Understanding Required Navigation Performance

4 Answers

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   Bryan on May 03, 2022

   I think your confusion is in the definition of navigation performance. Navigation performance measures how well actual navigation compares to some standard–like an approach chart. It needs a current chart and accurate GPS data to figure that out, but all it cares about is whether the chart and the aircraft’s actual position match.

   Navigation performance is not the accuracy of signals from navaids or navigation displays in the cockpit.

   Now that we know what navigation performance is, what is RNP? RNP is RNAV with a computer onboard that can tell if a procedure is being flown correctly AND alerts the pilot if it is outside of standards. It doesn’t do anything about verifying GPS signals (that’s RAIM’s job) or making sure cockpit displays are correct (that’s the avionics technician’s job) or ensuring that the system is loaded with up-to-date charts (pilot/avionics technician’s job).

   From the AIM, 1-2-2: “A critical component of RNP is the ability of the aircraft navigation system to monitor its ACHIEVED NAVIGATION PERFORMANCE…” As noted above, achieved navigation performance means how well the aircraft is following a given procedure. RAIM is important because the location of the aircraft is necessary to figure out whether the aircraft is following the procedure correctly. But RNP compares the actual location of the aircraft with whatever procedure is being flown to verify that the aircraft is where it is supposed to be AND alerts the pilot if the aircraft is not where it’s supposed to be.

   I hope that helps.

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   htf17 on May 03, 2022

   What does the alert look like? Will it say something like “INTEG” or “LOI?”

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   awair on May 04, 2022

   The Alert will depend on the aircraft/avionics fit. For example the B777 will display an EICAS “UNABLE RNP”, with an an associated checklist to be followed.

   The FMC database assumes a specific RNP value for the phase of flight, eg oceanic, cruise, arrival, approach or missed approach. Once the the ANP exceeds the RNP, you’ll get the alert.

   The pilot is then responsible for verifying if this is in fact an RNP segment, not just standard RNAV, and take approach action.

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   John D Collins on May 04, 2022

   RAIM or WAAS along with alerting is the method used to determine if the integrity of the position meets the RNP standards required for a specific navigation specification. Integrity is analogous to trustworthiness of the navigation guidance. On a conventional VOR or LOC Nav system, the Nav flag provides the equivalent function. If the integrity standard is not being achieved, an LOI or Integrity annunciation (alert) is generated. Other effects, such as Navigation flags, messages or downgrades to RNP capabilities that have a lower standard, example downgrade of an LPV or LP to LNAV.

   Accuracy is two sigma (95%) statistic. Integrity is the five sigma (99.999%) statistic usually defined as the Horizontal Protection Level (HPL) and is defined as: “The Horizontal Protection Level is the radius of a circle in the horizontal plane (the plane tangent to the WGS-84 ellipsoid), with its center being at the true position, which describes the region that is assured to contain the indicated horizontal position. It is based upon the error estimates provided by WAAS.”

   So on an LNAV or LNAV/VNAV the HPL lateral integrity requirement is 556 meters or 0.3 NM. For an LPV or LP, the HPL lateral integrity requirement is 40 meters. For RNP 1, HPL needs to be 1 NM or better. RNAV full scale deflection on a CDI is equal to the maximum value of the specification and typically, primary obstacle protection is provided out to twice the specification.

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