The flight crew can override
the RNP values, both lateral and vertical, for specific situations
in which the database RNP value is not applicable. (Page 4 of the
RNP PROGRESS pages, displayed on the CDU, reviews all relevant components
of total system error and flight crew override of RNP for lateral
and vertical navigation. It also gives a preview of the lateral
and vertical RNP values for the approach in the FMC flight plan.)
To determine whether
an airplane is eligible to fly in a given RNP-defined airspace,
the flight crew must know the RNP rating of the airplane. RNP ratings
vary by model and operational mode. The 737 has the lowest RNP rating
of all Boeing models for each of the three defined operational modes:
autopilot, flight director (FD), and hand flown. Lower RNP ratings
for an airplane, when overlaid onto RNP-defined airspace, result
in lower decision heights (fig.
If RNP is a measure of
how good the airplane's navigation system must be, then ANP is the
estimated real-time measure of how good the airplane's navigation
system actually is. Whereas RNP defines the allowable airplane error
in terms of distance and probability relative to the procedurally
defined path, ANP is based on probable airplane position determination
and on guidance errors.
On Boeing airplanes,
multiple sources of navigation data are integrated to determine
the system navigation solution. Inertial systems initially are very
accurate but may tend to drift if not updated accurately throughout
the flight. Global positioning system (GPS) units generally provide
exceptionally accurate data but must be monitored for undetected
failures and lack of satellite coverage. Ground-based radio navigation
aids vary in accuracy and availability.
These sources of data
are analyzed continuously by the FMC to calculate the best estimate
of current airplane position and estimated airplane position uncertainty.
If any one source is deleted, the confidence in the navigation position
will decrease. Thus, the ANP value will increase. Displaying ANP
can be a great help to the flight crew when trying to verify airplane
position because the crew no longer must tune, identify, and cross-plot
navigation aids. The FMC logic uses the best sources of data available
to provide the flight crew with a real-time navigation solution.
(See ANP Algorithms
and Navigation and Time.)