RNAV GNSS approaches
 

Hi all,

I understand that standard procedure for an RNAV approach is to be 'own nav'd' to the IAF, and that a period of level flight of 2nm is required. My questions are:
does this level flight have to be straight, or could it be in the turn?
Could you be sent to the final approach fix instead of the initial, or would the a/c FMS (or crew!) not like that?

Cheers

John
(curious ATCO!)

Hi John,

I will look for the reference but I am pretty sure that EU-Ops predicates that you must never be sent direct to the final descent point on an RNAV approach. Therefore you will be given direct to the IAF or given vectors to a point at least 2nm from the final descent point.

Hi John,

Here is an extract from an EU-OPS Airline's Operations Manual:

P-RNAV/RNP STARs and RNAV Approaches may utilise specific phrases which identify the method of intercept, navigation, climb or descent to be used. Details will be published on relevant approach charts. ATC must be advised of any deterioration or failure of navigation equipment below that required for entry into a scheduled navigation area or continued operation in such airspace.

Vectoring and Positioning
ATC tactical interventions in the terminal area may include radar headings, ‘direct to’ clearances which bypass the initial legs of an approach, interceptions of an initial or intermediate segments of an approach or the insertion of additional waypoints loaded from the data base. In complying with ATC instructions, the flight crew should be aware of the implications for the navigation system.

‘Direct to’ clearances may be accepted to the Intermediate Fix (IF) provided that it is clear to the crew that the aircraft will be established on the final approach track at least 2 miles from the FAF.

‘Direct to’ clearance to FAF is not acceptable. Modifying the procedure to intercept the final approach course prior to the FAF is acceptable for radar vectored arrivals or at other times with ATC approval.

The final approach trajectory should be intercepted no later than the FAF in order for the aircraft to be correctly established on the final approach course before starting the descent (to ensure terrain and obstacle clearance).

‘Direct to’ clearances to a fix that immediately precede an RF leg are not permitted.

For parallel offset operations enroute (in RNP 4), transitions to and from the offset track should maintain an intercept angle of between 30 and 45° unless specified otherwise by ATC.



So it appears that the 2nm segment that you refer to does not necessarily have to be level flight but should be aligned with the FAT.

As Right Way Up said, vectors should be given either to the IAF or to a point at least 2nm prior to the FAF.
Another option that I have seen is to be vectored onto an intercept heading and then told to intercept the final approach course. Again, this intercept point is at least 2nm prior to the FAF.

What if the entire procedure is coded from DEP to ARR? With these procedures, I am always curious how the clearances work.
or this one? (notice where the IAF's are located?)

http://i40.tinypic.com/14mys7k.jpg

On the above procedure, you would be cleared to the IAF with a 'descend via' clearance. Follow the whole procedure, beginning to end. You can't be cleared to a waypoint which is the beginning of a RF Leg, which in this approach is just about the whole procedure.

That approach is issued only to China Airlines. Also, there is very little traffic at that airport so there is not much in the way of ATC involvement once cleared for the approach. The crews also receive a lot of training before they are authorized that airport and approach.

Of course terpster, you are aware that I designed this?

Arr, that explains it! :p

underfire:

No, because I don't know who you are. Are you Steve Fulton?

Sorry, thought you were aware I had been with Naverus....

underfire:

I did not know that. It was a good company with a great CEO, pre GE. I don't know anything about the GE operation.

Steve did a great job for Alaska Airlines. I believe Alaska Airlines is/was anti-competitive making them all specials. But, that wasn't Steve's decision.

Air China,

China Airlines is a Taiwan based operator.

FINAL APP MODES (A330 FCOM DSC-22_30-80-30-20 P 1/8)

The FINAL APP mode includes the following modes:
• APP NAV mode for lateral guidance
• FINAL mode for vertical guidance.

FINAL APP ARMING CONDITIONS

The flight crew arms APP NAV and FINAL modes by pressing the APPR pb on the FCU, provided all
of the following conditions are met:
• The aircraft is above 400 ft AGL
• The active flight plan is valid (lateral and vertical profile)
• A non-precision approach has been selected in the active flight plan
• GA mode is not engaged.
The FMA displays "FINAL" and "APP NAV" in blue.
If NAV mode was already engaged, APP NAV engages immediately.

FINAL APP ENGAGEMENT CONDITIONS

APP NAV and NAV modes engage under the same conditions:
If NAV mode was engaged, APP NAV engages immediately. If HDG/TRK is engaged, APP NAV
engages when the intercept conditions are met (the aircraft’s heading or track intercepts the flight
plan active leg).
APP NAV will engage if the "TO" waypoint is displayed in white on the ND and MCDU.
FINAL mode engages, if:
• FINAL is armed (by pressing APPR on the FCU), and
• The aircraft intercepts a descending leg of the vertical flight path, or
• In V/S (FPA) or OPEN DES, the aircraft intercepts a level-off segment of the vertical flight
• profile, with an FCU selected altitude different from this level-off segment.
• APP NAV is engaged,
• Predictions are available,
• The APPR phase is active, and the deceleration point has been sequenced, and
• Note: APPROACH phase may have to be manually activated on MCDU PERF page if the
• approach starts at high altitude above aerodrome level (i.e. RNAV(RNP) approaches).
• The aircraft is within the capture area of the vertical profile
Laterally: ±1.5 NM from the Final Descent Point.
Vertically: +150 ft above the Final Descent Point.
• A blue arrow is displayed on NDs to indicate where the FINAL APP engagement conditions are met
• and where the final descent will begin automatically.
• If the same arrow is shown in white, at least one engagement condition is not fulfilled, FINAL APP
• will not engage and the aircraft will not descend automatically.

Definition of the Final Descent Point (also called Vertical Intersection Point "VIP" for
RNAV(RNP) approaches)

The Final Descent Point is the capture point of the final descent segment coded in the Navigation
Database. It is the latest point where FINAL APP is supposed to be engaged. For RNAV(RNP)
approaches, this point may be indicated on the chart as "VIP".
This point is defined in the Navigation Database by:
• A constant vertical flight path beyond this point,
• A coded altitude constraint that may be "at" or "at or above" (e.g. +3 000 ft). This Constraint
is displayed on ND (in magenta), next to the corresponding waypoint, when the CSTR key is
selected on the EFIS Control Panel. It is also shown on the F-PLN page at this WPT.
Note: The Final Approach Fix (FAF) is the position from where the obstacle clearance is defined
relative to an Obstacle Clearance Surface (OCS). Obstacle clearance is only ensured if
the aircraft is flying on the defined vertical flight path. Before the FAF, minimum altitudes
are published with fixed minimum Obstacle Clearance (MOC). The Final Descent Point is
the point in the procedure at or before the FAF from which a constant vertical flight path is
defined.

B-HKD:

Whatever the state airline is of mainland China. That wasn't really my point in any case. :)

miguel, ahhh yes, the infamous VIP....this was user preference (and seemed to cause more confusion than it was worth)

http://i.imgur.com/H6ikMnL.jpg

Yes, terpster, things were certainly much different pre-GE. I enjoyed the challenges with real RNP-AR, stuff with WestJet, PSP, SCC, all of the AUS procedures (with goofy waypoints) and especially the procedures in China, with the extreme altitudes.

In procedure design, just dabble with the real crazy stuff, like VNKT, CAT III autoland GBAS, offshore platform RNP, and RNP transition to GBAS final....

Currently working on several airports for Time Based Operations and wake turbulence mitigations.

What are you up to these days?

underfire:

Sounds great. I am not/was not a procedures designer. Rather an airline pilot, then retired airline pilot, who dug deep into TERPs from the beginning of my career.

Four years as a member of the PARC really helped my understanding of RNP AR. I am very familiar with VNKT. I thought that RNP AR was designed by Quovadis.

Similar to an ILS to allow the aircraft to be stabilised on final approach track before commencing descent.

John (OP)

You have not commented on the various replies, so I am not certain if your question has been answered.

I am not aware of any constraint where there has to be a 2nm level section prior to the IAF. Some procedures are on the STAR, and the TOD is in the procedure. Turns before/after IAF can be included, as the IAF is simply a coded waypoint, but the procedure does not have to begin with an IAF, and there can be turns before the IAF, especially when coding from the STAR.

Its a computer, make the damn thing do what you want,

Not really true any more.
One of the problems with implementation of 'RNAV approaches' is that the simple thing for regulators was to overlay them on ILS procedures. Beancounters then asked what are we doing all this extra training for? At which point some airlines ceased equipping for RNAV approaches.

The future is more likely to be curved approaches (sometimes multiply curved approaches) where the aircraft is 'established' once it starts the procedure regardless of whether the aircraft is flying a straight track on the ground.

See https://www.faa.gov/nextgen/snapshots/stories/?slide=6

When 4DTBO eventually becomes accepted the trajectory is likely to include 'Greener Skies' type approach paths as part of a continuous descent.

The criteria simply states 30 seconds of stabilised flight, and that can include turns.
FROP can be at 500 feet, with a FAF at the beginning of the turn.