For the first time in my life, I was established on both a localizer and glidepath side lobe.
This was last week in Alexandria, Egypt where I was vectored onto the ILS but at a much too high altitude. I was about 2500ft AGL and very close to the airfield (there was other traffic and ATC asked whether I would agree with a shorter ILS). Suddenly both the localizer and the glidepath came into view and I was established. Luckily I was in VMC already and the picture in front of me just didn’t look right. Then both needles started dancing but around the center.
I now understand how this side lobe business can get you killed… very impressive! Vectoring onto an ILS after the FAF takes some ATC knowledge which the controller didn’t have. When in IMC I will put more emphasis on looking at altitudes and I will from now on refuse any non standard ILS interception in IMC.
Thanks for sharing!
I have certainly seen LOC sidelobes. One doesn’t get a good clean intercept, but if you are sitting there with APR pressed on the autopilot, the aircraft will turn onto the sidelobe and it is the last thing you want.
That’s why when flying an ILS I always have the GPS set with a DCT to the airport, in OBC mode, and then the magenta line shows where the LOC should be, and press APR only just before reaching the line, when the LOC starts to move on the EHSI.
Isn’t the guidance sense reversed on the first LOC sidelobe?
Boy, you must have been waaaaay off the inbound course. In my day job I calibrate navaids, mostly ILS. For a localiser the clearance signal must have a minimum value of 150 uA to a point 35 deg displaced from the ILS centreline. The Course signal (i.e. +/- 10 deg of centreline) must have a signal strength of 175 uA. In other words, you should not get a flag indication until outside of 35 deg and there should be no lateral guidance until at the 10deg point. If the runway is really long the course signal will be narrower than 10 deg (all to do with signal width at landing threshold). False lobes (of which I’ve never seen on on a Loc) could only occur around the 35deg point.
Peter, I also use the DCT-OBS mode on the GPS as a back-up to the ILS and our fancy calibration kit. Only one word of warning, make sure you use DCT-OBS the ILS Loc rather than the airfield ICAO code. I fell into that trap only last week at Dubai World (OMDW) where the Airfield Reference Point (i.e. where the Garmin database places OMDW) is displaced 1.5nm North of the runway centreline! I guess they’re planning ahead for the extra runways. :)
Yes – I know that hack works only for an airport which has one instrument runway.
I looked up OMDW and it actually tells you the ARP is way off, but not many would have spotted it. I wonder if DCT OMDW takes you to the runway centre or the ARP in this case?
This is something to watch out for… thanks for the tip.
Boy, you must have been waaaaay off the inbound course.
Yes, the false localizer came during intercept. The GS has been there for some time but at one point I got the indicaton of being established although I was way too high and off the localizer course. The needles were jumpy so it was clear that something was not right (and being VMC, it was obvious the runway was too close and not straight ahead). Still, the ILS system is poorly designed with such failure modes and should have been replaced by LPV long time ago. Or at least MLS which I have only heard of during my IR training 
Peter, I can categorically confirm that DCT OMDW takes you to the ARP. 
The subsequent repositioning woke-up the copilot.
ILS is still very good if it is set-up correctly. MLS is fantastic but the world lost interest when GPS came along. Unfortunately, GPS is still not quite good enough for Cat III stuff; they’ve yet to sort out roll-out guidance.
A lot of GA avionics are a bit shagged especially when it comes to the flags. With an IFR4000 plugged in directly into the coax cabling you can check the flag levels but there is no really consistent way to do this with the antennae in the loop. How far back are you supposed to stand, and what to do about that hangar “over there”? And are you going to repeat that at all the compass bearings and plot the polar diagram to see if one of the two antennae has a dodgy connection?
And all you need is one of the two NAV (LOC) antennae, or one of the two GS antennae, to be broken, or maybe a shagged KN72 (or equivalent) and you can have lots of fun and games…
MLS must have been incredibly expensive piece of kit – definitely into 6 digits. The “box” contains 1 or more DME receivers and all kinds of fancy stuff.
It was at Lydd EGMD that I got the false LOC and now my ILS checklist says Press APR when close to the inbound track (as shown on the GPS) and when the LOC starts to come in.
I thought Cat 3 was basically
So I can’t see why you can’t have Cat 3 LPV I guess the emotional issue with 150+ passengers tracking a system controlled by the Pentagon is too much to bite off… same with IFR OCAS – a certain death
Isn’t MLS in use at London Heathrow? (Yes it is, I just checked the AIP)
I’m sure I have heard a couple of BA pilots asking for this approach rather than the ILS, so it is used occasionally. I assume that any approaches I heard being requested were simply for familiarisation and practice, rather than gaining any operational advantage over ILS users – or if indeed there are any.
You’re correct in the summary of how a Cat III works. The problem with (D)GPS on the roll-out guidance is that the update rate is relatively slow (normally about 5Hz) and the timing signals tend to jump about a bit, consequently leading to less than smooth correction demands. You get a similar effect if the Loc signal on an ILS is close to the limit for roll-out guidance. Under this circumstance, the A/P starts to hunt and the aircraft will fishtail down the runway.
I’m sure they’re not far off in smoothing-out the signal processing such that GPS derived centreline guidance could be used. At that point, I’m not sure how much time there is left for ILS although, as proven with NDB, legacy systems tend to have an awfully long period of legacy.
Finners, MLS allows reduced ATC separation and increased flow rates when LVP are in force as the signal does not require the level of protection that is demanded by ILS.
