Advisory U.S. Department of Transportation Circular Federal Aviation Administration Subject: AIRWORTHINESS APPROVAL OF Date: 9/12/88 AC No: 20-129 VERTICAL NAVIGATION (VNAV) SYSTEMS Initiated by: AIR-120 Change:
FOR USE IN THE U.S. NATIONAL AIRSPACE
SYSTEM (NAS) AND ALASKA PURPOSE. This advisory circular establishes an acceptable means, but not the only means, or obtaining airworthiness approval of an airborne vertical navigation system for use under VFR (visual flight rules) and IFR (instrument flight rules) within the conterminous United States, Alaska, and surrounding U.S. waters. Like all advisory material, this advisory circular is not, in itself, mandatory and does not constitute a regulation. It is issued for guidance purposes and to outline one method of compliance with airworthiness requirements. As such, the terms "shall" and " must" used in this advisory circular pertain to an applicant who chooses to follow the method presented.
2. RELATED FAR. Federal Aviation Regulations (FAR) Parts 23, 25, 43, and 91.
3. RELATED READING MATERIALS.
a. Federal Aviation Administration (FAA)/Technical Standard Order (TSO) C115, Area Navigation Equipment Using Multi-Sensor Inputs; ISO C60b, Area Navigation Equipment Using Loran-C Inputs; and TSO C120, Area Navigation Equipment Using Omega/VLF Inputs. Copies may be obtained from the Department of Transportation, FAA, Aircraft Certification Service, Aircraft Engineering Division (AIR-120), 800 Independence Avenue, SW., Washington, DC 20591.
b. Radio Technical Commission for Aeronautics (RICA), Document No.
RICA/DO-160B, Environmental Conditions and Test Procedures for Airborne Equipment, Document No. RICA/DO-178A, Software Considerations in Airborne Systems and Equipment Certification; RICA/DO-187, Minimum Operational Performance Standards for Airborne Area Navigation Equipment Using Multi-Sensor Inputs;
RICA/DO-194, Minimum Operational Performance Standards for Airborne Area Navigation Equipment Using Loran-C Inputs; and RICA/DO-190, Minimum Operational Performance Standards for Airborne Area Navigation Equipment Using Omega/VLF Inputs. Copies may be purchased from RICA Secretariat, One McPherson Square, Suite 500, 1425 K Street, NW., Washington, DC 20005.
4. BACKGROUND.
System Description. Vertical navigation (VNAV) equipment provides vertical path guidance computed as deviation from a desired ascending or descending path to a specified altitude at a waypoint. Vertical guidance is generally provided as a linear deviation from the desired track defined by a line joining two waypoints with specified altitudes or as a vertical angle from a specified waypoint. Such systems may be enhanced by inclusion of secondary
inputs such as optimized ciim or descent protiles, vertical acceleration and true airspeed. The desired vertical path may be pilot selectable or may be determined by the VNAV computer by computations based on the altitudes associated with successive waypoints.
b. General Operational Limitations. A vertical navigation system may be approved for en route, terminal and approach use within the conterminous United States and Alaska.
5. DEFINITIONS.
a. Approach Operations. Approach operations are those flight phases conducted on charted Instrument Approach Procedures (IAP's) commencing at the initial approach fix (IAF) and concluding at landing or the missed approach holding fix, as appropriate.
b. En Route Operations. En route operations are those flight phases conducted on charted Very High Frequency Omi Range (VOR) routes designated as high or low altitude routes (Jet or Victor), direct point-to-point operations between waypoints defined as part of these charted routes, or along great circle routes as described in Advisory Circular 90-82, Random Air Navigation Routes.
Flight Path Angle. The angle that the vertical flight path of the aircraft makes with the local horizontal.
d. Slant Range. The actual straight line distance between an aircraft in flight and a ground location. This distance is greater than the geographical surface range because of the aircraft altitude.
e. Slant Range Error. Slant range error is the difference between the distance of an aircraft to a Distance Measuring Equipment (DE) station on the surface and the distance from the station to a point directly beneath the aircraft on the surface. The error magnitude is a function of aircraft altitude above the station and the distance to the station.
f. Terminal Area Operations. Terminal area operations are those flight phases conducted on charted Standard Instrument Departures (SID), on charted Standard Terminal Arrivals (STAR), or other flight operations between the last en route fix/waypoint and an initial approach fix/waypoint.
g. Vertical Deviation. The deviation of the aircraft above or below the vertical profile as displayed on an indicator such that deflection is up when the aircraft is below the vertical profile.
h. Vertical Profile. A line or curve, or series of connected lines and/or curves in the vertical plane, defining an ascending or descending flight path either emanating from or terminating at a specified waypoint and altitude, or connecting two or more specified waypoints and altitudes. In this sense, a curve may be defined by performance of the airplane relative to the airmass.
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i. Vertical Profile Angle Error. The difference in degrees that the current aircraft flight path angle makes with the vertical profile.
j. Vertical Profile Intercept Point. The point at which the current aircraft flight path angle intercepts the vertical profile.
6. AIRWORTHINESS CONSIDERATIONS. Vertical navigation equipment has been certificated for VFR and IFR use as a vertical flight path guidance system for en route, terminal and approach navigation in the National Airspace System (NAS).
This paragraph establishes acceptable criteria for VNAV systems.
a. VNAV Equipment Installations Used for Operations Under Visual Flight Rules (VFR) Only. Operators wishing to use VNAV'equipment that is not coupled to an autopilot or flight director (except as noted in 6a (5) below) for operations limited to VFR may obtain approval of the installation by Type Certificate (IC), Supplemental Type Certificate (STC), data field approved by the FAA on an FAA Form 337, Major Repair and Alteration, or by the use of previously approved data. The approval for return to service should be signed by one of the entities noted in FAR 43; i.e., repair station, manufacturer, holder of an inspection authorization, etc. The installation verification should ensure, but is not limited to, the following:
(1) The VNAV Equipment Installation does not interfere with the normal operation of other equipment installed in the aircraft. This is accomplished by a ground test and flight test to check that the VNAV equipment is not a source of objectional electromagnetic interference (EMI), is functioning properly and safely, and operates in accordance with the manufacturer's specifications.
(2) The Structural Mounting of the VNAV Equipment is sufficient to ensure the restraint of the equipment when subjected to the emergency landing loads appropriate to the aircraft category.
(3) A Vertical Navigation Source Annunciator is Provided on or adjacent to the display if the VNAV equipment installation supplies any information to displays such as a horizontal situation indicator (HSI) or course deviation indicator (CDI) which can also display information from other systems normally used for aircraft navigation.
(4) The VNAV Equipment Controls and Displays are Installed with a placard(s) which states "VNAV Equipment Not Approved for IFR."
(5) The VNAV Equipment May Not Be Coupled to an autopilot or flight director unless the same installation has previously been made using the same VNAV equipment, autopilot, and/or flight director in the same aircraft under either TC or STC approval.
b. VNAV Equipment Installations Used for Operations Under IFR. Acceptable criteria for approval of VNAV equipment to be used under IFR are contained in this advisory circular. The initial certification of a VNAV system requires an engineering evaluation because of the need to verify performance, failure indications, environmental qualifications, etc. Subsequent installations of the same VNAV equipment system in other aircraft may require additional engineering Par. 5 3
evaluation, depending upon the degree of integration of the system with other aircraft systems, in particular autopilot and flight director systems. VNAV systems for use under IFR should provide the following:
(1) Flightcrew Inputs of:
(i) Altitude associated with the active waypoint in terms of feet at least to the nearest 100 feet for en route and terminal flight phases and 10 feet for the approach phase. If provided, waypoint horizontal position in increments not greater than 0.1 nautical miles (ri) or 0.2 degrees bearing from the waypoint.
(ii) Ascent or descent angle in terms of degrees at least to the nearest 0.1 degree, for equipment which specifies gradient angles.
(ili) Station elevation, if necessary for equipment employing slant range error correction, in terms of feet, at least to the nearest 1,000 feet for en route and terminal flight phases and 100 feet for the approach phase.
(iv) 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.
(v) A means to confirm correctness of input data prior to utilization of the new data by the system.
(2) The system displays should give no operationally misleading information and should provide:
(1) A continuous display of vertical path deviation with En Route/Terminal Approach (feet) (feet) Minimum Full-Scale Deflection >500 * >150 Readability <100 < 30 Minimum Discernable Movement ≤ 10 * NOTE: Smaller values of minimum full-scale deflection for approach may be acceptable provided the proposed value is found satisfactory by an engineering evaluation.
(ii) Entry of waypoint altitude in terms of feet to at least the nearest 100 feet for en route and terminal flight phases and 10 feet for the approach phase.
(iii) A vertical guidance presentation compatible with the aircraft's flight instrumentation such that the pilot is continuously furnished vertical deviation of the aircraft with respect to the preprogrammed ascent/ descent or level flight profile.
(iv) An annunciation of impending vertical waypoint crossing.
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(3) Caution Indication (s) for the System Should Be Located on or near the indicator specified in paragraph 6b(2) (i) and should provide a readily discernible caution indication(s) to the pilot (s) for any of the following:
(i) Inadequate or invalid navigation signals or sources.
(ii) The absence of primary power.
(i1i) Inadequate or invalid navigation displays or output sources.
(iv) Equipment failures.
NOTE: These failure/status indications shall 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 shall be annunciated by means of a flag displayed on the primary vertical navigation display. In other modes, an appropriately located annunciator may be used.
(4) The System Shall Be Capable of Providing Navigation Guidance to the accuracy specified in paragraph 8 within 20 seconds after input of the desired vertical track information (assuming sensor outputs are available).
(5) The System Shall Correct IME Range for Slant Range error unless it is demonstrated that the accuracies specified in paragraph 8 can be achieved without slant range error correction.
(6) Navigation Guidance Should Be Available within five seconds of waypoint data input.
(7) The Equipment Should Have the Capability to meet the criteria outlined in paragraph 6b(1) through 6b(6) throughout the range of environmental conditions which will be encountered in actual service. Exposure of the equipment to the environmental test conditions of ISO's C60b, C115, or C120 may be used to demonstrate this capability. For VNAV systems certified independent of an area navigation system, the environmental test conditions contained in RTCA Document No. DO-160B may be used to demonstrate this ability.
(8) The Equipment Should Provide a Means for the Flightcrew to determine system status prior to flight.
(9) The Equipment Should Provide the Navigation Accuracy specified in paragraph 8 for all groundspeeds up to a maximum value to be set by the manufacturer and for all ascent and descent rates up to a maximum value to be set by the manufacturer.
(10) The Equipment Should Provide Means to Alert the Flightcrew prior to arrival at a waypoint to permit anticipation of necessary vertical maneuvering. This indicator should be located on or near'the indicator specified in paragraph 6b(2) (i). For VNAV equipment that is not coupled to a flight director or autopilot, a procedural means based on a continuous and properly located distance to waypoint display may be used for vertical maneuver Par. 6 5
anticipation. Systems which provide steering signals for flight directors or autopilots should provide automatic vertical maneuver anticipation and a waypoint alert which 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. If this acknowledgment is stored by the equipment, acceptance of the acknowledgment should not precede the indication.
The timing of acknowledgment of an impending vertical path change is dependent 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.
If a Capability for Parallel Offset Tracks 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 in paragraph 6b(10).
(12) The Equipment Should Be Capable of Providing correct vertical path guidance when interfaced with lateral navigation equipment providing turn anticipation. The VNAV equipment should recognize the angle bisector of the lateral track change as the "zero distance to waypoint" location.
c. Software Changes. The provisions of this paragraph apply to VNAV equipment which utilizes a digital computer to provide navigation information for system monitoring. The computer program (software) operates the computer and provides the basic functions of these systems. The software for navigation functions of VNAV equipment described in paragraph 6b (for VNAV equipment used for IFR operations) should be verified and validated to at least the level 2 requirements as defined by RICA/D0-178A. Any changes to software which affects navigation functions are considered to be major changes to the equipment.
Unless software partitioning has been previously established, any change to Level 1 or level 2 software of VNAV equipment should be verified and validated to the appropriate level and should be demonstrated as not having inadvertently affected the remaining navigation functions. Changes to software used for VNAV equipment limited to VFR use or equipment having established partitioning from navigational functions software which provides navigational functions in IF systems are considered to be minor and do not require prior approval by the FAA, providing the manufacturer of the VNAV equipment has a software configuration management and quality assurance plan approved by the FAA. All software changes must be identified on the outside of the associated line replaceable unit in accordance with the criteria of RICA/DO-178A. Software changes in TSO approved equipment must be reported to the cognizant Aircraft Certification Office. If the equipment displays a software identifier to the flightcrew, the airplane or rotorcraft flight manual (or appropriate placard) should indicate the approved identitier. Software changes incorporated in equipment already installed in an aircraft may require additional evaluation and possible flight manual revision prior to returning the aircraft to service, depending upon the scope of the change.
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9/12/88 AC 20-129
7. EQUIPMENT INSTALLATION CONSIDERATIONS FOR USE UNDER IFR.
Location of the VNAV System Display. Each display element, used as a primary flight instrument in the guidance and control of the aircraft, should be located where it is clearly visible to the pilot with the least practicable deviation from the pilot's normal position and line of vision when looking forward along the flight path.
b. Failure Protection. Any probable failure of the airborne VNAV system should not degrade the normal operation of other required equipment or create a flight hazard. Normal operation of the VNAV equipment installation should not adversely affect the performance of other aircraft equipment.
c. Environmental Conditions. The aircraft environment in which the VNAV system is installed should be found to be compatible with the envirormental categories to which the equipment was tested.
d. Electromagnetic Interference. The VNAV system should not be the source of objectionable electromagnetic interference, nor be adversely affected by electromagnetic interference from other equipment in the aircraft.
e. Dynamic Responses. The system should indicate aircraft vertical position to the accuracy specified in paragraph 8 within five seconds after any maneuvering or changes in attitude encountered in normal operations.
f. System Controls. The system controls should be arranged to provide adequate protection against inadvertent system turnoff. The controls for system operation should be readily accessible to, and useable by, the flightcrew and be visible under all expected lighting conditions, including night and direct sunlight.
g. System Tests. The initial approval of a VNAV system for IFR use involves extensive testing to demonstrate operational performance, environmental qualifications, etc. When no autopilot/flight director interface is provided, subsequent installations of the same VNAV system in other aircraft need only be tested to the extent necessary to demonstrate proper operation of interfacing navigation and display equipment, satisfactory clearance of electromagnetic interference (EMI), and functional check of the VNAV equipment. For those systems interfaced with an autopilot/flight director system, each VNAV/aircraft combination should be carefully evaluated using the guidelines presented in this advisory circular.
h. Manufacturer's Instructions. VNAV equipment should be installed in accordance with instructions and limitations provided by the manufacturer of the equipment.
8. SYSTEM ACCURACY.
En Route, Terminal and Approach IFR Operation in the NAS. The error of the airborne VNAV equipment, excluding altimetry, should be less than that shown below on a 99.7 percent probability basis:
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Level Flight Segments Climb/Descent Along and Climb/Descent Intercept Specified Vertical Altitude Region of Specified Altitudes Profile (angle) (ft) (£Ł) At or below 5,000 ft 50 100 5,000 ft to 10,000 ft 50 150 Above 10,000 ft 50 220 NOTES:
1. Maximum operating altitudes to be predicated on compliance with total accuracy tolerance.
2. VNAV guidance may be used in level flight en route as in the case of altitude hold control laws, which are integrated with speed control laws to provide an energy trade. The incremental error component contributed by the VNAV equipment must be offset by a corresponding reduction in other error components, such as flight technical error, to ensure that the total error budget is not exceeded.
Altimetry Error. Refers to the electrical output and includes all errors attributable to the aircraft altimetry installation including position effects resulting from normal aircraft flight attitudes. In high performance aircraft, it is expected that altimetry correction will be provided. Such correction should be done automatically. In lower performance aircraft, upgrading of the altimetry system may be necessary.
4. VNAV Equipment Error. Includes all errors resulting from the vertical guidance equipment installation. Does not include errors of the altimeter system but does include any additional errors resulting from the addition of the VNAV equipment. This error component may be zero in level en route flight if the operation is limited to guidance by means of the altimeter only. It should not be disregarded in terminal and approach operations where the pilot is expected to follow the VNAV indications.
b. Flight Technical (Pilotage) Errors. With satisfactory displays of vertical guidance information, consensus estimates based upon experience and some limited testing indicate that flight technical errors can be expected to be less than the values shown below on a three-sigma basis.
Level Flight Segments Climb/Descent Along and Climb/Descent Intercept Specified Vertical Altitude Region of Specified Altitudes Profile (angle) (£t) (Et) At or below 5,000 ft 150 200 5,000 ft to 10,000 ft 240 300 Above 10,000 It 240 300 Par. 8
Sufficient flight tests of the installation should be conducted to verify that these values can be maintained. Smaller values for flight technical errors should not be expected, unless the VNAV system is to be used only when coupled to an autopilot; however, at least the total system vertical accuracy shown If an installation results in larger flight technical errors, the total vertical error of the system (excluding altimetry) should be determined by combining equipment and flight technical errors using the root sum square (RSS) method.
Level Flight Segments Climb/Descent Along and Climb/Descent Intercept Specified Vertical Altitude Region of Specified Altitudes Profile (angle) (£t) (£t) At or below 5,000 ft 158 224 5,000 ft to 10,000 ft 245 335 Above 10,000 it 245 372
9. IFR AIRWORTHINESS APPROVAL. There are two types of approval which differ greatly as to test requirements and data analysis.
a. First-Time Airworthiness Approval. This type of approval refers to the very first time an applicant presents a particular model VNAV equipment for FAA airworthiness installation approval and certification for an IFR navigation system. Any new models of VNAV equipment by the same manufacturer should undergo the same approval process as the original equipment unless it can be shown by analysis and tests that the new model will function as well or better than the approved equipment. A first-time approval is conducted in three phases:
(1) Lab/Bench Tests and Equipment Data Evaluation. This phase consists of the following:
(i) Analysis of the manufacturer's procedures for verification and validation of software and review of supporting documentation in accordance with the guidelines of RICA/DO-178A.
(ii) Verification of compliance with appropriate environmental qualification standards such as RTCA/DO-160B.
(iii) Examination of the equipment's display capabilities with emphasis on warning, caution, and advisory annunciations.
(iv) Analysis of failure modes.
(v) Review of reliability data to establish that all probable failures are detected.
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(vi) Evaluation of the ease of use of the controls and of the viewing ease of the displays and annunciations from a human factors point of
(vii) Review of installation and maintenance manuals.
(viii) Evaluation of operator's manual (pilot's guide).
(2) Aircraft Installation Data Evaluation. Nonnally the manufacturer of the VNAV equipment will provide an aircraft as a test bed for a first-time installation approval. The first-time installation approval will serve as a basis for subsequent installation approvals on the same type of aircraft incorporating the same autopilot/flight director system and for other aircraft installations not incorporating autopilot/flight director interface with the VNAV equipment. The following assessments are to be made:
(i) Review of installation drawings, wiring diagrams, and descriptive wiring routing.
(ii). Examination of a cockpit layout of the installed equipment with emphasis on equipment controls, applicable circuit breakers (labels and accessibility), switching arrangement and related indicators, displays, annunciators, etc.
(iii) Analysis of a data flow diagram in order to review which equipment transmits what data to which other equipment.
(iv) Review of a structural analysis of the equipment installation in order to ascertain whether all system components are satisfactorily attached to the basic aircraft structure.
(v) Examination of an electrical load analysis in order to verify that the added electrical power requirements of the system installation will not cause overloading of the aircraft's electrical generating capacity.
(vi) Evaluation of the integration with the associated lateral navigation equipment.
(3) Flight Test Evaluations. Flight tests are conducted in two stages:
(i) Functional Flight Tests consist of:
A Evaluation of all operating modes of the VNAV equipment.
B Examination of the interface (function) of other equipment connected to the VNAV system.
C Review of various failure modes and associated annunciations such as loss of electrical power, loss of lateral navigation signals, VNAV equipment failures, etc.
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9/12/88 AC 20-129 D Evaluation of steering response while autopilot is coupled to the VNAV equipment during a variety of lateral and vertical track changes including selection of VNAV modes while operating at the maximum and minimum speeds approved for VNAV use.
E Evaluation of displayed VNAV navigation parameters on interfaced flight deck instruments such as HSI, CDI, etc.
F Assessment of all switching and transfer functions pertaining to the VNAV equipment including high-power electrical loads and electrical bus switching.
G Evaluation to determine whether there exists any electromagnetic or radio frequency interference between the VNAV equipment installation and other onboard equipment, or vice versa.
H Evaluation of the accessibility of all controls pertaining to the VNAV system installation.
Evaluation of the visibility of the displays and annunciators pertaining to the VNAV system installation during day and night lighting conditions. No distracting cockpit glare or reflections may be introduced.
J Analysis of crew workload when operating the VNAV equipment.
certification of each VNAV system to be used for IFR operations should be based on a demonstration of system performance by recording the VNAV equipment vertical guidance and comparing it to the actual position of the aircraft along a pre-established vertical flight path. This evaluation can be made by use of theodolite observation, comparison against Instrument Landing System (or microwave landing system (MLS)) glide path signals, or by equivalent measuring techniques. Data should be gathered using a variety of descent/ascent rates, angles, and lateral navigation source inputs available to the VNAV system. The data should demonstrate that the appropriate accuracy criteria of paragraph 8 are met on a 99.7 percent probability basis. Flights into known poor performance areas, outside system angle and/or rate limits, using large (greater than 100 degrees) lateral track changes, etc. should be evaluated to verify proper operation of caution indications and lateral navigation interface.
Normal flight maneuvers should not cause loss of system sensor inputs and the system dynamic response should be confirmed. Any unusual flight technical errors or errors resulting from use of the autopilot and flight director should be evaluated and examined.
b. Follow-On Airworthiness Installation Approvals. This type of approval refers to installation approvals of the system in the same type of aircraft with the same autopilot/flight director integration and for other aircraft installations not incorporating autopilot/flight director interface with the VNAV equipment. Installation of VNAV systems in different types of aircraft where autopilot/flight director interface is provided or where a different autopilot/flight director is interfaced with the VNAV equipment require Par. 9 11
engineering approval. Follow-on approvals may use the first time airworthiness approval, which was either a TC or an STC, as a basis for installation approval.
Follow-on installation approvals may be accomplished by TC or STC, or may be in the form of a field approval on an FAA Form 337. The applicant or installing agency requesting a follow-on VNAV system installation utilizing this method of data approval should:
(1) Contact either the manufacturer or organization responsible for obtaining the first-time airworthiness approval in order to:
(i) Obtain a sample airplane flight manual (AFM) or rotorcraft flight manual (RFM) supplement (or supplemental flight manual, as appropriate).
(ii) Obtain verification of the equipment approval status, including the software program identification and lateral navigation equipment that may be interfaced with the system.
(iii) Discuss any problem areas and seek assistance in their solution.
(2) Conduct a similar data evaluation as outlined in paragraph 9a(2).
(3) Conduct flight evaluations similar to the flight tests outlined in , paragraph 9a(3) (i).
(4) Verify that the maximum expected groundspeed of the aircraft is less than the maximum operating speed for which the VNAV equipment is qualified.
(5) Spot check VNAV performance by conducting an evaluation flight incorporating at least three ILS approaches (or MLS comparing VNAV vertical guidance to that provided by the IIS glideslope) and three nonprecision approaches to a specitied minimum descent altitude (not a descent angle).
Accuracy should be verified by comparison to the ILS (MLS) glideslope indication and altimeter reading. The purpose of this test flight is not to validate VNAV navigation error values, but rather to verify that in the course of this installation nothing was done to compromise the accuracy of the system as determined by the first-time approval. Significant differences between the vertical path data provided by the ILS (MLS) glideslope and/or altimeter and VNAV equipment should be evaluated to determine possible causes. If a logical explanation is not available, additional flight test data points should be collected. If such error (s) persists, approval of the installation should be withheld.
10. OPERATIONAL CONSIDERATIONS.
a. Operation in the NAS. VNAV equipment may be used to provide supplemental vertical path guidance to aid in more efficient and precise operation of the aircraft. The altimeter shall always be the primary altitude reference for all flight operations.
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