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I am pretty sure the LNAV NPS pointer should appear from after takeoff, if LNAV is armed. But I am not sure about the VNAV NPS pointer. A VNAV climb is based on Speed, not Path, so I assume there is no NPS Pointer visible during a VNAV Climb. When will it first come on? Once cruise altitude is reached? What happens if one has to level-off before reaching his CRZ FL? Will the NPS pointer appear at that time?

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    $\begingroup$ Interesting question; offhand I want to say, when there is a defined vertical path - so upon reaching CRZ altitude... but I'll have to check that. Leveling in ALT HOLD doesn't create a vertical path, so that wouldn't do it; leveling at a crossing altitude on a SID, probably would. I'll try to remember to notice when they appear on my next trip & give you a real answer. $\endgroup$
    – Ralph J
    Oct 21 '20 at 14:52
  • $\begingroup$ Checked it on a departure & at cruise - no NPS while in VNAV PATH in either of those instances. Then went to the manual, which says that the vertical NPS is only active after Top of Descent. So, there it is. $\endgroup$
    – Ralph J
    Oct 23 '20 at 7:10
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The lateral NPS will be present if there is a defined LNAV track active, and the APPROACH mode (i.e. to track an ILS) is not active. The vertical NPS is inhibited until the FMC-computed Top-Of-Descent point, so it won't be present during an intermediate level-off during an RNAV departure. It's also inhibited after GS Capture, since the ILS glideslope is then primary & having the NPS would be distracting & unhelpful.

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  • $\begingroup$ Thanks for clarification, the manuals I have don't mention the bit about the vertical NPS being inhibited until the FMC-computed TOD. They are old so it may have been added later. $\endgroup$
    – Raffles
    Oct 23 '20 at 13:40
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The manual that I have says that

  • lateral NPS deviation scale represents current FMC lateral RNP.
  • vertical NPS deviation scale represents current FMC vertical RNP.
  • the NPS pointers are displayed if an approach mode is not engaged, and either TO/GA, LNAV or any VNAV mode is engaged.

The flight management manual says

Vertical Actual Navigation Performance

[Option - FMC U10.5 and later with vertical RNP enabled]

Vertical Actual Navigation Performance(VANP) is the FMC's estimate of the quality of its altitude determination. It is shown on RNP PROGRESS page 4/4. VANP represents the estimated maximum altitude error with 99.7% probability. That is, the FMC is 99.7% certain that the airplane's actual altitude lies within a vertical band equal to plus or minus the ANP value. The lower the VANP value, the more confident the FMC is of its altitude estimate.

Note: VANP is calculated from the baro-corrected altitude provided by the Air Data System. The pilot must set the baro setting reported by ATIS or provided in the approach clearance for the 99.7% confidence level to be valid.

Vertical Required Navigation Performance

[Option - FMC U10.5 and later with vertical RNP enabled]

The FMC uses 400 feet as a default Vertical Required Navigation Performance (VRNP) value for oceanic, en-route, and terminal phases of the flight.

Just as a matter of interest there is an interesting document by Boeing here (it doesn't answer the question in its entirety though).

The LNAV deviation scale is active anytime LNAV is in the engaged FD or autopilot mode.

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  • $\begingroup$ "the NPS pointers are displayed if an approach mode is not engaged, and either TO/GA, LNAV or any VNAV mode is engaged." In such a case I wonder what vertical "deviation" does the pointer show during a VNAV climb. The climb is not a fixed PATH, it is defined by Speed. Does the pointer indicate the deviation from the target speed? $\endgroup$ Oct 22 '20 at 8:36
  • $\begingroup$ @StamatisVellis It should show the FMC's estimate of the quality of its altitude determination, according to the information I added. $\endgroup$
    – Raffles
    Oct 22 '20 at 9:21
  • $\begingroup$ The lateral NPS shows if you're in HDG SEL + ALT HOLD... nice to see how far off your course this vector is taking you. $\endgroup$
    – Ralph J
    Oct 23 '20 at 7:13

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