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(7) Proper Interpretation of Pre-Departure Clearance (PDC) and printed routings.
Pilots of operators using PDC and printed routings must be able to properly interpret their assigned clearance. Specifically, pilots must be able to recognize direct routings, assigned altitudes, revised clearances, SIDs, and en route transitions. Pilots must understand their operator’s PDC notation and must request clarification from ATC if any doubt exists with regard to their clearance.
b. General Operating Procedures. Operators and pilots should not request or file U.S.
RNAV routes or procedures unless satisfying the criteria in this AC. If an aircraft not meeting these criteria receives a clearance from ATC to conduct an RNAV procedure, the pilot must advise ATC that he/she is unable to accept the clearance and request alternate instructions.
(1) The pilot should comply with any instructions or procedures identified by the manufacturer as necessary to comply with the equipment requirements of this AC.
(2) At system initialization, pilots must confirm the navigation database is current and verify the aircraft’s present position.
(3) RNAV DPs and STAR procedures must be retrieved by procedure name from the onboard navigation database and conform to the charted procedure.
(4) Whenever possible, RNAV routes should be extracted from the database in their entirety, rather than loading RNAV route waypoints from the database into the flight plan individually. Selecting and inserting individual, named fixes from the database is permitted, provided all fixes along the published route to be flown are inserted.
NOTE: This does not preclude the use of panel-mount GPS/GNSS avionics to meet the requirements of this AC to fly RNAV routes.
(5) Manual entry of waypoints using latitude/longitude or place/bearing is not permitted.
Additionally, pilots must not change any RNAV DP or STAR database waypoint type from a flyby to a flyover or vice versa.
(6) Flightcrews should crosscheck the cleared flight plan against charts or other applicable resources, as well as the navigation system textual display and the aircraft map display, if applicable. If required, confirm exclusion of a specific navigation aid. A procedure should not be used if doubt exists as to the validity of the procedure in the navigation database.
NOTE: Pilots may notice a slight difference between the navigation information portrayed on the chart and their primary navigation display.
Differences of 3 degrees or less may result from equipment manufacturer's application of magnetic variation and are operationally acceptable.
(7) Verification of assigned route and correct entry of transitions into RNAV System/FMS.
(a) DPs. Prior to flight, pilots must verify their aircraft navigation system is operating correctly and the correct runway and DP (including any applicable en route transition) are
Par 10 Page 13 entered and properly depicted. Pilots who are assigned an RNAV DP and subsequently receive a change of runway, procedure or transition must verify the appropriate changes are entered and available for navigation prior to takeoff. A final check of proper runway entry and correct route depiction, shortly before takeoff, is recommended.
(b) Routes. Pilots must verify proper entry of their ATC assigned route upon initial clearance and any subsequent change of route. Pilots must ensure the waypoints sequence depicted by their navigation system matches the route depicted on the appropriate chart(s) and their assigned route.
(c) STARs. Pilots must verify their aircraft navigation system is operating correctly and the correct arrival procedure and runway (including any applicable transition) are entered and properly depicted.
(8) Use of navigation map displays. Prior to takeoff, pilots of aircraft with a navigation map display should verify the relationship of the aircraft position symbol to their assigned runway (if available) and route on their display matches external visual cues, as well as charts.
Specifically, once on or near their assigned runway, pilots should ensure their navigation display reflects the same relative position to the runway and the route depiction reflects that of the respective chart. During flight, these displays should be used in concert with textual displays for route verification.
(9) Pilots must use a lateral deviation indicator (or equivalent navigation map display), flight director and/or autopilot in lateral navigation mode on RNAV 1 routes. Pilots are encouraged to use a lateral deviation indicator (or equivalent navigation map display), flight director and/or autopilot in lateral navigation mode on RNAV 2 routes. Pilots of aircraft with a lateral deviation indicator (e.g., a standalone GNSS receiver) must ensure that lateral deviation indicator scaling (full-scale deflection) is suitable for the navigation accuracy associated with the route/procedure (i.e., ±1 NM for RNAV 1, ± 2.0 NM for RNAV 2).
NOTE: Some TSO-C129 equipment automatically goes to ±5.0 NM beyond 30 NM from the airport reference point. This is acceptable provided the pilot maintains the required minimum performance standard associated with the procedure.
All pilots are expected to maintain route centerlines, as depicted by onboard lateral deviation indicators and/or flight guidance during all RNAV operations described in this AC unless authorized to deviate by ATC or under emergency conditions. For normal operations, cross-track error/deviation (the difference between the RNAV system computed path and the aircraft position relative to the path) should be limited to ± ½ the navigation accuracy associated with the procedure or route (i.e., 0.5 NM for RNAV 1, 1.0 NM for RNAV 2). Brief deviations from this standard (e.g., overshoots or undershoots) during and immediately after procedure/route turns, up to a maximum of 1 times the navigation accuracy (i.e., 1.0 NM for RNAV 1, 2.0 NM for RNAV 2), are allowable.
NOTE: Some aircraft do not display or compute a path during turns. As such, pilots of these aircraft may not be able to adhere to the ± ½ lateral
Page 14 Par 10 navigation accuracy during procedural/route turns but are still expected to satisfy the standard during intercepts following turns and on straight segments.
(10) If ATC issues a heading assignment taking the aircraft off a procedure, the pilot should not modify the route in the RNAV system until a clearance is received to rejoin the procedure or the controller confirms a new route clearance. When the aircraft is not on the published procedure, the specified accuracy requirement (subparagraph 8a) does not apply.
(11) Manually selecting aircraft bank limiting functions may reduce the aircraft’s ability to maintain its desired track and are not recommended. Pilots should recognize manually selectable aircraft bank-limiting functions might reduce their ability to satisfy ATC path expectations, especially when executing large angle turns. This should not be construed as a requirement to deviate from AFM procedures; rather, pilots should be encouraged to limit the selection of such functions within accepted procedures.
(12) DPs and STARs are flown as RNAV 1 procedures. RNAV routes are flown as RNAV 2 unless otherwise specified.
(13) Pilots operating RNP-approved aircraft under the provisions of this AC are not required to modify manufacturer's RNP default values established in the Flight Management Computers.
c. RNAV DP and STAR Specific Requirements.
(1) RNAV DP Engagement Altitudes. For DPs, the pilot must be able to engage RNAV equipment to follow flight guidance for lateral RNAV no later than 500 feet above airport elevation.
(2) Pilots must use a lateral deviation indicator (or equivalent navigation map display), flight director and/or autopilot in lateral navigation mode on RNAV 1 routes. The full-scale course deviation indicator (CDI) deflection value of ±1 NM is acceptable.
(3) DME/DME/IRU (D/D/I) Aircraft. Pilots of aircraft without GPS/GNSS, using DME/DME/IRU, must ensure the aircraft navigation system position is confirmed, within 1,000 feet, at the start point of takeoff roll. The use of an automatic or manual runway update is an acceptable means of compliance with this requirement. A navigation map may also be used to confirm aircraft position, if pilot procedures and display resolution allow for compliance with the 1,000-foot tolerance requirement.
(4) GNSS Aircraft. When using GNSS, the signal must be acquired before the takeoff roll commences.
NOTE: For aircraft using TSO-C129/C129a, the departure airport must be loaded into the flight plan in order to achieve the appropriate navigation system monitoring and sensitivity. For aircraft using TSO-C145a/C146a avionics, if the departure begins at a runway waypoint, then the departure
Par 10 Page 15 airport does not need to be in the flight plan to obtain appropriate monitoring and sensitivity.
d. Contingency Procedures. The pilot must notify ATC of any loss of the RNAV capability, together with the proposed course of action. If unable to comply with the requirements of an RNAV procedure, pilots must advise ATC as soon as possible. For example, “. . .N1234, failure of GPS/GNSS system, unable RNAV, request amended clearance.” The loss of RNAV capability includes any failure or event causing the aircraft to no longer satisfy the criteria of this AC. Example failures include, but are not limited to, loss of autopilot/flight director (if required), or reversion to navigation other than GNSS or DME/DME/IRU (even though no pilot monitoring of navigation updating source is required). In the event of communications failure, established lost communication procedures should be followed.
11. PILOT KNOWLEDGE REQUIREMENTS AND TRAINING. The pilot is expected to be knowledgeable in the following areas. Also, for parts 121, 125, 129, 135, and 91K operators, the approved training program should address the elements listed below. This training program should provide sufficient training (for example, simulator, training device, or aircraft) on the aircraft’s RNAV system to the extent that the pilots are not just task oriented. Training need not be repeated if it addresses specific items listed below.
a. The information in this AC.
b. The meaning and proper use of Aircraft Equipment/Navigation Suffixes.
c. Procedure characteristics as determined from chart depiction and textual description.
(1) Depiction of waypoint types (flyover and flyby) and path terminators (provided in Appendix 3 and any other types used by the operator) as well as associated aircraft flight paths.
(2) Required navigation equipment for operation on RNAV routes, DPs, and STARs (for example, DME/DME/IRU and GPS/GNSS).
(3) Phraseology. Some RNAV procedures may incorporate the use of “Descend via” clearances. Pilots should be familiar with the correct use of the terminology and procedures as mentioned in AIM (refer to Air Traffic Procedures, Arrival Procedures).
d. RNAV system-specific information:
(1) Levels of automation, mode annunciations, changes, alerts, interactions, reversions, and degradation.
(2) Functional integration with other aircraft systems.
(3) The meaning and appropriateness of route discontinuities as well as related flightcrew procedures.
(4) Monitoring procedures for each phase of flight (for example, monitor PROG or LEGS page).
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(5) Types of navigation sensors (for example, DME, IRU, GPS/GNSS) utilized by the RNAV system and associated system prioritization/weighting/logic.
(6) Turn anticipation with consideration to speed and altitude effects.
(7) Interpretation of electronic displays and symbols.
e. RNAV equipment operating procedures, as applicable, including how to perform the following actions:
(1) Verify currency of aircraft navigation data.
(2) Verify successful completion of RNAV system self-tests.
(3) Initialize RNAV system position.
(4) Retrieve and fly a DP or STAR with appropriate transition.
(5) Adhere to speed and/or altitude constraints associated with a DP or STAR.
(6) Make a runway change associated with a DP or STAR.
(7) Verify waypoints and flight plan programming.
(8) Perform a manual or automatic runway update (with takeoff point shift, if applicable).
(9) Fly direct to a waypoint.
(10) Fly a course/track to a waypoint.
(11) Intercept a course/track.
(12) Be vectored off and rejoin a procedure.
(13) Determine cross-track error/deviation.
(14) Insert and delete/clear route discontinuity.
(15) Remove and reselect navigation sensor input(s).
(16) When required, confirm exclusion of a specific navigation aid or navigation aid type.
(17) Insert and delete a lateral offset.
(18) Change the arrival airport and alternate airport.
(19) Insert and delete a holding pattern.
Par 11 Page 17
f. Operator-recommended levels of automation for phase of flight and workload, including methods to minimize cross-track error to maintain procedure centerline.
g. Contingency procedures for RNAV failures.
12. AC FEEDBACK FORM. For your convenience, the AC Feedback Form is the last page of this AC. Note any deficiencies found, clarifications needed, or suggested improvements regarding the contents of this AC on the Feedback Form.
Appendix 1 APPENDIX 1. CRITERIA FOR APPROVAL OF BASELINE AREA NAVIGATION (RNAV) SYSTEMS USING DISTANCE MEASURING EQUIPMENT (DME)
1. PURPOSE. The FAA is responsible for evaluating DME/DME coverage and availability against a minimum standard DME/DME RNAV system for each route and procedure. Detailed criteria defining DME/DME RNAV system performance as it relates to the DME infrastructure is needed. This appendix defines the minimum performance and functions (baseline) for DME/DME RNAV systems intended to support the implementation of RNAV 1 and RNAV 2 routes, as well as RNAV 1 departure procedures (DP) and Standard Terminal Arrivals (STAR).
These criteria may be applied under an airworthiness approval for new equipment or used by the manufacturer for self-certification of existing equipment.
2. MINIMUM REQUIREMENTS FOR DME/DME RNAV SYSTEM.
a. Tuning and Updating Position of DME Facilities. The DME/DME RNAV system must:
(1) Position update within 30 seconds of tuning DME navigation facilities.
(2) Auto-tune multiple DME facilities.
(3) Provide continuous DME/DME position updating. (Given a third DME facility or a second pair has been available for at least the previous 30 seconds, there must be no interruption in DME/DME positioning when the RNAV system switches between DME stations/pairs.)
b. Using Facilities in the Chart Supplements. The FAA cannot ensure all DME signals within reception distance of U.S. airspace meet ICAO standards. These could include non-U.S.
DME facilities, or Department of Defense (DOD) maintained DME facilities excluded from the National Airspace System (NAS) database. DME/DME RNAV procedure design will only use DME facilities listed in the Chart Supplements. Although a procedure design issue, applicants may mitigate this restriction by:
(1) Having the DME/DME RNAV system only use DME facilities listed in the Chart Supplements.
(2) Requiring exclusion of non-NAS DME facilities from the aircraft’s navigation database when the RNAV routes or procedures are within reception range of these non-NAS DME facilities.
(3) Demonstrating to the FAA that their RNAV system performs reasonableness checks to detect errors from the non-NAS DME facilities and excludes these facilities from the navigation position solution when appropriate (e.g., using the ARINC 424 coding to preclude tuning co-channel DME facilities when the DME facilities signals-in-space overlap). See Appendix 1, paragraph 3 for guidance on testing of reasonableness checks.
c. DME Facility Relative Angles. When needed to generate a DME/DME position, the DME/DME RNAV system (referred to as FMS hereafter) must use, as a minimum, DMEs with a
Appendix 1 relative include angle between 30 degrees and 150 degrees. The FMS may use DME pairs outside these angles (for example, 20 degrees to 160 degrees).
d. RNAV System Use of DMEs. The RNAV system may use any receivable DME facility (listed in the Chart Supplements) regardless of its location. When needed to generate a DME/DME position, as a minimum, the RNAV system must use an available and valid low-altitude and/or high-altitude DME anywhere within the following region around the DME facility:
(1) Greater than or equal to 3 nautical miles (NM) from the facility;
(2) Less than 40 degrees above the horizon when viewed from the DME facility; and
(3) For facilities with an ARINC 424 figure of merit (FOM), the RNAV system may use the FOM value as the acceptable, usable region:
If the ARINC 424 FOM is: The aircraft’s DME/DME RNAV system must be:
Less than or equal to: And less than:
0 40 NM from the facility 12,000 ft above facility elevation 1 70 NM from the facility 18,000 ft above facility elevation 2 130 NM from the facility -- 3 160 NM from the facility -- NOTE 1: RNAV systems may use additional DME facilities (for example, a Localizer (LOC) DME facility may be used but is not required to be used for positioning). RNAV systems are not required to use the FOM value.
NOTE 2: RNAV routes and procedures may include new FOMs with Expanded Service Volumes (ESV).
(4) A valid DME facility:
(a) Broadcasts an accurate facility identifier signal,
(b) Satisfies the minimum field strength requirements, and
(c) Is protected from other interfering DME signals according to the co-channel and adjacent channel requirements.
e. No Requirement to Use Very High Frequency Omni-Directional Range (VOR), Non-Directional Beacon (NDB), LOC, Inertial Reference Units (IRU) or Attitude and Heading Reference System (AHRS). There is no requirement to use VOR, LOC, NDB, IRU or AHRS during normal operation of the DME/DME RNAV system.
f. Position Estimation Error (PEE). A minimum of two DME facilities satisfying the criteria in paragraph 2, and any other valid DME facilities not meeting that criteria, the
Appendix 1 95 percent PEE must be better than or equal to 1.75 NM. A Flight Technical Error (FTE) contribution not exceeding 1.0 NM (95 percent) may be assumed for RNAV 2 operations.
NOTE 1: This performance requirement is met for any navigation system that uses two DME stations simultaneously, limits the DME inclusion angle to between 30 and 150 degrees and uses DME sensors that meet the accuracy requirements of Technical Standard Order (TSO)-C66c. If the RNAV system uses DME facilities outside the range identified above, the DME signal-in-space error can be assumed to be 0.1 NM 95 percent.
NOTE 2: When using a minimum of two DME facilities satisfying the criteria in Appendix 3, subparagraph 2d, the 95 percent PEE must be better than or equal to the following equation:
𝟐𝟐𝟐𝟐𝑫𝑫𝑫𝑫𝑫𝑫 | 𝑫𝑫𝑫𝑫𝑫𝑫 ≤ 𝟐𝟐 ��𝟐𝟐𝟐𝟐𝟏𝟏,𝒂𝒂𝒂𝒂𝒂𝒂 + 𝟐𝟐𝟐𝟐𝟏𝟏,𝒔𝒔𝒂𝒂𝒔𝒔� + �𝟐𝟐𝟐𝟐𝟐𝟐,𝒂𝒂𝒂𝒂𝒂𝒂 + 𝟐𝟐𝟐𝟐𝟐𝟐,𝒔𝒔𝒂𝒂𝒔𝒔� 𝒔𝒔𝒂𝒂𝒔𝒔( 𝜶𝜶) Single facility reference, where: σsis = 0.05 NM σair is MAX {(0.085 NM, (0.125% of distance)} α = inclusion angle (30o to 150o)
g. Preventing Erroneous Guidance from Co-Channel Facilities. The RNAV system must ensure co-channel DME facilities do not cause erroneous guidance. This could be accomplished by including VOR reasonableness checking when initially tuning a DME facility or excluding a DME facility when there is a co-channel DME within line-of-sight. See Appendix 1, paragraph 3 for guidance on testing of reasonableness checks.
NOTE: The DME assessment cannot use a DME facility when there is a co-channel DME facility within line-of-sight.
h. Preventing Erroneous VOR Signals-in-Space. The RNAV system must ensure an erroneous VOR signal-in-space does not cause the position accuracy to exceed 1.75 NM for RNAV 2 and 0.87 NM for RNAV 1. This could be accomplished by not using VOR signals when DME/DME will be available or weighting and/or monitoring the VOR signal with DME/DME to ensure it does not mislead position results (for example, through reasonableness checks). See paragraph 3 for guidance on testing of reasonableness checks.
i. Ensuring RNAV Systems Use Operational Facilities. The RNAV system must use operational DME facilities. DME facilities listed by Notice to Airmen (NOTAM) as unavailable (for example, under test or other maintenance) could still reply to an airborne interrogation.
(Therefore, nonoperational facilities must not be used.) An RNAV system may exclude nonoperational facilities by checking the identification or inhibiting the use of facilities identified as not operational.
Appendix 1
j. Operational Mitigations. Operational mitigations defined to qualify equipment with this AC will not require pilot action during critical phases of flight, pilot monitoring of the RNAV system’s navigation updating source(s), or time intensive programming/blackballing of multiple DME stations prior to executing a procedure.
NOTE 1: Blackballing single facilities listed by NOTAM as out-of-service and/or programming route/procedure-defined “critical” DME is acceptable when this mitigation requires no pilot action during a critical phase of flight.
A programming requirement also does not imply the pilot should complete manual entry of DME facilities not in the navigation database. Instead, this allows RNAV systems to tune a critical DME, as appropriate to a specific route or procedure.
NOTE 2: The critical phase of flight is normally from the final approach fix on an approach procedure through missed approach, or from field elevation to 2,500 ft above airport elevation on a departure.
3. REASONABLENESS CHECKS. Many Flight Management Systems (FMS) perform a reasonableness check to verify valid DME measurements. Reasonableness checks are very effective against database errors or erroneous system acquisition (such as co-channel facilities), and typically fall into two classes:
• Those the FMS uses after it acquires a new DME. The FMS compares the aircraft’s position before using the DME to the aircraft’s range to the DME, and
• Those the FMS continuously uses, based on redundant information (for example, extra DME signals or IRU data).
a. General Requirements. The reasonableness checks are intended to prevent navigation aids from being used for navigation update in areas where the data can lead to radio position fixing errors due to co-channel interference, multipath, and direct signal screening. In lieu of using radio navigation aid published service volume, the navigation system should provide checks, which preclude use of duplicate frequency Navigational Aids (NAVAID) within range, over-the-horizon NAVAIDs, and use of NAVAIDs with poor geometry.
b. Assumptions. Under certain conditions, reasonableness checks can be invalid.
(1) Do not assume a DME signal remains valid just because it was valid when acquired.
(2) Do not assume extra DME signals are available. The intent of this baseline is to support operations where the infrastructure is minimal (for example, when only two DMEs are available for parts of the procedure).
c. Use Stressing Conditions to Test Effectiveness. When an applicant uses a reasonableness check to satisfy any requirement in this AC, they must test the effectiveness of the check under stressing conditions. An example of this condition is a DME signal that is valid at acquisition and ramps off during the test (similar to what a facility under test might do), when there is only one other supporting DME or two signals of equal strength.
Appendix 1
4. PERFORMANCE CONFIRMATION PROCESS. New systems may demonstrate compliance with these criteria as part of the airworthiness approval. For existing systems, the manufacturer should determine compliance with the equipment and aircraft criteria in this appendix. Manufacturers who have achieved their compliance should provide this information by letter to their customers. Operators/pilots may use this approval as a basis for their operations.
Manufacturers are also requested to provide a copy of this letter to the Flight Technologies and Procedures Division, (202) 267-8790, to facilitate making this information available to all operators. Guidance is provided below for both an airplane manufacturer and FMS and DME manufacturers.
a. Airplane Manufacturer (Type Certificate (TC) Holders Incorporating FMS and DME/DME Positioning). The manufacturer should review the available data for the integrated navigation system, and obtain additional data as appropriate, to determine compliance with the criteria in this AC. Those manufacturers who have achieved compliance with the criteria should provide this information by letter to their customers. Manufacturers are also requested to provide a copy of this letter to the Flight Technologies and Procedures Division to facilitate making this information available to all operators.
b. Equipment Manufacturers (Typically Separate TSO DME and FMS Holders).
(1) DME Sensor. The only requirement in this appendix that needs to be considered for a DME sensor is the accuracy requirement. DME sensors have been demonstrated to a variety of performance requirements per TSO-C66, Airborne Distance Measuring Equipment (DME) Operating within the Radio Frequency Range of 960-1215 Megahertz.
(a) TSO-C66 performance standards have evolved as follows:
1. TSO-C66: (August 1960) RTCA/DO-99.
2. TSO-C66a: (September 1965) RTCA/DO-151, accuracy requirement as total error with 0.1 NM attributed to ground facility, airborne equipment accuracy of 0.5 NM or 3 percent of distance, whichever is greater, with a maximum of 3 NM.
3. TSO-C66b: (November 1978) RTCA/DO-151A, accuracy requirement as total error with 0.1 NM attributed to ground facility, airborne equipment accuracy of 0.5 NM or 1 percent of distance, whichever is greater, with a maximum of 3 NM.
4. TSO-C66c: (September 1985) RTCA/DO-189, accuracy requirement as total error for the airborne equipment of 0.17 NM or 0.25 percent of distance, whichever is greater.
(b) The accuracy required by TSO-C66c is adequate to support the criteria in this appendix, and DME equipment manufacturers under these versions of the TSO do not need to further evaluate their equipment for RNAV 1 and RNAV 2 operations. DME sensor manufacturers may use the following process to establish more accurate performance than originally credited.
1. Determining Achieved Accuracy. Rather than relying on original demonstrated performance, the applicant may elect to review the original TSO or
Appendix 1 TC/Supplemental Type Certificate (STC) test data to determine the demonstrated accuracy and/or make any appropriate changes to qualification tests to determine achieved accuracy.
NOTE: When conducting accuracy analysis, the DME signal-in-space error can be assumed to be 0.1 NM 95 percent (both inside and outside the published service volume). If demonstrating accuracy under bench or flight test conditions the actual accuracy of the bench equipment or ground facility should be considered.
2. Accomplishing New Testing. New testing should be performed under the same conditions used to demonstrate compliance with the original TSO-C66 standard.
3. Manufacturers who have demonstrated more accurate DME performance should state the demonstrated accuracy in a letter to their customers. Manufacturers are also requested to provide a copy of this letter to the Flight Technologies and Procedures Division to facilitate making the information available to all operators.
(2) Multi-Sensor Systems (FMS). The manufacturer should review the available data for the integrated navigation system, and obtain additional data as appropriate, to determine compliance with the criteria in this appendix. Manufacturers who have determined compliance should state such in a letter to their customers, along with any operational limitations (for example, if the pilot is expected to manually inhibit the use of facilities which are listed by NOTAM as unavailable). The manufacturer’s certification may limit the compliance to specific DME systems, or may reference any DME qualified to the accuracy requirements of TSO-C66c.
Manufacturers should also provide a copy of this letter to the Flight Technologies and Procedures Division.
(a) FMS accuracy is dependent on a number of factors, including latency effects, the selection of DME facilities, the method of combining information from multiple DMEs, and the effects of other sensors used to determine a position. For FMSs using two (or more) DMEs at the same time and limiting the DME include angle to between 30 and 150 degrees, the accuracy requirement can be met if the DME sensors meet the accuracy requirements of TSO-C66c. For FMSs without these characteristics, the accuracy should be evaluated under poor DME geometry scenarios and should consider the demonstrated DME sensor accuracy. Poor geometry scenarios may include angles at the limits specified earlier, with or without additional DME facilities available outside those conditions.
(b) Identify those conditions that would result in failure to meet the accuracy requirement, and the means to preclude those identified conditions.