(RNP AR)
Have you ever looked at an approach plate and wondered what was up with the curved lines on the plan view? These approaches actually allow you to fly an instrument approach with a curved path and also provide a glide path down to a Decision Altitude (DA). We have made amazing strides with GPS approaches. The first approaches provided lateral guidance down to a Minimum Descent Altitude (MDA). Sometime later, manufacturers created approaches with vertical guidance using baro-aiding. The next leap occurred in the United States when the Satellite Based Augmentation System (SBAS) and the Wide Area Augmentation System (WAAS) provided vertical guidance. WAAS approaches provide approach minimums down as low as ILS CAT I minimums, 200-foot DH and 1800 RVR.
AR = Authorization Required
The “AR” in the notes section indicates that special aircraft and aircrew authorization is required. Pilots need to attend specialized training, much like you would for an ILS CAT II authorization. The training will include flying at least two approaches in a full-motion simulator, both as pilot flying and as pilot monitoring, for a multi-pilot crew. The approaches will include normal approaches and being vectored off the approach for re-sequencing. You will also need to perform a published missed approach and complete a landing out of an approach. You will subsequently need to request authorization from the FAA, which will be granted in the form of a Letter of Authorization or Operations Specifications.
The airplane also needs to be certified for RNP AR approaches. The Bombardier Challenger CL-650 with the Pro Line 21 is approved for approaches with an RNP as low as 0.3. This 0.3 is the minimum lateral approach sensitivity. It indicates that a full-scale deflection of the CDI from about the Final Approach Point (FAP) to the Missed Approach Point (MAP) will be about 1/3 of a mile. The approach minimums will actually be based on the sensitivity of the approach capability. Boeing’s 777 is certified for RNP AR approaches down to minimums of 0.12. The lower RNP minimums will provide lower approach minimums.
RNP = Required Navigation Performance
RNP defines performance-based navigation to operate between two points in space or in a block of airspace. In addition to the RNP lateral sensitivity requirement, the approaches also require specific equipment just like when flying a CAT II Approach or flying in RVSM airspace.
Required Equipment
An interesting observation from an RNP AR “required equipment list” indicates that “two GNSS sensors must be available to commence the approach” and “one GNSS sensor is required to continue the approach.” This differs from the Flight Management System limitations that require suitable navigation information from “one GNSS; or two DMEs; or one VOR/DME; or one IRS”.
RNP AR approaches need to have a clear, unobstructed GNSS signal from the satellites when conducting an approach such as along the ridgeline of mountains in Palm Springs, CA. It is also worth noting that RNP AR approaches do not use or require WAAS. This also means that the lateral distance remains linear from the FAP (yes, Final Approach Point, not Final Approach Fix) to the MAP unlike the angular guidance provided by an LPV WAAS approach.
RF = Radius to a Fix
The curved portions of the approach are referred to as Radius-to-a-Fix legs (RF). In addition to the obvious curved lines, RF legs can be identified on the plan view of the approach charts by the lack of the magnetic course printed on the chart. The profile view identifies the curved path by “LT Arc” or RT Arc” for left turning arc or right turning arc respectively.
Speed Limits
To keep the airplane within the required lateral distance, there may be speed limits. Posted speed limits, such as “Max 210 KIAS” may be printed on the plan view next to an intersection. The manufacturer may establish additional restrictions (e.g. the CL-650 must be fully configured by the Intermediate Fix (IF) on the approach,” and the FAA imposes a maximum airspeed from the FAP to the DA of 140 knots for a CAT C category airplane.
Flying the Approach
Let’s use the Rockwell Collins Pro Line 21 on the CL-650 for demonstration purposes.
When cleared for the approach, proceed directly to the cleared fix, select Approach and VNAV Modes on the Flight Control Panel (FCP), and remember to verify that the appropriate modes were accepted on the Flight Mode Annunciator (FMA). Set a lower altitude in the altitude selector to the FAP altitude or MDA to ensure the aircraft continues on the descent path for the approach. When the Flight Mode Annunciator captures the VGP (Vertical Glide Path), the approach will no longer recognize or capture altitudes in the altitude selector or altitude alerting device. You may now set the missed approach altitude in the altitude selector to prepare for a missed approach.
Lastly, confirm that the message on the Primary Flight Display (PFD) switches from “RNP AR ARM” to RNP AR APPR” and verify that the RNP APPR CDI Scale indicates “0.3 nm” prior to crossing the final approach point (FAP).
Now you can pretty much just sit back and watch the magic happen. The airplane will fly curved paths around cities, mountains, or airspace while descending via an FMS-generated glide path down to a decision altitude.
Tolerances
The maximum allowed lateral deviation is two dots, and vertically one dot. The one dot vertical is 75 feet. An amber deviation alert is displayed for deviations beyond the above limits.
Missed Approach
During a missed approach the RNP will ramp up from 0.3 back to the 1.0 that is required for the terminal area. A few approaches, such as KSDL RNAV (RNP) Y Rwy 3, have a note stating: “Missed approach requires RNP less than 1.0.” Most airplanes will not be capable of conducting this approach due to this restriction.
Non-normal
The FMS also provides internal monitoring and alerting of the approach and as stated earlier, does not use WAAS. Non-normal indications include: “UNABLE RNP,” NO APPR,” “EFIS MISCOMP,” and “Loss of GNSS.” A missed approach is also required during training to practice sequencing the FMS through the missed approach.
Miscellaneous
- You must have and use the local altimeter setting.
- The primary altimeters must be within 100 feet of each other; if you have two.
- You may not proceed direct-to-waypoint immediately preceding an RF (curved) leg; you must start on a straight leg.
- You must also monitor TAWS (Terrain Awareness equipment to ensure obstacle and terrain clearance.
Titles and Minimums
The titles of the approach plates prove to be one of the most difficult areas to understand. They are inconsistent between Jeppesen, FAA, TERPS, and Pans Ops. Just because “RNP” is in the title does not necessarily indicate that it is an RNP AR approach. All FAA government RNAV charts indicate “RNP Apch” in the notes, even for the normal approach minimums of LNAV, LNAV/VNAV, and LPV approaches. You must actually look at the minimums section to determine the type of approach.
GPS Approaches have come a long way since their conception. I never envisioned that I would be flying curved approaches with vertical guidance to a decision altitude. If your flight simulator is set up for these approaches, you might be able to persuade your instructor to let you fly one during your next recurrent training to see if they’re right for you or your operation.
