So You Fly RNP-0.1 ? That Don’t Impress Me Much !
Aviation is technology and change resistant. It’s all about certification, validation safety studies, and so on. Given the expected results, this is not that bad, but this also leads to strange perceptions. One of the hot topics these days is Required Navigation Perfomance, a.k.a. RNP.
The concept is simple: if a plane can fly with a precision of 1 nautical mile 99.99999% of the time, it can be certified as RNP-1. 0.3 NM ? RNP-0.3, and so on. Flying a precision approach requires an RNP-0.1 certification. The whole story is more complex, but I made it shorter for you: it’s GPS guidance.
Planes flying ILS approaches are guided by electronic signal to a precision of a few meters, and this can be done with autopilot or manual flying. So what is that crucial in flying RNP-0.1 ? 0.1 Nautical mile is 185.2 meters, and GPS can provide positioning with an accuracy of 3 to 30 meters. So what ?
Yes, there are integrity, and failure mode questions. But the same questions to exist with current systems. An aircraft flying down a valley on an ILS and suddenly losses its electrical power is in a serious emergency situation. A similar aircraft, flying the same valley but guided by GPS will have the same problems in the same conditions.
Now that instruments have been “virtualized” via glass-cockpits, and systems like “highways in the sky”, the navigation source no longer really matters. It can be GPS, ILS, DME-DME, inertial platforms, gnomes or elves, as long as it can be converted to lateral and vertical deviation signals, it can drive an ILS-like indicator and an autopilot.
Flying an RNP route is usually simpler as a traditional navigation, as points can be defined anywhere, independently from beacon locations, so the procedural aspect is no big-deal, emergency situations put aside.
So why is a plane flying a GPS-guided route with a precision 0.1 nautical mile a big event ? As a technical guy, I don’t see any big innovation in that. So to paraphrase Shania Twain: that don’t impress me much ! The only “magic” I can see in that it could be certified, given how picky (seen how polite I can be ?) the regulatory bodies can be…
4 Comments, Comment or Ping
fred
What is new is the ability to monitor the accuracy and alert the crew if RNP capabilities decrease. This is what differentiate RNP from basic RNAV. The challenge is there.
By the way, RNP1 is a 1 NM precision 95% of the time, not 99,999%.
fred
May 18th, 2008
PlasticPilot
Fred, I agree that accuracy monitoring is a serious challenge, as it is in any ground navaid equipment. However, knowing how VORs and ILSs are monitored (couple of monitors at known positions), cross-checking a GPS position with inertial or other positioning system does not seems like a big deal to me. But I could be totally wrong
May 18th, 2008
fred
well, I’m not a specialist to answer …
May 19th, 2008
PlasticPilot
As far as I know, the VOR and ILS monitoring is based on receivers (a.k.a. monitors) located near the station. If the received signal is not as expected, the control system switches to the second transmitter. If the problem is not solved, the system automatically switches the transmitter off.
This offers relatively good protection against transmitter problems, or local interference, but nothing prevents wider range disturbances. Hearing and checking the morse code is also kind of an integrity check, but it’s not that robust.
May 19th, 2008
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