View Full Version : Capturing LOC/GS

6th Jun 2007, 23:16
Was wondering,what the max distance out from touchdown is, where we can lock onto the LOC or LOC/GS, knowing they will give us the correct indications.

6th Jun 2007, 23:23
on the a320 (and i suppose most other types) auto loc capture has to happen at less than 25nm. g/s when it's active and you're on the loc at the right altitude


6th Jun 2007, 23:40
In the US the service area seems to be not less than 20nm.

Ndicho Moja
7th Jun 2007, 00:49
The rated coverage, where the ILS signal can be considered reliable, is usually 20-25 nautical miles.

7th Jun 2007, 02:13
The numbers I was given a long time ago were LOC 24 miles and less,
GS 9 miles and less, for safe capture. In otherwords inside 25 for LOC and 10 for GS.

Dan Winterland
7th Jun 2007, 03:22
Protected ranges for LOC and GS signals are published in Jepps.

7th Jun 2007, 05:29
Yes, 'tis true.

It really depends on how well the particular ILS installation is engineered.

Yeras ago, it was common practise to have ATC inform the flight to intercept the R25L localizer at KLAX at 100 miles.
Did this work?

Actually, quite well indeed.

In fact, I flew into KLAX not all that long ago, and the exact same instructions were received.

So much for max 24 miles....:ugh:

7th Jun 2007, 08:37
I'm talking about protected distances that make it safe to 'lock on' not how far out you can receive a flyable signal. Early one morning over Costa got a "turn left, intercept the localiser 27L" useable but you certainly wouldn't lock on to it at that distance.

7th Jun 2007, 09:31
Keep in mind that you can enter an extended runway centerline into the FMS, meaning you could effectively intercept the "virtual" localizer 500 miles away, if you wanted to.

7th Jun 2007, 10:53
I'm talking about protected distances that make it safe to 'lock on' not how far out you can receive a flyable signal.

Well, lets see.

Intercept, capture, and fly inbound.
Oddly enough, this is precisely what was done at greater than normal distances I mentioned above.
I don't refer to manually flying inbound, using the heading select mode of the autopilot, but actually capturing and tracking the localizer at extended distances.

Older aircraft could do this just fine.

It could well be that new(er) generation aircraft are not so adaptable, and if this is the case (rather than individual airline SOP's, which sometimes are not that well thought out), one would have to ask....why not?

Good gosh, even the 'ole B707 could capture/track the localizer at extended distances, provided of course that the ground installation was up to snuff.

7th Jun 2007, 11:02
ILS facilities are protected according to the following ICAO standard:
17NM within 35° of LLZ course
25NM within 10° of LLZ course
10NM within 8° of RWY centreline

Some airport may differ which should be in the AIC

7th Jun 2007, 12:21
Flew an ILS into Marham from 70nm out once, in a Victor K2 ......... cos we could and it was a VMC day!

7th Jun 2007, 13:32
The differnece is SAFE to lock-on and ABLE to lock on. No two aircraft, even of the the same model, will necessarily be the same outside the protected distances.

Dan Winterland
7th Jun 2007, 13:43
A NAVAID should not be used for primary navigation outside it's protected range. When on the LOC and GS, you are following the ILS and therefore it shhould be considered as the primary NAVAID.

Mr B. I remember doing that as well. The Marham ILS localiser was transmitted at higher power, something to do with it being used as an emergency navaid to get battle damaged aircraft safely back across the North Sea.

7th Jun 2007, 15:28
The differnece is SAFE to lock-on and ABLE to lock on....The Marham ILS localiser was transmitted at higher power...
Well now, imagine that
Clearly a few of the old(er) timers know the score.

Of course, the KLAX 25L localizer is/was the same, and indeed there were STAR's predicated on its use....fly inbound from 100 NM.

What a shame that some of the younger guys/gals just have not had the exposure to the way procedures are handled elsewhere...and yes, quite safely as well.
Such a narrow view of things aviation.

However, what really is surprising is that some of the younger folks insist they are correct when in actual fact many simply have not need exposed to ....something different.

'Why, THAT positively is not allowed' I have heard some of 'em say, when in actual fact, a particular procedure has been in use for years and years, well before the young 'uns came along, and proudly proclaimed...'it just ain't so'.
Gives me a laugh every time.

7th Jun 2007, 16:35
and an ils signal (loc + g/s) is controlled and calibrated very regularly by an aircraft loaded with equipment. you never see those flying the ils's from 100 nm away.

and the difference between safe and able, cited above, is the same as certified and demonstrated. a good 'ole 707 was able to do a good barrel role but wasn't certified and safe to. and the young'uns have (i think) as good an idea of what's possible with an aircraft and what's safe with one (we're carrying pax remember), as the 'ole ones when they were young'uns.


7th Jun 2007, 17:20
and an ils signal (loc + g/s) is controlled and calibrated very regularly by an aircraft loaded with equipment. you never see those flying the ils's from 100 nm away.

Doesn't mean they couldn't calibrate it that far out though does it?

Just like Boeing could have certified flap extension on the 73 above 20000 but didn't....

Personally, I can't see the problem especially if backed up with another navaid whose coverage extends out the relevant distance.

Yet again, common-sense ought to prevail else they'll throw the rule-book at you....just like the detractors on this thread!


7th Jun 2007, 19:56
don't take me wrong, i guess we all do ils captures out at 30 or more miles. it works and even well. but it won't stand in front of a court if comes to that. i'd take the ils at 75 miles if i had only that to find a field. the certification procedure is based on rates of occurence of failures. out to 25 miles the failure rate is acceptable (for the government, the insurances, ...). up to 20000 ft the rate of occurence of a flight destabilisation is acceptable (for the same people).
again if flying at 22000 ft and absolutely (for what reason i don't know ...) needed to set some flaps then i wouldn't hesitate (knowing that my flight envelope is not demonstrated to the same reliability).

the original q was about a safe capture of loc & glide signals.

as certified it's safe to 25 nm.


7th Jun 2007, 23:21
I have a question for you, airseb...

Let us, for the sake of discussion, presume you are following a standard published arrival route, and that arrival (STAR) clearly indicates that your flight was to transistion from the enroute airway, and intercept a localizer at a DME distance of 100nm.

There it is, printed right on the chart.
Offical, and all.
Keep in mind that this routing is flight checked on a regular basis, by the regulatory authority concerned

Now, the approach controller says to follow this routing.
What do you do?
Refuse the clearance?
Ask for another routing?
Divert to your alternate?

Drop back five yards and punt?


8th Jun 2007, 10:22
SMOC, parabellum et al (who's al?!!) are all on the right lines. I was one of the engineers responsible for the ILS at Heathrow some years ago and I can tell you that the EGLL ILS was flight checked only within the promulgated range and no more.

It did amuse us when we received complaints from crews about g/s + loc deviations outside the promulgated range...and we carried on drinking our tea...

The point is well made by many posts, it's all very well to say that the ILS receivers in the aircraft can pick up the signal but it's not certified to work beyond its promulgated range. By all means use it but don't expect it to give you a faultless signal and make sure you have a back up - as one of the previous posts suggested. Like it or not, as professional aircrew, we have to follow the rules, that's the nature of the job. Boring, maybe, but that's the label on the tin.

I could pontificate about other ILSs in other countries which may be different but I'll stick to what I know and what I've actually stuck my screwdriver/oscilloscope probe into - ooh er missus!!

Does anyone KNOW whether these 'long range' ILSs are actually promulgated/flight tested to these incredible ranges? Getting the lobes and the integrity of the signal out to such a long range would depend on a whole host of factors - nature of the terrain would be a prime problem for propagation of the signal that far.....


8th Jun 2007, 11:22

I don't know if the Marham ILS was promulgated/flight tested in any manner you are familiar with. I DO know that approaches on Marham's Rwy 24 (it was 24 at the time anyway) came over a small chunk of flat old Norfolk and miles and miles of North Sea. In this particular instance, not a lot to interfere with it.

8th Jun 2007, 11:54
Nothing better than a little bit of Nahrfolk and miles and miles of ocean for the propagation of RF!

As I said in my post, I'm going to stick to talking about the systems I actually KNOW about so I'm not in a position to talk about military ILS. Promulgated ranges for the Marham ILS were surely published somewhere?It's got to be a different ball-game from a regulatory viewpoint anyway hasn't it? The CAA/ICAO lay down our standards in civilian life but I don't know what applies in the military.

If I remember correctly we only had an output power of about 25watts on the localiser array at Heathrow so plenty of room for improvement with a decent RF amplifier.

How is flight checking achieved in the military? I imagine that FRA up at Teeside are involved? Anyone from FRA care to add their h'apenny's worth?

8th Jun 2007, 12:15
And... just to come back to the original question, which worries me slightly as I re-read it....

As one of the blokes who used to be responsible for the engineering of the (civilian) systems that so many seem to be following outside the promulgated range... it is immaterial how far outside the correct range you can pick it up, it doesn't matter which aircraft type you are flying, if you use it then you are operating outside the certified parameters of the system. I know it goes on and I know it (mostly) works, but you ain't in a good position if something goes awry.

What can go wrong?

Well, the signal can bend all over the place, you could suffer from all manner of abberations and command reversals... I'm not suggesting it's going to fly you into a mountain but...

Just be careful now...

Now if you'll excuse me Mrs Doyle is coming in with some tea!!

8th Jun 2007, 14:28
Well now, 8846, at least you seem to know what you are talking about, specifically, the ILS installations that you are familiar with.
I would certainly agree, in that context.

However, some who read here should realise that in other parts of the world, extended range high power ILS installations are available, and KLAX is one of them.
Arrival routes predicated on their use are published, and aircrews are expected to comply, by ATC.

If they do not....well, I suppose the concerned flight could be diverted somewhere else.
Anywhere else, for the betterment of all concerned.

8th Jun 2007, 15:07
Extract from the FAA Aeronautical Information Manual
Aeronautical Information Manual
Official Guide to Basic Flight Information and ATC Procedures
Chapter 1. Navigation Aids
Section 1. Air Navigation Aids
K. ILS Course Distortion
1. All pilots should be aware that disturbances to ILS localizer and glide slope courses may occur when
surface vehicles or aircraft are operated near the localizer or glide slope antennas. Most ILS
installations are subject to signal interference by either surface vehicles, aircraft or both. ILS CRITICAL
AREAS are established near each localizer and glide slope antenna.
2. ATC issues control instructions to avoid interfering operations within ILS critical areas at controlled
airports during the hours the Airport Traffic Control Tower (ATCT) is in operation as follows:
(a) Weather Conditions. Less than ceiling 800 feet and/or visibility two miles.
(1) Localizer Critical Area. Except for aircraft that land, exit a runway, depart or miss
approach, vehicles and aircraft are not authorized in or over the critical area when an
arriving aircraft is between the ILS final approach fix and the airport. Additionally, when
the ceiling is less than 200 feet and/or the visibility is RVR 2,000 or less, vehicle and
aircraft operations in or over the area are not authorized when an arriving aircraft is
inside the ILS MM.
(2) Glide Slope Critical Area. Vehicles and aircraft are not authorized in the area
when an arriving aircraft is between the ILS final approach fix and the airport unless the
aircraft has reported the airport in sight and is circling or side stepping to land on a
runway other than the ILS runway.
(b) Weather Conditions. At or above ceiling 800 feet and/or visibility two miles.
(1) No critical area protective action is provided under these conditions.
(2) A flight crew, under these conditions, should advise the tower that it will conduct an
AUTOLAND or COUPLED approach to ensure that the ILS critical areas are protected
when the aircraft is inside the ILS MM.
EXAMPLE - Glide slope signal not protected.
3. Aircraft holding below 5,000 feet between the outer marker and the airport may cause localizer signal
variations for aircraft conducting the ILS approach. Accordingly, such holding is not authorized when
weather or visibility conditions are less than ceiling 800 feet and/or visibility 2 miles.
4. Pilots are cautioned that vehicular traffic not subject to ATC may cause momentary deviation to ILS
course or glide slope signals. Also, critical areas are not protected at uncontrolled airports or at airports
with an operating control tower when weather or visibility conditions are above those requiring
protective measures. Aircraft conducting coupled or autoland operations should be especially alert in
monitoring automatic flight control systems.
Note: Unless otherwise coordinated through Flight Standards, ILS signals to Category I runways are
not flight inspected below 100 feet AGL. Guidance signal anomalies may be encountered
below this altitude.

8th Jun 2007, 15:15
1.2.1 In the UK, ILS Critical Areas are protected at all times that an ILS is in use. Additionally, the larger ILS Sensitive Areas are protected through ATC procedures when aircraft are making an ILS approach. The size of the Sensitive Area increases as the Category (I, II or III) of operation changes in response to decreasing visibility.

1.2.2 In the United States the ILS Critical Area is not protected if the weather conditions are better than 800 ft cloud ceiling and two miles visibility, as described in the extract from the FAA Aeronautical Information
Manual (AIM) at Appendix A.

1.3 Recommendation

1.3.1 Operators should bring this information to the attention of all their crews who are likely to fly to airfields in the United States.

1.4 Additional information


1.4.2 JAA Temporary Guidance Leaflet ‘Section Four, Operations, Part 3: Temporary Guidance Material -

Leaflet No. 23: Use Of Autoland System On ILS Category I Facilities Or Category II/III Facilities When Low Visibility Procedures (LVP) Are Not In Force.’

1.4.3 FAA Aeronautical Information Manual


8th Jun 2007, 23:04
Yup, that's all fine and dandy and thanks 411 for expanding my knowledge of ILS systems in other parts of the world.

Agent Orange is going up a slightly different path though...

To explain all the legaleese in the documents that you have reproduced...

The ILS signal is subject to interference from anything close to the aerial. To protect the signal from corruption we establish areas around the aerials into which a vehicle may not go without permission from ATC.

It is adjudged that in good weather the extent of these areas is sufficient to protect the signal from significant corruption to allow an aircraft to follow the guidance down to Cat 1 minima. There may also be slight deviations in the signal at greater range, even when the critical area is not being infringed, but in Cat 1 weather this is considered acceptable.

If the weather worsens then a greater protective area comes into force - the Cat 2/3 holds e.t.c. to ensure that the signal is reliable down to Cat 2/3 minima.

This is different again to the promulgated range question posted originally, which refers to the signal in space at the extent of its range.

All very interesting..or perhaps not! You be the judge?!!


Dan Winterland
9th Jun 2007, 02:41
The Marha 24 LOC (not GS) did have an extended protected range published. it was in the order of 60 miles, I think.

The Cranwell 27 ILS had a set of power lines running under it at about 4 miles. When you got the about 1200', you had to increase the rate of descent slightly to stop you going above the GS!

Gash ILS Checker
12th Jul 2007, 19:21
Military checking almost identical to civvy in the UK but the ranges are the same.

LOC- 25nm +/- 10 degrees of centreline
17nm +/- 35 degrees of centreline

Glide 10nm +/- 8 degrees of centreline


Previous post stated that they follow the glide as soon as they intercept it, just check that range!!!!

13th Jul 2007, 11:52
A few years ago, Airbus A300-B4 0400 ish. arrival at Dublin Rwy 28 Cat III, we intercepted the LOC at about 40 miles. Everything looked good until G/S capture about 9 miles.
V/L, G/S green, Land 3, etc. Aircraft then seemed to wander off the LOC by about 1 dot and did not correct itself. We left it to about 1500ft to see what happened and then decided to go around and have another go.
2nd approach from 9 miles was perfect.
So did the black boxes forget what they were doing having been on the LOC for so long? Maintenance said there were no faults.
Is this a problem with capturing the LOC from so far out?

14th Jul 2007, 02:06
One difference between an ILS course over empty ocean and one in a crowded airspace is that the latter can be more vulnerable to interference from another ILS using the same frequency.

It's something like receiving two FM stations on the same frequency.