False glideslope capture on a high, close ILS intercept
An aircraft is cleared for the ILS runway 27 at a coastal airport (aerodrome elevation 500 ft, glide path 3.0°, DME at the threshold). The crew is high and close: at 6 DME they are cleared to 4,000 ft and told to intercept the localizer. At 5 DME the glideslope needle centres at 3,950 ft and the autopilot starts down. At 3 DME the needle is still centred at about 3,300 ft, but the charted 3° height at 3 DME is about 1,955 ft. The radio altimeter reads 2,800 ft. The localizer is steady, no GS warning flag is displayed, and ATC has just cleared a preceding departure to climb through 3,000 ft. The aircraft is in IMC; the missed approach is a climbing right turn to 4,000 ft. Fuel before diversion is 35 minutes, and the diversion airport is 25 minutes away. The captain has flown the approach many times and says the glideslope is 'always a bit high here'.
1. What is the most likely explanation for the glideslope indication, and what cross-checks confirm it?
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The aircraft is about 1,345 ft above the charted 3° path at 3 DME. A centred glideslope at that height and distance corresponds to a path of roughly 8°, not 3°. A normal glide path cannot be centred there. The most likely explanation is a false glideslope lobe, typically formed above the true path, captured because the aircraft intercepted from above and close in. A genuine GS transmitter anomaly is also possible; either way the signal is not usable for a 3° approach.
Confirm with DME versus altitude: at 3 DME the 3° height above the threshold is about 3 x 6,076 x tan 3° = 955 ft, plus 500 ft aerodrome elevation, about 1,455 ft above mean sea level, or about 1,955 ft if using the charted figure. The aircraft is far above that. Cross-check the radio altimeter, the published profile, the raw GS deviation, and an independent source such as RNAV or the ILS DME arc. A terrain awareness display also shows the aircraft is not on a normal descent profile. The absence of a warning flag does not prove the signal is valid. The captain's 'always high here' expectation is precisely the kind of belief that must be overridden by instrument cross-check.
2. State the immediate actions and the ATC call.
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Disconnect the autopilot, apply go-around power and fly the published missed approach (climbing right turn to 4,000 ft). Do not follow the glideslope down and do not attempt to 'salvage' the approach by descending steeply; the aircraft is high, close and possibly in conflict with the departing traffic climbing through 3,000 ft.
Call ATC promptly: 'Going around, false glideslope, request vectors for another approach.' Tell them the aircraft is high on the missed approach and ask for traffic information on the departure. If the missed approach track conflicts with the departure, say so and request an alternative heading or altitude.
Then decide whether a second approach is realistic. A proper intercept from below the glideslope is needed. With 35 minutes before diversion and a 25-minute diversion, there is little margin for a second attempt plus a missed approach. If the crew is not fully re-established, divert. The localizer being steady is not a reason to continue; the vertical path is the unsafe element.
3. What reports should follow, and what human factors are involved?
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Report the event to ATC and to the operator: time, DME, altitude, glideslope indication, weather, and the fact that the approach was abandoned. File the occurrence report required by the operator's SMS, and notify the navaid maintenance organisation so the GS transmitter and its critical area can be checked. If a departing aircraft or vehicle was in the critical area, that detail matters.
The human factors are expectation bias and automation confidence. The captain expected the glideslope to be high, and the autopilot appeared to capture and track a valid signal. The monitoring pilot's DME/altitude cross-check was the defence that broke the error chain. A good crew brief before intercept should state the expected altitude at each DME and the action if the GS is not within tolerance. Stabilised-approach criteria should be applied early, not at 500 ft.