Definition
A secondary glideslope signal radiated by an ILS glideslope antenna at an angle steeper than the published glidepath, typically near 9 degrees or higher, caused by the harmonic radiation pattern of the ground-based transmitter. A receiver intercepting this signal will display valid-looking glideslope indications even though the aircraft is not on the correct approach path.
Plain English
An extra, fake glide path that an ILS antenna also sends out at a much steeper angle than the real one. Your cockpit instrument can lock onto it and look normal, but following it would put you on a much steeper descent than the real approach.
Context Anchor
Encountered during ILS approaches, especially when intercepting the glideslope near the final approach fix altitude or checking that the glideslope is captured at the published altitude.
Derivation
False' here means 'misleading' or 'not the genuine one,' not 'broken.' The signal is real and stable — it's simply not the one the pilot is supposed to use.
Why Pilots Care
Following a false glideslope produces an unstabilized, excessively steep descent that can result in landing short of the runway or a missed approach.
Grounding Statement
If the glideslope indication appears to center while the aircraft is higher than the published intercept altitude, the pilot should suspect that it may not be the correct glidepath.
Intuition Check
False does not mean the whole ILS has failed. It means the receiver may show a believable “on path” indication that is not the correct, published glidepath.
Example Sentence 1
To avoid capturing a false glideslope, the crew intercepted the glidepath from below at the published altitude.
Example Sentence 2
If the glideslope is captured well above the published intercept altitude, the crew should suspect a false glideslope and fly the published vertical path instead.