How does the ILS glide slope guide aircraft down to the runway?
A mast beside the touchdown zone sends two tones that tell the aircraft receiver whether it is above or below the descent path, and the AGL layout keeps vehicles and aircraft off the ground in front of it.
The ILS glide slope is the part of the instrument landing system that gives vertical guidance down to the runway. An antenna mast stands beside the runway, level with the touchdown zone, and radiates two signals on one UHF frequency, modulated at 90 Hz and 150 Hz. Above the glide path the 90 Hz tone is stronger, below it the 150 Hz tone is stronger, and on the path they are equal. The pilot or autopilot keeps the glide slope pointer centred, which gives a steady descent of usually about 3°. The frequency is paired with the localizer, so one channel selection tunes both. ICAO documents say glide path; glide slope is the term many airports and the FAA use.
The glide slope is not part of the AGL system, but the two work as a pair. The glide slope brings the aircraft down in cloud, and the approach lights and touchdown zone lights take over visually when the pilot sees the runway. At many runways the PAPI is set to an angle that agrees with the ILS glide path, so the instrument and visual guidance lead to the same touchdown area. The localizer gives the lateral guidance that goes with it.
Most glide slope systems depend on the signal reflecting from the ground in front of the mast, so that ground is kept smooth and clear. Vehicles, aircraft, snow, standing water and tall vegetation can bend the path, so the ILS critical area is kept clear whenever the ILS is in use, and the larger sensitive area is controlled during low visibility operations, using holding positions, signs and markings and, where provided, stop bars. The mast stands close to the runway, so it is made frangible and sited under the obstacle rules. Lighting, signs, cables and civil works near the mast are reviewed with the ILS owner, and the ILS may need a flight check after changes.
Frequently Asked Questions
Q1What does an ILS glide slope do?
It gives the pilot or autopilot vertical guidance down to the runway. An antenna mast beside the runway, level with the touchdown zone, sends signals that the aircraft receiver turns into a fly up or fly down indication, so the aircraft can follow a steady descent path in cloud or poor visibility.
Q2Which tone is above and which is below the glide path?
The glide slope radiates 90 Hz and 150 Hz tones on one UHF frequency. Above the glide path the 90 Hz tone is stronger, below it the 150 Hz tone is stronger, and on the path the receiver finds them equal. The difference is shown on the glide slope pointer in the cockpit.
Q3Why does the ground in front of the glide slope antenna matter?
Most glide slope systems rely on the signal reflecting from the ground in front of the antenna to form the descent path. Vehicles, aircraft, snow, standing water, tall vegetation or new structures in that area can change the reflection and bend the path, so the area is kept smooth and clear while the ILS is in use. The exact area is set by the navaid owner and the airport.
Q4Is the glide slope part of airfield ground lighting?
No. It is a radio navigation aid owned and maintained by the air navigation service provider or the airport's navaid team. It guides the aircraft down by radio, and the approach lights, touchdown zone lights and PAPI take over visually when the pilot sees the runway. They meet at the airfield layout and at the rules that protect the ILS signal.
Q5Does the PAPI have to match the ILS glide slope?
At many runways the PAPI is set to an angle that agrees with the ILS glide path, so the instrument and visual guidance lead the aircraft to the same touchdown area. The angles and the eye height over the threshold are set in the design documents, and the PAPI is checked against the ILS when either is changed.
Q6Can lighting or civil work near the glide slope affect it?
Yes. Metal structures, signs, fences and cable work close to the mast, or earthworks that change the ground in front of it, can alter the signal. New or moved lighting, signs and civil works near the glide slope are reviewed with the ILS owner, and the ILS may need a flight check afterwards.
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