Locator false lobes
 

Can the ILS locator also have false lobes, like the Glide Slope?

I once armed approach mode too early when being vectored to an ILS, and the plane turned around four miles early. I don't know whether it was a "false lobe", or some other extraneous signal, or perhaps simply a fault with the autopilot.

that virtual rwy centre line signal is called "localizer". and YES, it can have those side-beams. actually i'm not so sure about the GS having those alike.
usually its known which airports/rwys have those and they are fairly easy to avoid.

Yes.

The LOC (LLZ) uses the same 90MHz/150MHz signal modulation technique as the G/S signal.

A LOC carrier signal is a VHF signal in the 108-112MHz frequency range. That signal is modulated into a 90MHz signal for the left lobe, and 150Mhz modulation on the right lobe.

A G/S carrier signal is a UHF signal in the 329-335MHz frequency range. That signal is modulated into a 90MHz signal for the upper (high) lobe, and 150MHz for lower (low) lobe.

So, yes, its a similar technique of frequency modulation of the ground station signal. One tells you "left" or "right", the other tells you "high" or "low". Provided that you have navigated yourself to the right starting point, the zones of equal strength of 90/150 modulations have you "on track" and "on slope". Navigating to the a commencement point will be a fix or ground based locator, a lead bearing off an arc, or an ATC vector.

If you did not start at the right point, that is, outside the designed capture zone, you can track a "false" LOC lobe, and capture a "false" G/S.

You will get course reversals of the LOC bar > ±45 degrees of Localiser course.

False indications/capture is possible if LOC is used/armed > ±25 degrees of Localiser course.

Useful info here: http://www.casa.gov.au/pilots/download/ILS.pdf

Use published lead bearings as a guide to when you expect 'true' LOC capture. Use point-to-point mental nav calc/DR to anticipate proper capture of LOC.

Eg: Worst case, turning 90 degrees onto a LOC at 10nm to run to threshold. If you are doing 300kt GS, rule of thumb = 1% of groundspeed = 3nm required for the right angle turn onto the inbound track. 240kt GS = 2.4nm for the 90 degree turn.

1/60 rule applied to 10nm to run = 1 degree of azimuth @ 10 = 6 degrees of azimuth for 1nm.
3nm required for turn = 3 x 6 = 18 degrees of azimuth (bearing change) for that 300kt GS right turn.
Or
2.4 x 6 = 14.4 = roughly 15 degrees.

Well within the 25 degrees rated coverage of the LOC lobes. And thats for a right angle turn. Hopefully your STAR or vector has a shallower intercept.

Published LOC inbound course = 090M
Intercepting from the south?
Commence your Rate 1 turn crossing the (270-18) = 252 radial/bearing from.:ok:

Indeed, if the reference to "ILS Locator" was actually intended to mean LOC (LLZ), there's an excellent chance of latching onto a false LLZ signal. In many countries, the AIP warns about this and those warnings are reflected in 3rd party supplied charts, such as those from Jeppesen.

A false GP is, normally, a different situation because the most usual cause is undeclared maintenance on the GP - read the story of the way it was encountered by an Air NZ crew a few years ago. Still, all bets are off if you try to intercept the LLZ beyond about 20-25 degrees.

Most educated crews will find another way to determine when they are within about 20 degrees of the actual LLZ course. If there's no Locator (i.e. NDB or, more usually, low powered NDB) or a suitably located VOR, a certified RNAV system should provide a good indication as to when you're within the real zone of the real Localiser course.

Crews are actually paid to use their intelligence because they know that, if they capture a localiser lobe, a serviceable GP signal could take them to the scene of the prang. Rules of thumb are good for avoiding the problem! :ok:

Yes, they can occur. They do occur, actually
To trannsform a non-directional wave from a into a beam or lobe (directional) usually produces "side lobes" (usually at right angles with the intended main lobe, with less power). It happens with radar lobes, as well as in Localizers (not locator, by the way, locators are NDBs).
Think of the reflection of a light beam in a wall. You will se "side lobes".
If you arm LOC too early you can intercept one of these, instead of the main one.
I have never had an experience with a fales G/S, or I do not recall it.

Does any one of you who has had such an experience?

Localisers have sidelobes. There's usually a reverse-sense null just outside of the +/- 35 degrees azimuth service volume.

False glide slopes are pretty much part of the design and they are there. However, the first false glide slope is likely to be found at about twice the height of the normal glide slope and, in addition, reverse-sensing. If you manage to fly down a reverse-sensing glide slope at twice the nominal angle and not notice that something is amiss, it's impressive! Flying down a normal-sensing false glide slope will be a crash dive, as the first such null is found at about three times the nominal angle.

Still, that's good reason to always catch the GP from below, to do your altitude checks and be conscious of the descent rate when descending on the GP.

The Air NZ flight was the victim of something else entirely, as part of the signal normally forming the GP was turned off for maintenance purposes.

A few years back, the localiser at TBPB had a false lobe about 4 to 5 miles north of the localiser. Several reported cases of APPR Mode being armed too early (Dash 8s) and the aircraft trying to intercept the localiser midway down the west coast of Barbados (airport on the south coast).
Haven't heard anything recently, so they may have sorted it out.

ITCZ

Surely those modulating frequencies should be 90 Hz and 150 Hz IIRC.

After an excellent landing you can use the airplane again!

Yes,
90/150 Hz, not MHz.

Thanks for the replies.

My experience occured intercepting the locator at Cannes, being vectored in from the East. I'll see if I can repeat it next time I do the approach to get a better fix on where exactly it occured.