I got a question regarding the connection between RNP (required navigational performance) accuracy and cdi sensitivity of the HSI.

According to the instrument flying handbook the standard for RNP on route would be 2Nm from the centerline.

So for me this means, that I will be at least 95% of my flight in that corridor (given, if raim is available all the time)

So let's say I am off by 1 Nm enroute due to inaccuracy.

According to Rnp that would be still ok, however the question now is, what would my CDI show me?

The gps obviously does not know that it is 1Nm off course (if so why would we have such a thing like rnp of 2Nm) so the CDI should be centered even though I am actually 1Nm off the correct course.

If this is true I theoretically could be e.g. 4 Nm off course even though I got only a 2 dot deflection on my HSI given that a full deflection would represent 5 Nm because it is not WAAS capable (2Nm rnp + 2 Nm for the two dots)

Thanks for you help in advance... 😊

  • $\begingroup$ "The gps obviously does not know that it is 1Nm off course". The RNP software knows where is the centerline of your RNP corridor. A GPS receiver without WAAS knows where you are with a practical precision of 15 m times the GDOP value (< 3 in normal conditions, and often < 2). Your position can be even more accurate as it is computed from the INS, the GPS and the available ground aids. The CDI only shows the difference between your known position and the RNP centerline. There is no cumulative error. $\endgroup$
    – mins
    Commented Sep 8, 2017 at 11:19
  • $\begingroup$ So the cdi refereses to the actual centerline which can be provided through the RNP software and shows me the distance to the RNP centerline with a inaccuracy of about 50ft (GPS) and about 10 ft (WAAS). Without RNP the reference to the accurate centerline would not be given and I could be way off because of currupt signal/ not enough satellites?do I got that right? Thanks $\endgroup$
    – Mani
    Commented Sep 8, 2017 at 17:55
  • $\begingroup$ The GPS position accuracy is not dependent on RNP. A possible cause of deviation is the inability for the autopilot to follow the intended RNP route. An overall presentation of errors is available at 2.2.1 Lateral navigation (page 100) in ICAO PBN Manual (Doc 9613). $\endgroup$
    – mins
    Commented Sep 8, 2017 at 19:24
  • $\begingroup$ Actually we have two types of systems: either the ADIRUs doesn’t receive the GPS and it is the FMS which will combine the ADIRUs positions, the GPS position and the radio position, or the ADIRUs positions are GPS corrected inside de ADIRUs and the FMS will combine the resultant corrected ADIRUs positions with radio position. In all cases the autopilot is not concerned, this applies to manual flight too. If actual GPS detected precision is less precise than the RNP the FMS will neglect the GPS to calculate your position which becomes a combined radio/ ADIRUs position calculated by the FMS $\endgroup$
    – user40476
    Commented Jun 15, 2019 at 7:47
  • $\begingroup$ Please refer to this document for more details, it applies to AIRBUS but also generally to other manufacturers, starting page 21 more precisely : theairlinepilots.com/forumarchive/aviation-regulations/… $\endgroup$
    – user40476
    Commented Jun 15, 2019 at 7:58

2 Answers 2


The Performance Based Navigation concept allows to optimize the instrument procedure design with the aircraft navigation performance. This concept is used en route, to reduce aircraft separation, and in terminal area to optimize arrival and departure procedures. The utmost development of Performance Based Navigation for approach, missed approach and departure is known under different names. FAA initially referred to RNP SAAAR Operations, SAAAR standing for Special Aircraft and Aircrew Authorization Required. ICAO now refers to RNP Operations with Authorization Required (RNP AR).

The estimation of the aircraft position is performed by the Flight Management System based on different sensors: Inertial (ADIRU), GPS, radio navigation (DME and VOR). The FMS computes the best aircraft position estimate based on the following hierarchy of navigation modes:

  • GPS/inertial,
  • DME/DME/inertial,
  • VOR/DME/inertial,
  • Inertial only.

The RNP capability depends on the availability of GPS/IRS integrity.

The GPS integrity mainly depends on the number of received satellites, the FMS will calculate the GPS estimated error as a function of the number of received satellites and which satellites among the most appropriate for the aircraft position.

The ADIRUs are capable to evaluate their own estimated error that they send to the FMS, similarly the FMS itself is able to calculate the ADIRUs drift error based on previous ILS landing(if any) as he compares the ADIRU position to the real aircraft position while on the ILS, This error is reduced to a mean average drift per hour of flight.

The FMS therefore is able to calculate an Estimated Position Error (EPE), thus allowing the CDU to display both the Required Precision, and the Estimated precision

With respect to the HSI, it displays a route as well the approach calculated by the FMS, within the estimated precision which normally is better than the required precision, the pilot is supposed to be on the route and not one dot or 2 dots apart. When the CDI is centered you are within the Estimated Error; However indeed if the CDI is not centered the error is augmented as you say in the question, but you should not add the RNP but the Estimated NP that shows on the CDU, because the FMS calculations are normally correct.

Of course if your equipments are faulty and mysteriously the multiple faults are not detected you may be neither within the RNP nor within the ENP.

Please refer to the following website for more details:



The CDI shows how far you are from where the navigator thinks you should be.

RNP refers to how wrong the navigator itself is allowed to be. ANP is how wrong it thinks it may be at any given moment, which is a weird concept. As long as ANP is better (lower) than RNP, you're legal.

CDI deviation and ANP are additive, as you might expect, but note that they may have different signs. For instance, if the CDI shows you're a mile east and the ANP is one mile, you might be in the correct position (if the nav error is a mile west) or two miles east (if the nav error is also a mile east). You just don't know. Obviously, the lower (better) the ANP, the more certain you can be about your location.

Thosr two cases will look the same to the pilot because the navigator has no clue in which direction it is wrong; if it did, it would simply adjust its calculated position to have no error at all.


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