy do a LNAV/VNAV approach.

For departure the only reason I can possibly think of for this question is to track the centerline in case of an engine failure. Just use track select, done deal!

My original questions were posed simply to find out if I could get normal backcourse indications sitting at the gate. I was hoping someone could confirm that what I would see on the ground would be what you were seeing in the the air (being a landlubber, I didn't know what you were seeing in the air)

According to my training notes, the EIU's have the capability of automatically changing the sense of the loc indications on the PFD when following a backbeam (I guessed, to allow you to follow the loc pointer, in the usual fashion, to the beam centre).

If the indications simply relied on current track and entered ILS course, then I could quite easily reproduce the indications on the ground. If there was some additional logic (such as air-ground signals or aircraft movement or aircraft position relative to the Loc antenna), then I wouldn't be able to reproduce those signals.

Anyway, for info, the runway is on the right of the aircraft. If the indications in the photo are indeed correct for backcourse operation in the air, then I could simply fly towards the pointer/deviation bars to get to the centreline.

If this display is different from the one you were expecting, then I am starting to worry again :bored:

To Otterman: Yes, there are indeed many ways to skin a cat. Somewhere in the dim dark past, however, I did see a departure routine which used the words follow ILS backbeam until... something... I can't remember what. However, it was probably not for an aircraft like the 744 :)

Again, thanks for your insight.

Cheers.
NSEU.

How old are your training notes? In the good old days of steam driven instruments, the standard VOR/ILS display had a fixed needle presentation:

http://darko.hytron.net/Aircraft/nav...m/vors_453.jpg

With this display, it was necessary to have a "backcourse" switch, in order to reverse the sense of the needle when travelling "backwards" down the beam on a backcourse approach. When the HSI presentation was introduced:

http://www.esmofly.net/HSI-esm.jpg

Which is the same presentation graphically reproduced on the EFIS, the backcourse switch became redundant, as the entire deviation display is "upside down" when flying the backcourse - and this naturally reverses the sense of the deviation indicator.

This was a recent B744 sim exercise for us.
On selecting the LOC/BAC approach from the FMC database, it auto-tuned the ILS with its front-course, which were shown on the nav-rad page.
LNAV and VNAV were used to fly an overlay approach.
On approach the ND showed the localiser in a reversed sense. i.e. you would have to disobey its indications in order to regain the centerline.
We used LNAV but the LOC/BAC localiser is the overriding information for the approach.

Zeus,

You're allowed to fly a back course in LNAV? Here in the US, you can't fly a LOC based approach using the FMS. Raw data only.

That news to me CA..do you have a source?

FWIW on the rare occasions we do a back course (777, using Boeing's own published procedures) as long as the procedure is in the FMC the approved horizontal mode is LNAV, of course monitoring the raw loc display throughout.

Prohibited on my US Global, too. Any LOC-based approach must be flown using the LOC, not the FMS.

GF

Note that Back Course ILS is a lot more sensitive than a front course. Worth defining a Final Approach Fix then RW MAP or THOLD coordinate in the FMS and flying LNAV, whilst using the raw display as a reference for the procedure.

Every BC ILS I have witnessed as PNF in the sim, flown using the heading bug, results in the PF blowing through 1 dot during the latter stages of the approach.

Suggest you have a go on your next Sim training or request if your training program would allow for it.

Gents,

Without knowing exactly what options have been ticked off for this particular airplane configuration I would be very careful with your assumptions, as there are so many FMC/PFD/ND options available that it can get you in the dirt quickly.

What airports do you fly into that use BC approaches?

Is this an aircraft thing or a State thing then?

What is the situation in Canada?

We had a 744 in our fleet with a separate BC pushbutton. Never used it though.

For other aircraft, would consider using LNAV in order to keep the needle(which is the primary reference) centered.

Of course you can fly a BC using LNAV, or front course, or VOR, or NDB. LNAV is just being used to hold a track while you monitor the raw data indications which are the primary reference. You could use HDG SEL as well and vary it as required to maintain an appropriate track to stay on the localizer. I don't remember any rule or regulation stating how you stay on the localizer, just that the deviation limits are for the localizer.

The BC buttons were on our Classics, but none on the 400s.