Chapter 11. Equipment Performance - Baro-VNAV.
11-1. General Limitations for Operations under IFR.
a. A baro-VNAV system may be approved for enroute, terminal, and approach use within the contiguous U.S., Alaska, and Hawaii. VNAV equipment may be used to provide advisory vertical path guidance to aid in more efficient and precise operation of the aircraft.
However, unless compensated for temperature, baro-VNAV equipment can only be used within the limitations for temperature published on approach procedure charts. The barometric altimeter must always be the primary altitude reference for all flight operations (see paragraphs 12-7, 18-2.b and 23-3.1).
b. Systems providing automatic temperature compensation to the baro-VNAV guidance must comply with RTCA/DO-283B, appendix H. This enables baro-VNAV operations outside of the temperature limits published on approach procedure charts.
(1) Equipment manufacturers may include compensation for temperatures above and below the limits published on approach charts.
Note: Any equipment limitations must be documented in the installation manual.
(2) Temperature compensation may be provided on segments other than the final approach and missed approach (i.e., the segments prescribed by RTCA/DO-283B, appendix H) if the equipment includes a method for the flight crew to inhibit the compensation function.
Equipment manufacturers providing baro-VNAV temperature compensation on other segments must specify language in the installation instructions/manual for an AFM(S)/RFM(S) caution that the flight crew/pilot must coordinate use of temperature compensation with ATC prior to employing this function. The reason for an AFM(S)/RFM(S) caution is to ensure there is no loss of separation between an aircraft employing temperature compensation and an aircraft not employing this function.
Note: Aviation authorities in different countries may or may not authorize temperature compensation outside the final approach segment.
c. Procedures for manual altitude corrections must be established for systems that do not provide automatic temperature compensation. A pilot workload evaluation must be performed for all segments in the approach, including the missed approach holding waypoint.
d. Baro-VNAV equipment may also be used to provide advisory vertical guidance to aid in more efficient and precise operation of the aircraft for enroute, terminal, and/or approach operations (see chapter 4). The primary barometric altimeter will be the primary altitude reference for all flight operations.
e. Baro-VNAV performance is not adequate for vertical guidance on LPV or GLS approaches.
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11-2. Baro-VNAV Equipment Performance for Operations under IFR.
a. The initial certification of a baro-VNAV system requires an engineering evaluation because of the need to verify performance, failure indications, environmental qualifications, etc.
Subsequent installations of the same baro-VNAV equipment system in other aircraft may require additional engineering evaluation, depending upon the degree of integration with other aircraft systems, in particular autopilot and FD systems.
b. For enroute, terminal, and approach IFR operations, the airborne baro-VNAV system must have TSE components in the vertical direction that are less than those shown in Table 6 below, 99.7 % of the flying time (reference RTCA/DO-283B, paragraph 2.2.2.6.1).
Misleading guidance during LNAV/VNAV approach operations is considered a major failure condition (see Table 8). A baro-VNAV system providing vertical guidance during the final approach segment of an LNAV/VNAV approach operation must meet the design error and failure mode probability requirements that satisfy a major failure condition (see paragraph 12-2).
Note 1: Baro-VNAV vertical guidance is independent from the lateral guidance. Loss of baro -VNAV guidance during an LNAV/VNAV approach results in a fail-down to LNAV or a missed approach. Therefore, loss of navigation for baro-VNAV is considered a minor failure condition.
Note 2: Baro-VNAV systems certified before 11/20/1989 may have been certified to a lesser standard that is not consistent with a major failure condition. Operators of these aircraft should apply the final approach segment step-down fix to the LNAV/VNAV line of minima and should use the primary barometric altimeter to ensure meeting the step-down fix altitude constraint.
(1) Baro-VNAV TSE includes all errors (except horizontal coupling error and the effects of temperature) resulting from the vertical guidance equipment installation. This includes altimeter system errors.
(2) During the final approach segment only (i.e., from the final approach fix to the missed approach point) baro-VNAV TSE must also include horizontal coupling error.
Note: GPS/SBAS Class 2 and Class 3 as well as GPS/GBAS vertical positioning performance exceeds the accuracy requirements of a barometric sensor used for final approach VNAV. The GPS/SBAS 50m Vertical Alert Limit (VAL) used for LNAV/VNAV is consistent with baro-VNAV and may also be used to comply with RNP approach.
c. Table 6 is a means to identify the degradation in altimeter performance as altitude increases. But Table 6 is not intended as an exact representation of expected performance 87
characteristics with distinct steps. Instead, altimeter performance degradation is curvilinear as altitude increases. Therefore, to qualify a baro-VNAV system, the 99.7% aircraft altimetry system error for the aircraft (assuming International Standard Atmosphere temperature and lapse rates) must be less than or equal to the following:
𝐴𝐴𝐴 = −8.8 ∗ 10−8 ∗ 𝐻2 + 6.5 ∗ 10−3 ∗ 𝐻 + 50 (ft) where H is the true altitude of the aircraft.
Altitude Region (MSL) Level Flight Segments Flight Along Specified Vertical Profile At or below 5000 ft.
150 ft. 160 ft.
5000 ft. to 10000 ft 200 ft. 210 ft.
10000 ft. to 29,000 ft 200 ft. 210 ft.
Above 29,000 ft to 41,000 ft 200 ft.
(See Notes 1 and 2) 260 ft.
Table 6. Vertical Path Performance Limit for Baro-VNAV Note 1: These requirements are based upon airborne altimetry and avionics systems that provide performance consistent with reduced vertical separation minimums (RVSM) requirements. Aircraft meeting RVSM requirements provide acceptable vertical total system error in level flight for the last row in table 6. No additional demonstration or compliance evaluation is required.
However, the altimetry system error cannot be extrapolated to the other altitude blocks.
Note 2: For aircraft type designs prior to April 9, 1997, the value is 200 ft in the cruise flight envelope and is not to exceed 250 ft over the full aircraft operating envelope.
Basic height-keeping parameters for 1000 ft. vertical separation minimum between flight levels 290 and 410 are in ICAO Document 9574, Manual on Implementation of 1000 ft. Vertical Separation Minimum between FL 290-FL410 Inclusive.
Note 3: Table 6 does not address altitudes above FL 410 because current altimetry and baro-VNAV systems are adequate for the 2,000 ft air traffic control vertical separation criteria.
d. Flight crew Inputs.
(1) The altitude associated with the active waypoint should be to the nearest 100 feet for enroute and terminal flight phases, and nearest 10 feet for the approach phase. If provided, waypoint horizontal position should be in increments not greater than 0.1 nautical miles.
(2) The ascent or decent angle should be to at least the nearest 0.1 degree for equipment that specifies gradient angles.
(3) The station elevation, if necessary for equipment employing slant range error correction, should be at least to the nearest 1,000 feet for enroute and terminal flight phases, and to the nearest 100 feet for the approach phase.
(4) For terminal and approach operations, the ability to enter the altitude associated with at least eight (for TO-FROM equipment) or nine (for TO-TO equipment) successive waypoints.
(5) A means to confirm input data correctness prior to the system using the new data.
e. System Displays.
(1) There should be a continuous display of vertical path deviation with the performance indicated in Table 7 below.
Enroute/Terminal (feet) Approach (feet) Full-Scale Deflection (see note 1) ≤ 500 150 (see note 2) Readability ≤ 100 ≤ 30 Minimum Discernable Movement ≤ 10 ≤ 5 Table 7. Vertical Path Deviations Note 1: This is the minimum standard for vertical deviation display scaling and does not preclude using a scale of other than the values listed (e.g. large PFD display) provided that the scaling is suitable to control the aircraft on the intended path.
Note 2: Other values of full-scale deflection for approach may be acceptable provided the proposed value is found satisfactory by an engineering evaluation.
(2) The vertical guidance presentation should be compatible with the aircraft’s flight instrumentation such that the pilot is continuously furnished the aircraft’s vertical deviation with respect to the pre-programmed ascent/descent or level flight profile.
(3) Scale change for the final approach must be done in a manner that supports operational suitability for transitioning onto the final approach segment.
(4) Baro VNAV systems using angular vertical scaling must meet the following:
(a) The deviation scaling supports the FTE monitoring and bounding (75 ft deviation below path).
Note: This may require limiting the length of the approach to exclude operating where the angular deviations no longer support FTE monitoring and bounding.
(b) The deviation limits are equivalent to the operational limits for glideslope deviations during an ILS approach.
f. Alert Indications.
Alert indications for the system should be located on or near the vertical path deviation indicator and should provide a readily discernible annunciation to the pilot(s) for any of the following:
(1) Inadequate or invalid navigation signals or sources;
(2) Absence of primary power;
(3) Inadequate or invalid navigation displays or output sources; and
(4) Equipment failures.
Note: These failure/status indications must occur independently of any operator action. Power or navigation equipment failures may be indicated in a common manner.
In the approach mode, the lack of adequate navigation signals or sources will be annunciated by means of a flag displayed on the primary vertical navigation display. In other modes, an appropriately located annunciator may be used.
g. Additional Constraints.
(1) The baro-VNAV equipment should indicate the aircraft vertical path position to the accuracy specified in paragraph 11-2.b within five seconds after any maneuvering or changes in attitude encountered in normal operations.
(2) The system should be capable of providing navigation guidance to the accuracy specified in paragraph 11-2.b within 20 seconds after input of the desired vertical track information (assuming sensor outputs are available).
(3) Navigation guidance should be available within five seconds of waypoint data input.
(4) The equipment should have the capability to meet the criteria outlined in paragraph 11-2 throughout the range of environmental conditions that will be encountered in actual service. Exposing the equipment to the environmental test conditions of TSO-C115b (or later revision) may be used to demonstrate this capability. The environmental test conditions contained in RTCA/DO-160F may be used to demonstrate this ability for baro-VNAV systems certified independent of a multi-sensor RNAV system.
Note: Previously certified baro-VNAV systems may have used an earlier RTCA/DO-160 revision. This equipment may still be installed within the provisions of its original certification.
(5) The equipment should provide a means for the flight crew to determine system status prior to flight.
(6) The equipment should provide the navigation accuracy specified in paragraph
11-2.b for all groundspeeds up to a maximum value, and for all ascent (if applicable) and descent rates up to a maximum value to be set by the applicant.
(7) The equipment should provide a means to alert the flight crew prior to arrival at a waypoint for anticipating necessary vertical maneuvering. This indication should be located on or near the vertical path indicator. A procedural means based on a continuous and properly located distance to waypoint display may be used for vertical maneuver anticipation for baro-VNAV equipment that is not coupled to a FD or autopilot.
(a) Systems that provide steering signals for FDs or autopilots should provide automatic vertical maneuver anticipation and a waypoint alert that occurs prior to the initiation of the vertical maneuver. Systems that are coupled to the automatic guidance/control system(s) should not cause the aircraft to depart an assigned altitude until the impending altitude change is indicated to the crew within the pilot’s primary field of view, then acknowledged by timely crew action unless the system is mechanized to the altitude selector in such a manner that a transition to descent or climb will not occur unless the altitude selector has been reset by the flight crew to the new altitude.
(b) Acceptance of the acknowledgment should not precede the indication if this acknowledgement is stored by the equipment. The acknowledgement timing of an impending vertical path change depends on operational procedures and aircraft/equipment design;
however, past practice has shown that this acknowledgment should not occur more than five minutes prior to the impending vertical path change.
(8) If parallel offset track capability is provided, waypoint alerting and vertical maneuver anticipation should be provided prior to arrival at the point where the offset track intersects the angle bisector of the parent track. These functions should operate as described above in paragraph 11-2.d(7).
(9) The equipment should be capable of providing correct vertical path guidance when interfaced with lateral navigation equipment providing turn anticipation. The baro-VNAV equipment should recognize the angle bisector of the lateral track change as the “zero distance to waypoint” location.
h. The software should be verified and validated to at least level C as defined in RTCA/DO-178B or ‘C’ for baro-VNAV equipment that uses a digital computer.
Chapter 12. General Installation Considerations.
12-1. Introduction.
The following information applies to the installation of all positioning and navigation equipment intended for IFR use.
12-2. Failure Classification.
a. From enroute through Category I precision approach, the loss of navigation function is typically considered to be a major failure condition for the aircraft (see AC
25.1309-1, AC 23.1309-1, AC 27-1, or AC 29-2 as applicable). Other aircraft navigation systems should be considered when determining GNSS loss of navigation, which could be major (no other navigation systems) or minor (other applicable navigation systems).
Navigation data is considered to be misleading when un-annunciated position errors exist. For enroute, terminal, LNAV, and LNAV/VNAV approaches, presenting misleading information to the flight crew is considered to be a major failure condition for the aircraft. For LP/LPV and GLS approaches, presenting misleading information to the flight crew is considered to be a hazardous failure condition. These failure classifications are summarized in Table 8.
Advisory Vertical Guidance Enroute/ Terminal Area/ Non-precision Approach (LNAV or RNP
0.3) Non-precision Approach with Vertical Guidance (LNAV/VNAV) LP/LPV Approach GLS Approach Loss of Navigation No Effect Major Major Major Major Misleading Information Minor Major Major Hazardous Hazardous Table 8. Typical Hazard Classifications Note 1: For RNP values less than 0.3, losing RNP capability constitutes loss of navigation. Refer to appendix 2 for further RNP AR airworthiness information and AC 90-101 (latest revision) for complete guidance on RNP AR operations.
Note 2: Baro-VNAV vertical guidance is independent from the lateral guidance. Loss of baro -VNAV guidance during an LNAV/VNAV approach results in a fail-down to LNAV or missed approach. Therefore, loss of navigation for baro-VNAV is considered a minor failure condition.
b. The applicant must conduct a safety assessment of the positioning/navigation equipment installation to verify that design errors and failure modes meet the probability requirements for that failure class. AC 23.1309-1, AC 25.1309-1, AC 27-1, or AC 29-2, per the latest revision levels, provide an acceptable means for showing that the hardware complies with 93
---- pertinent airworthiness requirements. See paragraph 15-2.a for GPS/SBAS and GPS/GBAS retrofit installation guidance related to navigation displays previously approved for ILS.
c. Any probable failure of the positioning/navigation equipment must not degrade or adversely affect the normal operation and performance of other required equipment or create a flight hazard. The interfaces with other aircraft equipment must be designed such that normal or abnormal positioning/navigation equipment operation must not adversely affect the operation of other equipment nor must operation of other equipment adversely affect the positioning/navigation equipment operation.
12-3. Software Considerations.
a. In December 2011, RTCA published RTCA/DO-178C, Software Considerations in Airborne Systems and Equipment Certification. AC 20-115C Airborne Software Assurance, defines an acceptable means of qualifying software and provides guidance for applicants with software qualified to previous RTCA/DO-178 revisions. Applicants may choose to use RTCA/DO-178C instead of RTCA/DO-178B provided they declare their intention to the applicable aircraft certification office for the project.
b. The applicant is encouraged to submit the Plan for Software Aspects of Certification (PSAC) early in the software development process. Early submittal will allow timely resolution of issues such as partitioning and determining software levels. Software certification is normally addressed during the design approval for positioning/navigation equipment, but should be confirmed at installation to ensure the certification level is appropriate for the installation intended function.
Note: It may be possible to obtain approval for navigation functions that have hazardous level effects with FMS software developed to RTCA/DO-178B or ‘C’ level C criteria. AC 20-174, Development of Civil Aircraft and Systems provides an acceptable method for establishing design assurance levels. This guidance may be helpful for navigation architectures containing independent functions.
12-4. Hardware Considerations.
AC 20-152 (latest revision) RTCA, Inc., Document RTCA/DO-254, Design Assurance Guidance for Airborne Electronic Hardware, defines an acceptable means of qualifying complex electronic hardware devices. The applicant is encouraged to submit the Plan for Hardware Aspects of Certification (PHAC) early in the development process. Hardware certification is normally addressed during the design approval for positioning/navigation equipment; but, should be confirmed at installation to ensure the certification level is appropriate for the installation intended function.
12-5. Installed Equipment Compatibility.
There is no one, single, standardized interface for positioning and navigation equipment to other equipment on-board the aircraft such as displays, autopilots, terrain warning, etc.
Positioning and navigation equipment manufacturer’s installation manuals/instructions should define what other equipment is compatible with the positioning and navigation system.
Equipment manufacturers are encouraged to develop compatible equipment lists and/or detailed interface requirements for their installation manuals/instructions. The installer must ensure that other on-board equipment interfaced with the positioning and navigation systems is compatible, and the airworthiness approval holder is ultimately responsible for defining the installation. But the installation burden is greatly eased when equipment manufacturers define compatible systems in their installation manual/instructions, including software version and/or part number if necessary.
12-6. Installation Instructions.
a. The equipment must be installed in accordance with the instructions and limitations provided by the positioning and navigation equipment manufacturer. The airworthiness approval will address any additional installation specifics.
b. Equipment should be installed with separation between wires of redundant equipment (see AC 43.13-1 latest revision). Wire separation will mitigate the possibility of loss of navigation due to a single event. During installation approval of the system, when wire separation cannot be achieved, the following questions must be evaluated and the potential for single event upsets minimized:
(1) Is it possible for a cable harness to be exposed to wire chafing in a manner that both channels fail simultaneously?
(2) Is the cable harness located near flight control cables, high electrical capacity lines or fuel lines?
(3) Is the cable harness located in a protected area of the aircraft (isolated from engine rotor burst)?
(4) Is any electromagnetic interference (EMI) between systems caused by cable routing?
12-7. Equipment Capability and Installed Limitations.
a. Positioning and navigation avionics might have optional TSO functions that are not supported at the aircraft level after installation. The avionics must have the functions inhibited through configuration settings (e.g., strapping, software, etc.) or through database management processes if the aircraft is not qualified to perform those functions. The AFM(S)/RFM(S) must contain an appropriate entry for any limitations. An example to illustrate the point is avionics with the capability to perform RF legs. The installed avionics may have approved RF leg capability, but the aircraft lacks one or more necessary capabilities such as a roll-steering 95
autopilot or displays able to depict RF legs. For this installation, the avionics would need to have the RF leg capability inhibited.
b. Airworthiness approval holders should include a comprehensive PBN statement of capabilities and qualification in the AFM(S)/RFM(S) describing the aircraft’s PBN capabilities and operations limitations as an easy reference for flightcrews and authorities that grant operations approvals (see appendix 5 for an example). Doing so can greatly ease the process for operators that need operations approvals and enhance flightcrew awareness of the aircraft’s PBN capability.
12-8. Final Approach Segment Step-Down Fixes in Navigation Databases.
a. In the U.S. when RNAV (GPS) approach procedures are produced or updated, the procedure designer names the final approach segment (between the final approach fix and missed approach point) step-down fixes applicable to the LNAV and LP lines of minima. But, some previously published RNAV (GPS) approach procedures still contain unnamed final approach segment step-down fixes, meaning there will be a mix of named and unnamed stepdown fixes published with RNAV (GPS) approach procedures until all the procedures receive their periodic update. ARINC 424 specifications provide data elements for encoding both named and unnamed step-down fixes in a navigation database. Including final approach segment step-down fixes in the aircraft’s onboard navigation database is entirely optional.
However, the database must include all named waypoints that make up the instrument approach procedure except for final approach segment step-down fixes. Showing step-down fixes on a vertical profile display can enhance flight crew situation awareness, but can also complicate installation issues (see paragraphs 12-8.b through 12-8.d). Airworthiness approval applicants should contact their ACO early when seeking an approval for equipment with step-down fixes in the onboard navigation database.
Note 1: Final approach segment step-down fixes are commonly used on U.S. RNAV (GPS) approach procedures to support an LP or LNAV line of minima.
Note 2: Pilots are expected to use distance information to comply with step-down fixes when they are not included in the equipment database.
Note 3: ARINC 424 is not specified by any positioning or navigation TSO. Nor is there any TSO requirement to include final approach segment or unnamed step-down fixes prior to the final approach fix in navigation databases for approach procedures.
b. Equipment manufacturers providing final approach segment step-down fixes in their onboard navigation databases for RNAV (GPS) approach procedures must either provide a method to remove them or provide an installation limitation for cockpit configurations that cannot properly support them (i.e., define the requirements in the installation instructions/manual). If removing final approach segment step-down fixes, manufacturers may choose to employ installation-specific configurations (i.e., software, strapping, etc.), or they 96
may offer a navigation database solution (i.e. a database with final approach segment step-down fixes and a database without final approach segment step-down fixes).
c. Many U.S. RNAV (GPS) approach procedures include LNAV, LNAV/VNAV, and LPV lines of minima. However, a final approach segment step-down fix does not apply to the LNAV/VNAV or LPV minima. The airworthiness applicant must ensure the displayed RNAV(GPS) approach is in the primary field of view, in the proper sequence, unambiguous, and does not create detrimental clutter.
Note: For LPV, some ICAO States may prohibit displaying step-down fix information from the onboard navigation database. Navigation system manufacturers should take this into consideration during their navigation system design and airworthiness applicants should consider this during equipment installation.
(1) The cockpit design and layout can greatly influence an airworthiness approval when it comes to displaying final approach segment step-down fixes. Integrating an RNAV (GPS) approach with LNAV/VNAV or LPV capability in an older cockpit design can be challenging when the onboard navigation database includes final approach segment step-down fixes due to limited display capability and little or no labeling flexibility.
(2) When installing equipment with final approach segment step-down fixes in the onboard navigation database, the display integration must:
• Be accessible and readable by all required flight crewmembers;
• Cleary depict and label the aircraft’s track;
• Clearly label displayed waypoints, fixes and numeric information with sufficient detail to enable a pilot to discern a step-down fix from other fixes on a procedure; and,
• Clearly display distance/bearing/track to the landing threshold point/fictitious threshold point (LTP/FTP) during the final approach segment of an LPV approach.
(3) The suggested optimum implementation is showing final approach segment step-down fixes for LNAV and LP approaches, but not showing final approach segment stepdown fixes during an LNAV/VNAV or LPV approach. When displaying distance/bearing/track to a step-down fix on the LPV final approach segment (i.e., after crossing the final approach fix), the equipment must also provide a readily available, clear, unambiguous indication of distance/bearing/track to the LTP/FTP. Due consideration should be given to the cockpit configuration and display capabilities during the airworthiness approval process. Additionally, equipment manufacturers must ensure final approach segment step-down fixes do not interfere with LPV path construction via the FAS datablock.
d. When using any advisory vertical guidance during an approach to LNAV or LP minimums, or when using GPS substitution on a conventional approach procedure, the flight crew must use the primary barometric altimeter to ensure compliance with any altitude restrictions. RNAV(GPS) procedure design criteria puts final approach segment step-down 97