does a 5 degree ILS scan cause problems in strong headwinds? I
What's that?
there was a strong crosswind and I it appeared that we came in at an angle of more than 5 degrees to the runway (although that may have been an optical illusion) or do pilots have to rely on a visual approach in such circumstances?
Ah....there's nothing scanning, in the radar sense..the ILS localiser defines an electronic centreline using a couple of fixed signals. The combination of those two signals is picked up by a very simple non-scanning VHF aerial on the aircraft and decoded by a magic box to give the pilots a left/right/ on centreline indication. It doesn't matter within very broad limits where aircraft is pointing, if you are physically somewhere near the centreline, even if flying through it at 90 degrees, or perhaps even flying away from the airport you'll still get the Left/Right/Centre indications on the Flight Instruments.
Moving to the case you describe, crosswind, yes the aircraft will be pointing off at an angle to the runway but you/or the autopilot still tries to fly (track) along the ILS/runway centreline. As an extreme example the likes of a 777 will autoland, using the ILS, with a crosswind of 40 knots - in that case the aircraft just tracks along the centreline, despite the nose pointing away from the runway by very roughly 20 degrees..(that said, you don't really want to actually touchdown with that amount of drift still applied, but that's a gear/tyre issue rather than a problem with the ILS).
BTW having operated into LHR for over 20 years the ATC pros here are coming up with a lot of "blimey, I didn't know that" stuff - thanks ladies/gents
