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Old 18th Jun 2006, 17:32   #1 (permalink)
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Locator approach vs NDB approach

Does anyone know the full difference between a locator/DME approach and an NDB/DME approach when the aid in use appears to be the same NDB, but there are separate approach plates (and also tracks) for the two approaches.

I've used NDB approaches many times, but never a locator approach. I understood that a locator is a loess-powerful NDB, which is used as a middle or outer marker.

Any practical and useful tips welcome.
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Old 18th Jun 2006, 17:43   #2 (permalink)
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Interesting question. Speaking as a controller I thought they were the same thing. Can you give an example where both are available on the same beacon - maybe looking at the plates will give a hint?

I've always thought that the difference between an NDB and LOC was that the former was certified and flight checked as both an en-route aid and a let-down aid (i.e. does it give a good signal close to the ground) whilst the latter was only checked for en-route nav.
Old 18th Jun 2006, 17:50   #3 (permalink)
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Can you give details of the two approaches you mention? I'd be interested to take a look.

If the beacon in use is the same for both the L/DME and NDB/DME approaches, perhaps the use of one track rather than another allows lower minima to be used (ie better obstacle clearance), but the other is aligned with the Runway QFU (as could be the case when the beacon is specified as an 'L' or 'OM' for another approach).

I can't see any operational implications of using one rather than the other though; if the weather is [email protected] pick the one with the lowest minima, if it's nice pick the one with the minimum runway/FAT offset. Or as cleared by ATC, natch.
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Old 19th Jun 2006, 08:40   #4 (permalink)

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As I understand it, the difference is a certification issue related to output power of the beacon and its rated reception distance. The common, garden-variety NDB has greater range and is used for enroute track guidance as well as instrument approach. In the case of the LOC, there is normally no requirement to use the aid for enroute track guidance so the output power is lower and, as a result, the rated reception distance (sometimes called rated coverage) is a lot less.

The LOC is often associated with an ILS as either the OM or MM, or both, which might be cheaper than installing marker beacons - though I could stand to be corrected on that point.

There's no difference from a procedure design perspective, except perhaps to ensure that the procedure keeps aircraft within the rated coverage of the LOC. Pilot tracking standards will be the same, whether using NDB or LOC.

I can't think of any situation where 2 different approaches are based on the same navaid and, in one, it's referred to as a LOC and in the other as a NDB. About the only thing that comes to mind is that, maybe, one procedure was designed some years ago when the aid might've be classified as, for example, a LOC, and the other procedure is more recent, perhaps as the result of an upgrade of the navaid from LOC to NDB.

Hmmm... having just re-read that paragraph, I'll bet it's as clear as mud...
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Old 19th Jun 2006, 13:35   #5 (permalink)
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Could it be that a Locator App uses a NBD located say 4 miles away from the MAP, usually alligned with the FATrk (ie East Midlands/Cranfield) and a NDB App uses a NDB Co-Located on the airfield, usually off set to the FATrk (ie Gloucester/Exeter).

Mr L.
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Old 19th Jun 2006, 17:36   #6 (permalink)
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Here is a brief description of the NDB and when it is used in conjunction with an ILS approach

Compass Locator
1. Compass locator transmitters are often situated at the MM and OM sites. The transmitters have a power of less than 25 watts, a range of at least 15 miles and operate between 190 and 535 kHz. At some locations, higher powered radio beacons, up to 400 watts, are used as OM compass locators. These generally carry Transcribed Weather Broadcast (TWEB) information.
2. Compass locators transmit two letter identification groups. The outer locator transmits the first two letters of the localizer identification group, and the middle locator transmits the last two letters of the localizer identification group.
Nondirectional Radio Beacon (NDB)
a. A low or medium frequency radio beacon transmits nondirectional signals whereby the pilot of an aircraft properly equipped can determine bearings and "home" on the station. These facilities normally operate in a frequency band of 190 to 535 kilohertz (kHz), according to ICAO Annex 10 the frequency range for NDBs is between 190 and 1750 kHz, and transmit a continuous carrier with either 400 or 1020 hertz (Hz) modulation. All radio beacons except the compass locators transmit a continuous three-letter identification in code except during voice transmissions.
b. When a radio beacon is used in conjunction with the Instrument Landing System markers, it is called a Compass Locator.
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Old 19th Jun 2006, 22:27   #7 (permalink)
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Originally Posted by OzExpat
The LOC is often associated with an ILS as either the OM or MM, or both, which might be cheaper than installing marker beacons - though I could stand to be corrected on that point.
The OM is a 75 MHz Fan Marker modulated to 400 Hz. The MM is identical but modulated to 1300 Hz and the IM to 3000 Hz. A Fan Marker gives no useful azimuth cues but a fairly precise distance from touch down indication, with a wide spread either side of the Approach Centre Line. As captjns mentions, CLs and NDBs are often collocated with OMs and MMs.

For what it's worth,

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Old 20th Jun 2006, 06:58   #8 (permalink)
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Just to cover all the bases:

Is it possible that one of the approaches is in fact a LOC/DME approach - i.e. a Localiser / DME? The two charts would have different radials outbound from the same NDB beacon for the Base Turn, but the LOC/DME would then involve turning to track the Localiser inbound, rather than tracking the NDB.

A Locator / DME approach would (I think) be written Lctr / DME.

I only mention this because I, too, have never seen two different approach charts for the same NDB beacon (so far as I recall). I gather from your post you are quite experienced, but we can all make a mistake reading a chart, particularly if you were just flicking through the JEPP book and glanced at these particular approaches in passing.

If not, then you might be about to give me both barrels for being patronising, which is certainly not my intention. However, I have found that, sometimes, it is the simplest errors of perception that are the hardest to spot and correct!

Last edited by Linton Chilcott; 20th Jun 2006 at 18:25.
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