Current pilot mindset makes it very difficult the distrust the ILS, despite conflicts with other instrument readings. But the ILS is not infallible. The recent MyCargo 6491 crash was a case of 'false glideslope'. Another potential threat is 'erroneous glideslope'.
The difference is that, with 'false glideslope', you are encountering a 'mirror' set of side lobes AFTER the actual glideslope at an angle steeper than the actual glideslope (though potentially reversed in phase), whereas with 'erroneous glideslope' you are encountering only the glideslope carrier signal, which carries both the 90HZ and 150HZ signals in balanced proportion. The carrier signal alone will always result in an 'on glideslope' indication no matter where you capture it, so you would tend to capture it from below (or immediately upon selecting APP) and begin your descent BEFORE the actual glideslope. 'Erroneous glideslope' is caused by a lack of side lobe signals, which, in this case, was due to a maintenance procedure where the back-up system was improperly left in bypass mode. That makes this an extremely rare and unlikely scenario. Still, without TAWS or EGWPS, only visual cues very near to disaster alerted this crew to go-around...
Very interesting video recreation of the incident here: (note: abundance of acronyms may make 3WE's head explode)
The difference is that, with 'false glideslope', you are encountering a 'mirror' set of side lobes AFTER the actual glideslope at an angle steeper than the actual glideslope (though potentially reversed in phase), whereas with 'erroneous glideslope' you are encountering only the glideslope carrier signal, which carries both the 90HZ and 150HZ signals in balanced proportion. The carrier signal alone will always result in an 'on glideslope' indication no matter where you capture it, so you would tend to capture it from below (or immediately upon selecting APP) and begin your descent BEFORE the actual glideslope. 'Erroneous glideslope' is caused by a lack of side lobe signals, which, in this case, was due to a maintenance procedure where the back-up system was improperly left in bypass mode. That makes this an extremely rare and unlikely scenario. Still, without TAWS or EGWPS, only visual cues very near to disaster alerted this crew to go-around...
Very interesting video recreation of the incident here: (note: abundance of acronyms may make 3WE's head explode)
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