RNAV approach DCT to IF

[Sue Ridgepipe]

For an RNAV (GNSS) approach Airbus says that "direct to" clearances to the IF may be accepted provided the resulting track change at the IF does not exceed 45°.

I've seen some approaches where, if you fly the whole approach starting at the IAF, the track change at the IF is close to 90°. What's the reason then that we can't accept a direct to if it's more than 45°?

[Check Airman]

We've got tons of those here in the US. Known as a TAA (Terminal Arrival Area)- the're used to negate the need for procedure turns. As the link (http://155.178.201.160/d-tpp/1610/00973R29.PDF) shows, the course change at the IF is 90 degrees, which is typical.

I'd imagine the recommendation is there to prevent an overshoot, but as you can see, you have 6nm fromm the IF (TUKSE) to the FAF (JONTU). More than enough room to get settled.

[underfire]

The ac all have different anticipation built in. In the procedures shown by check, lets say you clear to BUSJY (how the f is that pronounced?) the waypoints are flyby, so the ac will anticipate what it takes to connect the dots and get to JONTU. Some will begin the turn straight away, and others will take different radius turns, the only constraint is to flyby JONTU.

On that procedure, you can be direct to from the STAR to 3 different places.

If you tried to put in a 'direct to' heading to an IF more than 45 degrees, what if it is 90 or 120, what would the ac do?

[Sue Ridgepipe]

It will still provide the necessary turn anticipation, as long as it's not coded as fly-over. As far as I can tell, it will do the same regardless of whether it's a direct to or it's part of the programmed approach procedure, but there must be something I'm missing.

[Check Airman]

If you tried to put in a 'direct to' heading to an IF more than 45 degrees, what if it is 90 or 120, what would the ac do?
I've seen that situation. We went direct to the FAF with an intercept heading of ~110 degrees on a visual approach. Fifi handled it flawlessly.

[FE Hoppy]

When does the FD change to APPR status? If it's triggered by sequencing the first fix of the approach then there is a difference between sequencing the IAF then turning and going direct to the IF then turning.
May well be due to AP/FD gain scheduling.

[aterpster]

Sue Ridgeline:

For an RNAV (GNSS) approach Airbus says that "direct to" clearances to the IF may be accepted provided the resulting track change at the IF does not exceed 45°.

In the U.S. ATC can clear you direct to the IF so long as the course change doesn't exceed 90 degrees, and other conditions (set forth in the AIM) are met.

[aterpster]

Check Airman:

We've got tons of those here in the US. Known as a TAA (Terminal Arrival Area)- the're used to negate the need for procedure turns. As the link shows, the course change at the IF is 90 degrees, which is typical.

Criteria changes now see revised TAA approaches where a course reversal is required for arriving from the opposite direction. It wasn't this way for a long time, but criteria now has more stringent DTA requirements.

[Sue Ridgepipe]

Thanks aterpster, that's good to know. The only place I see this comment from Airbus is on their PDP, perhaps it's outdated now as it's not a limitation as far as I can tell from our FCOM. Can anyone shed more light on where this might have come from? Is it an outdated thing?

[FlightDetent]

Sue, if you provide version and verse of your PDP somebody may have a look at a latter release. On top of that, I suspect, maybe the author was just being reasonably conservative...

[Amadis of Gaul]

I've seen that situation. We went direct to the FAF with an intercept heading of ~110 degrees on a visual approach. Fifi handled it flawlessly.



On the other hand, two days ago we went direct to Exban for the RNAV 36L at MCO from all of about a 40deg angle, and FiFi screwed it up so bad, it was embarrassing.

[Escape Path]

I remember our training material (A320) said it's not advised to fly direct to the FAF. At a minimum, fly a heading that intercepts the approach track before the FAF and fly a radial-in to the FAF. I think I remember it being because the airplane should be as stabilized as possible both laterally and vertically, for the final descent

[galaxy flyer]

EP,

We're discussing direct to the IF, not the FAF, which is a no-no.

GF

[Capn Bloggs]

AIP Australia for GNSS procedures:
An aircraft which is not required to hold or to lose height in a holding pattern may commence the approach without entering the holding
pattern if:

c. for procedures using GNSS:

(2) in controlled airspace, the aircraft is being vectored to intercept the initial approach segment or is tracking direct to the intermediate fix.

Note to c.(1): The first track of a GNSS procedure must be joined using the tracking guidance provided by the GNSS receiver.

Note to c.(2): “direct to” clearances may be requested to the intermediate fix (IF) provided that the resultant track change at the IF does not exceed 45°.
So, a regulatory requirement (here, at least), not only Airbus.

[Escape Path]

EP,

We're discussing direct to the IF, not the FAF, which is a no-no.

GF
My mistake. I thought they could bear some relation

In our daily operation we do quite a bit of flying DTO X waypoint in a RNAV approach, most cases it would be to the "vertical intercept point", though I reckon none of those approaches actually makes us intercept the RNAV flight path in a turn of more than 90 degrees

[MD83FO]

While we're here, what's the right time to arm the approach, and why shouldn't we arm it before that?

[Amadis of Gaul]

I arm it as soon as I hear the words "cleared for the approach" or shortly thereafter. As for why not before that, why would I arm an approach for which I am not yet cleared?

[CaptainMongo]

Most of our clearances are, "Fly HDG xxx, maintain xxxx until established, cleared ILS xx approach"

When cleared for the approach, we arm only LOC. When LOC captures we arm approach. This prevents descending on the GS prior to being on a published section of the approach.

Another point - many approaches have step down fixes prior to GS intercept. Arming approach prior to GS intercept altitude could cause non compliance with a step down fix altitude. In this case, after being cleared for the approach, we set FAF altitude and (as long as FMGC fixes/altitudes are correctly coded/confirmed) push for managed descent, after the last step down is complied with, we arm the approach.

(Another aside - when cleared for a visual, we maintain last assigned altitude until on approximately a 3 degree glidepath to the runway. Previously our pilots were descending to FAF altitude and these early descents were occasionally causing GPWS warnings.)

[xerxesdk]

For an RNAV (GNSS) approach Airbus says that "direct to" clearances to the IF may be accepted provided the resulting track change at the IF does not exceed 45°.

I've seen some approaches where, if you fly the whole approach starting at the IAF, the track change at the IF is close to 90°. What's the reason then that we can't accept a direct to if it's more than 45°?

where exactly in Airbus documentation did you find this?

[Capn Bloggs]

Airbus may simply be repeating the regulatory requirement (AIP ENR 1.5):

Note to c.(2): “direct to” clearances may be requested to the intermediate fix (IF) provided that the resultant track change at the IF does not exceed 45°.