I have never experienced any side lobes that deserve the name. Occasionally, the needles will briefly centre (sort of), but they then wiggle around a lot, making it obvious that you are in fact not on the loc.
Also, the problem (if there ever was one) has disappeared many years ago with the advent of GPS and moving maps, always showing you where you are in relation to the loc.
Most ATC who can vector somewhere between IAF and FAF should have a radar for a position fix which has own limitations but those are unrelated to ILS errors (except for pilot/atc human limitations)
Can someone reassure me, it seems to me you can’t be on a side/fake lob for both LOC+GS with stable neddles? If this can happen, I think a graphical representation would help…
In the calibration world we always check the signal at the edges of the ‘main’ lobes. I cannot recollect ever having two concurrent false signals, even when we have various alarm modes set (high, low, left, right, wide and narrow for both glide and localiser). Most false signals (lobes) are spurious, relatively short lived and will be outside the +/-35deg arc laterally and 0.45θ to 1.75θ, where θ is the notional GP angle (usually about 3 degs). If you are unlucky enough to get a LOC false lobe, you are outside 35degs of the centreline (10 deg if between 17-25nm range). A normal glide path signal has a 3-5 deg width. To be precise, a GP width is the angle derived from 2 * INV TAN (105/distance from LOC to threshold in meters).
PS. The first GP false lobe is at about 8.5-9 deg. If you don’t notice an anomaly at that angle/rate of descent, you should consider taking-up golf as an alternative hobby. :)
Interesting. I reckon I was 30-40 degrees off. The deviation bar was stable; it moved towards the centre, crossed it (about 1nm before that pic was taken) and moved off it, smoothly. No flag. The GS was flagged and that is a key safeguard, but someone who was completely confused could fly a LOC-only approach on this.
The first GP false lobe is at about 8.5-9 deg
Do you mean the first lobe, or the first lobe which is “the right way up”? AIUI there should be a lobe at 6 degrees but it is the wrong way up. I once tried to fly that lobe and rapidly got really fast 
As to Bosco’s point about having sufficient situational awareness from other (non “classical IFR”) sources to ignore sidelobes, sure, but that’s a different discussion. In classical IFR you use the ADF for this; when the needle is on the right bearing you can expect to see the real LOC signal, and only then you press APR on the autopilot. The other day I spoke to an Easyjet pilot who told me they still do this, or did recently. The modern way is to use a moving map GPS, like my pic above, in the OBS mode, with a DCT to the airport, which gives you a magenta line where the LOC should lie (approximately, since a DCT to the airport gives you a DCT to the ARP …) but AFAIK very few people do it that way (most pilots have no idea about the OBS mode).
I’ve never seen a 6 deg false lobe. What you may get is a dark area between the main lobe and the first false one. On a system that is pushing-out too much power I suppose you could get an apparent reversal of the 90 & 150hz signals in this dark area. The system would have to be way out of specified tolerances though.
OBS – In the scenario you have there I would have tuned NAV 2 to the Jersey VOR and set-up a single needle pointer to give me a real-time angular displacement from the centreline – a second cross check which also release the GPS for some more fancy stuff.
I would not consider localizer side lobes to be a risk factor. If you are flying in a non radar environment, you should be following airways, stars, transitions, and procedure routes. If you are in a radar environment, you should not be vectored to intercept the localizer anywhere close to a side lobe. Since I don’t fly these ancient systems any longer, they are irrelevant to flying an LPV. :)
The key is when one is “expecting” the localiser.
If, on the flight shown, I had pressed the APR button shortly before the moment I took that photo, it would have intercepted and turned. Good signal and no flag → it’s gonna go for it
So one has to “somehow know” from other sources when one is close to the localiser.
NCYankee wrote:
Since I don’t fly these ancient systems any longer, they are irrelevant to flying an LPV. :)
I guess you don’t bother with any CAT II or CAT III approaches then? I’ll take a CAT III ILS ahead of an LPV every time.
Dave_Phillips wrote:
I guess you don’t bother with any CAT II or CAT III approaches then? I’ll take a CAT III ILS ahead of an LPV every time.
True, but both I and my Bonanza are not authorized to fly anything other than CAT I ILS. ILS are too fickle and more work to fly than the LPV. Other than for practice and training, I haven’t had a need to fly an ILS since 2008.
“Since I don’t fly these ancient systems any longer, they are irrelevant to flying an LPV”
Still an ILS have a signal error/precision with well defined geometry compared to an LPV signal, the errors from the latter will be hard to debug or understand while flying (one can get convinced by looking at the amount of critical items to be checked before starting an ILS vs an LPV)
