A Few RNAV Approach Questions

[ark 1,288]

Just for the sake of understanding, in a B737NGX or B777 class of a/c:

1.    My understanding is that during an RNAV LPV approach, the lateral and vertical cues on the PFD are very similar to that during an ILS approach (e.g., pink diamonds), so can a RNAV LPV approach be hand flown like an ILS approach can be hand flown (either with or without the FD, but no autopilot in control)?

2.   Can an RNAV (RNP) approach that incorporates RF segments be hand flown? If so, what kind of guidance is provided on the PFD  during the RF segments?

3.  In reading about RNAV (RNP) approaches, it seems the requirements (equipment, pilot training, etc) are more demanding than for RNAV LPV type approaches (I assume because the "protected air space" is smaller) yet the DA minimums for LPV approaches seem to be lower than for RNP approaches  (Example: 35L at KDEN).   Why, I would have expected it to be just the opposite?

   Thanks for any insights or corrections,
   Al Klayton

[Paul_Smith 92]

1) All approaches can be hand flown, and usually are. Letting the computer fly the approach is the exception not the norm. 

 

2) RF segments often must be hand flown as a lot of FMS are not capable of automating them. RNP (required navigaional performance) simple means your equipment must have the stated minimum positional accuracy to fly the approach.

 

3) LPV provides additional guidence increasing the accuracy of the information presented, so reducing the minimums required for the pilots decision. Additional equipement requires additional training and certification.

 

The basic rule is always the same, aviate, navigate, comunicate. In that order. The pilot flies the plane, other stuff can be used to help, but the pilot flies the plane.

[skelsey 765]

I would slightly disagree with you here, Paul:

 

 

1) All approaches can be hand flown, and usually are. Letting the computer fly the approach is the exception not the norm. 

 

Almost all landings are hand-flown. But I would suggest that in airline ops, hand-flying the initial approach is certainly not the norm.

 

There is no absolute requirement to use the autopilot on an RNAV RNP procedure, but there is a requirement to use the FD and airlines may specify RNP values below which it is mandatory to use the autopilot. Boeing certainly recommend the use of autopilot in LNAV for RNAV RNP operations.

 

According to the FAA (AC-90-101A):

Autopilot and Flight Director (FD). RNP AR procedures with RNP values less than 0.3, or with radius to fix (RF) legs, require the use of autopilot or FD driven by the RNAV system in all cases. The autopilot/FD must operate with suitable accuracy to track the lateral and vertical paths required by the RNP AR procedure. ​

Requirements for using lines of minima less than RNP 0.3. The AFM or aircraft qualification documentation should identify whether the aircraft is capable of using lines of minima associated with RNP associated with RNP less than 0.3, and the required equipment configuration to achieve this capability. For example, dual autopilots may achieve a smaller RNP capability than dual FDs. ​​​​

 

I'd be surprised if there were many airlines routinely hand-flying RNP procedures, but that's not to say they don't exist.

 

However -- having said all of that, I do wholeheartedly agree with this line:

 

The pilot flies the plane, other stuff can be used to help, but the pilot flies the plane.

 

 

​

[scandinavian13 5,192]

Just for the sake of understanding, in a B737NGX or B777 class of a/c:

 

1.    My understanding is that during an RNAV LPV approach, the lateral and vertical cues on the PFD are very similar to that during an ILS approach (e.g., pink diamonds), so can a RNAV LPV approach be hand flown like an ILS approach can be hand flown (either with or without the FD, but no autopilot in control)?

 

2.   Can an RNAV (RNP) approach that incorporates RF segments be hand flown? If so, what kind of guidance is provided on the PFD  during the RF segments?

 

3.  In reading about RNAV (RNP) approaches, it seems the requirements (equipment, pilot training, etc) are more demanding than for RNAV LPV type approaches (I assume because the "protected air space" is smaller) yet the DA minimums for LPV approaches seem to be lower than for RNP approaches  (Example: 35L at KDEN).   Why, I would have expected it to be just the opposite?

 

   Thanks for any insights or corrections,

   Al Klayton

 

1. Yes. The only limitation here is that if you're not using the AP, you need an FD.

 

2. Yes. An approach is an approach is an approach. The FDs provide guidance and will guide you along the path like they would guide you along the path up at cruise.

 

3. Careful what you're reading into here. RNAV comes in many varieties (fun fact: RNAV, in principle, exists at cruise, too). It's simply a concept that conveys "an ability to navigate from point A to point B without using ground-based nav radials." What you're running into here is that you're missing the ILS-like nature of the LPV, versus the nature of a basic RNAV path. Sure, RNAV RNP 0.1 is more difficult to certify/maintain (crew/aircraft-wise) than RNAV LPV, but that doesn't mean it is more accurate. LPV uses WAAS to verify your position against a known location to provide an additional level of certainty versus a ground-based, known location. It also provides increasing sensitivity as you approach the runway (more specifically, as you approach a fixed point) just like an ILS would. For this reason, despite how technically wonderful RNAV RNP 0.3/0.1 are, LPV is more accurate, and will result in lower mins.

[Copper. 78]

Everyone has answered your question extremely well... This forum is great!

 

I'll just add to what everyone has already said:

- the track on the ND is considered raw data for RNP

- Boeing have certified the 737NG to be hand flown down to 0.1 RNP for some airlines.

- most airlines make policies around who can fly what RNP, and when.

 

I am a huge fan of PBN, but I've always wondered what would happen to the world if gps was lost. I guess the industry has already considered it though before implementing these new flying specs.

 

Anyway, I digress.

[ark 1,288]

Thanks very much for the inputs. What I didn't realize is that an RNP approach doesn't require WAAS, and forgot the vertical deviation limit during the final approach is linear and fixed at + - 75 ft all the way to the DA.

 

Since WAAS is not required for a RNP approach, how is ANP calculated -- what inputs are used other than the "standard" GPS signal to insure the a/c remains within + - 2 x RNP? 

 

BTW, is RNP strictly a lateral requirement?  In the RNP obstacle clearance diagrams I've seen, it has always been depicted as a horizontal requirement.

 

Thx,

Al

 

 

 

 

Al Klayton   

[scandinavian13 5,192]

Since WAAS is not required for a RNP approach, how is ANP calculated -- what inputs are used other than the "standard" GPS signal to insure the a/c remains within + - 2 x RNP? 

 

BTW, is RNP strictly a lateral requirement?  In the RNP obstacle clearance diagrams I've seen, it has always been depicted as a horizontal requirement.

 

ANP is calculated internally based on some pretty complex math by the machine.

 

The question it's asking itself, though, is pretty simple:

Based on all available position inputs, how certain am I that I am in this spot? (Metric is then given in nm of error.)

 

There are vertical components. This document may be helpful for you.

[Paul_Smith 92]

...

 

I am a huge fan of PBN, but I've always wondered what would happen to the world if gps was lost. I guess the industry has already considered it though before implementing these new flying specs.

 

Anyway, I digress.

 

You would imagine so, but I am not convinced that they have. The fact that we have not been hit with a severe solar storm in the last 15 or so years that we have become dependent on GPS does not mean we wont be. I suspect that there are a lot of pilots out there who would feel more than a little uncomfortable if they lost their satnavs and yet a really severe CME like the Carrington event or the one that just missed the Earth in 2012 wouldn't just take out satilites, it could take out ground based and airborn computers, phone lines and power grids.  That would not be a good day to fly.

[downscc 1,327]

I am old enought to have used VOR-based RNAV (Random Navigation) with the old King super duper computer that essential made a waypoint by "moving" the VOR with a bearing/distance selection. The exercised involved laying out sectionals on the floor for the route, draw a straight line, then using a divider and compass find the nearest VOR offsets. The enter those one by one into the little gizmo on the panel. Volia! You had a straight line from point A to B until ATC clearance delivery haha.

[DavidP 1,410]

I am a huge fan of PBN, but I've always wondered what would happen to the world if gps was lost. I guess the industry has already considered it though before implementing these new flying specs.

 

 

Good question. Next year here in Australia we will be decommissioning a significant number of ground based aids (we still have LOTS of NDB's here). Many VOR's will go too. They intend maintaining a certain number of "strategic" aids, but we are basically hanging our hats on GPS. It is a good and a bad thing, but I laugh talking to younger real world pilots and asking them how to "home" to a DME (skill long lost which I used to spend hours practicising in Link Trainers and was a separate endorsement on our Instrument Ratings at the time).

[downscc 1,327]

Austrialia has a huge advantage of the US in my opinion, implementing a modern system is much easier because you have much less 1950's technology and wide open spaces. Same for Canada, again just my opinion.  We will get there, but there's a lot of union jobs at stake so it will just take a little longer.

[DavidP 1,410]

Austrialia has a huge advantage of the US in my opinion, implementing a modern system is much easier because you have much less 1950's technology and wide open spaces. Same for Canada, again just my opinion.  We will get there, but there's a lot of union jobs at stake so it will just take a little longer.

 

What we see as the good thing with the US system is the US actively promotes General Aviation, where our regulator wants to see its demise - we do not, and never look like getting LPV as they won't even implement full WAAS!

[downscc 1,327]

Ouch!  To be fair to your regulators, ours seems to tolerate GA and the airlines want to kill business aviation but we have a couple of lobby groups like AOPA and NBAA that help. Of course, we have given political candidates free rides on occasion to help in a small way. I cannot imagine getting around Texas without GA let alone someplace as huge as Austrialia.