Appendix 5 APPENDIX 5
OBSTACLE ASSESSMENT FOR RNP FOR CATEGORY I OR CATEGORY II
1. Obstacle Assessment for Standard Required Navigation Performance (RNP) Types (e.g., Linear Values of RNP).
1.1. Obstacle Assessment for RNP Approaches and Missed Approaches.
1.1.1. General. This Appendix provides criteria that may be used by procedure designers in the development of RNP approaches for suitably equipped aircraft together with any necessary operational mitigations and procedures.
These criteria should be used in conjunction with other considerations in this AC. When authorized by AFS-400, approaches developed in accordance with this appendix may be issued as special non-14 CFR part 97 procedures issued through OpSpecs or a letter of authorization (LOA). These criteria may be used in conjunction with airworthiness demonstrations of airborne equipment, or in the assessment of other States criteria used in international operations for U.S. Operators.
The approach RNP is specified from the Final Approach Fix (FAF) to the point along the final approach segment at which the lowest applicable DA(H) typically is applied. There may be one or more levels of RNP specified on a final segment. Missed approach RNP, or levels of RNP if more than one level or RNP is specified, is typically specified from a point related to the lowest applicable DA(H), and typically continues to a missed approach holding fix or missed approach waypoint. RNP also may be applied to a “go-around safety” assessment.
When applied to a “go-around safety assessment,” the RNP level and associated obstacle clearance start at the end of the touchdown zone with an expanding lateral area that widens to match the level of RNP used, and then continues at the RNP level(s) specified. The expanding lateral area starts on the centerline for the approach at the end of the touchdown zone and widens at a 7.5 degree splay. Splay criteria based on ICAO PANS-Ops may alternately be used at the discretion of the procedure designer or operator (e.g., 1:8 splay/ 7.125 degrees). A go-around safety assessment is applicable from the end of a touchdown zone to reaching the missed approach holding fix or applicable missed approach waypoint (see below for specific criteria). When conducting a “go-around safety assessment,” the potential growth of ANP following pertinent failures should be appropriately considered, relative to the designated level(s) of RNP in approach or missed approach segments.
Procedures for U.S. air carrier operations (operations conducted IAW Title 14 of the Code of Federal Regulations (14 CFR) part 121 or part 135 should address application of RNP to “go-around safety” (see paragraph 4.3.1.8 of the main body of this AC). It is recommended that other operators also address “go-around safety.” A go-around safety assessment is intended to assist operators in assuring safe operations in the rare event of a low altitude go-around with certain failures. It is not intended to preclude or limit operations necessary at any particular location.
Provisions of this appendix may be used for levels of RNP specified in the AFM or for other levels of RNP as authorized by the FAA.
NOTE: The United States Standard for Terminal Instrument Procedures (TERPS) is the basis for Standard Instrument Approach Procedures formulation within the United States and its territories.
1.1.2. Final Approach (FAS), Missed Approach (MAS) and other Related Segments. The criteria presented in this Appendix apply to the Final Approach (FAS) and Missed Approach segments (MAS). The FAS is defined as that segment of an approach extending from the GPIWP or APIWP, whichever occurs later, to GIRP. However, for the purpose of defining RNP obstacle clearance in this appendix, the Final Approach segment (FAS) is considered to begin at the FAF and ends at the FPCP (runway Datum Crossing Height (CH)), or missed approach point (e.g., DA(H)). No specific minimum or maximum length is assigned to the FAS, but the FAF must be located such that
Appendix 5 consideration is given to how the FMC VNAV operation may be constrained in certain ways at the point the FAS commences. In addition, consideration should be given in the placement of the FAF recognizing that a continuous VNAV descent may be intended to the FAF, instead of a level intermediate segment with a minimum VNAV intercept altitude. The Missed Approach segment is defined as beginning at a point coincident with the lowest applicable DA(H) and ending at a specified missed approach waypoint (e.g., Initial Missed Approach WP, Missed Approach Holding WP). No minimum or maximum length is assigned to the MAS, but consideration should be given to having the aircraft established on an en route transition. Definitions for various segments used in procedure construction are as specified in Table A5-1 below (Also see AC main body paragraph 4.6, and Appendix 1):
Approach and Missed Approach Segments Applicable To RNAV Instrument Procedures Using RNP Table A5-1 Final Approach Segment (FAS) The segment of an approach extending from the Glidepath Intercept Waypoint (GPIWP) or Approach Intercept Waypoint (APIWP), whichever occurs later, to the Glidepath Intercept Reference Point (GIRP). For the purpose of procedure construction, The Final Approach segment is defined as beginning at the FAF and ending at the Flight Path Control Point (FPCP) or point at which the missed approach segment starts (e.g., point of lowest nominal DA(H)).
Extended Final Approach Segment (EFAS) That segment of an approach, co-linear with the Final Approach Segment, but which extends beyond the Glidepath Intercept Waypoint (GPIWP) or Approach Intercept Waypoint (APIWP).
Runway Segment (RWS) That segment of an approach from the glidepath intercept reference point (GIRP) to Flight Path Alignment Point (FPAP).
Initial Missed Approach Segment (IMAS) That segment of an approach from the Glide Path Intercept Waypoint (GIRP) to the Initial Missed Approach Waypoint (IMAWP).
Missed Approach Segment (MAS) That segment of an instrument approach procedure from a point on the FAS corresponding to the position where the lowest DA(H) occurs under nominal conditions, to the designated IMAWP, or missed approach holding WP, as specified for the procedure.
1.1.3. Approach and Missed Approach Conditions To Be Assessed. Three basic conditions are considered in the development of obstacle clearance criteria for RNP approaches and missed approaches:
a. The aircraft arrives at the DA(H), continues with visual reference to a landing on the runway.
b. The aircraft arrives at the DA(H), initiates a missed approach, and experiences an engine failure.
c. The aircraft arrives at the DA(H), continues with visual reference to the runway, initiates a rejected landing at the end of the touchdown zone, and experiences an engine failure.
Each of these conditions has associated criteria for lateral and vertical obstacle clearance protection. In addition to these normal and non-normal conditions, rare-normal conditions must be assessed. Unless wind limitations are specified, these rare normal conditions should be considered as a wind from the most adverse direction at the certificated limit for landing, increasing to 50 knots at 1000 ft. AGL. This rare-normal wind condition shall increase
Appendix 5 at a gradient of 10 knots per 1000 ft. up to a maximum of 100 knots from the most adverse direction (i.e., tailwind).
However, such conditions need not be considered in combination with non-normal events (e.g., engine failure).
In instances, the normal missed approach path and non-normal missed approach path may be different laterally. In such an event, transition from the normal path to the non-normal path should be considered, including performance or energy state of the aircraft, for engine failures that could occur at various critical points along the normal flight path.
1.1.4. Touchdown Zone. A touchdown zone (TDZ) typically is considered to be the first 3000 ft. of a designated landing runway. When appropriate for the purposes of this provision, Operators may propose to use a different designation for a touchdown zone. For example, alternate consideration of a (TDZ) may be appropriate for runways that:
/elevenoclock Are less than 6000 ft. in length and which do not have standard TDZ markings, /elevenoclock Short runways requiring special aircraft performance information or procedures for landing, /elevenoclock Runways for STOL aircraft, or /elevenoclock Runway where markings or lighting dictate that a different TDZ designation would be more appropriate.
1.2. Obstacle Criteria.
1.2.1. Obstacle Identification Surface Between Point Of Lowest DA(H) and the Runway. For condition 1.1.3a, described above, an obstacle identification surface is defined for the visual segment between the DA(H) and the TDZ on the runway. This surface originates at the runway threshold and is inclined at an angle 1 degree less than the VNAV angle for the FAS. This surface is bounded laterally by two rays which originate from the center of the runway at a point 1000 ft. from the threshold, splay at an angle of 10 degrees relative to the runway centerline, or FAS, to the DA(H), or the point at which the lateral limit of 2XRNP is reached. This area should be free of fixed or movable obstacles (regardless of whether they are or are not present by their aeronautical purpose) at the time an instrument approach is conducted inside the FAF. A procedure should not be authorized with an obstacle in this area unless the presence of the obstacle(s) is specifically reviewed and authorized by FAA, and the flightcrew of the landing aircraft is provided information on the location and nature of the obstacle. Other options to resolve a penetration include increasing a VNAV angle, removing the obstacle, displacing the runway threshold, not implementing the approach, adjusting a lateral path, or implementing various combinations of the above options (Figure A5-1).
Figure A5-3 shows a method for determination of RNP obstacle clearance for a final segment controlling obstacle between DA(H) and the runway.
1.2.2. Obstacle Identification Surface Between the Point of Lowest DA(H) and a Missed Approach Waypoint.
For the condition described in paragraph 1.1.3b, above, the lateral containment surface is centered on the FAS and bounded on either side by two parallel lines located at a distance of 2XRNP (Figure A5-2). Within the limits of this containment surface, a variable Required Obstacle Clearance (ROC) must be provided which is a function of altitude and temperature. This ROC is established by a Vertical Navigation Error Budget (VEB) evaluation that characterizes the vertical navigation accuracy of the system and provides a parametric methodology to evaluate procedures and assess the impact of obstacles. For example, the Root-Sum-Square (RSS) of the VNAV performance variables that contribute to errors in the vertical axis include, but are not limited to, horizontal along-track navigation system errors, temperature induced barometric altimetry errors, flight technical errors, static source errors, minimum waypoint resolution, minimum vertical path angle resolution, etc. ROC increases along the FAS from a lower reference point up to the upper elevation reference point typically at the FAF. By subtracting the ROC from the VNAV elevation at defined locations, a sloping Obstacle Identification Surface (OIS) beneath the VNAV path is established. The OIS is anchored by the lower reference point at the path’s 250 ft. height above touchdown point
Appendix 5 and by the upper elevation reference point typically 2000 ft. above field elevation. The DA(H) is defined as the Required Obstacle Clearance plus 50 ft. above the point on the OIS where the aircraft must be established in a climb to clear all obstacles. The climb gradient used for this analysis is established for a particular aircraft by evaluating the worst case condition. This may include one-engine inoperative, maximum permissible tailwind, maximum permissible landing weight, icing/temp/altitude degradations, etc. A variable DA(H) may be employed if certain conditions are specifically excluded (e.g., no icing). For Instrument Approaches other than ILS, GLS, or MLS (see
4.3.3), developed by a VEB evaluation, the minimum ROC is 250 ft.
The methodology for determining the DA(H) is the same regardless of whether the controlling obstacle is in the FAS or MAS.
Figure A5-4 shows a method for determination of RNP obstacle clearance for a missed approach segment controlling obstacle.
Figure A5-5 shows the normal instrument approach case that has neither an approach or missed approach controlling obstacle.
1.2.3. Obstacle Identification Surface Between the End of the TDZ and a Missed Approach Waypoint. For the condition 1.1.3c, described above, a lateral containment surface is centered on the MAS and bounded on either side by two rays which originate from a point 200 ft. either side of the runway centerline at the end of the TDZ (typically 3000 ft. from the approach end of the runway - see 1.1.4 above). These rays splay at an angle of 7.5 degrees out to a maximum distance from the MAS centerline of 2XRNP. Within the lateral limits of this containment surface, a minimum of 35 ft. ROC must be provided below the one engine inoperative net flight path of the aircraft (Figure A5-6). Splay criteria based on ICAO PANS-Ops may alternately be used at the discretion of the procedure designer or operator (e.g., 1:8 splay/ 7.125 degrees). For curved initial missed approach segments (e.g., segments based on an ARINC 424 “RF” leg type), an equivalent lateral splay providing equivalent lateral clearance along the path arc length may be used.
Extreme cold temperature considerations should be assessed for VNAV angles, and safe obstacle clearance assured for any initial or intermediate segments (see paragraph 6.2.13).
1.2.4. FAS Turn Construction. Final Approach Segment (FAS) turns are constructed using appropriate lateral path guidance algorithms of the navigation system for which the procedure is designed, or by using generic algorithms which take numerous navigation system characteristics representative of the range of systems to be used into consideration.
Navigation database-defined turns defined through short leg WP sequences or ARINC 424 “RF” Leg types may also be used. If used, appropriate consideration should be made for anticipated ground speeds to be used, leg sequencing, and for “roll in' and “roll out” of an RF leg. Normally, an RF leg should not be based on an assumed nominal bank angle greater than 25 degrees, to allow for path recovery in the event of path displacement disturbances.
1.2.4.1. FAS Turn Construction for Fly-by Waypoints. For turns on the FAS (other than for an RF leg), the outside (of the turn) lateral containment surface is constructed via an arc of radius 2XRNP, which is centered on the turn waypoint. For the inside lateral containment surface, the ground speed condition which results in the greatest amount of turn anticipation (earliest departure from and latest return to the FAS centerline) is used for construction.
For this condition, the containment surface can be constructed in two ways. The first method uses a straight line which extends between the intersections of the two perpendiculars, located at the start and end points of the turn anticipation arc, and the 2XRNP containment surface which is parallel to the segments before and after the turn waypoint. The second method uses an arc of radius equal to the turn anticipation arc minus 2XRNP (Figure A5-7).
For RF legs, the RNPX2 surface is as defined by the specified RNP level.
1.2.4.2. FAS Turn Construction for Fly-over Waypoints. In the event that this type of turn is required (rare use), the ground speed which results in the greatest amount of overshoot and latest return to the FAS centerline should be determined. For this condition, the outside containment surface is constructed as an arc and straight segment
Appendix 5 combination parallel to and at a distance of 2XRNP from the computed flight path. The inside containment surface is constructed using the conservative assumption of no overshoot. Given this condition, the containment surface is simply defined as the intersection of the 2XRNP surfaces parallel to the Final Approach Segments (Figure A5-8).
1.2.5. MAS Turn Construction. MAS turns are constructed in a manner identical to turns in the FAS, unless the turn occurs prior to the point at which the containment surfaces are fully expanded to the 2XRNP value (e.g., balked or rejected landing). In this event, only fly-by waypoints should be used because of the complexity which results from constructing the outside containment surface for the fly-over waypoints.
1.2.5.1. MAS Turn Construction for Fly-by Waypoints. For turns on the MAS, prior to the point at which the containment surfaces are fully expanded to the 2XRNP value, the containment surface should be constructed in the following manner:
• The outside lateral containment surface is constructed by transferring the width of the splay abeam the turn waypoint via an arc to the following segment.
• The arc is of radius equal to the attained half-width of the preceding segment and is centered at the turn waypoint.
• The arc is extended to a line perpendicular to the centerline of the following segment and passes through the turn waypoint.
• The splay is continued from that point by an angle of 7.5 degrees to a distance of 2XRNP from the centerline. To simplify the containment surface construction for the inside of the turn, a straight line is drawn between the earliest point of departure and the latest point of return back to the following segment for the fly-by of the turn waypoint.
• For other than RF legs, the containment surface expands by a 7.5 degree splay angle using the simplified inside turn approximation as the reference centerline. This splay is continued until reaching the 2XRNP displacement from the reference centerline (Figure A5-9). Splay criteria based on ICAO PANS-Ops may alternately be used at the discretion of the procedure designer or operator (e.g., 1:8 splay/ 7.125 degrees).
• For RF legs, the RNPX2 surface is as defined by the specified RNP level.
1.2.5.2. MAS Turn Construction For Fly-over Waypoints. Fly-over waypoints are not used for a MAS.
1.2.6. RNP Reductions. RNP reductions would normally be expected to occur at waypoints marking the transition from the enroute airway to a transition feeder route to an approach (typically at the IAF). Upon reaching the IAF, there are typically no further RNP reductions throughout the approach and missed approach. RNP reductions should be considered based on the anticipation of the first longitudinal point where the lower level of RNP is required and assurance that appropriate alerting can be provided prior to the time that the lower level of RNP is needed.
If required, RNP reductions on the FAS should be considered based on anticipation of the first longitudinal point where the lower level of RNP is required, and assurance that appropriate alerting can be provided prior to the time the lower level RNP is needed. No transition area is required. However, the RNP reduction should be located such that consideration is given to the maximum latency of RNP alerting messages, maximum ground speed, crew response time, height of any obstacles immediately beyond the change in RNP, and the one-engine inoperative climb gradient. This distance, “d,” is shown in Figure A5-10. RNP increases, particularly on a MAS or at the beginning of a MAS, do not require this special consideration, thus distance “b” in Figure A5-10 could be zero.
RNP reductions are not typically used on a MAS.
1.2.7. Coordinate Systems. Waypoint coordinates shall be defined in the WGS-84 or NAD-83 coordinate system (or equivalent international system for locations outside the US). Waypoint resolution shall be provided to at least
0.01 arc minutes.
1.2.8. Obstruction and Terrain Charts. The best source(s) of topographical or obstruction charts that are available should be used.
Appendix 5
1.2.8.1. Recommended Use of USGS Charts. Use of USGS 1:25,000 or 1:24,000 charts (or equivalent) is recommended wherever possible.
1.2.8.2. Vertical Clearance Adjustments for Certain Topographical Charts. FAA Order 8260.19C assigns an accuracy code of “2C” to the 1:24,000 topographical charts. This does not meet the minimum accuracy standard for a precision final segment of an approach. For this reason, a 40 ft. horizontal and 20 ft. vertical adjustment is required to the obstacle values taken directly from the topographical chart. These adjustments are applied in the horizontal and vertical direction that most adversely affects the procedure (i.e., the range is reduced by 40 ft. and the height increased by 20 ft.).
1.2.8.3. Tree Heights. Tree heights consistent with the maximum found in the area must be added to all contour elevations, unless specific survey heights are used in areas of interest.
1.2.8.4. Assumptions for Terrain Elevations. Assumptions for terrain elevations should be conservative. If an obstacle of interest falls between two gradient lines, the obstacle should be assigned a height equal to the next higher gradient line minus one unit of elevation. For terrain elevations which are critical (or controlling), the terrain should be assumed to rise to a height equal to the next higher gradient line minus one unit of elevation, at an incremental distance beyond the gradient line in question.
1.2.9. Man-Made Obstacle Data. Man-made obstacle data may be obtained from the U.S. Department of Commerce Quarterly Obstacle Memo Digital Obstacle File, Airport Obstruction Chart, FAA IAPA database, or ICAO equivalent. Horizontal and vertical adjustments are applied as a function of the accuracy code assigned to each obstacle. For areas of interest beyond 14 CFR part 77 (or ICAO equivalent) surfaces (e.g., initial and intermediate segments), proper consideration should be made for obstacles which would not be part of the official obstacle records. This consideration may be an appropriate additive to all terrain contours or some other equivalent means (e.g., flight inspection or survey).
1.2.10. Wheel To Navigation Reference Point or Longitudinal Navigation Reference Points. Aircraft which have a wheel to navigation reference point (e.g., altimeter reference) vertical height less than 19 ft., or a longitudinal navigation reference point (e.g., altimeter reference point) to lowest and most aft wheel distance of 125 ft. or less at the normal approach pitch attitude and speed need not account for altimeter vertical and longitudinal displacement from wheel height. Aircraft, which have vertical or longitudinal distances that exceed these values, should include suitable correction factors along with any RSS analysis of potential vertical path displacement errors.
1.3. Examples of completed RNP Forms. Examples of completed FAA Forms 8260 for RNP Procedures are shown in Figures A5-11 and A5-12 for an “RNAV” Procedure with RNP-based minima and for an “xLS and RNAV” procedure with RNP-based minima.
Appendix 5
VNAV-1 oo DA(H) o
OBSTACLE IDENTIFICATION SURFACE 2 x RNP 10 DA(H)
OBSTACLE IDENTIFICATION SURFACE 1000'
OBSTACLE IDENTIFICATION - VISUAL SEGMENT Figure A5-1
Appendix 5 DA(H) 2XRNP 2XRNP IMAWP
CONTAINMENT SURFACES
RNP LATERAL AREA TO CONSIDER - MISSED APPROACH FROM DA(H) Figure A5-2
Appendix 5 Figure A5-3
OBSTACLE IDENTIFICATION SURFACE IS
DEFINED AS THE NOMINAL VNAV FLIGHT
PATH REDUCED BY THE VNAV ERROR BUDGET
RNP OBSTACLE CLEARANCE - FINAL SEGMENT
CONTROLLING OBSTACLE (BETWEEN THE DA(H) AND THE RUNWAY) DA(H) DETERMINED BY AIRCRAFT PERFORMANCE .
ASSUMING WORST-CASE CUMULATIVE VNAV ERRORS AIRCRAFT WOULD BE
STARTING MISSED APPROACH FROM THE OIS AND CLIMBING ON THE ENGINE
INOPERATIVE MISSED APPROACH CLIMB GRADIENTSURFACE THE ‘ENGINE INOPERATIVE MISSED APPROACH’ CLIMB GRADIENT SURFACE
MUST CLEAR ALL OBSTACLES ROC 2000 Upper elevation reference point 2000 ’ above field elevation Momentary 50’’ (max) Descent Below DA(H) Approach Surface Baseline Minimum 250’ ROC Height Above Touchdown
OBSTACLE IDENTIFICATION SURFACE
VERTICAL PATH ANGLE ROC 250 Lower elevation reference point = 250’ above Field Elevation Lower OIS ref point RW XX
CONTROLLING
OBSTACLE
USING BAROMETRIC
ALTIMETRY ROC DECREASES
WITH DECREASING ALTITUDE
AS RUNWAY IS APPROACHED DA(H)
NOMINAL MISSED APPROACH
FLIGHT PATH Performance Determined Engine Inoperative Missed Approach Climb Gradient Surface
Appendix 5 Figure A5-4 Approach Surface Baseline ROC 250 Performance Determined Engine Inoperative Missed Approach Climb Gradient Surface DA(H) DCH (50’ HAT) ROC 2000 Upper elevation reference point 2000 ’ above field elevation
RNP OBSTACLE CLEARANCE - MISSED APPROACH SEGMENT CONTROLLING
OBSTACLE Lower OIS ref point
VERTICAL PATH ANGLE Minimum 250’ ROC Height Above Touchdown Momentary 50’’ (max) Descent Below DA(H) RW XX
USING BAROMETRIC ALTIMETRY
ROC DECREASES WITH
DECREASING ALTITUDE AS
RUNWAY IS APPROACHED
OBSTACLE IDENTIFICATION SURFACE IS
DEFINED AS THE NOMINAL VNAV FLIGHT
PATH REDUCED BY THE VNAV ERROR BUDGET
NOMINAL MISSED APPROACH
FLIGHT PATH
CONTROLLING
OBSTACLE
Upper OIS ref point Lower elevation reference point = 250’ above Field Elevation DA(H) DETERMINED BY AIRCRAFT PERFORMANCE .
ASSUMING WORST-CASE CUMULATIVE VNAV ERRORS AIRCRAFT WOULD BE
STARTING MISSED APPROACH FROM THE OIS AND CLIMBING ON THE ENGINE
INOPERATIVE MISSED APPROACH CLIMB GRADIENTSURFACE THE ‘ENGINE INOPERATIVE MISSED APPROACH’ CLIMB GRADIENT SURFACE
MUST CLEAR ALL OBSTACLES Obstacle Identification Surface
Appendix 5 Figure A5-5
OBSTACLE IDENTIFICATION SURFACE IS
DEFINED AS THE NOMINAL VNAV FLIGHT
PATH REDUCED BY THE VNAV ERROR BUDGET
RNP OBSTACLE CLEARANCE - NO CONTROLLING OBSTACLE Minimum 250’ ROC Height Above Touchdown
OBSTACLE IDENTIFICATION SURFACE Upper OIS ref point ROC 2000 Upper elevation reference point 2000 ’ above field elevation
USING BAROMETRIC ALTIMETRY
ROC DECREASES WITH
DECREASING ALTITUDE AS
RUNW AY IS APPROACHED
VERTICAL PATH ANGLE Momentary 50’’ (max) Descent Below DA(H) Approach Surface Baseline ROC 250 RW XX
NOMINAL MISSED APPROACH
FLIGHT PATH Performance Determined Engine Inoperative Missed Approach Climb Gradient Surface DCH (50’ HAT) DA(H) DETERMINED BY AIRCRAFT PERFORMANCE .
ASSUMING WORST-CASE CUMULATIVE VNAV ERRORS AIRCRAFT WOULD BE
STARTING MISSED APPROACH FROM THE OIS AND CLIMBING ON THE ENGINE
INOPERATIVE MISSED APPROACH CLIMB GRADIENTSURFACE THE ‘ENGINE INOPERATIVE MISSED APPROACH’ CLIMB GRADIENT SURFACE