In theory the answer should be Never. You should fly through the localiser.
I notice that ATC use various terminologies to make it varying obvious what you are supposed to do.
If you get “cleared for the ILS” that is clear.
If you get “report localiser established” but not “cleared for the ILS” (the old UK way, and sometimes used elsewhere) that is also clear.
If you get “turn left 230, base leg” but not “cleared for the ILS”, that is definitely not a clearance to intercept, but it’s “obvious” what the ATCO means. I would intercept, but would say “confirm cleared for the ILS” before that, but what if the radio is busy?
If you get a series of headings, the last of which is is obviously an intercept, but none of the above phrases are used, you should fly through the localiser. But it is so rare for ATC to vector traffic through a localiser (without saying anything) that this is almost certainly an ATC mistake, and (if the ATCO doesn’t notice what has happened) at many airports you will end up in a mountain. Obvious cases with a mountain nearby and way above the ILS platform are Split and Dubrovnik.
So in that case you fly through, but then it’s obvious that the ATCO has forgotten you, which means he is probably not monitoring your obstacle clearance (which he has a duty to do if vectoring you) so you are on your own and have to navigate yourself, which is going to put you where he for sure doesn’t want you…
Just ask before you have to decide whether to fly through or intercept?
Happened many times to me, always a case of ATC having forgotten about me, never had the case where they intentionally wanted me to fly through the localizer.
In the US, if a vector is going to take you thru the final approach course. the controller is supposed to inform you of the fact. Also in the US, if there is any confusion as to what is expected, the pilot is required to get it clarified with the controller. Me: “Charlotte approach, 83N do you want me to join the localizer?”
What I was thinking is what you do when there either is not enough time to make the clarifying call by the time you realise you are approaching the localiser, or the radio has been busy the whole time. Both have happened to me.
I’d say one more fact to pay attention to is whether you are above or below the glideslope when crossing the localizer. If you are above, don’t even think of intercepting. For example, a couple of weeks ago at EBCI I was vectored approximately like this:
What I was thinking is what you do when there either is not enough time to make the clarifying call by the time you realise you are approaching the localiser, or the radio has been busy the whole time.
Even though it’s not the controller’s intention most of the time, the answer is very clear: fly through the localizer unless it’s unsafe in which case ATC instructions can be disregarded.
If not cleared to intercept, you should not intercept. There may be a perfectly good reason why ATC did not clear you. (although they may also have forgotten all about you)
That is one of the things I vividly remember was stressed during my IR TK in the US.
You do however need to maintain situational awareness. Imagine being vectored more or less to ILROU for the Calvi LOC/DME at about a 45° intercept. If you go through the LOC you will rapidly find yourself in cumulus granitus.
So I totally agree with @achimha:
Even though it’s not the controller’s intention most of the time, the answer is very clear: fly through the localizer unless it’s unsafe in which case ATC instructions can be disregarded.
This actually happened to me on my IR skills test. I realised what was happening but didn’t know whether it was deliberate. As we were at Bournemouth and heading south-west there was no immediate danger, so I held heading. My examiner eventually requested a radio check which woke up the controller and we were vectored round for another go, with many apologies.
Ironically, I’d gone to some considerable lengths to reconfigure the transponder so that I could use an EXAM call sign, in the hope that it might get me some consideration from ATC.
When I commented on the examiner’s terminology after the test, he retorted “I’d have used a much more fruity phrase if it wasn’t an exam.”
Another insignificant anecdote…
During my IR training in California, I did my IR x-c (with instructor) from Hayward to Santa Monica by night. While being vectored around for the approach by Van Nuys APP, I suddenly realised that the frequency had become extremely quiet. Several attempts at raising Van Nuys remained unanswered. Sure enough, on the hour they had closed the facility and left us hanging. A couple of frequency changes later we landed at KSMO.
Being forgotten by ATC is not infrequent. I now try to anticipate these frequency changes to avoid surprises.
Another thing that can happen during an approach is getting strange turns. During vectoring for an NDB approach in Northern California I got a turn to intercept the bearing to the station the “long away around”, i.e. more than 180 degrees. I happily complied thinking that the controller had realised I would be too close to the FAF unless he turned me the long way around… It turned out to be his mistake, but nevertheless worked out well.
Sometime in the next year or so, we hope to get access to the minimum vectoring charts in a form they may be viewed as a layer on an iPad application. I have always wanted this as a means of situational awareness when being vectored. It saves the question “what is minimum vectoring altitude at our location?”
