"John Harper" wrote
There's been at least one case where a controller vectored someone
through the localizer because he'd forgotten him, and they flew into
a mountain. It's best to avoid that. OTOH the other posters correctly
say that just turning onto the localizer without an instruction is
wrong too. The controller should already have warned you if
he is going to fly you through the loc though. I've been flown through
the loc several times, once was for spacing and I was warned, the rest
were all being forgotten (maybe 3-4 times).

My take is this. You need to know the terrain around you. If you're
clear of terrain, just fly the heading, and ask as soon as possible what's
going on... "21Z has flown through the localizer". They'll soon wake
up when they hear that. But if there's the slightest risk of CFIT
then 91.3 applies. If you're headed for the hills (e.g. at San Jose)
and you can't get a word in within 30 secs or so (at spamcan speeds)
then vector yourself away from terrain. They've got you on radar.
But 30 secs is a looooong time, even Norcal 135.4 (Oakland approach)
is rarely solid busy for that long.

Norcal has once vectored me into the hills that were straight ahead of me
for "traffic avoidance". Well, sure, there wasn't any other traffic there.
The U word ("unable") has its place too.

I think this is about the best answer I've seen so far. I would only
add that a vector TOWARDS terrain is not the same thing as a vector
INTO terrain. At low alitudes, almost any vector takes you towards
terrain - eventually.

IMO the thing to do is decide in advance how close you're willing to
get to hard stuff, and not get any closer. A corollary to this (as
stated above) is that you must know where you are. Legally, you are
being vectored and the controller is responsible for terrain
avoidance, but that won't mean much to your pax.

The only other point I would make is this - you need not be in the
mountains for this to be an issue. For example, on the West side of
Houston, most of us go to SGR to shoot ILS approaches. For those
playing along on the home game,
http://www.myairplane.com/databases/...l/SGR_ir35.pdf

Typically, you get vectors to final at 2000 MSL, and are vectored on
the West side of the localizer. I would give the controller about 2
minutes max after crossing the loc before I took action on my own.
The towers are about 8 miles past the loc on a typical vector, and
some of them stick up above 2000.

On the flip side, SGR is a major reliever, and when the weather is
scuzzy the traffic on the ILS ranges from spam cans to bizjets.
Vectors for spacing are often necessary, and typically the same
controller is working SGR and the airliners going into HOU, so the
frequency gets busy. Deciding to turn in on the loc without being
cleared for the approach is bad juju.

Michael