What altitude should I be at to intercept an ILS glideslope?
Find the correct ILS glideslope intercept altitude on the approach chart, capture from below, and avoid MSL, AGL, altimeter and ATC mistakes.
Intercept an ILS glideslope at the published glideslope-intercept altitude shown on the instrument approach chart, while established on the localiser and level below the glidepath. There is no universal altitude. Use the chart’s MSL altitude, not a guessed height above the airport, and obey ATC restrictions.
For Aviation & Real-World Flying, that published figure is the only reliable answer. An altitude such as 2,000 or 3,000 feet might be correct for one approach and dangerously wrong for another.
Where is the glideslope intercept altitude shown?
The glideslope intercept altitude appears in the approach chart’s profile view, normally beside the glideslope intercept point or precision final approach fix. Depending on the chart format, it may be associated with a lightning-bolt-style symbol, a named fix or an altitude constraint.
Approach altitudes are normally published as altitude above mean sea level. If the chart shows 2,500 feet and the airport elevation is 500 feet, fly an indicated altitude of 2,500 feet with the correct local pressure setting—not 2,500 feet above the airport.
Check the chart’s units and notes. A height in parentheses may be an AGL reference, while cold-temperature corrections may be required under the applicable national or operator procedure. The ILS beam remains fixed in space; an incorrect pressure setting or required correction can change where your indicated altitude meets it.
How should I position the aircraft for glideslope capture?
Arrive level at the published intercept altitude, establish on the localiser and let the glideslope descend towards the aircraft from above.
- Brief the chart: identify the localiser frequency, inbound course, intercept fix, published altitude and every preceding altitude restriction.
- Set the altimeter: use the applicable local pressure setting and confirm that feet, metres and barometric units have not been confused.
- Capture the localiser first: use a sensible intercept angle and become established before glideslope capture. Our guide to intercepting and tracking an ILS localiser covers that part of the procedure.
- Hold the intercept altitude: the glideslope pointer should approach the centre from above. Do not climb to chase it.
- Begin descent only after valid capture: confirm the indication, aircraft mode annunciation and clearance before following the published glidepath.
Arming approach mode does not authorise descent and does not repair poor positioning. The indications and capture logic are easier to diagnose once you understand how the ILS glideslope signal guides the aircraft.
Why must an ILS glideslope be intercepted from below?
Intercepting from below provides a predictable capture and avoids chasing the beam with a steep, unstable descent. If the glideslope pointer is already below centre before capture, the aircraft is above the glidepath.
Conventional ILS transmitters can also produce false glidepath lobes above the intended beam. Published procedures, ATC vectors and aircraft capture logic are designed around intercepting the correct glideslope from below. Do not descend aggressively to catch a pointer that has passed underneath the aircraft; request new vectors or discontinue the approach if a stable intercept cannot be made.
Which altitude numbers should I not confuse?
The glideslope-intercept altitude serves a different purpose from the other figures printed on an ILS chart.
| Chart figure | What it means | Use for glideslope intercept? |
|---|---|---|
| Published glideslope-intercept altitude | The barometric altitude at which the final glidepath should be joined | Yes |
| Airport or runway elevation | Ground elevation above mean sea level | No |
| Decision altitude or decision height | The decision point near the bottom of the approach | No |
| Localiser-only step-down altitude | A constraint used when flying the non-precision localiser procedure | Only if the chart specifically makes it part of the ILS procedure |
| ATC-assigned altitude | An instruction that remains binding until its limit or clearance condition is satisfied | Follow it and query any conflict |
What if ATC assigns a different intercept altitude?
Fly the assigned altitude until the instruction permits you to leave it, then follow the published procedure and restrictions. An approach clearance does not erase wording such as “maintain 3,000 until established”. Rules governing when descent may begin vary by jurisdiction, so clarify any assignment that appears to place the aircraft above the glideslope or below a charted minimum.
In a flight simulator, built-in ATC, navigation data and airport scenery can disagree. If the aircraft reaches the charted intercept point but the beam is displaced, verify the pressure setting, ILS frequency and identification, inbound front course, localiser capture and data compatibility. Our practical simulator guide to setting up and flying an ILS explains the complete sequence without replacing the approach chart.