ILS Approach with a Glide Path Anomaly at Minimums
You are flying a Category I ILS to a runway with a 3.0° glide path. Weather is at the Cat I limits: RVR 600 m, ceiling 200 ft. On final, established on the localizer, the glide path pointer centres normally until about 1,200 ft AGL. Below that, the glide path pointer begins to ride full-scale fly-up while the localizer remains centred. Your DME distance (co-located with the glide path) reads 3.1 NM and decreasing steadily. The marker beacon has not yet sounded. The aircraft is stable, on speed, and the autopilot is off. You are the pilot flying; the other pilot is monitoring. ATC has cleared you for the approach and there is no reported traffic on the runway.
1. Interpret the glide path indication. What physical causes produce a fly-up indication below about 1,200 ft on a serviceable-looking ILS, and how do you discriminate between them using the information available?
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A glide path pointer that goes full-scale fly-up means the receiver believes the aircraft is far below the path. At 3.1 NM DME on a 3.0° path the nominal height is about 3.1 x 6076 x tan(3.0°) ≈ 990 ft above the glide path origin, i.e. roughly 950-1,000 ft above runway level. If your altimeter and DME agree with that geometry, the aircraft is not actually low, so the indication is false.
Candidate causes:
- False glide path / side lobe capture: the receiver has locked onto a reflected or spurious lobe. These are usually above the true path and produce fly-down, not fly-up, so this is less likely here.
- Ground reflection multipath from a large object (hangar, terrain, aircraft on a taxiway) in the first Fresnel zone of the glide path array. This is the classic cause of a smooth but wrong fly-up or fly-down at low height.
- Glide path transmitter or monitor fault with the monitor not yet tripping the changeover.
- Receiver/antenna or processing fault in the aircraft.
- Terrain/obstruction in front of the glide path array producing a distorted lower path.
Discrimination: cross-check the barometric altimeter against DME geometry (3.1 NM at 3.0° should be about 990 ft above the path origin). If altitude and DME agree, the aircraft is on the correct geometric profile and the indication is false. A quick check of the localizer (centred), the marker (not yet sounded, consistent with being above the outer marker), and any other aircraft reports helps confirm whether the problem is airborne or ground-based. The fact that both pilots see the same indication and the aircraft is stable rules out a handling problem.
2. State your decision at this point and the reasoning, referencing the applicable minima and the approach ban concept.
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Decision: go around now, do not continue below the point where the glide path became unusable.
Reasoning:
- A Cat I approach requires a usable glide path to the decision altitude. Once the glide path indication is full-scale fly-up and demonstrably false, the approach is no longer a valid ILS approach; continuing would be flying a localizer-only approach with Cat I ILS minima, which is not permitted unless the procedure is published and authorised as a localizer-only approach with its own higher minima.
- The weather is at the Cat I limits (RVR 600 m, ceiling 200 ft). There is no margin to absorb a degraded vertical profile. At 200 ft ceiling you will not see the runway environment early enough to salvage a localizer-only descent.
- The approach ban principle applies: if the reported RVR/visibility falls below the applicable minima, or the approach cannot be conducted to the required standard, the approach must not be continued. Here the equipment, not the weather, has failed the standard.
- Announcing the go-around early, while still above 1,000 ft AGL with a stable aircraft, is the lowest-risk option. Continuing to "see if it recovers" burns the margin and puts you at decision altitude with no vertical guidance and no visual reference.
Tell ATC immediately: "Going around, glide path unreliable." This also protects the next aircraft.
3. After the go-around, what would you report and what would you expect ATC to do?
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Report specifics, not just "go-around":
- What failed: glide path indication went full-scale fly-up below approximately 1,200 ft AGL while the localizer remained centred and DME/altitude geometry confirmed the aircraft was on profile.
- When and where: on final at about 3.1 NM DME, inside the outer marker.
- Aircraft state: stable, autopilot off, no aircraft malfunction suspected, localizer normal.
Expect ATC to:
- Protect the runway and suspend further ILS approaches to that runway pending a check.
- Notify the ILS ground maintenance/engineering so the glide path monitor and transmitter can be checked; a monitor that has not alarmed is itself a finding.
- Offer an alternative: a different runway, a different approach type (e.g. localizer-only with its higher minima if published, or an RNAV approach with vertical guidance if available and authorised), or holding for weather improvement.
- Pass the report to following aircraft so they are not surprised.
You should also file an occurrence report, because a glide path anomaly at low height is a serious safety event and the engineering follow-up depends on accurate details.