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(b) A direct path to a fix (DF);
(c) A specified track to a fix, defined by a course (CF), and
(d) A specif ied track to an altitude (FA).
Note 1: Industry standards for these paths can be found in RTCA/DO-236B and ARINC Specification 424, which refer to them as TF, DF, CF, and FA path terminators.
Also, certain procedures require RF legs as described in paragraph 4 of this appendix. EUROCAE ED-75A/ RTCA DO-236B and ED-77/RTCA DO-201A describe the application of these paths in more detail.
Note 2: The navigation system may accommodate other ARINC 424 path terminators (e.g., heading to manual terminator (VM)); and the missed approach procedure may use these types of paths when there is no requirement for RNP containment.
(2) Fly-By and Fly-Over Fixes.
The aircraft must have the capability to execute fly-by and fly-over fixes. For fly-by turns, the navigation system must limit the path definition within the theoretical transition area defined in RTCA/DO-236B under the wind conditions identified in Table 11.
Since the fly-over turn is not compatible with RNP flight tracks, an RNP AR procedure design will use a fly-over turn at a fix only when there is no requirement for RNP containment.
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Turn Height Above Airport (Fee
t) Standard Tailwind Component (Knots)* 500 25 Standard AFS-400 Approval 1,000 37.5 30 1,500 50 35 2,000 50 40 2,500 50 45 3,000 50 3,500 55 4,000 60 4,500 65 5,000 70 5,500 75 6,000 80 6,500 85 7,000 90 7,500 95 8,000 100 8,500 105 9,000 110 9,500 115 10,000 120 10,500 125 ≥11,000 130 Table 11. Tailwind Component (VKTW) for Turn Calculations
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*Other tailw ind gradients may be used after a site-specific determination of wind based on that location’s meteorological history (using available information from other sources). Document the source and value used on the procedure form (the FAA uses Form 8260-9).
Note: A new tailwind component may be interpolated for turns initiated at an altitude located between the values in Table 11. The 0 ft value for wind is 15 knots if an interpolated value is ever used below 500 ft.
(3) Waypoint Resolution Error.
The navigation database must provide sufficient data resolution to ensure the navigation system achieves the required accuracy. Waypoint resolution error must be less than or equal to 60 feet, including both the data storage resolution and the RNAV system computational resolution used internally for flight plan waypoint construction. The navigation database must contain vertical angles (flight path angles) stored to a resolution of hundredths of a degree, with computational resolution such that the system-defined path is within 5 feet of the published path.
(4) Capability for a “Direct-To” Function.
The navigation system must have a “Direct-To” function the flight crew can activate at any time. This function must be available to any fix. The navigation system must also be capable of generating a geodesic path to the designated “To” fix, without “S-turning” and without undue delay.
Note: The applicant should identify any limitations associated with the operational use of the aircraft navigation system’s “Direct-To” functions. For example, if there are limitations associated with intercepting an RF leg segment, then the flight manual or aircraft qualification guidance should identify those limitations.
(5) Capability to Define a Vertical Path.
The navigation system must be capable of defining a vertical path by a flight path angle to a fix. The system must also be capable of specifying a vertical path between altitude constraints at two fixes in the flight plan. The navigation system must also define fix altitude constraints as one of the following:
(a) An “AT or ABOVE” altitude constraint (for example, 2400A, may be appropriate for situations where bounding the vertical path is not required);
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Appendix 2
(b) An “AT or BELOW” altitud e constraint (for example, 4800B, may be appropriate for situations where bounding the vertical path is not required);
(c) An “AT” altitude constraint (for example, 5200); or
(d) A “WINDOW” constraint (for example, 2400A3400B).
(6) Altitudes and/or Speeds.
The navigation system must extract the altitudes and/or speeds defined in published terminal procedures from the onboard navigation database.
(7) Path Construction.
The navigation system must be able to construct a path to provide guidance from the current position to a vertically constrained fix.
(8) Capability to Load Procedures from the Navigation Database.
The navigation system must have the capability to load the entire procedure(s) to be flown into the RNAV system from the onboard navigation database. This includes the approach (including vertical angle), the missed approach, and the approach transitions for the selected airport and runway.
(9) Means to Retrieve and Display Navigation Data.
The navigation system must provide the ability for the flight crew to verify the procedure to be flown through review of the data stored in the onboard navigation database.
This includes the ability to review the data for individual waypoints and for navigation aids.
(10) Magnetic Variation.
For paths defined by a course (CF and FA path terminators), the navigation system must use the magnetic variation value for the procedure in the navigation database.
(11) Changes in RNP Value.
RNP changes to lower RNP values must be complete by the first fix defining the leg with the lower RNP value. Any operational procedures necessary to meet this requirement must be identified.
Note: One acceptable means to meet this requirement may be to require the flight crew to manually set the lowest RNP value the RNP AR procedure uses before commencing the procedure (i.e., prior to the initial approach fix).
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(12) Autom atic Leg Sequencing.
The navigation system must provide the capability to automatically sequence to the next leg and display the sequencing to the flight crew in a readily visible manner.
(13) Altitude Restriction Display.
A display of the altitude restrictions associated with flight plan fixes must be available to the pilot. If there is an RNP AR procedure with a flight path angle associated with any flight plan leg, the equipment must display the flight path angle for that leg.
c. Demonstration of Path Steering Performance.
The applicant must complete the demonstration in accordance with AC 120-29 (latest revision), paragraphs 5.19.2.2 and 5.19.3.1 when the demonstration of RNP capability includes a demonstration of the aircraft's path steering performance (i.e., flight technical error).
d. Displays.
(1) Continuous Display of Deviation.
(a) The navigation system must provide the capability to continuously display to the pilot flying, on the primary flight instruments for navigation of the aircraft, the aircraft position relative to the RNAV defined path (both lateral and vertical deviation). The display must allow the pilot to readily distinguish if the cross-track deviation exceeds the RNP value (or a smaller value) or if the vertical deviation exceeds 75 feet (or a smaller value).
(b) The aircraft should have an appropriately-scaled, non-numeric deviation display (i.e., lateral deviation indicator and vertical deviation indicator) in the pilot’s primary optimum field of view. A fixed-scale CDI is acceptable as long as the CDI demonstrates appropriate scaling and sensitivity for the intended RNP value and operation. With a scalable CDI, the scale should derive from the selection of RNP, and should not require the separate selection of a CDI scale. Alerting and annunciation limits must also match the scaling values.
If the equipment uses default RNP values to describe the operational mode (e.g., enroute, terminal area and approach), then displaying the operational mode is an acceptable means from which the flight crew may derive the CDI scale sensitivity.
(c) In lieu of appropriately scaled lateral and vertical deviation indicators in the pilot’s primary optimum field of view, a numeric display of deviation may be acceptable depending on the flight crew workload and the numeric display characteristics.
(2) Identification of the Active (TO) Waypoint.
The navigation system must provide a display identifying the active waypoint either in the pilot’s primary optim um field of view, or on a readily accessible and visible display to the flight crew.
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(3) Display of Distance and Bearing.
The navigation system must provide a display of distance and bearing to the active (TO) waypoint in the pilot’s primary optimum field of view. Where not viable, a readily accessible page on a control display unit, readily visible to the flight crew, may display the data.
(4) Display of Groundspeed and Time.
The navigation system must provide the display of groundspeed and time to the active (TO) waypoint in the pilot’s primary optimum field of view. Where not viable, a readily accessible page on a control display unit, readily visible to the flight crew, may display the data.
(5) Display of TO/FROM the active waypoint.
The navigation system must provide a TO/FROM display in the pilot’s primary optimum field of view.
Note: See paragraph 8-3.g(2) for TO/TO navigation computer displays.
(6) Desired Track Display.
The navigation system must have the capability to continuously display to the pilot flying the aircraft the RNAV desired track. This display must be on the primary flight instruments for aircraft navigation.
(7) Display of Aircraft Track.
The navigation system must provide a display of the actual aircraft track (or track angle error) either in the pilot’s primary optimum field of view or on a readily accessible and visible display to the flight crew.
(8) Failure Annunciation.
The aircraft must provide a means to annunciate failures of any aircraft component of the RNAV system, including the navigation sensors. The annunciation must be visible to the pilot and located in the primary optimum field of view.
(9) Slaved Course Selector.
The navigation system must provide a course selector automatically slaved to the RNAV computed path.
(10) RNAV Path Display.
Where the minimum flight crew is two pilots, the navigation system must provide a readily visible means for the pilot monitoring to verify the aircraft’s RNAV defined path and A2-12
Appendix 2
the aircr aft’s position relative to the desired path.
(11) Display of Distance to Go.
The navigation system must provide the ability to display distance to go to any waypoint selected by the flight crew.
(12) Display of Distance Between Flight Plan Waypoints.
The navigation system must provide the ability to display the distance between flight plan waypoints.
(13) Display of Deviation.
The navigation system must provide a numeric display of the vertical deviation with a resolution of 10 feet or less, and the lateral deviation with a resolution of 0.01 NM or less.
(14) Display of Barometric Altitude.
The aircraft must display barometric altitude from two independent altimetry sources, one in each pilots’ primary optimum field of view to support an operational cross check of altitude sources.
Note 1: If the aircraft includes the ability to automatically compare the output of the independent altitude sources, including independent aircraft static air pressure systems, and the aircraft can provide an alert in the pilot’s primary optimum field of view when deviations between the sources exceed 100 feet, then the applicant should document this comparator monitoring function in the AFM/RFM or aircraft qualification guidance.
Note 2: As stated in paragraph A2-3.a(1)(c) note 3 of this appendix, using GPS/SBAS vertical guidance precludes the need for independent monitoring as prescribed by AC 90-101 (latest revision), appendix 4, paragraph 3.h.
(15) Display of Active Sensors.
The aircraft must display the current navigation sensor(s) in use. The aircraft should provide this display in the primary optimum field of view.
Note: The flight crew can use this display for operational contingency procedures. Flight crew procedures may mitigate the need for this display if the applicant and/or operator can demonstrate the flight crew workload is A2-13
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acceptable.
e. Design Assurance.
(1) The system design assurance must be consistent with at least a major failure condition for the display of misleading lateral or vertical guidance on an RNP AR approach.
(2) The system design assurance must be consistent with at least a major failure condition for the loss of lateral guidance and a minor failure condition for loss of vertical guidance on an RNP AR approach.
Note 1: Loss of vertical guidance is considered a minor failure condition because the pilot can take action to stop descending or climb when guidance is lost.
Note 2: The guidance in paragraph A2-3.e of this appendix is consistent with the intent of AC 90-101A. However, AC 90-101A contains an editorial error that could cause confusion among RNP AR 0.3 and less than 0.3 for misleading guidance versus loss of guidance.
f. Navigation Database.
The aircraft navigation system must use an on-board navigation database that can:
Receive updates in accordance with the AIRAC cycle; and
Allow retrieval and loading of RNP AR procedures into the RNAV system.
g. Database Protection.
(1) The aircraft's navigation system must not permit the flight crew to modify the stored data in the onboard navigation database.
Note: When the flight crew selects and loads a procedure from the onboard navigation database, the RNAV system must execute the procedure as published. This does not preclude the flight crew from having the means to modify a procedure or route already loaded into the RNAV system.
However, no modification of the procedures stored in the onboard navigation database may occur, and the procedures must remain intact within the onboard navigation database for future use and reference.
(2) The aircraft must provide a means to display the validity period for the onboard navigation database to the flight crew.
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A2-15 A2-4. Requirements for RNP AR Approaches with RF Legs.
This paragraph defines additional requirements to conduct approaches with RF legs. The AFM/RFM or aircraft qualification guidance should identify whether or not this capability is provided.
a. The navigation system must have the capability to execute leg transitions and maintain tracks consistent with an RF leg between two fixes.
Note: The AFM/RFM or aircraft qualification guidance should document limitations if: 1) the aircraft cannot proceed “Direct-To” the initial fix defining an RF leg segment, 2) the aircraft cannot accept a radar vector (a heading assigned by ATC) to the middle of an RF leg segment for intercepting and completing the RF leg segment from that point while sustaining the desired level of performance.
b. The aircraft must have an electronic map display of the selected procedure.
c. The flight management computer, the FD system and autopilot must be capable of commanding a bank angle up to 25 degrees above 400 feet above ground level (AGL) and up to 8 degrees below 400 feet AGL.
d. Upon initiating a go-around or missed approach (through activation of take-off/go around (TOGA) or other means), the flight guidance mode should remain in LNAV.
Note: If the flight guidance does not remain in LNAV upon initiation of a go-around or missed approach, then the applicant and/or operator should define flight crew contingency procedures for maintaining compliance with the desired track and re-engaging LNAV as soon as possible. These contingency procedures should clearly address flight crew actions should a go-around or missed approach begin with the aircraft established on or having just completed an RF leg segment.
A2-5. Requirements for RNP AR Approach values less than 0.3.
The AFM/RFM or aircraft qualification documentation should identify if the aircraft is capable of using lines of minima associated with RNP less than 0.3, and which equipment configuration is required to achieve this capability (e.g., dual autopilots may achieve a smaller RNP capability than dual FDs).
a. No single-point-of-failure can cause the loss of guidance compliant with the RNP value associated with the approach. Typically, the aircraft must have at least the following equipment: dual GNSS sensors, dual FMSs, dual air data systems, dual autopilots, and a single
Appendix 2
iner tial reference unit (IRU).
b. The system design assurance must be consistent with at least a hazardous (severe-major) failure condition for the display of misleading lateral or vertical guidance on an RNP AR approach where the procedure requires RNP less than 0.3 to avoid obstacles or terrain while executing an approach.
Note: AC 20-174, Development of Civil Aircraft and Systems, provides an acceptable method for establishing design assurance levels and may be used to show compliance with this requirement. This guidance provides an acceptable method for establishing a design assurance process by taking into account the overall aircraft operating environment and the independent functions of the aircraft’s systems.
c. The system design assurance must be consistent with at least a hazardous (severe-major) failure condition for the loss of lateral guidance and a minor failure condition for the loss of vertical guidance on an RNP AR approach where the procedure requires RNP less than 0.3 to avoid obstacles or terrain while executing an approach.
Note 1: Directly meeting this requirement can substitute for the general requirement for dual equipment (described above).
Note 2: Loss of vertical guidance is considered a minor failure condition because the pilot can take action to stop descending or climb when guidance is lost.
Note 3: The guidance in paragraphs A2-5.b and A2-5.c of this appendix is consistent with the intent of AC 90-101A.
However, AC 90-101A contains an editorial error that could cause confusion among RNP AR 0.3 and less than
0.3 for misleading guidance versus loss of guidance.
d. Upon initiating a go-around or missed approach (through activation of TOGA or other means), the flight guidance mode should remain in LNAV. If the aircraft does not provide the ability to remain in LNAV, then the following requirements apply:
(1) If the aircraft supports RF legs, the lateral path guidance after initiating a go-around (given a minimum 50-second straight segment between the RF end point and the DA) must be within 1 degree (1°) of the track defined by the straight segment through the DA point (see Figure 1). The prior turn can be of arbitrary angular extent and radius as small as 1 NM, with speeds commensurate with the approach environment and the radius of the turn.
(2) The flight crew must be able to couple the autopilot or FD to the RNAV system (engage LNAV) by 400 feet AGL.
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e. After initiating a go-around or m issed approach following loss of GNSS, the aircraft must automatically revert to another means of navigation that complies with the RNP value.
Note: Since loss of GNSS is unlikely, one means of compliance for this requirement is to show when GNSS is lost (i.e., when annunciation of loss of RNP capability occurs prior to the aircraft reaching the DA):
1) The conditional probability the aircraft exits the final approach obstacle clearance volume should be less than
0.001 (one in a thousand). This will ensure that 999 times out of 1000 the aircraft can complete an RNP AR approach should a loss of GNSS occur; and
2) The conditional probability the aircraft exits the missed approach obstacle clearance volume should be less than
0.01 (one in a hundred). This will ensure that 99 times out of 100 the aircraft can complete a missed approach procedure from the lowest minimums should a loss of GNSS occur. Since executing a missed approach is unlikely under normal weather conditions, this conditional probability is less stringent than the requirement for the final approach segment.
A2-6. Requirements for Approaches with a Missed Approach Less Than RNP 1.
The AFM/RFM or aircraft qualification guidance should identify whether or not the aircraft can achieve less than RNP 1.0 when executing a missed approach procedure. The AFM/RFM or aircraft qualification guidance should also specify the aircraft configurations or modes of operation necessary to achieve RNP values less than 1.0 (e.g., dual autopilots may achieve a smaller RNP capability than dual FD).
a. No single-point-of-failure can cause the loss of guidance compliant with the RNP value associated with a missed approach procedure. Typically, the aircraft must have at least the following equipment: dual GNSS sensors, dual FMSs, dual air data systems, dual autopilots, and a single IRU.
(1) The system design assurance must be consistent with at least a major failure condition for the loss of lateral guidance on an RNP AR approach when the missed approach procedure requires RNP less than 1.0 to avoid obstacles or terrain.
(2) For RNP AR missed approach operations requiring less than 1.0 to avoid obstacles or terrain, the loss of the lateral guidance display is a hazardous (severe-major) failure condition.
Note 1: The AFM/RFM should document system designs A2-15
Appendix 2
consistent w ith this effect. This documentation should describe the specific aircraft configurations or modes of operation achieving RNP values less than 1.0 during a missed approach procedure. Meeting this requirement can substitute for the general requirement for dual equipment (described above).
Note 2: It may be possible to obtain approval for navigation functions that have hazardous level effects with FMS software developed to DO-178B level C criteria. AC 20-174 provides an acceptable method for establishing design assurance levels. This guidance may be helpful for navigation architectures containing independent functions.
b. Upon initiating a go-around or missed approach (through activation of TOGA or other means), the flight guidance mode should remain in LNAV to enable continuous track guidance, particularly during an RF leg. If the aircraft does not provide this capability, the following requirements apply:
(1) If the aircraft supports RF legs, the lateral path after initiating a go-around (TOGA) (given a minimum 50 second straight segment between the RF end point and the DA) must be within 1 degree of the track defined by the straight segment through the DA point (see figure 1). The prior turn can be of arbitrary angular extent and radius as small as 1 NM, with speeds commensurate with the approach environment and the radius of the turn.
(2) The flight crew must be able to couple the autopilot or FD to the RNAV system (engage LNAV) by 400 feet AGL.
c. After initiating a go-around or missed approach following loss of GNSS, the aircraft must automatically revert to another means of navigation that complies with the RNP value.
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