RNP and full-scale deflection
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RNP and full-scale deflection
We have a little discussion here at my school regarding RNP. As I understand it, RNP is based on the accuracy of the equipment in the airplane, the navaids on the ground (or GPS) and flight crew errors. So, if you are flying a GPS approach with RNP 0.3 and you have a full-scale deflection or less, you should not be further than 0,3 NM from the centerline. Is this correct?
Another theory I've heard is that the system can "lie" to you with 0.3 NM. So if you are on the same approach with a full-scale deflection, you could be as far 0.6 NM from the centerline. This doesn't seem to make sense, but some of the stage checkers here thinks that is correct.
Another theory I've heard is that the system can "lie" to you with 0.3 NM. So if you are on the same approach with a full-scale deflection, you could be as far 0.6 NM from the centerline. This doesn't seem to make sense, but some of the stage checkers here thinks that is correct.
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Cut and paste from my AOM Hope it helps
CDI
When the FMS is selected as the navigation source on the PFD, the scale of the CDI
will be based on the displayed active RNP value. The two-dot full scale deflection
will always be equal to the RNP value displayed on the PFD.
Excessive Lateral Deviation
The FMS does not provide an indication to the crew of an excessive lateral (cross
track) deviation. Lateral deviation is not considered in the display of the RNP
integrity alert. It is the pilot’s responsibility to monitor lateral deviation, and take
appropriate action to ensure that the aircraft follows the flight plan with minimal error.
Integrity Alert
The steering sources required for each RNP type assume worst-case position
determination by the FMS. Regardless of the sensors in use, the RNP integrity alert
will always be provided at any time the current position determination mode of the
FMS does not yield satisfactory accuracy and integrity to satisfy the containment
requirements for the active RNP type.
CDI
When the FMS is selected as the navigation source on the PFD, the scale of the CDI
will be based on the displayed active RNP value. The two-dot full scale deflection
will always be equal to the RNP value displayed on the PFD.
Excessive Lateral Deviation
The FMS does not provide an indication to the crew of an excessive lateral (cross
track) deviation. Lateral deviation is not considered in the display of the RNP
integrity alert. It is the pilot’s responsibility to monitor lateral deviation, and take
appropriate action to ensure that the aircraft follows the flight plan with minimal error.
Integrity Alert
The steering sources required for each RNP type assume worst-case position
determination by the FMS. Regardless of the sensors in use, the RNP integrity alert
will always be provided at any time the current position determination mode of the
FMS does not yield satisfactory accuracy and integrity to satisfy the containment
requirements for the active RNP type.
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Greetings Marius,
RNP is a required performance that your aircraft has to be able to match, it is lets say a tube, with a diameter, in our case the diameter is equal to 0.3, so if the sensors plus the other errors of your aircraft display 0.3 or less, then the aircraft, that we assimilate to a ball, can enter the tube and go through it.
sensor budget is call ANP (Actual Navigation Performance) in Seattle, and EPU Estimated Position Incertainty in Toulouse, in fact say are both the same.
Now if due to a system degradation, loss of GPS, bad GPS geometry inducing a high DOP, then the ball diameter will grow, and when it becomes greater than the RNP it will not be able to go through the tube anymore.
SO the RNP doesnot change, only your ANP/EPU does.
THe RNP has different values depending on where you are, for exemple it is RNP 1 until the FAF, then RNP 0.3 from your FAF to your MAP, then it is RNP 1 from the MAP for the Go Around segment, so as long as you have an ANP/EPU smaller than the RNP you can continue to fly the procedure, if the ANP/EPU becomes bigger than the RNP, you Go Around or, you vacate the RNP airspace.
Systems dont lie, but human do
ANP/EPU are mesured values using very sophisticated algorythms, the most popular on is referenced as a Kalman filter, Google it, and read.
The most probable case where the EPU could be, lets say optimistic, is during reception of a GPS solution that has a Bad Geometry, inducing a DOP, Dilution Of Precision, in that case the FMS/FMC will use the DOP to correct the precision, just by multiplying the precision by the DOP.
RNP is a required performance that your aircraft has to be able to match, it is lets say a tube, with a diameter, in our case the diameter is equal to 0.3, so if the sensors plus the other errors of your aircraft display 0.3 or less, then the aircraft, that we assimilate to a ball, can enter the tube and go through it.
sensor budget is call ANP (Actual Navigation Performance) in Seattle, and EPU Estimated Position Incertainty in Toulouse, in fact say are both the same.
Now if due to a system degradation, loss of GPS, bad GPS geometry inducing a high DOP, then the ball diameter will grow, and when it becomes greater than the RNP it will not be able to go through the tube anymore.
SO the RNP doesnot change, only your ANP/EPU does.
THe RNP has different values depending on where you are, for exemple it is RNP 1 until the FAF, then RNP 0.3 from your FAF to your MAP, then it is RNP 1 from the MAP for the Go Around segment, so as long as you have an ANP/EPU smaller than the RNP you can continue to fly the procedure, if the ANP/EPU becomes bigger than the RNP, you Go Around or, you vacate the RNP airspace.
Systems dont lie, but human do
ANP/EPU are mesured values using very sophisticated algorythms, the most popular on is referenced as a Kalman filter, Google it, and read.
The most probable case where the EPU could be, lets say optimistic, is during reception of a GPS solution that has a Bad Geometry, inducing a DOP, Dilution Of Precision, in that case the FMS/FMC will use the DOP to correct the precision, just by multiplying the precision by the DOP.
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RNP is a required performance that your aircraft has to be able to match, it is lets say a tube, with a diameter, in our case the diameter is equal to 0.3, so if the sensors plus the other errors of your aircraft display 0.3 or less, then the aircraft, that we assimilate to a ball, can enter the tube and go through it.
Im not sure the tube theory is a particularly good analogy for approaches. Being 0.3 vertically from design track centreline is not acceptable. 0.3 laterally and 75' vertically is the limit for my company.
Confusion about the 0.6 bit could come from the idea that for procedure design, obstacles are considered out to 2 X RNP value from the design centreline (.3 X 2, in this case).
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Thanks for your answers!
I'm still not sure if I understand this 100 % yet. A full scale deflection is 0.3 NM from the centerline, and the calculated position should be within 0.3 NM of your actual position? RNP, is, as I understand, based on the accuracy of the navigational equipment and how accurate the crew can navigate, flight crew errors. So how accurate should the flight crew be able to navigate? Be able to maintain within half scale deflection? Three-quarter scale deflection?
I'm still not sure if I understand this 100 % yet. A full scale deflection is 0.3 NM from the centerline, and the calculated position should be within 0.3 NM of your actual position? RNP, is, as I understand, based on the accuracy of the navigational equipment and how accurate the crew can navigate, flight crew errors. So how accurate should the flight crew be able to navigate? Be able to maintain within half scale deflection? Three-quarter scale deflection?
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Required Navigation Performance - Wikipedia, the free encyclopedia
Have a look here. It explains RNP in more general terms.
Have a look here. It explains RNP in more general terms.
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Greetings
The B777 is equipped with the NPS Navigation Performance System, along with a Navigation Performance scale for lateral, and vertical RNP, and a directional Indicator.
the full scale represents the RNP value regardless of its value, i.e., RNP 1, RNP4, RNP 0.3, so the RNP value doesnot not change the scale width ( this is why I refered to the diameter of the tube, and not to its Radius)
In the middle of the scale, you have a mark (I cannot find the word) now everytime the pointer is in the middle it is showing you that you exactly on track, if the pointer goes to the full deflection to the left, it means that you are on the right of the track, with a Xtrack error equal to half of the RNP value, the ANP will turn amber, and the pointer will flash, showing you that you have to fly left to regain the published track, and vice versa.
The AMC 20.27 states that for an RNP approach the maximum deviation should not exceed half of the published RNP, for that particular segmentg,i.e., IAF to FAF RNP1 so max dev is 0.5, FAF to MAP RNP 0.3 so max dev is 0.15, and from MAP and GoAround segment, RNP1 so max dev is 0.5.
FAA.GOV website has all the literature, have look.
Regards
The B777 is equipped with the NPS Navigation Performance System, along with a Navigation Performance scale for lateral, and vertical RNP, and a directional Indicator.
the full scale represents the RNP value regardless of its value, i.e., RNP 1, RNP4, RNP 0.3, so the RNP value doesnot not change the scale width ( this is why I refered to the diameter of the tube, and not to its Radius)
In the middle of the scale, you have a mark (I cannot find the word) now everytime the pointer is in the middle it is showing you that you exactly on track, if the pointer goes to the full deflection to the left, it means that you are on the right of the track, with a Xtrack error equal to half of the RNP value, the ANP will turn amber, and the pointer will flash, showing you that you have to fly left to regain the published track, and vice versa.
The AMC 20.27 states that for an RNP approach the maximum deviation should not exceed half of the published RNP, for that particular segmentg,i.e., IAF to FAF RNP1 so max dev is 0.5, FAF to MAP RNP 0.3 so max dev is 0.15, and from MAP and GoAround segment, RNP1 so max dev is 0.5.
FAA.GOV website has all the literature, have look.
Regards
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Simple explanation of RNP
If you are RNP x then when the equipment in your aircraft indicates that you are exactly on the centreline - 95% of the time you will be x or less from the actual centreline.
submit 0.5, 1, 5, 10 or whatever the RNP figure you want to use is.
I think that you may be getting confused with the scaling of the display as one proceeds from enroute to terminal to approach and the different values of full scale deflection in each situation.
As an example if the best you could do is RNP 5 then regardless of what full scale deflection was, when the indication is on track i.e. no deflection then as I said above, 95% of the time you will be in this case +/-5nm of track.
RNP not just left/right and is not simply a circle. The shape of the area round the indicated position in which the aircraft could be will depend on the navigation systems used for the RNAV - GPS or DME DME or VOR DME or IRS or whatever. The RNP figure will be the worst case and for example in the case of IRS alone, there will be a time limit.
Regards,
DFC
If you are RNP x then when the equipment in your aircraft indicates that you are exactly on the centreline - 95% of the time you will be x or less from the actual centreline.
submit 0.5, 1, 5, 10 or whatever the RNP figure you want to use is.
I think that you may be getting confused with the scaling of the display as one proceeds from enroute to terminal to approach and the different values of full scale deflection in each situation.
As an example if the best you could do is RNP 5 then regardless of what full scale deflection was, when the indication is on track i.e. no deflection then as I said above, 95% of the time you will be in this case +/-5nm of track.
RNP not just left/right and is not simply a circle. The shape of the area round the indicated position in which the aircraft could be will depend on the navigation systems used for the RNAV - GPS or DME DME or VOR DME or IRS or whatever. The RNP figure will be the worst case and for example in the case of IRS alone, there will be a time limit.
Regards,
DFC
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Airbus has...
Airbus has a "Getting to Grips with Modern Navigation" document...rather
lengthy....but, it may provide an answer to your question. Certainly,
it will dispel many of the myths out there.
Fly safe,
PantLoad
lengthy....but, it may provide an answer to your question. Certainly,
it will dispel many of the myths out there.
Fly safe,
PantLoad
Well not quite, I think what they are asking is:
If the CDI is full scale deflection then the Flight Technical Error or ability of the aircraft to fly the RNP track or in Boeing speak the cross track error is at its maximum. That means that the aircraft is not following the track centerline due to some aircraft flight control reason, such as strong winds, AP steering error etc.
What you are discussing is if you have a CDI deflection showing "off track" and a navigation solution that is not great at the same time giving an ANP that could also be at the limit.
ANP (EPU) is a confidence of position statement not a measurement of actual error, so the assumption that a cross track error at full scale and an ANP at maximum is an error of 2 x RNP is incorrect.
If the CDI is full scale deflection then the Flight Technical Error or ability of the aircraft to fly the RNP track or in Boeing speak the cross track error is at its maximum. That means that the aircraft is not following the track centerline due to some aircraft flight control reason, such as strong winds, AP steering error etc.
What you are discussing is if you have a CDI deflection showing "off track" and a navigation solution that is not great at the same time giving an ANP that could also be at the limit.
ANP (EPU) is a confidence of position statement not a measurement of actual error, so the assumption that a cross track error at full scale and an ANP at maximum is an error of 2 x RNP is incorrect.
Last edited by c100driver; 15th Apr 2009 at 09:10.
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Ok, I think I got it know. So, when they came up with RNP, they expected that the ANP and the pilots (or AP) would be that accurate that 95 % of the time, the plane would be within the limits for that particular RNP level?
BTW, thanks for all the answers!
BTW, thanks for all the answers!
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Check out this Boeing Aero article on their nav displays, especially illustrations #3 and #4.
As 737Jock has noted, as EPU (Boeing = ANP) increases, the allowance for Flight Technical Error is reduced.
Having the CDI precisely centered does not necessarily mean that the airplane is "on course" with respect to the surface of planet earth.
As 737Jock has noted, as EPU (Boeing = ANP) increases, the allowance for Flight Technical Error is reduced.
Having the CDI precisely centered does not necessarily mean that the airplane is "on course" with respect to the surface of planet earth.
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The RNAV deviation scales introduced by Boeing make the picture much more clear if you are doing pure RNAV procedures, one glance tells you how much manouvering space you have at your current ANP and the required RNP.
And in general it is true that any RNAV approach deviation is linear unlike an ILS, however if you use boeings preferred method for non precision approaches (IAN) you will have a build in funnel similar to the ILS, even so far that you get GPWS call outs for glidepath deviation, that method is available on all 737 that have those nice RNAV deviation scales. The aim is not only to have the same procedure as for an ILS but comparable indication and deviation scaling as well. The same is of course even more true for GBAS-Approaches which for all practical means are identical to a normal CAT I ILS, including the funnel narrowing down when you approach the runway.
And in general it is true that any RNAV approach deviation is linear unlike an ILS, however if you use boeings preferred method for non precision approaches (IAN) you will have a build in funnel similar to the ILS, even so far that you get GPWS call outs for glidepath deviation, that method is available on all 737 that have those nice RNAV deviation scales. The aim is not only to have the same procedure as for an ILS but comparable indication and deviation scaling as well. The same is of course even more true for GBAS-Approaches which for all practical means are identical to a normal CAT I ILS, including the funnel narrowing down when you approach the runway.
