Very interesting article by Nav Canada on RNP (2010).
SUMMARY
RNP (Required Navigation Performance) is an advanced form of GPS navigation which is revolutionizing the way instrument approaches are carried out due to the fact that it does not rely on ground based navigation aids such as ILS (Instrument Landing System).
Although RNAV and GPS are currently used in everyday operations of commerical air carriers, their application has generally been limited to straight-line tracks between waypoints during en route navigation. This has led to rigid use of instrument approach procedures (ILS) which often lead to multiple step down non-precision and circling approaches, with 90 degree squared off turns from base leg to final. Since no aircraft can execute a true 90 degree turn over the ground, these "fly-by way points" serve as a general guide while the actual flight path flown is much different then the perpendicular angles plotted on the approach plate.
RNP or Performance-Based Navigation allows the aircraft to accurately navigate along flexible linear surfaces rather than point-to-point as it does on standard GPS. Departure and instrument approach procedures can therefore be designed to quite literally "curve" around terrain and obstacles. Such innovation has given rise to the short transition - a procedure built into the RNP/RNAV approach whereby the aircraft may navigate the final approach course by means of a fixed-radius turn as it approaches the airport from a downwind or base leg. Colloquially referred to by pilots and controllers as the “Short Gate,” these arced transitions to final have the potencial to save the airline hundreds of thousands of liters fuel per year by drastically shortening the gear/flaps down approach flight time which utilizes a 3.5 times higher fuel burn then normal cruise.
This capability is already being used on the Boeing 737 at WestJet, has opened up new routes and improved WJ service to airports surrounded by mountainous terrain.
I have often wondered, the "what if" scenario that one day, when they make NDB's, VOR's and ILS's obsolete because of new GPS based technology, the sun fart's out the mother of all solar flare's and wipes out most satellites. Then what ?
I don’t believe there is any time savings with an RNP approach. WestJet has some “short gates” which are part of the arrival transition to the approach and I believe these can save them significant track miles. The real advantage of an RNP approach is where terrain constraints prohibit an ILS to low minima – or prevent the approach entirely. An RNP approach can be designed to wind around terrain allowing a constant descent with proper obstacle clearance. Kelowna would be a perfect example.
Minor editorial point – RNP approaches although very precise, are actually still considered non-precision approaches.
Sounds like a lot of people posting dont know diddly squat about RNP's..
But to answer one question, if the sun 'farts' both the IRS's and the conventional VOR navigation have more than enough navigation to exicute a safe missed approach, no different than a VOR, NDB losing power..
RNP approaches are very easy to navigate by using the LNAV/VNAV capabilities in the aircraft sytem, as easy or easier than a traditional localizer/glideslope. The key is airspeed to keep it within 1x RNP. RNP is spherical not conical, so throughout the approach it is required to be within the 'circle', unlike conventional approaches where there is more error allowed the further you are from the nav source.
double-j wrote:Sounds like a lot of people posting dont know diddly squat about RNP's..jj
3 people is a lot? I see a couple of legitimate questions from people who are curious. Who doesn't know "diddly squat"? If you're the expert you present yourself as, why don't you just try to educate the masses, and not to be a dick about it.
I had a chance to fly the GE tech's as we flight checked all the RNP approaches and missed approaches for WJ into YXU and YHM.
We flew along each of the arcs and missed approaches; taking video of the approach and photos of the limiting obstacles on approaches. The flight checks are done every 3 years. The RNP loosely follows a number of the RNAV approaches already at YHM but instead of a T, it is a descending, curving approach inside the FAF.
I think the approaches are brilliant and are an amazing way to utilize the technological capabilities of the 737 to save both time and money.
@ True North - Thanks for the post! Indeed you are right, I am just curious to learn more about RNP after reading the Nav Canada article. As someone not currently in aviation but seriously considering making the switch, I have always been facinated by anything related to aviation. I thought this thread would be a good way for people who are interested to learn more.
@ Culshawm - Can you elabourate more on how the RNP system actually works when flying in the cockpit? I'm interested to hear what you see/exprience as you do an RNP approach versus a traditional ILS approach. Is the workload during approach higher or lower with RNP?
@ double-j - When you say that a traditional ILS approach is conical, I can envision that as the localizer signal gets wider and wider as it travells further away from the antenna array which makes a conical approach path the aircraft must fly in. For an RNP approach, I envision that the flight path is defined by a more "virtual cylinder" in space that dictates the track an aircraft must take on approach. This "cylindrical" flightpath can be bent around obstacles to create the most efficient approach possible and removes the need for rigid 90 degree banks used in the downwind/base/final turns of a traditional ILS approach. Is this a general idea of how it works?
My apologies about the 'diddly squat' comment, it was uncalled for..
CJ555 you are indeed correct. Traditional approaches use the TERPS method of obsticle clearance, so basically the MDA is based on a 'cone' from either side of the aid. RNP however uses a tube shaped approach throughout the approach and missed approach using the accuracy of the navaids onboard and requires stricter RNP the lower you go on the approach. The airplane must have a nav accuracy to remain 2X RNP I think about 9999.9% of the time (If I recall.)
There is a great video outthere, Ill try to find it.
double-j wrote:My apologies about the 'diddly squat' comment, it was uncalled for..
CJ555 you are indeed correct. Traditional approaches use the TERPS method of obsticle clearance, so basically the MDA is based on a 'cone' from either side of the aid. RNP however uses a tube shaped approach throughout the approach and missed approach using the accuracy of the navaids onboard and requires stricter RNP the lower you go on the approach. The airplane must have a nav accuracy to remain 2X RNP I think about 9999.9% of the time (If I recall.)
There is a great video outthere, Ill try to find it.
jj
9999.9%. That is a lot! I think you meant 99.999%.
double-j wrote:The airplane must have a nav accuracy to remain 2X RNP I think about 9999.9% of the time (If I recall.)
jj
What does 2 X RNP mean? Is that in relation to the speed & altitude on the approach? So the closer you get, the more strict it gets with respect to allowable "deviations"?
double-j wrote:My apologies about the 'diddly squat' comment, it was uncalled for..jj
Good boy.
cj555, you have pretty good vision of it. On an ILS the a/c flys into a cone. During RNP ops the a/c flys inside a tube, the dimensions of which are determined by the RNP value selected by the crew. For example, WestJet is one of a handful of operators in the world authorized to fly approaches down to RNP .1 minima. There is also RNP 1, 4 and 10 airspace. RNP 10 typically being for NAT tracks and WATRS airspace.
Flying an RNP approach is not much different for the crew than flying an ILS. There are some very subtle differences but basically the crew follows the vertical and lateral commands of the flit director, same as an ILS.
Westjet has their own company RNP approaches they have been using for several years now. In the US, RNP approaches have been published with the other charts for some time. Get hold of some Jepp or NOS charts and they are there. Look up some charts on fltplan.com for US airports and you will see RNP approaches.
Somebody with experience with them could confirm this, but I believe that when you do you simulator training that you have to practice and be tested on the RNP approaches just like an ILS or a GPS.
What does 2 X RNP mean? Is that in relation to the speed & altitude on the approach? So the closer you get, the more strict it gets with respect to allowable "deviations"?
If you were shooting an approach to RNP 0.3 minima, the airplane is guaranteeing it's lateral accuracy to 0.3NM 95% of the time, and 2X (two times, not 2 'the letter X') RNP, also known as the containment area, the airplane has to guarantee accuracy within 2X RNP (0.6nm) 99.999% of the time. This strictness remains the same from the moment we select the RNP value on the FMC (usually 100NM back ish from the top of descent when we're loading and setting up the approach and doing briefings).
so 2X RNP is another tube around the airplane that's twice the radius.
Uh, we've been doing RNP-AR approaches for years in a/c with a lot more inertia than a 737 and it's really not a big drama. You have to make sure the technical and operational requirements are met but it's as easy as an ILS, just allows more flexibility for approach design.
And yes, you do sim practice for RNP approaches, to train for the aforementioned unique requirements over ILS approaches. As far as the "crew following the vertical and lateral commands of the flt director", it's more like the autopilot that's doing the following. The crew is more in a monitoring role, managing any failures or discrepancies.
complexintentions wrote:Uh, we've been doing RNP-AR approaches for years in a/c with a lot more inertia than a 737 and it's really not a big drama. You have to make sure the technical and operational requirements are met but it's as easy as an ILS, just allows more flexibility for approach design.
And yes, you do sim practice for RNP approaches, to train for the aforementioned unique requirements over ILS approaches. As far as the "crew following the vertical and lateral commands of the flt director", it's more like the autopilot that's doing the following. The crew is more in a monitoring role, managing any failures or discrepancies.
A bit OT but what aircraft are you referring to. Since Alaska airlines was the first in the world to be authorized to do rnp approaches, and we know they have nothing bigger than 37s, i am curious to which aircraft you are referring to.
B777, A330/40 and now A380 (Emirates). Approved for RNP-AR (Authorization Required) approaches into places like SEZ and JFK, off the top of my head. Some other newer ones as well. US approval took forever but the capability has been there for awhile.
I only commented because of the thread title making it sound like a bigger deal than it is. (Although the tech is cool).
If ATC gave us short gates we would save time and fuel. Not every arrvial has a shortgate though.
As for How they look - aside from turns they display the same as any other approach. The modes are different that's all.
Flying into and out of YLW VFR doing the RNP shows just how useful they are. Also makes life much less difficult in the event you lose one.
both the IRS's and the conventional VOR navigation have more than enough navigation to exicute a safe missed approach, no different than a VOR, NDB losing power..
