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When to descent at a fly-by waypoint

When on a RNP approach (RNAV) you usually see quite some fly-by waypoints. When can you start descending to the next lower altitude?

1. When the GPS unit switches over to the next leg
2. When you start the turn to the next waypoint
3. Something else?

Waypoint

Bushpilot C208/C182
FMMI/EHRD, Madagascar

I don’t know the correct answer. I descend mid-turn.

Last Edited by loco at 12 Nov 19:51
LPFR, Poland

I think that when the next leg becomes active (ie turns magenta) is that earliest you can start. That is normally about mid turn.

EGKB Biggin Hill

I would also advise to ask the question on a forum where turns take long enough to notice (haha).

Last Edited by loco at 13 Nov 09:40
LPFR, Poland

loco wrote:

I would also advise to ask the question on a forum where turns take long enough to notice (haha).

How long a turn takes (under IFR) is the same, regardless of speed (if that’s what you meant).

EGKB Biggin Hill

Even when limited by max bank angle?

Let me quote Wikipedia on that one:

During a constant-bank level turn, increasing airspeed decreases the rate of turn, and increases the turn radius. A rate half turn (1.5° per second) is normally used when flying faster than 250 kt.

Last Edited by loco at 13 Nov 10:16
LPFR, Poland

In the real world these would typically be a CDFA aiming to be configured and at platform altitude by the FAF. In effect step down issues should not apply, you are typically passing above the intermediate altitudes. The CDFA may have started from top of descent, in fact.

Training environments may require driving to the IAF, assuming above MSA, and then descending to the intermediate fix once the leg switches.

Oxford (EGTK), United Kingdom

Someone else suggested to me that you can only start descending when established on the next track (wings straight and level). When flying higher speeds that sometimes might be a bit late and will require a higher rate of descent on the next leg.

Personally I agree with the mid-turn option as the GPS unit switches to the next waypoint.

Usually you will be above the published altitudes, but some approaches have a 3 degrees angle on the whole RNP and with faster unpressurized aircraft’s that means going down with 1000 ft/min or more on the descent rate. Not all passengers seem to like that.

Bushpilot C208/C182
FMMI/EHRD, Madagascar

Logically, according to the arrival/approach procedure construction, as long as your XTK from the next segment track as drawn on the chart does not exceed the RNP prescribed for this segment, you can be considered flying that segment and thus can descend.

LKBU (near Prague), Czech Republic

Ultranomad wrote:

as long as your XTK from the next segment track as drawn on the chart does not exceed the RNP prescribed for this segment, you can be considered flying that segment

I understand your logic, but if there is no course change at the waypoint, and you are following nominal track, then your XTK for the next segment will have been zero for all of the previous segment.

huv
EKRK, Denmark
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