G/S engage before LOC?

[Will]

I know there is an AFDS option in PSX to prohibit the glideslope from engaging before the localizer mode is engaged. I'm curious to know more about the history of this. It's hard to envision a scenario in which you would want the autopilot to follow the glideslope down without the localizer. But are there uses for this? Or is it an older technology that was superseded by a standard in which descent on glideslope is prohibited without LOC guidance? Are older 747s different than newer ones? Which variant is more popular among operators?

Any insights, whether historical, technical, or operational, are welcome. Thanks.

[Jeroen Hoppenbrouwers]

I think it may have ended since the Turkish Airlines crash, but when listening to Schiphol ATC in the early 2000s I regularly heard "KLM123, turn left heading 2 1 0 to intercept the localizer for 1 8 right, descend with the glide." Obviously because they would be too high on loc intercept otherwise.

This may be one of those things that proved very convenient and also one more hole in the cheese.

Obviously in the Netherlands, hitting something while descending off-localizer is not very likely, but keeping your engines at flight idle far longer definitely was a major factor leading to the crash.


Hoppie

[IefCooreman]

I think it may have ended since the Turkish Airlines crash, but when listening to Schiphol ATC in the early 2000s I regularly heard "KLM123, turn left heading 2 1 0 to intercept the localizer for 1 8 right, descend with the glide." Obviously because they would be too high on loc intercept otherwise.

As I understand it, that phrase relates to minimum intercept altitudes for the glideslope and is sometimes used when the depicted "platform" altitude on the chart is lower than the cleared altitude by the ATCO.

If somebody is "cleared for the approach" it means the pilot has more "control/freedom" over altitudes. Imagine an ATCO gave a descent clearance to 4000f and the intercept altitude for the glide on the chart is depicted as 2000ft. When "cleared for the approach", the pilots would be allowed to descend on their own to 2000ft once they are localiser established, even when they are below the glide. To avoid that "early" descend below the glide according to the depicted procedure, the ATCO would not state "cleared for the approach", but would state "descend with the glide" to avoid the aircraft descending below the cleared intercept altitude.