I imagine checking ILS accuracy by measuring ILS signals at various points along the approach path is routine maintenance, although I don't know how it is done. How often must this kind of check be done to ensure the ILS is still accurate enough?

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    $\begingroup$ Related: aviation.stackexchange.com/questions/35341/… $\endgroup$ Commented Feb 2, 2020 at 14:14
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    $\begingroup$ I don't know the exact answer (hence why this is just a comment) but from experience the ILS is monitored continuously, checked by a vehicle driving down the runway weekly and calibrated (by using an aircraft) twice yearly. $\endgroup$ Commented Feb 2, 2020 at 14:15
  • $\begingroup$ I think there is a difference between inspection and monitoring. Your use of the word “checked” may be too ambiguous. $\endgroup$
    – Dean F.
    Commented Feb 2, 2020 at 14:33

1 Answer 1


The FAA has a Flight Inspection Manual which describes the checks that must be done and how often. Table 4.1 shows that an ILS must be flight checked every 540 days. Section 15 describes what happens during one of these checks. It's quite an extensive inspection!

  • $\begingroup$ Note that many countries (at least in Europe) seem to be working with a 180 day interval for ILS calibration. $\endgroup$ Commented Feb 2, 2020 at 14:31
  • $\begingroup$ @J. Hougaard - Are there any specifics on routine monitoring? $\endgroup$
    – Dean F.
    Commented Feb 2, 2020 at 14:34
  • $\begingroup$ @DeanF. Been digging around a bit, doesn't seem like the ICAO provides that many specific intervals (although they do describe quite extensively what exactly has to be done - see DOC 8071). So it would seem it is largely up to member states to determine how often calibration is needed. As for monitoring alone, that is done continuously $\endgroup$ Commented Feb 2, 2020 at 14:37
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    $\begingroup$ @PerlDuck They probably don't do all runways at the same time, hence the large number of announcements $\endgroup$ Commented Feb 2, 2020 at 17:48
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    $\begingroup$ @J.Hougaard When possible / needed all runways are done in one go. Individual runways then are done after adjustments to the ILS system or modifications to the direct environment (to assess impact of possible signal reflection interference). Some runways in Frankfurt have dual ILS installations with two different glide slopes (3.00° & 3.20°), so there is quite a lot to calibrate. $\endgroup$
    – DeltaLima
    Commented Feb 4, 2020 at 16:05

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