Glideslope Fluctuation After a New Hangar

The most probable cause is multipath reflection. The hangar is a large steel structure placed close to the runway and within the region where the lower glideslope lobe can illuminate it, so aircraft at around 4 NM see the direct signal plus a reflected signal arriving at a slightly different angle. That produces the fly-up then fly-down oscillation the pilots report. The geometry explains why the localizer is unaffected: the localizer uses a much larger aperture and a different frequency band, and the reflection geometry differs. The monitor did not detect it because a monitor samples the field at one or a few fixed points near the runway, not along the approach path at 4 NM. A reflection that is strong along the approach but weak at the monitor antenna is invisible to the monitor. A normal monitor reading is evidence of signal integrity at the monitor point only, not proof that the guidance is correct along the whole approach. The crane NOTAM is a secondary candidate but is east, not south, of the centreline.

First, warn the tower and approach that the glideslope is suspect and ask them to pass the report to the next arrival. Second, pull the maintenance logs and the as-built survey for the hangar to confirm its height, position and bearing from the threshold, and compare it with the glideslope critical and sensitive areas. Third, check the monitor antenna location and the recorded monitor history for any small simultaneous variation that was dismissed as noise. Fourth, request a flight inspection; a ground check alone cannot resolve a reflection that appears several miles out. Fifth, review whether the hangar was included in the obstacle assessment for the ILS, and if not, record that as a finding. Sixth, keep a written log of the three pilot reports with times, aircraft types and distances. The coordination must include tower, approach and the airport operator, because the airport operator may be able to restrict work near the hangar or confirm a change in its structure.

Do not keep the ILS in service as a full CAT I procedure while three independent crews report the same glideslope instability. The safe action is to take the glideslope out of service and publish a NOTAM. Options are a localizer-only approach with a higher minima, or a non-precision approach if one is available and the minima are appropriate for the weather. In VMC with 10 km visibility, a visual approach is the practical choice for the next arrival, but the decision must be recorded and the approach controller must not offer an ILS glideslope clearance. Withdraw the glideslope from service rather than issuing a 'use at your own risk' caution, because the fluctuation at 4 NM could induce a destabilised approach at a critical height. Restore the glideslope only after a successful flight inspection, with the hangar either modified or the procedure formally assessed against the new obstacle.