RNAV

[ElNull]

I have a couple of questions that might look dumb. I know the A/C get it’s position from GPS,IRS,RADIO and FMS. Does the FMC generates the LNAV route/airway based on it’s position calculation?


Also if FMS position is not accurate for any reason (with regard to ATC) and there is a big position shift. Does this mean the aircraft will fly to next way point at a wrong geographical location as well (thinking it is the correct location lat/long)?

[bigduke6]

Yes, exactly. The aircraft is navigating using the FMC position.

So if that position is in error, then the navigation is that much in error.

However, as you point out, with multiple sources, that error is likely small or essentially none with GPS. And there is an alerting/monitoring feature of the FMC that calculates if its possible navigation error will exceed the tolerance for the airspace or the approach being flown.

[ElNull]

Yes, exactly. The aircraft is navigating using the FMC position.

So if that position is in error, then the navigation is that much in error.

However, as you point out, with multiple sources, that error is likely small or essentially none with GPS. And there is an alerting/monitoring feature if the FMC predicts that its possible navigation error will exceed the tolerance for the airspace or the approach being flown.

Thanks for the quick answer!

[QuarterInchSocket]

bigduke - kudos. That is the most succint explanation ive ever seen!

ElNull - refer this report (unofficially translated version from russian). it represents an lnav map shifted (sic map shift) flight path, as compared to geographic flight path. the aircraft crashed at the end of a mismanaged go around

https://skybrary.aero/bookshelf/books/3640.pdf

Unadulterated russian version
https://reports.aviation-safety.net/2013/20131117-0_B735_VQ-BBN.pdf

[MarkerInbound]

AirAsia had a little screw up in SYD a few years back

https://www.theguardian.com/australia-news/2016/sep/07/airasia-pilot-flies-melbourne-malaysia-navigation-error

[turbidus]

Read up on:

HPL/HIL: Horizontal Protection Limit and Horizontal Integrity Limit. this is shown on the FMC

and
VPL: Vertical Protection limit.
DOP: Horizontal/vertical dilution of precision
IRU/IRS drift

The horizontal integrity limit (HIL) or horizontal protection level (HPL) is a figure which represents the radius of a circle which is centered on the GPS position solution and is guaranteed to contain the true position of the receiver to within the specifications of the RAIM scheme.