Turkish A330 incident, Kathmandu

[8che]

ATC Watcher,


Accuracy 95% of the time has nothing to do with GPS. That lost 5% can not be affected by a USAF general !


ANP is a measure of the FMC POSITION accuracy not GPS. The FMC generates its position using a combination of inertial/DME-DME/LOC-DME/VOR-DME/GPS etc. The GPS is just one sensor that it can use but the ANP is a measure of FMC ability 95% of the time not GPS. Latest Boeing FMC's are authorised to 0.3 without GPS even being fitted !


and without GPS a bad radio update can certainly affect position even if they are cemented into the ground. That's why radio inhibit/purge functions are fitted to FMC's.


Anything less than 0.3 RNP (RNP-AR) and yes we currently have to use GPS but that's because ILS's don't bend around corners. Yes most of us have GPS fitted these days but lets not forget its the FMC that ultimately produces the position not the GPS.


I understand the argument between ground based and shall I say "aircraft based approaches" but ground based systems are far more expensive and limited in there geographical options and dependent on variable servicing standards throughout the world. There are almost daily reports of scalloping/interference etc. The FMC/GPS approaches are statistically more reliable and we never have and never will operate to risk free standards. All we do is manage the risk. Every approach in Pans-OPS or TERPS has a calculated probability of crashing. It may be small but its there.

[aterpster]

The 95% probability of being within 1 X RNP is a statistical probability. The 5% excursion may occur on the first RNP AR approach or it may not occur until the 100,000 approach. And this probability is for the entire fleet (world-wide) of RNP AR aircraft, not any given aircraft. So, it will occur to some aircraft in the fleet and not to others.

Further, this probability is not for GPS, or for that matter GNSS, because performance based navigation is considered to be sensor independent. As some others have said, the position used for the navigation solution goes through quite a process in an RNP AR certified aircraft.

This is not like lousy VOR radials are even more lousy NDB bearings. Odds are that you could fly the VNKT RNAV (RNP) Rwy 2 approach every day of your 35 year career and be on, or within a few feet of centerline every time.

And, to not be on, or within a few feet of center without having received an "Unable RNP" message is quite remote.

But, the engineers must deal with statistical theory when providing RNP containment areas without any secondary obstacle clearance areas that conventional PANS-OPS and TERPS have. Thus, the RNP AR containment area is 2 X RNP. And, with lower RNP values other mitigations must be included in the navigation solution, including EGPWS with enhanced database capabilities in an obstacle rich environment.

Finally, RNAP AR approaches are not "all weather" procedures, unlike fail-active CAT III autoland. RNP AR approaches have as their primary purpose the avoidance of terrain at terrain-rich locations. And, there are airports where it simply is not possible to design an effective RNP AR approach because the terrain is simply too onerous.

[kinteafrokunta]

8che...right on the money! ATC Watcher, please state your points but kindly refrain from mocking pilots operating the system " for not knowing ". The eggs can come in flying fast! Incoming.......

[silvertate]

8che

ANP is a measure of the FMC POSITION accuracy not GPS. The FMC generates its position using a combination of inertial/DME-DME/LOC-DME/VOR-DME/GPS etc.

Sorry, but I can only assume you have never seen the system in operation.

The fact of the matter is that the FMC position will closely shadow the GPS fixes, to the detriments of all other aids. The IRS and VOR-DME fixes can be miles away, and the FMC will still follow the GPS. I think the system bias must be 95% to the GPS fixes.

And therein lies another problem, because if the GNS detects a RAIM error it will stop using the GPS and 'revert to the IRS/VOR fixes', as the manual says. Yet the IRS/VOR positions could be miles away. The manual rather unhelpfully does not say how fast this transition will take, but if it is within a minute or so, you could end up with a dramatic map-shift just when you do not want it.

Regards the 95% probability, this is a sum of ALL the errors that can effect the system, especially when on the approach. It is not a measure of GPS accuracy per-se. It includes errors from the VOR-DME fix, GPS signal ghosting or reception errors, satellites not in optimal positions, insufficient satellites, atmospheric refraction, hardware noise, IRS heading errors and much else besides. Grind all that together, and you have only a 95% probability of your position being within 0.3nm. And don't complain to me about this - that is what the system says in the manual. If you don't like it, complain to the OEM manufacturer.

And finally, regards Capt Blogs. Sorry, but if you are doing a 0.3nm rnp GNS approach, that is all you are going to get. You cannot assume that the runway will always be in front of you. Why? Because that is what the system says on the tin. The 0.3 nm rnp is a pseudo non-precision approach, and not a pseudo ILS. Especially if you have no WAAS system (which Europe does not have) and don't know if someone has a $50 jammer on the airfield boundary. And if you don't like the 15 degree turn to achieve the runway, speak to the CAA who designed that approach, or speak to your flight manager and get an extra 'company 200ft' added to the minima. But whatever you do, don't always expect the GNS approach to be perfect, if you do not have WAAS augmentation and checking.

[silvertate]

aterpster

The 5% excursion may occur on the first RNP AR approach or it may not occur until the 100,000 approach. And this probability is for the entire fleet (world-wide) of RNP AR aircraft, not any given aircraft.

You obviously don't know what a probability is.

Tell you what - get one of those 20-sided dice that they have in modern board games. Now throw it 100,000 times, and see how often your chosen number comes up. See the problem now?

[aterpster]

Silver:

You obviously don't know what a probability is.

Tell you what - get one of those 20-sided dice that they have in modern board games. Now throw it 100,000 times, and see how often your chosen number comes up. See the problem now?

Two different issues. You flip a coin and over many flips it will be heads 50% of the time. Same idea with your 20-sided dice.

If it is like the roll of a dice, we better get rid of RNP AR before someone hits a mountain.

[aterpster]

From the FAA AIM:

Section 2. Area Navigation (RNAV) and Required Navigation Performance (RNP)

1-2-1. Performance-Based Navigation (PBN) and Area Navigation (RNAV)

a. Introduction to PBN. As air travel has evolved, methods of navigation have improved to give operators more flexibility. Under the umbrella of area navigation, there are legacy and performance-based navigation (PBN) methods, see FIG 1-2-1. The legacy methods include operations incorporating systems approved under AC 90*45, Approval of Area Navigation Systems for Use in the U.S. National Airspace System, which allows two *dimensional area navigation (2D RNAV) within the U.S. National Airspace System (NAS). AC 90*45 describes 2D RNAV in terms of both VOR/DME dependent systems and self* contained systems such as Inertial Navigation Systems (INS). Many operators have upgraded their systems to obtain the benefits of PBN. Within PBN there are two main categories of navigation methods: area navigation (RNAV) and required navigation performance (RNP). For an aircraft to meet the requirements of RNAV, a specified RNAV accuracy must be met 95 percent of the flight time. RNP is an RNAV system that includes onboard performance monitoring and alerting capability (for example, Receiver Autonomous Integrity Monitoring (RAIM)). PBN also introduces the concept of navigation specifications (Nav Specs) which are a set of aircraft and aircrew requirements needed to support a navigation application within a defined airspace concept. For both RNP and RNAV designations, the numerical designation refers to the lateral navigation accuracy in nautical miles which is expected to be achieved at least 95 percent of the flight time by the population of aircraft operating within the airspace, route, or procedure. This information is introduced in International Civil Aviation Organization's (ICAO) Doc 9613, Performance*based Navigation (PBN) Manual (Fourth Edition, 2013) and the FAA Advisory Circular (AC) 90*105A, Approval Guidance for RNP Operations and Barometric Vertical Navigation in the U.S. National Airspace System and in Remote and Oceanic Airspace (expected publication date in late 2014) further develops this story.

[whitelabel]

So if we check RNP agains EPE how reliable is that info then? Is that position error really the figure its indicating or can that still be very different 5% of the time? Seems highly unlikely with vor/dme/ils/irs/gps updating. The system has to warn us if data input from the gps sensors is not correct but how can it do that if it does not know that the data is false or unreliable..

[LeadSled]

The 5% uncertainty are still there and will remain there as long as GPS ( as opposed to EGNOS , etc..) is used.

I am intrigued.
Why will EGNOS change this situation, particularly if you consider the new generation GPS going up now. What is "etc.", Galileo, sundry Russian or Chinese systems? Beware the world's most accurate PR claim.

[ATC Watcher]

LeadSled

Ah EGNOS ! like a beautiful lady promising you things but always failing to show up at the rendez-vous!
It is an augmentation system ( similar to WAAS in the U.S. ) supposed to give you meter precision . Initially planned for 2009 , now 2020 for aviation if I am correct. But by coincidence I am going to a meeting with normally them in it tomorrow . I will ask their current status .
The "etc." . In my earlier post was referring to the various augmentation systems avail or planned , including fixed ones , to allow precision APP .

[sarge75]

Would love to hear the recording from tower, along the lines of:

ATC: "are you visual?"

Capt/FO "No"

About 20 seconds later, everyone at the airport hear a loud bang...

[silvertate]

aterpster

Two different issues. You flip a coin and over many flips it will be heads 50% of the time. Same idea with your 20-sided dice.

If it is like the roll of a dice, we better get rid of RNP AR before someone hits a mountain.

Unfortunately the rnp error probability is very nearly the same as a simple dice roll. The only difference is that a 20-sided dice has a definite 5% probability of giving any one particular number. Conversely, the rnp has an indeterminate 5% probability of being wrong at any one time. The rnp probability cannot be definite, because there are too many variables involved, and each variable has its own probability of being in error. And you will probably need a series of incorrect variables to coincide before you find yourself off track. But the total probability of error is still up to a maximum 5% chance of being off track at any one time.

And your highlight of 'population of aircraft' does not help with the probabilities, because each individual aircraft is a part of the population and therefore a part of the probability matrix. Having 5% of aircraft being probably off track, is exactly the same as saying your individual chance of being probably off track is 5%. You cannot say that any one 20-sided dice, out of 1,000 such dice, is excluded from the probability matrix.


Put it this way. If the probability of an excursion away from 0.3 nm rnp were one in 100,000 flights, as you intimated, the manufacturer of GNS systems would say:

... rnp 0.3 is expected to be achieved at least 99.999% of the flight time.
but they do not, they say:
... rnp 0.3 is expected to be achieved at least 95% of the flight time.

There is a good reason for that very large 95% caveat. The total GNS system, including all the many variables and disturbances that go together to determine the probability of your position, is not as reliable as many pilots seem to think. Especially if you are weaving through high terrain.

1,000 ft decision, anyone....?

[silvertate]

OK465

So if your only options are between a VOR NPA approach with at 6 degree descent angle or an RNP-AR APV with a 3 degree descent angle, which will you prefer to fly?

That depends, doesn't it.

If we had 20 years of experience of successful GNS approaches, we could narrow the probabilities down a bit. But launching into a new system and pretending that it is the same or better than an ILS is somewhat premature. Especially when it is only has a guaranteed 95% probability of being correct.

I was reasonably happy with the 1,000 ft decision we used in training, but less so with the 300 ft decision at this airport. And I am still not happy with a pseudo-glideslope that does not call out vertical profile errors. Do you know what the max vertical error is for the approach? And was the PNF monitoring it, all the way down the approach? Do you always know the escape procedure, if the GPS loses the plot and deletes the trackline while on the approach?

I give you the example of TCAS, which was launched as the latest thing in improved flight safety. And then the first thing TCAS does, is cause an inflight collision. The same difference exists here. Let's think through the possibilities and pitfalls a bit more, and walk before we run.

[Capn Bloggs]

Silvertate, I think you doing a bit of raving.

Please explain the GNSS error "satellites not in optimum positions" you mentioned a few posts back.

And I am still not happy with a pseudo-glideslope that does not call out vertical profile errors. Do you know what the max vertical error is for the approach? And was the PNF monitoring it, all the way down the approach?

Ever heard of, or seen, Nav Performance Scales?

Do you always know the escape procedure, if the GPS loses the plot and deletes the trackline while on the approach?

Ever heard of an IRS (as explained by aterpster earlier but which you obviously missed).

And then the first thing TCAS does, is cause an inflight collision.

What nonsense.

[320goat]

RNP-AR

There are issues with terrain masking and RAIM outages, but you look at these prior to dispatch.

As has been pointed out, if the GPS goes you revert to standard drift rate of the IRS so not a major problem immediately.

We have a procedure of checking the altimeters at the FAF and they have to be within a certain tolerance (can't remember just now as we don't have approval just yet).

If you want a gross error check on vertical profile, I tend to use 320' per nautical mile for a 3 degree glide.........not too difficult.

[aterpster]

ok465:

A Powerpoint Honeywell presented a few years ago:

https://aerospace.honeywell.com/~/me...20Benefit.ashx

Not exactly a roll of the dice.

[silvertate]

Capt Blogs etc:

>>Satellites not in optimal positions.

The satellite constellation is fixed, while the Earth rotates inside it. Thus the number of satellites you can see, and the declination and azimuth of those satellites, will change during the day (and with your location). Some locations and times are better than others - especially if there is a satellite outage in your area.


>>Nav Performans Scales
>>and vertical profiles

Not fitted in our steam driven Boeing. But we do have the approaches in the FMC. Beginning to see the problem of the lack of regulatory oversight?


>>IRS backup.

Worse than useless. As I said before, our IRS fix can easily be a couple of miles away from the GPS fix, and the VOR/DME is not much better in mountainous terrain. So if you lose the GPS on the approach, and the FMC position reverts to IRS fix position, you get a rapid 2nm map-shift. Ok, so the nearest mountain peak is only a couple of nm away, and the autopilot is trying to recapture the new IRS trackline which is 2nm away - your move, as they say...



>>TCAS assisted incidents.

The Swiss collision would not have happened without TCAS. It was a total system usage error that had not been fully thought through and promulgated to all airlines and crews before the incident.



>>RAIM outages.

RAIM is real-time monitoring of the satellite constellation. You cannot determine RAIM outages by NOTAMS. Or are you telling me that the NOTAMS know exactly when the next satellite will be hit by some space debris, or lose its IRS stability system?



>>300ft per nm

Brilliant. The max altitude deviation on a 0.3 nm rnp approach is just 75ft, and the hills are all around you. Is that a satisfactory check? Do you really monitor ILS glideslope excursions by doing a 3x crosscheck?

[de facto]

Troll alert.

Aterpester..good link

[aterpster]

de facto:

Troll alert.

At the least, hasn't been through the training.

[silvertate]

aterpster

A Powerpoint Honeywell presented a few years ago:
https://aerospace.honeywell.com/~/media/Brochures/Gulfstream%20PlaneView%20RNP%20Economic%20Benefit.ashx

Not exactly a roll of the dice.

Sorry, but did you read that Powerpoint link? It says (p10), quote:

RNP ... indicated by RNP-X is a value in nautical miles from the intended horizontal position, within which an aircraft would be at least 95% of the total flying time.

That means with rnp 0.3 you will be within 0.3 nm of your intended position 95% of the time, which is what we have been saying. So what point were you trying to make with this link?

ok465

Would you personally fly one without authorization just because nobody was watching?

I think you can gather from my posts, that I would not (except in VFR as a pseudo visual). But that did not stop half the fleet doing these approaches, because no guidance had been given. And it did not stop other operators either, because one inbound would do the GNS approach and the next from the same operator would say 'we don't have authorisation'. Confusion seems to be the order of the day.

And the multiplicity of notation does not help. The USA might have RNP-AR, but in Europe we generally have RNAV GNSS. EPKT Katowice in Poland or EGBB Brum in the UK are GNSS approaches. There are a few around. LTBS Dalaman GNSS in Turkey is more pertinent to this thread, as it goes through the hills and is offset.

And then when you get to Spain we have the LEMG Malaga GLS approach. Now a GLS is actually a GBAS Landing System (GLS) - an approach that uses a WAAS or an EGNOS. I hope you got all that. In other words, Malaga uses the new EGNOS system, which is the European WAAS lookalike, and therefore eradicates most of the problems discussed here.

EGNOS augmentation:
EGNOS in Aviation: Vertical Precision for Improved Approaches | European GNSS Agency

de facto

Troll alert. Aterpester..good link

I see it is play time at school again.

If the THY A330 had not been perfectly aligned with the centerline, due what has been discussed here, then the resulting landing may have been more tricky. Just sayin'.