Instrument approaches: the outbound leg

[FlyingForFun]

A question which came up at work recently, and which we couldn't agree on an answer to:

Have a look at the plate for the ILS/DME RWY 28 approach at Blackpool (http://www.ais.org.uk/aes/pubs/aip/pdf/aerodromes/32NH0804.PDF) (AIS website registration required to access to the link), although the question could probably apply to any ILS approach with a procedure turn (rather than a base turn) in it.

In this approach, you reach the beacon (which is on the airfield), and possibly enter the hold, before leaving the beacon on a track of 099 - the reciprical of the ILS inbound track, which is 279. You then fly a 45/180 procedure turn to establish on the localiser.

What should you be tracking whilst flying this outbound leg? Do you track 099 from the NDB? Or do you track the localiser in the reverse direction?

Myself and a colleague disagreed on this one. I explained my point of view, he explained his, and we decided that we both had valid points and neither of us knew which was correct. So my colleague e-mailed Steve Oddy (CAA staff examiner, and author of RANT), who very kindly replied extremely promptly. I'll post later and let you know what his reply was, but I'm curious what the concensus of the forum is first.....

FFF
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[High Wing Drifter]

I think you should be tracking BPL(NDB) outbound as that is the IAF. I've not thought about this before, but tracking an ILS outbound would require backcourse equipment otherwise the ILS does not operate as a command instrument. That and 2deg sensitivity.

[Megaton]

I reckon you're tracking outbound from the IAF which is based upon the BPL; therefore, track 099 deg.

[Kiltie]

If the NDB is the IAF that is what you must use to fly the outbound leg. LOC back-courses are not approved in the UK.

[A and C]

I think that you are making this all far too complicated.

First what dose it say on the top of the plate ? ILS DME rwy 28 so the approach is NOT based on any NDB but you must have a minimum of LOC & DME to fly the approach if you have GS so much the better.

The only function of the NDB is for the hold and to get you overhead the field to start the ILS/DME approach if you can find your way to overhead the field some other way (radar or area nav) then you dont need to use an ADF.

I think it is time for some common sence to be applied the LOC back course takes a lot less cockpit workload to track than an NDB and is far more accurate so why make life hard for yourself, keep the cockpit workload down to a minimum level and use your brain power for the important stuff.

I am fully aware that some might want to turn this into a black art but it's just flying !

KiltieYou are quite correct that LOC backcourse approaches are not approved in the UK but this is an ILS/DME and the critical part of the approach is not being flown as a backcourse.
It is common sence to fly the aircraft in the most accurate way and using the LOC to position outbound is doing just that if the approach required the use of an NDB the it would say NDB ILS DME rwy 28 at the top of the plate!!

[Megaton]

Keep the workload down? I don't think that flying effectively a backcourse outbound and then normal sensing inbound is reducing the workload. You should have the NDB tuned anyway for IAF so all you have to do is track outbound. It's not that the back cse is inherently difficult but at times of high workload, fatigue, wx etc it has often reverse sensing can be confusing.

Also, it may be an ILS/D approach but the MSA are based upon BPL sectors and the IAF (BPL again) is part of the instrument approach so you cannot execute the approach without it.

[FlyingForFun]

So far, 3 replies in favour of the NDB and 1 in favour of the localiser.

One interesting point regarding the localiser, though: if you choose to use this, you are not using the back-course. The back-course would be on the approach to the other runway, runway 10. What you are doing is using the front of the localiser, but backwards.

Back-course approaches are a completely different subject. (And - HWD - they don't require any special equipement, just a knowledge of how to interpret the instruments.)

FFF
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[High Wing Drifter]

A and C,
I'm not sure I agree that the NDB complicates matters in this case. In my simple mind the easiest approach I think would be to have the HSI/VOR already on on the ILS track and the HSI/DG bugged for intercept and follow the RMI/RBI. In other words have two instruments for different parts for the approach and setup accordingly.

FFF,
HWD - they don't require any special equipement, just a knowledge of how to interpret the instruments.
Indeed, I don't think I contradicted that. The point I am making is simply that unless you have a backcourse button the thing that looks like VOR/HSI no longer operates like one. Surely a case of Murphy's law if ever there was one :uhoh:

[DFC]

First to correct some misconceptions;

Balckpool only has an ILS on 28. The signals transmitted along the approach to 28 are what is knows as the "front course" regardless of which dirction the aircraft is travelling. If one travels along the approach to 10, the ILS LOC signal received will be what is known as the "back course". It is in this area that the signals are not to be used.

To answer the question;

Annex 10 Volume 1 is the reference.

The upper boundary of the useable localiser signal is defined as a surface with a slope of 7deg from the horizontal starting at the localiser antenna.

Thus an aircraft at 3000ft will have to be about 4nm from the antenna to enter the useable signal area. At 2000ft this reduces to about 2.7nm (if my trig is correct!).

Add to the situation the fact that while the NDB is on the field in this case, it is not on the runway centerline and the allowed position of the aircraft with respect to the NDB when in the overhead.

So the answer is that the NDB must be used to track outbound until the aircraft is within the localiser coverage (between 3.7 and 5 dme outbound (the runway is about 1nm long!) depending on height) when the localiser could be used. But of course, things like DME slant range has to be taken into account when close in at such heights!

From the parctical point of view - track the NDB with an eye on the LOC as the procedure turn is reached to ensure that the procedure turn is started from a position as close to as possible the ideal correct fix.

I know the LOC would provide more accurate tracking (when used correctly) but the procedure designer has allowed for the errors in the NDB system.

Regards,

DFC

[A and C]

The first issue that you raise is workload...... well that depends on the equipment fitted in the aircraft I see no increase in workload with a HSI, with an OBI it is different and I can see the increase in workload over a HSI but it would still be less than back tracking an NDB and more importantly far more accurate.

You are correct that the MSA is based on the NDB but as I said if you can locate the NDB position some other way such as area nav then staying within the MSA in not an issue ( the approach radar could also help you with this).

The GA would undoubtedly be harder to fly without the NDB but the area nav would also work for this if you read the plate is says climb straight ahead to 2000ft then left to the locator if a course was required it would say climb on 279 from lctr.

[High Wing Drifter]

Balckpool only has an ILS on 28. The signals transmitted along the approach to 28 are what is knows as the "front course" regardless of which dirction the aircraft is travelling. If one travels along the approach to 10, the ILS LOC signal received will be what is known as the "back course". It is in this area that the signals are not to be used.
Good point DFC! A little bit of confusion there on my part :O The sense would be correct for the localilser regardless of what radial was selected, unlike a VOR.

[FlyingForFun]

HWD,

Sorry to keep picking up on your posts - nothing personal!

First of all, you seem to think that a "back course button" would make things easier on a back course. I quote you: "The point I am making is simply that unless you have a backcourse button the thing that looks like VOR/HSI no longer operates like one".

My understanding (having never flown a back-course) is that there is no back-course button which affects the information which is displayed to the pilot. If the aircraft is equipped with an HSI, it will read in the correct sense when flying a back-course regardless. If it is equipped with an OBI, it will read in the reverse sense regardless, and you will need to fly in the oposite direction to what the needle indicates.

However, many autopilots do have a "back course" button. This does not alter the display which the pilot sees - it only determines how the autopilot interprets the ILS data.

None of this is at all relevant to the question I asked, though, because the entire procedure is flown on the front course, which you did recognise in your last post.

But again, you're understanding of what the instruments would show is wrong. You say: "The sense would be correct for the localilser regardless of what radial was selected".

In fact, if you have an OBI, the sense will be incorrect regardless of what radial is selected. The reason for this is nothing to do with whether you are on the front-course or the back-course of the localiser. It is because your heading is approx 180 degrees out from the localiser's final approach course that the OBI will read the wrong way. An HSI would read the correct way.

If you don't believe me, go and draw some diagrams showing the aircraft off-track, and work it out from first principles. Or, even better, go and fly it!




DFC,

The second part of your post is exactly the answer I was looking for, giving a reference (which I will check out when I get a chance), an answer, and a reason. It does, however, make Steve Oddy's reply to the question seem very interesting..... but I'll let the debate go on for a little longer first!

FFF
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[High Wing Drifter]

FFF,

I know that we have established that this has nothing to do with the debate but you did decide to pick up on it, so here I go :D

However, many autopilots do have a "back course" button. This does not alter the display which the pilot sees - it only determines how the autopilot interprets the ILS data.
I don't really know for real. I was a remark born from the teachings of BGS ground school, that the indications for a backcourse are corrected when backcourse is selected. Obviously the real world is a little different to BGS. Therefore, I stand corrected.

In fact, if you have an OBI, the sense will be incorrect regardless of what radial is selected. The reason for this is nothing to do with whether you are on the front-course or the back-course of the localiser. It is because your heading is approx 180 degrees out from the localiser's final approach course that the OBI will read the wrong way. An HSI would read the correct way.
Some confusion here due to my undetailed response. I did mean the sense relative to the ILS track and that it doesn't change by changing the selected radial so things can be a little confusing if you treat it like a VOR. Point taken aboout the HSI. Roll on my IR!

[funfly]

Forgive me for sounding dumb but isn't it impossible to track out on an NDB? Surely all you will get is a 180 fix as long as you are facing away from it whatever your bearing from the NDB!
Don't you just get overhead the beacon at the correct height then head out on the outgoing bearing specified on the plate (allowing for wind) then rate 1 turn to get 'established' at the correct distance and at the correct height.
I must be missing something.

[benhurr]

Track outbound from the NDB.

Funnily enough I only know this cos it is what I did on my initial IR test and I passed!

Funfly. You fly out on the specified bearing relative to the NDB, on a heading which allows for the wind, to ensure the correct track.

[justinmg]

Forgive me for sounding dumb but isn't it impossible to track out on an NDB? Surely all you will get is a 180 fix as long as you are facing away from it whatever your bearing from the NDB!
Don't you just get overhead the beacon at the correct height then head out on the outgoing bearing specified on the plate (allowing for wind) then rate 1 turn to get 'established' at the correct distance and at the correct height.
I must be missing something.

What would happen if the wind blew you off course....Once on the outbound, the ADF reading taken with your current heading tells you what "radial" of the NDB you are on.

[Dr Eckener]

ILS DME rwy 28 so the approach is NOT based on any NDB but you must have a minimum of LOC & DME to fly the approach if you have GS so much the better.


I don't think so. The boxed navaids include the ndb so you need it for the procedure. No brainer i'm afraid - outbound ndb, inbound ils.

[Keef]

There's a lot of confusion here!

Yes, you can track outbound from an NDB and know where you are (within the margin of error of an ADF). If you're tracking 090 degrees, and the ADF needle is pointing directly behind you and stays there, you are on a bearing of 090 from the NDB. If the ADF needle is pointing 10 degrees to the left of directly behind, you're on a bearing of 100 degrees from the NDB.

Yes, there are Back-Course procedures in the USA: the VOR display even has a little light labelled "BC" that lights up when you're on one. I had to do one for my FAA IR checkride.

[Send Clowns]

Errrrmmmm surely it is DR, or am I missing something? As far as I can see it doesn't use the NDB, as then surely it would be an ILS/NDB/DME procedure which is not how it is described. Use of back course of LOC is not approved in the UK, so cannot be part of the procedure. So the approved procedure is to DR outbound, and if the procedure turn doesn't give you enough space to correct any errors onto the localiser and get established you'd have to bin the approach. Since a 10° error at 6 nm is only 1 nm error, this should not be too much of a problem!

You can of course use any navigation that you are competent to use, I would suggest the 277° radial of the POL VOR (setting 097° of course!) would be best assuming you have a second NAV box. Surprised it has not come up as a favourite, as it is far the easiest!

So that is what the "BC" button means on my autopilot!

[RatherBeFlying]

Agreed I'd track to the NDB and turn OB to 099 -+ wind correction and hold that course for a bit.

My suspicion is that the loc signal will give good data before the ADF will on a back bearing on many SE a/c.

I've done a bunch of VFR back bearing tracks in various SEs and have yet to find one I would trust on an NDB approach:uhoh:

[Keef]

The plate is here (http://www.ais.org.uk/aes/pubs/aip/html/egnh.htm).

Maybe I'm missing something.

You leave the NDB BPL on 420, heading 099 degrees (almost straight towards the POL VOR) with your ILS and DME tuned to I-BPL on 108.15. When you get to 6D you make a procedure turn left onto 054, then back round onto the final track inbound. Looks pretty standard to me.

It looks as if you do need an ADF to make the approach, unless you ask for clearance to join direct from POL.

[Keygrip]

Shouldn't that be TRACKING 099 (not Heading 099).

The link to the plate didn't work but my guess is that the NDB is for track reversal, not approach (which is why its name is not listed as an ILS/NDB/DME, or whatever, as some of you suggest it would have to be).

[FullyFlapped]

I think the legally and technically correct answer is to track OB from the NDB, but I must admit I'd use the POL radial in a heartbeat if I could !

I actually heard this very procedure being attempted a little while ago, whilst flying in the area. At the risk of taking this off-topic a little, and just for my own curiosity, would anyone feel brave enough to detail the calls you should make to ATC, from start to finish, whilst flying this (assuming you're arriving from outside the zone) ?

FF :ok:

[FlyingForFun]

Funfly - you most certainly can track from an NDB, by comparing the direction the ADF needle points to your current heading. It is covered on the IMC course and the IR course, or if you want to read up on it, buy a copy of Trevor Thom volume 5.

Send Clowns - as I said earlier, the use of the localiser in this case would not be using the back course. The back course is to the west of the field, you would be remaining to the east of the field, using the front course of the localiser in reverse.

Fully Flapped - which procedure is it that you want the radio calls for? The "Direct Arrival From VOR DME POL" procedure? It would go like this:

You: "G-CD Pole Hill 3400', request direct arrival for the ILS DME runway 28"
ATC: "G-CD You are cleared for the direct arrival for the ILS DME runway 28, descend with the procedure and report localiser established"
You: "Cleared for the direct arrival, wilco, G-CD".

From here, you would track POL (probably on Nav 2), and have the ILS tuned on Nav 1. You would descend to not below 2600'. At 10nm, you would expect to be on the ILS, and if not you would take up an intercept heading. Then, report "localiser established", descend to not below 2000', and continue as if for the normal procedure.

For those of you who are suggesting using the POL radial to fly beacon outbound, that radial does not pass over the airfield. It intercepts the final approach track at 10nm, but it is 2 degrees out from the final approach track, and so will pass a little way to the south of the airfield. Sorry, but I don't think that will work.






And here's Steve Oddy's reply. (Not word-for-word, because I don't have it in front of me.)

He says that he would use the localiser. He then says that on a skills test, the examiner may choose to "fail" the localiser during the beacon outbound leg if he needs to see NDB tracking. He therefore implies that either is acceptable.

However, DFC's reply has convinced me that Steve Oddy is wrong, and that the localiser must not be used, because, as DFC explains, at the height at which you would commence the procedure you will be outside of the area which is covered by the localiser.

Interesting how something so apparently simple can generate so much discussion!

FFF
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[Northern Highflyer]

Having flown the BPL procedures as part of my IR training and test, I was never taught to (or told that I could) use the localiser when tracking outbound.

The procedure begins from the IAF at BPL. It's already there tuned and identified so why not use it ? I would track 099 from the IAF (BPL) until 6d on the DME before making the turn.

[High Wing Drifter]

The Danish CAA have all the ICAO docs (inlc that which DFC is referring to) here (http://dcaa.slv.dk:8000/icaodocs/)

[Send Clowns]

FFF

Ah, of course. Had been lazily answering other people, not thinking too hard about that aspect of the approach. However I would suspect it is not approved to follow a LOC backwards either, although all indications would be correct assuming the final approach track is on the HSI CDI.

[baldwinm]

FFF,

"My understanding (having never flown a back-course) is that there is no back-course button which affects the information which is displayed to the pilot"

There is on our Apollo SL30 - so the indications can be reversed if desired.

If equipped with a Garmin 430 the answer is to follow the magenta line. The only problem with that comes on the procedure turn which takes a lot longer than you would expect at normal light aircraft speeds. One solution is to fly the 054 degree bit for the usual minute or so and then use the "direct to" button to move onto the next waypoint.

Do many people do the direct approach from POL in preference to getting vectors?

[FlyingForFun]

Ah - useful information here. I stand corrected about the lack of a "back-course" button! I've certainly seen several autopilots with a back-course button, but never anything like you describe on the Apollo - but obviously different aircraft have different systems and I was wrong to generalise!Do many people do the direct approach from POL in preference to getting vectors?No - vectors is the norm for "real" flights, and the full procedure is the norm for training flights. I've done the direct arrival a couple of times, on training flights where I'm teaching VOR tracking. If the weather is VMC when I do this exercise, I rejoin visually (no benefit to student in doing the ILS when I haven't taught it to him yet), but if it's IMC and I'm teaching VOR tracking, it's quite handy to get the student to track 277 from POL, then at 10d I take over the controls and fly the ILS. The student gets far more benefit from this than he would from radar vectors.

Can't think of any reason other than training to choose to use the direct arrival over radar vectors, since the vectors allow ATC to fit you in with other traffic if necessary, and the routing will be pretty much identical if there is no other traffic. But radar is u/s from time to time, and the radar frequency is closed at certain times of day, so it might be necessary to do it from time to time. In fact, I was once arriving from the south-east (on radar vectors from Manchester which took me roughly over the LPL if I remember correctly) when radar was closed, and I was cleared to self-position on the ILS, so intercepted the POL 277 radial at around 12 miles on I-BPL and then continued as per the direct arrival procedure.

FFF
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[slim_slag]

You don't need one of those sissy buttons when flying the back course, just make sure you remember the needle is reverse sensing and you will be fine. You can see how it works if you depart from an ILS equipped runway with the localiser tuned it and look at the needle. Why are back courses not allowed in the UK? It's another option and has got to be better than a circle to land.

Regarding that plate, for reasons given by DFC you need to track outbound using the NDB. However, the POL VOR 277 is only 2deg off, which is well within the allowed limits of VOR navigation, and my mental calculation says you will be about 1/3rd nm off the localiser when at the airport flying outbound, which is pretty good and probably better than you could obtain by using the NDB on the airport. You want to stay alive more than anything else, and if anybody asks just say you used the NDB.

[High Wing Drifter]

Slim_Slag,

Sorry to switch into uber picky mode, but if you used POL wouldn't the potential error be greater than 1/3nm - upto about 2nm I reckon? I guess that could make intercepting the localiser more difficult than the max error of about 1nm for the NDB.

[bookworm]

I think the legally and technically correct answer is to track OB from the NDB, but I must admit I'd use the POL radial in a heartbeat if I could !

Would you? POL is 32 nm from Blackpool. The potential error is a good couple of miles either side of the track you want. Of the 3 options, using POL would seem to be the worst choice.

DFC puts a very strong case for using the NDB. In these days when tracking an NDB is about making sure that the ADF needle is pointing approximately the same way as your GPS pointer, the case for using that rather than the LOC only gets stronger. ;)

[slim_slag]

HWD,

My logic is that the POL radial intercepts the localiser 10nm from the airfield at 2deg. Getting out MS calculator in trigonometry mode, Tan = opposite/adjacent. Opposite = Tan(2deg) x 10nm = 0.34 miles. It's been a long time since I used this stuff in anger though, could be well wrong. Bookworm's GPS answer is of course the correct one, but didn't want to open can of worms.

[DFC]

FFF

However I would suspect it is not approved to follow a LOC backwards either, although all indications would be correct assuming the final approach track is on the HSI CDI.

There still seems to be confusion about what the backcourse really is. Here is the exact description;

One is on the front course when if one faces the antenna, the 90hz area is on the left and the 150hz area is on the right of the centerline.

One is on the back course when if one faces the antenna, the 90hz area is on the right and the 150hz area is on the left.

When within the approved coverage of an identified localiser front course, one can use the information to either;

travel towards the antenna, travel away from the antenna or obtain a crosscut while travelling across the approach.

In the UK, one is not permitted to make use of any signals received in the back course area (see above for where this is).

As for using POL. Not a good idea. As one moves away from the VOR, the possible error magnitude increases. For this reason, one will see from tha procedure that one can not descend below 2600 using POL until established inbound on the localiser at 10dme or less IBPL.

Those that would use POL would be further limited by the fact that the vor leg for the direct arrival terminates at 10dme IBPL on the LOC. Between the Blackpool overhead and that point, one would have to use the MSA as a minimum level and at 3500ft, this is above the starting level for the procedure.

As I said previously use the NDB.

-------

Interesting what Steve Oddy said i.e. he would fail you for using the BPL outside the DOC but would be happy for the IBPL loc to be used outsideof it's coverage. :uhoh:

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How do people get IRs or even IMCs without having to know the coverage of the various navaids?

Sometimes I think perhaps we need to add some bits to the sylabus rather than taking things out!

----------

Regards,

DFC

[bookworm]

The upper boundary of the useable localiser signal is defined as a surface with a slope of 7deg from the horizontal starting at the localiser antenna.
Thus an aircraft at 3000ft will have to be about 4nm from the antenna to enter the useable signal area. At 2000ft this reduces to about 2.7nm (if my trig is correct!).

Nothing wrong with your trig but worth bearing in mind that the antenna is about a mile beyond the threshold. Picking up the LOC at about 2 DME seems reasonable, therefore.

[bookworm]

How do people get IRs or even IMCs without having to know the coverage of the various navaids?

Procedures are designed in such a way that a detailed knowledge of all 459 pages of Annex 10 Volume 1 is not required. So I don't think knowing coverage is a vital piece of flight crew knowledge. But there are some snippets, like the one you cite, that are more useful than others!

[DFC]

Bookworm,

You are correct! My trig was OK but my simple addition was crap - I added the aprox runway length when I should have subtracted it. :)

Procedures are designed in such a way that a detailed knowledge of all 459 pages of Annex 10 Volume 1 is not required. So I don't think knowing coverage is a vital piece of flight crew knowledge. But there are some snippets, like the one you cite, that are more useful than others!

I am no way suggesting that people should have any detailed knowledge of Annex 10. But while every IMC and IR holder will have learned about the lateral coverage of the LOC, very few training notes cover the vertical coverage.

Regards,

DFC

[bookworm]

But while every IMC and IR holder will have learned about the lateral coverage of the LOC, very few training notes cover the vertical coverage.

A very good point. I was ever so slightly embarrassed that I had to look it up. ;)

[FlyingForFun]

I have to admit to not being in the slightest bit embarassed at not knowing the vertical coverage of the localiser, because it is quite simply not something that I can ever think of any real reason for needing to know.

However, I am embarassed that I didn't realise that there would be some kind of vertical coverage issue when tracking the localiser outbound, because simple common sense says that you can't receive it in the overhead, and the natural question then is where can you receive it. Had this occured to me, I'm sure I would have been able to find the relevant information, but DFC posted the necessary data here before it even occured to me!

FFF
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[Piltdown Man]

Let's throw my brick in the pond on "Back Coursing". FACT: If you set up you inbound course of the ILS and then track outbound, the needle will still give command information to get you back to the centreline of the LLZ. It is the classic of two wrongs making a right! However, if the procedure says use the NDB to go outbound, do so - the people who put these charts together know more about these things than we will ever do. But practice with the wretched (NDB) things enough and they will, believe me, become your friends. High up and enroute, yes I like little white line on the display. Low down and in the weeds, give me a needle please.

[martinidoc]

The procedure requires outbound tracking of the 099 NDB radial.
The other important bit of the procedure which requires the NDB is the missed approach which requires climb ahead and turn left to BPL. So even if doing the published Direct arrival from POL, it would seem to me that an ADF is required.

[boomerangben]

Interesting. I would use a combination of both localiser and NDB, but there is no note to say that the procedure is unavailable if the NDB is u/s.

[marsman]

The NDB is to be used to track outbound on. Where on the plate does it mention anything about using the loc??:confused: (whilst tracking outbound)

Looking at the profile it shows you overhead the BPL at 3000' tracking outbound (099)