Centred Glideslope That Cannot Be True
On an ILS approach to runway 33L at Kuwait (threshold elevation 206 ft, glideslope 3.0°), you are cleared to intercept the localizer at 3,000 ft. At 8.0 DME from the threshold, the localizer is centred, the glideslope needle is exactly centred, and the altimeter reads 3,500 ft with QNH 998. The DME is tuned to the ILS. The chart shows a procedure altitude of 3,000 ft at 8 DME and a 3° path. The autopilot is off. ATC has just sent a preceding aircraft around for 'glideslope indications'. The first officer says the glideslope is perfectly centred and suggests continuing because the localizer is also centred. You are in IMC and the runway is not yet in sight.
1. Calculate the nominal ILS path height at 8.0 DME and compare it with the indicated altitude. Convert the difference into approximate glideslope dot deviation.
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For a 3° path, height above threshold = DME × 6,076 × tan 3°. tan 3° = 0.05241, so one nautical mile gives about 318 ft of path height.
At 8.0 DME: path height = 8 × 318 = 2,547 ft above threshold. Add threshold elevation 206 ft: nominal altitude = 2,753 ft.
Indicated altitude is 3,500 ft, so the aircraft is about 3,500 − 2,753 = 747 ft above the nominal path. The angular error is arctan(747 ÷ (8 × 6,076)) = arctan(0.0154) ≈ 0.88°.
A typical glideslope full-scale deflection is 0.7°, so 0.88° is beyond full scale. One dot is about 0.35°, roughly 297 ft at 8 DME, so 747 ft is about 2.5 dots fly-down. A centred glideslope needle is therefore incompatible with the altimeter and DME.
2. What are the likely causes, what cross-checks would you make, and what immediate action is required?
Read the worked answer
The likely causes are a false glideslope lobe, a faulty glideslope receiver or display, or an incorrect altimeter setting/DME reference. The localizer being centred only confirms azimuth, not vertical path.
Cross-checks: confirm QNH 998 and the altimeter reading, compare DME against the chart distance/altitude table, check the localizer and any marker or RNAV distance, and compare with the preceding aircraft's go-around. Do not use the glideslope as the primary vertical guide when it conflicts with DME/altitude by more than full scale.
Action: go around. Apply takeoff/go-around power, set the missed-approach attitude and climb, follow the published missed-approach procedure or ATC instructions, and tell ATC that the glideslope indication is unreliable. Do not continue to descend on a centred needle that puts you 747 ft above the 3° path at 8 DME.