Learn when to arm autopilot approach mode for an ILS, why LOC should capture before GS, and how to prevent common glideslope failures.
In real-world aviation, arm autopilot approach mode after the ILS is tuned and identified, the front course is set, and the aircraft is cleared to intercept from below on a sensible heading. Arm it before glideslope interception—normally as the localizer comes alive—so LOC captures first and GS captures from below.
What is the correct ILS arming sequence?
The safe sequence is tune, identify, intercept from below, arm APP, and then verify localizer and glideslope capture on the flight mode annunciator.
- Set up the ILS. Tune the correct frequency, identify the station where that facility is available, select the appropriate navigation source and set the published inbound course. If the flight management system auto-tunes the ILS, verify its selection rather than assuming it is correct.
- Join the intercept. Fly the published track or the heading assigned by air traffic control while maintaining the authorised altitude. The aircraft should be below the glideslope, using an intercept angle within the autopilot’s limits; about 20–30° is common during radar vectors.
- Arm APP or APR. Select approach mode when the localizer indication starts moving towards the centre. Do not wait until the glideslope is centred.
- Check the annunciations. Confirm that localizer and glideslope modes are armed. A lit APP button alone does not prove that the guidance modes are ready to capture.
- Monitor the capture order. The localizer should become active first, followed by the glideslope as it moves down towards the centre. Intervene if the aircraft turns or pitches unexpectedly.
For a simulator example covering the whole process, including tuning and mode selection, follow our complete MSFS 2024 ILS sequence.
Should APP be armed before or after localizer capture?
On most autopilots, APP should be armed before localizer capture once the aircraft is cleared and correctly positioned. Approach mode waits for valid capture conditions; selecting it does not normally command an immediate turn or descent.
Some aircraft procedures instead call for LOC mode first, followed by APP after the localizer is captured. This is also appropriate when ATC has cleared the aircraft only to intercept the localizer. LOC supplies lateral guidance without glideslope capture, as explained in our comparison of LOC-only and full ILS guidance.
If using LOC first, arm APP as soon as the localizer is established and while the aircraft remains below the glideslope. Aircraft manuals and operator procedures take precedence because capture logic differs between autopilot installations.
Why must the glideslope be intercepted from below?
An ILS glideslope should be intercepted from below because the published procedure and normal autopilot logic are designed around the beam descending towards the aircraft. Many systems will not capture reliably after the aircraft has crossed above it.
Approaching from above also risks chasing the indication with an excessive descent rate. ILS transmitters can produce false glideslope lobes above the normal path, another reason never to descend steeply in an attempt to force capture. If the aircraft is too high, correct the approach under the applicable clearance or abandon and re-establish it.
How can I tell whether APP is armed or captured?
The flight mode annunciator is the definitive indication of what the autopilot has armed and captured. Labels and colours vary, but many glass cockpits show armed modes in white and active modes in green.
| Approach phase | Typical lateral indication | Typical vertical indication |
|---|---|---|
| Before localizer capture | Current mode active; LOC armed | Current altitude mode active; GS armed |
| After localizer capture | LOC active | Current vertical mode active; GS armed |
| After glideslope capture | LOC active | GS active |
Some systems do not display glideslope armed until the localizer is captured. Treat the aircraft’s flight manual and annunciator logic as authoritative.
Why does the localizer capture but not the glideslope?
A missed glideslope capture is usually caused by interception geometry, tuning or mode selection rather than an autopilot failure.
- The aircraft is above the glideslope. Level flight will not make a beam below the aircraft move into a valid capture zone.
- APP was selected too late. The aircraft may already have passed through the glideslope while altitude hold remained active.
- LOC mode is active by itself. Localizer mode captures lateral guidance but does not arm vertical ILS guidance.
- The wrong source or frequency is selected. Check the active NAV receiver, runway, ILS frequency and inbound course.
- The intercept is too close or too aggressive. The autopilot may be unable to turn onto the localizer before crossing it.
- Another mode cancelled APP. Selecting heading, navigation or a different guidance source can cancel an armed mode. The annunciator reveals this immediately.
Simulator aircraft can add model-specific faults or incomplete capture logic. Anyone seeing this behaviour in Flight Simulator X can use our FSX-specific glideslope capture checks.
Does APP mode authorise descent or guarantee autoland?
No—arming APP neither authorises descent nor guarantees an automatic landing. In real-world flying, remain at the assigned or published altitude until the approach clearance and procedure permit descent; on many autopilots, glideslope capture can cause the aircraft to leave altitude hold.
Approach mode only captures the available lateral and vertical guidance. Autoland requires a suitably equipped aircraft, an eligible ILS, the correct autopilot configuration and the required cockpit annunciations. Otherwise, disconnect and land manually in accordance with the applicable procedure and minima.