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Asked by: Dan Chitty
I am trying to get clarity on RNP (required navigational performance).
Example from approach plate: "GPS or RNP 0.3". Does the RNP reference only pertain to a non GPS unit that uses RNAV (such as VOR/DME RNAV)?
John D. Collins
on Mar 17, 2013
Good luck on getting clarity regarding RNP. The available information is lacking in my opinion and the terminology usage is changing.
The term does not include VOR/DME RNAV units such as the KNS80. For all practical purposes it includes just GPS for GA users. Certain high end FMS systems may be approved for RNP .3. Enroute and terminal usage of DME-DME based area navigation equipment predominantly used by the airlines is permitted for certain RNP operations.
The approaches that we define in the US as RNAV (GPS) are designated by ICAO to be RNP .3 approaches for the LNAV minimum line. In the US, an appropriately installed IFR GPS may fly these approaches. If the navigation system uses Baro-VNAV or WAAS, in the US the LNAV/VNAV minimums may also be flown and are also considered RNP .3 by ICAO. ICAO is still working on defining its specification for LP and LPV minimums, but once again, an IFR WAAS GPS is permitted to fly these procedures if the AFMS supports them. The instrument procedures that we title RNAV (RNP) are considered by ICAO as RNP AR (AR stands for Authorization Required) and may only be flown by approved operators with approved pilots. The RNAV (RNP) approaches are not available to GA users. A series of AC’s are available that provide guidance for operations in the US Airspace, including AC 90-100A (Use of RNAV on RNAV SID, STAR and ODP), AC 90-105 (RNP Approaches LNAV – LNAV/VNAV), AC 90-107 (LP and LPV), and AC 90-108 (Use of RNAV on conventional Routes).
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on Mar 18, 2013
Thank you for the excellent and insightful feedback. The information you have provided is very helpful.
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on Mar 19, 2013
At my employer, we train RNAV/RNP approaches and our aircraft are capable of RNP .11 (if on the approach chart) in the terminal area and the accuracy is amazing. We have done RNP approaches with tight RF legs in 50 knot crosswinds and the jet never strays off the magenta line. The lateral containment area is double that of the RNP value (a .3 RNP would have .6 miles of protected airspace on each side of the approach, etc.). The ANP of the FMS is usually .01 or .02! The vertical RNP is 125 feet but usually has an ANP of about 30 feet. Pretty cool stuff.
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Dan Chitty on Mar 20, 2013
Thank you for this additional information.
I have an additional question: Can you explain to me why after passing the FAWP a GPS approach is allowed to operate for 5 minutes if there is a satellite problem? Why would the GPS receiver not immediately alert the pilot that there is satellite problem?
In other words, the GPS receiver may indicate all is normal but in actuality there is a satellite problem. If I am flying a GPS approach, I want to know immediately if there is a satellite problem. Excerpt from AIM below.
If a RAIM failure occurs after the FAWP, the receiver is allowed to continue operating without an annunciation for up to 5 minutes to allow completion of the approach (see receiver operating manual). If the RAIM flag/status annunciation appears after the FAWP, the missed approach should be executed immediately.
on Mar 29, 2013
The GPS has many function going on internally. Many pilots do not even realize there are almost as many ground stations as satellites helping the GPS unit navigate.
When the GPS enters the final approach mode, sensitivity scaling to RNP 0.3, the flight path is “stored in memory”… to keep it simple. Then the GPS is allowed to fly off that memory for the next 5 minutes. Even if all the satellites fall out of the sky.
Why? Give the pilot an opportunity to land and not have to go back and try the whole approach over again.
Dan Chitty on Mar 30, 2013
Thank you for your correspondence. Your feedback is greatly appreciated and certainly gives me a better understanding, especially the “stored in memory” statement of which makes good logical sense.
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