View Full Version : Required Navigation Performance Questions
28th Apr 2009, 08:23
When a RNP value is described as providing 95% probability of containment, does that literally mean that 95% of the time, an aircraft can be expected remain within this space? (e.g. .1NM)
Does the same apply to RNPx2 providing a 99.9% containment rate?
RNP is a required navigation performance level described by the specification of a numeric value indicating the required navigation accuracy for a specific operation, typically specified laterally in nautical miles - e.g., RNP 1 is a Required Navigation Performance of ± 1 nautical mile (95% Probability). RNP containment is specified as RNP(X) times two (i.e., RNP(X) x 2).
Also, does a horizontal RNP value (e.g. .1NM) refer to .1NM left and .1NM right or .05NM left and .05NM right? (a total of .1NM)
28th Apr 2009, 08:31
maybe this could be of help.
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28th Apr 2009, 08:31
Think of RNP as the radius of a circle around the FMS present position. The Aeroplane must really be within that circle 95% of the time.
28th Apr 2009, 08:58
So, RNP containment = RNP(X) x 2 = RNP(radius) x 2.
28th Apr 2009, 09:32
This might suggest that there is a chance that 5% of the time the a/c is not within RNP. When making LNAV/VNAV NPA approaches will crews advise what their airline SOP's are for cross checking raw data during final stages. I know raw data should be displayed somewhere and monitored, but what calls if any are made, and where is the raw data monitored. Do you use just the ND needle repeaters, or does PNF have VOR display up? Of course on NDB's only the blue needle repeaters + RMI are available. (B NG)
When executing a VOR/DME approach do you intercept in VOR mode then reselect LNAV, or just monitor that the correct radial is being tracked in LNAV.
For the VNAV, does everyone still call ALT v DME. The RNP is a radius and thus effects longtitudinal dispalcement as well as lateral.
Accidents happen often becasue of the unexpected; subtle changes that are not picked up. Aligning 2 degrees off the correct radial could bring you out a little off track at MDA and on a nasty night cause some interesting low level manoeuvring.
28th Apr 2009, 22:35
A few incorrect assumptions are made there.
ANP is just a confidence measure not an actual error. If you like it is more accuratley called estimated position uncertanty or EPU. This is a really important point to understand. For example an approach has an RNP of 0.3 and our ANP is 0.29 this does not mean that the position solution is in error of 0.29 NM it is that the FMC only has the confidence that it is within 0.29 NM. It may be out by that much but it also may be exactly on the centerline in the correct position.
Containment is ANP less than RNP. ANP greater than RNP is a missed approach. Cross track error greater than RNP is a missed approach. Obstacle clearence is calculated at 2 x RNP (for a total of 4 x RNP i.e. 2 x either side of track) is a 4 Standard Deviation probability of 99.999. So a missed approach is flown at 1 x RNP but you are protected out to 2 x RNP.
RAT5 highlights a good point, ANP is not just left and right it can also be forward and back so using a circle is the best description.
Alt v DME is only a check on flight path gradient and gives no protection that you are on the correct slope unless you are flying an ILS/MLS etc. For RNP, VOR or ADF approaches it will not save you from an Altimeter Error
Vert RNP is not in wide use yet.
There is no requirement for "raw data" in RNP approaches (assuming you have a full dual FMC GNSS aircarft) in fact most RNP AR approaches "raw data" does not exist.
The real threat to RNP operations is not lateral but the altimeter used for Baro VNAV.
29th Apr 2009, 01:40
When making LNAV/VNAV NPA approaches will crews advise what their airline SOP's are for cross checking
I'm not going to repeat what c100driver has said, as he's just about covered all of it. But regarding monitoring, one of the big differences between RNP operations and RNAV operations is that RNP has to have a warning that you are exceeding the 'limits', whilst RNAV doesn't. Boeing gives the 'Unable RNP' message (I don't know what the Airbus does). So, for monitoring purposes, RNP will do it for you, hence the lack of necessity of cross-checking. Of course, airmanship might well dictate that you keep and eye on other sources of info. This is just the letter of the 'law', as opposed to how I would fly.
29th Apr 2009, 01:51
Just to be clear - any RNP value described pertains to the radius length from the center of a virtual containment sphere?
And containment is rated at 95%?
And RNP(X) x 2 containment is rated at 99.999%?
By the way, are these 2 popular publications trying to describe the same RNP capability? Have they fallen prey to the same confusion as myself?
The second figure (18 meters or 59 feet) seems to be approximately half of the first figure (.02 nautical miles or 121.5 feet).
WAAS also supports required navigation performance (RNP) operations, says Raytheon, providing a precision navigation capability down to RNP 0.02 (an accuracy of 0.02nm).
Naverus's precise satellite-guided flight paths look like discrete pipelines. The routes pilots follow never vary more than 18 meters.
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29th Apr 2009, 03:25
RNP is the design requirements of a procedure and does not change.
ANP is the ability of an FMC to project accurately the desired course. The confidence that the FMC has that it is accurately projecting the course is measured in NM. This is measured as a CIRCLE with a 95% confidence that the aircraft true position is inside that circle. The reason it is not a sphere is that most aircraft flying these approaches are using Baro VNAV for vertical projection which in not subject to a Vertical ANP. Vert ANP is coming but it is not here yet as it does not use Baro-altimetery.
Crosstrack Error and Vertical Deviation are the aircrafts ability to fly the FMC projection.
What they are actually saying is that using WAAS the FMC has a capability of very low ANP. The press and most line pilots usually gets ANP and RNP mixed up because of a lack of understanding.
At present the B737 NG have the lowest RNP demonstrated values for jet transports of 0.10 which in a low threat terrain RNP AR approach is close to 250 feet AGL DA. With WAAS or GBAS a lower RNP may be able to be flight demonstated, this should allow an aircraft with a low ANP ability to fly a low (below 0.10) RNP approach. The world of RNP is moving fast so the G650 and other new designs may be able to achieve lower soon.
Note the bold, we use that in our airline to describe the three components that make up Performance Based Navigation;
Design, Project, Fly = RNP, ANP and Crosstrack Error + Vert DEV
30th Apr 2009, 09:22
Great explanation. Thanx.