There is no mystery about occurrences of wrong instrument landing system (ILS) indications (Flight International, 25-31 March). I had an identical experience at a UK airport some years ago in an Airbus A310. There was a perfectly centralised glideslope indication, with identification and without any warning flags, while at the same time the aircraft was in fact deviating from the glideslope.
It was explained that this happens as a result of maintenance, where one part of the glideslope beam is switched off and the other part is left transmitting. In these circumstances during maintenance, the identifier should also be switched off to indicate the ILS is unserviceable. After maintenance, both parts of the beam should be switched back on. It appeared that one of these things had been forgotten.
I should also point out, on an associated subject, that modern automatic direction finders, receiving signals from imperfect old non-directional beacons, can be caused to have their needles do a 180° turn up to 3.5km (2nm) before passing the beacon.
Both incidents have been the subject of mandatory occurrence reports so it is disappointing that the subjects are not general knowledge. Why is it that the system for disseminating safety information worldwide doesn't work effectively?
Capt T A Davies (retired) Cranleigh, Surrey, UK
Source: Flight International
