ILS Glideslope Warning at Decision Altitude
You are flying a Category I ILS to a runway with a 200 ft decision altitude and 550 m visibility. On short final the localizer is centred, but at about 600 ft AGL the glideslope pointer drops to full-scale fly-up and the warning flag appears. The aircraft is stable, on speed, and the autopilot is off. Tower reports the runway visual range as 600 m and the wind as 8 kt down the runway. You have 25 minutes of fuel above final reserve, and there is a suitable alternate 20 minutes away.
1. What does the glideslope flag and full-scale fly-up most likely indicate, and what is the immediate handling action?
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A glideslope warning flag means the receiver is no longer receiving a valid glideslope signal; the pointer's full-scale fly-up position is then meaningless and must not be followed. Likely causes are a ground transmitter fault, loss of the glideslope signal due to shielding or interference, or a receiver failure. It is not a genuine fly-up command, because a real fly-up would not normally be accompanied by a valid-signal flag.
Immediate action: ignore the glideslope pointer, maintain the current flight path using the localizer and altimeter, and do not chase the needle. Announce the failure, cross-check the localizer and DME/altitude against the published profile, and prepare to revert to a localizer-only approach or a missed approach. The aircraft is stable and the localizer is centred, so there is no need for abrupt control input.
2. Decide whether to continue to the decision altitude or go around, and justify it.
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With the glideslope unusable, the approach can no longer be flown as a Category I ILS to 200 ft. The correct decision is to go around unless a published localizer-only approach with a higher minimum is available and can be flown. Continuing to 200 ft using altimeter and localizer alone would breach the approach minimum for the aid actually available and would remove the vertical guidance that protects obstacle clearance.
A go-around is also the better fuel decision: 25 minutes above final reserve is ample to fly the missed approach, and the alternate is 20 minutes away. Continuing risks an unstable approach at minimums with no vertical guidance in 600 m visibility. The disciplined choice is to break off, advise ATC of the glideslope failure, and either request a localizer-only approach with its higher minima or divert.
3. What would change your decision if the flag had appeared at 1 500 ft AGL instead?
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An earlier failure at 1 500 ft gives more time and height to react but does not change the principle: a flagged glideslope is unusable. It would, however, allow a smoother transition. You could level off, notify ATC, and request either a localizer-only approach or vectors for another aid. The extra height also allows a check of whether the failure is receiver-related by selecting another navigation source or another ILS if available.
The key point is that the decision rests on the validity of the guidance, not on the height at which it fails. Earlier failure merely widens the options; the prohibition on using a flagged glideslope to 200 ft remains.