RNP and RNAV Operations
This section describes the basic concepts of RNP and provides information on RNAV operations by phase of flight to include terminal (SIDs and STARs), en-route, and approach.
Basic RNP Concept
RNP has been developed through international cooperation as a means of identifying the navigation performance required for a specified area, route, airspace, procedure or operation. RNP supports use of RNAV systems that use multi-sensor position updating methods such as the flight management computer (FMC). This accuracy is specified in NM (e.g., RNP 0.3 means the performance required is “within 0.3 NM”). RNP values are usually smaller for terminal area procedures (SIDs, STARs, approach transitions, and approaches) than en-route procedures. Small RNP values normally require navigation performance that is more precise than current VOR/ADF navigation, allowing lower weather minima for departures and/or approaches.
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1.44 FCT 737 (TM) October 31, 2006
737 Flight Crew Training Manual
The FMC uses one of the following as the displayed RNP:
.
default RNP - FMC default values are set by the FMC and are displayed if
no RNP is available from the navigation data base or one has not been
manually entered
.
navigation Data Base RNP - RNP values (if available) are displayed
based on values associated with the procedure. These values may be
unique for certain segments or terminal procedures
.
manually entered RNP - remains until changed or deleted.
The crew may need to make a manual RNP entry if the displayed RNP for the route or procedure is incorrect. Setting an RNP smaller than what is specified for the procedure, airspace, or route, may cause nuisance crew alerts. If the RNP is set larger than that specified for a procedure or segment, crew alerting may occur at the incorrect RNP (if the specified RNP is exceeded). Operators should select FMC default values that meet the requirements of their route structure or terminal area procedures. However, AFM requirements may specify RNPs for certain approaches. (For example, RNP 0.3 is required for RNAV (GPS) approaches).
The FMC calculates and displays its Actual Navigation Performance (ANP) as described in the FCOM. When the ANP exceeds the RNP a crew alert is provided. When this occurs on a route or terminal area procedure where an RNP is published, the crew should verify position, confirm updating is enabled, and consider requesting an alternate clearance. This may mean changing to a non-RNP procedure or route or changing to a procedure or route with a RNP higher than the displayed ANP value. Crews should note that ANP is only related to the accuracy of FMC position. Lateral deviation from the route or procedural track is indicated by the XTK ERROR (cross-track error) value shown by the FMC. LNAV is required for all RNP operations. Use of the autopilot is recommended to minimize cross-track error. An excessive cross-track error does not result in a crew alert.
737-600 - 737-900
Note: The NPS system (as installed) provides an alert on the PFD when lateral or
vertical deviation exceeds preset limits. Reference the FCOM for specific
NPS system indications and description.
The following illustration shows the FMC position and displayed ANP and shows the relationship of ANP to RNP for a particular route segment. Normally, a route segment or procedural leg is defined by its required width (the illustration shows
2.0 NM either side of the centerline). RNP is normally set at 50% of the allowed maximum deviation from the route centerline. Required width is determined by minimum terrain or traffic clearance requirements. The probability of exceeding this maximum deviation while in LNAV with the autopilot engaged is very small. For each airplane type, minimum demonstrated RNPs are given in the AFM. These minimum values vary depending on LNAV, flight director and autopilot use, and whether GPS is the active source of position updating.
737 Flight Crew Training Manual
Low RNP operations such as RNP 0.15 and below require precise path tracking. Use of the autopilot and LNAV normally provide the required path tracking accuracy. Some RNAV (RNP) approach procedures also require use of VNAV PTH for vertical guidance. These procedures show only LNAV/VNAV approach minima and do not allow use of LNAV only. Use of the flight director alone may not provide sufficient guidance to maintain the path accurately.
737-600 - 737-900 Note: If the autopilot is not available, flight crews should use the flight director and the additional cues displayed on the navigation display (position trend vector, airplane symbol, and digital cross track deviation) with at least one map set at a range of 10 NM or less.
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