ILS question

[james ozzie]

(From someone who has not used one)

I assume it is normal to capture the localiser first and then the glide slope. I was wondering if it is possible or ever done to pick up the glide slope first and then the localiser? (I believe the GS transmitter sends out ghost lobes which I guess could complicate things considerably?)

[c100driver]

Yes it is.

On some Boeing B737 the GS can capture prior to LOC.

Most that I have flown the GS cannot be captured until LOC captured.

It is a pin selectable option from Boeing however I am unsure which is the standard and which is the option.

[Dash8driver1312]

If you get a sporty intercept heading, the controller may clear you to "descend with the glide path," but our equipment (Dash 8) won't capture the glide before the localiser: you have to set a vertical speed mode until the capture.

[Oktas8]

It is usually preferable to capture LOC first, then capture the GS from below.

However, if the GS bar is centered whilst descending on to the ILS, it is perfectly normal to adjust vertical speed to keep it centered whilst continuing to intercept the LOC.

In all aircraft I've flown (light aircraft & turboprops), the autopilot won't "capture" the GS first - but the pilot is quite likely to manage the flight path to achieve the same goal.

[james ozzie]

Thank you for these replies.

[AerocatS2A]

The BAe 146 will capture the GS first but our SOPs prohibit it.

[ShyTorque]

Yes, it's quite possible. But to descend with it until established on the localiser is very unwise because the aircraft is not flying over the "protected area" of the approach and may encounter an obstruction, or the ground in some locations.

[Dufo]

When approaching runways from a very shallow horizontal angle, we can track the glidepath even from 60 Nm out but flight director does not lock on it until established on LOC. Below or above - no difference.

[Matey]

Use of an appropriate vertical mode such as vertical speed is recommended to initially fly a continuous descent approach. The glide slope should not be armed until on the localiser and also within the glide slope protected area. On my current type ( B737NG) the glide slope will not capture until localiser established. Anticipation cues on the instrumentation (PFD) and standby instruments will indicate position relative to the glide slope prior to localiser capture to assist in achieving a continuous descent approach.

[reynoldsno1]

GS is only flight inspected to a max of 12NM ... outside that performance cannot be guaranteed.