APP capture failure

Flight path

Maintain a safe heading, altitude and speed while you diagnose the approach.

Check the chart

Verify the correct ILS, runway, localizer frequency and front course.

Check the receiver

Confirm the appropriate NAV receiver is tuned and that the displayed frequency and course match the chart. FSS says that manually changing the frequency or course disconnects auto tune.

Check source and preview

With FMS selected, confirm that the correct LOC1 or LOC2 preview is displayed before pressing APP. FSS warns that APP has no effect in FMS source without the corresponding NAV1 or NAV2 preview.

Check geometry

Confirm that the aircraft is positioned to intercept the signal and is at an appropriate altitude. An armed mode cannot capture a signal the aircraft is not in a position to intercept.

Read the FMA

Look for armed and then active lateral and vertical modes. Check the deviation indications rather than relying on the button light.

Approach discontinuation

If the approach cannot be made stable or guidance is uncertain, go around. Record the airport, procedure, navdata cycle, build, source and FMA before filing a report.

FSS says automatic ILS tuning requires an ILS approach selected in the MCDU and the aircraft within 150 NM of the destination. Automatic preview also requires FMS as the selected source. With FMS selected and auto tune active, the documented source change occurs when LOC is armed and the localizer needle begins to move. Check the receiver, preview and FMA rather than assuming the transfer has occurred. ^[9]