In case of a single runway with two separate ILS systems (one in each end, facing localizers) operating on two different frequencies, can both localizers be on and transmitting at the same time, or must one be switched off if the other is on?

Please refer to ICAO recommendations if possible.


1 Answer 1


ICAO Annex 10 Volume 1 has the answer: At those locations where two separate ILS facilities serve opposite ends of a single runway, an interlock shall ensure that only the localizer serving the approach direction in use shall radiate, except where the localizer in operational use is Facility Performance Category I — ILS and no operationally harmful interference results.

In plain language: only one localizer can be on at a time, unless the ILS in use is only CAT I and it has been confirmed that no harmful interference will happen.

  • $\begingroup$ Thanks for all but to make sure I understand the answer it is not possible to leave the two ils operating in the same time although the two ils have adifferent frequencies in CAT 2 $\endgroup$
    – user36963
    Commented Jan 29, 2019 at 19:45
  • $\begingroup$ @user36963 Correct. It doesn't matter that they are on different frequencies, the localizer beams (90 Hz and 150 Hz) may still interfere. $\endgroup$ Commented Jan 29, 2019 at 19:48
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    $\begingroup$ @user36963 I suggest you give Annex 10 a read. There is some additional guidance in Attachment C. $\endgroup$ Commented Jan 29, 2019 at 19:49

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