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When to descent at a fly-by waypoint

huv wrote:

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.

Actually, there is no need to use XTK, either, it can be even easier: as soon as you have approached the waypoint closer than RNP, you are already within the allowable margin of error for the next segment.

LKBU (near Prague), Czech Republic

Ultranomad wrote:

as soon as you have approached the waypoint closer than RNP, you are already within the allowable margin of error for the next segment.

Subject to approaching the waypoint where the next track requires less than 110 degrees in the turn.

Most training GNSS RNAV seem to violate this rule as they proceed from the NDB hold to the IAF.

Oxford (EGTK), United Kingdom

RobertL18C wrote:

Most training GNSS RNAV seem to violate this rule as they proceed from the NDB hold to the IAF.

I train people to anticipate by flying outside the straight line so that they approach the IAF from within the TAA “cake slice” (which is normally 90°, rather than 110°; I think that the EASA standard is 100°, anyway.)

EGKB Biggin Hill

Timothy wrote:

I train people to anticipate by flying outside the straight line so that they approach the IAF from within the TAA “cake slice”

Apart from making sense, isn’t that what ICAO Doc 8168 says also (I-2.4.4):
An aircraft established within a TAA area may enter the associated approach procedure at the IAF without conducting a procedure turn provided the angle of turn at the IAF does not exceed 110°. In most cases, the design of the TAA will not require a turn in excess of 110°; if necessary, the aircraft should be manoeuvred within the TAA to establish the aircraft on a track prior to arrival at the IAF that does not require a procedure turn.

huv
EKRK, Denmark

I descend mid turn.

Administrator
Shoreham EGKA, United Kingdom

There is another bit in Doc 8168 that is also vaguely relevant to the original question about when a descent is permitted when flying a procedure. In the section about final approach, I-5.2.4 says:
The descent should be initiated prior to the FAF, in order to achieve the prescribed descent gradient/angle.

When I first read this, I was quite relieved, as this is not what I was trained to do. I was trained to never leave the platform altitude before positively having passed the FAF or intercepted the glide, which of course always caused me to be high on the glide initially.
Note that 8168 does not indicate how much prior to FAF the descent should – or may – be initiated. Of course, the procedure designers do not provide unlimited clearance for an early final descent, but in practise I guess it is just a matter of what works for you.

Last Edited by huv at 14 Nov 12:10
huv
EKRK, Denmark

huv wrote:

Apart from making sense, isn’t that what ICAO Doc 8168 says also (I-2.4.4):
An aircraft established within a TAA area may enter the associated approach procedure at the IAF without conducting a procedure turn provided the angle of turn at the IAF does not exceed 110°. In most cases, the design of the TAA will not require a turn in excess of 110°

Can anyone find the EASA equivalent, which is, I think, 100°?

EGKB Biggin Hill

In the US AIM, there is a definition of “established”. It reads as:

11. Definition of “established” for RNAV and RNP operations. An aircraft is considered to be established on-course during RNAV and RNP operations anytime it is within 1 times the required accuracy for the segment being flown. For example, while operating on a Q-Route (RNAV 2), the aircraft is considered to be established on-course when it is within 2 NM of the course centerline.

The maximum turn permitted from an airway to a feeder or IAF on an approach is 120 degrees for a conventional procedure and 90 degrees for a RNAV procedure.

KUZA, United States

Timothy wrote:

Can anyone find the EASA equivalent, which is, I think, 100°?

Don’t EASA countries apply Doc 8168 as is? AFAIK there are no “European” design rules like the US TERPS.

Last Edited by Airborne_Again at 14 Nov 22:05
ESKC (Uppsala/Sundbro), Sweden

Airborne_Again wrote:

Don’t EASA countries apply Doc 8168 as is?

That’s why I was asking for help on the source of the 100° rule, which several people have told me about, but I have never seen in real life.

NCYankee wrote:

The maximum turn permitted from an airway to a feeder or IAF on an approach is … 90 degrees for a RNAV procedure.

And for direct to the IF? I believe that to be 90°, but 100° and 110° have also been quoted to me.

EGKB Biggin Hill
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