Appendix A A-4 A.1.3 An Explanation of W/δ.
A.1.3.1 Throughout this Appendix, there are multiple references to the performance parameter W/δ. The following discussion is provided for the benefit of readers who may not be familiar with the use of this parameter.
A.1.3.2 It would be difficult to show all of the gross weight, altitude, and speed conditions constituting the RVSM envelope(s) on a single plot. This is because most of the speed boundaries of the envelopes are a function of both altitude and gross weight. As a result, a separate chart of altitude vs. Mach would be required for each aircraft gross weight. Aircraft performance engineers commonly use the following technique to solve this problem.
A.1.3.3 For most aircraft with RVSM altitude capability, the required flight envelope can be collapsed to a single chart, with good approximation, by use of the parameter W/δ (weight divided by atmospheric pressure ratio). This fact is due to the relationship between W/δ and the fundamental aerodynamic variables M and lift coefficient as shown below.
W/δ =1481.4 CL M2 SREF where δ = ambient pressure at flight altitude divided by sea level standard pressure of 29.92126 inches Hg.
W/δ = Weight over Atmospheric Pressure Ratio.
CL = Lift Coefficient (CL = L/qSREF).
L = Lift (in cruise flight L is equal to W).
q = Dynamic Pressure, q = 1481.4M2 δ.
Dynamic pressure is in the form of lbs/ft2.
M = Mach number.
SREF = Reference Wing Area in square feet.
W is the weight in pounds.
A.1.3.4 As a result, the flight envelope may be collapsed into one chart by simply plotting W/δ, rather than altitude, versus Mach number. Since δ is a fixed value for a given altitude, weight can be obtained for a given condition by simply multiplying the W/δ value by δ.
A.1.3.5 Over the RVSM altitude range, it is an accurate approximation to assume that position error is uniquely related to Mach number and W/δ for a given aircraft.
Appendix A A-5 A.2 RVSM Flight Envelopes.
A.2.1 General. For the purposes of RVSM approval, the aircraft flight envelope is considered in two parts: 1) the full RVSM envelope, and 2) the basic RVSM envelope. The basic RVSM envelope is the part of the flight envelope where aircraft operate the majority of time. The full RVSM envelope is the entire range of operational Mach numbers, W/δ, and altitude values over which the aircraft is operated within RVSM airspace. In general, the full RVSM envelope comprises parts of the flight envelope where the aircraft operates less frequently and where a larger ASE tolerance is allowed.
A.2.2 Full RVSM Envelope. The full RVSM envelope will comprise the entire range of operational Mach number, W/δ, and altitude values over which the aircraft can operate within RVSM airspace. Table A.2-1 establishes the parameters to consider.
Table A.2-1. Full RVSM Envelope Boundaries Lower Boundary is defined by:
Upper Boundary is defined by:
Altitude FL 290 The lower of the following:
• FL 410
• Airplane maximum certified altitude
• Altitude limited by: cruise thrust; buffet; other aircraft flight limitations Mach or Speed The lower of the following:
• Maximum endurance (holding) speed
• Maneuver speed The lower of the following:
• MMO/VMO (maximum operating speed (Mach/velocity))
• Speed limited by: cruise thrust; buffet; other aircraft flight limitations Gross Weight The lowest gross weight compatible with operation in RVSM airspace The highest gross weight compatible with operation in RVSM airspace A.2.3 Basic RVSM Envelope. The boundaries for the basic RVSM envelope are the same as those for the full RVSM envelope except in regard to the upper Mach boundary.
A.2.3.1 For the basic RVSM envelope, the upper Mach boundary may be limited to a range of airspeeds over which the aircraft Group can reasonably expect to operate most frequently. The manufacturer or design organization should define this boundary for each aircraft Group. It may be defined as equal to the upper Mach/airspeed boundary defined for the full RVSM envelope or a specified lower value. This lower value should not be less than the Long
Appendix A A-6 Range Cruise Mach number plus .04 Mach unless limited by available cruise thrust, buffet, or other aircraft flight limitations.
A.2.3.2 Long Range Cruise Mach number is the Mach for 99 percent of best fuel mileage at the particular W/δ under consideration.
A.3 Group and Non-Group Aircraft.
A.3.1 Group Aircraft. Aircraft comprising a Group must be of nominally identical design and build with respect to all details that could influence the accuracy of the height-keeping performance. The following conditions should be met:
1. Aircraft should be approved by the same TC, TC amendment, or Supplemental Type Certificate (STC), as applicable.
2. For derivative aircraft, it may be possible to use the database from the parent configuration to minimize the amount of additional data required to show compliance. The extent of additional data required will depend on the nature of the changes between the parent aircraft and the derivative aircraft.
3. The static system of each aircraft should be installed in a nominally identical manner and position. The same SSEC should be incorporated in all aircraft of the Group.
4. The avionics units installed on each aircraft to meet the minimum RVSM equipment requirements (paragraph A.4) should be manufactured to the manufacturer’s same specification, and have the same equipment part number and software part number (or version and revision).
Note: Aircraft which have avionic units which are of a different manufacturer or equipment part number, software part number (or version and revision) may be considered part of the Group if the applicant demonstrates to the appropriate FAA office this standard of avionic equipment provides identical or better system performance.
5. The airframe manufacturer or design organization produced or provided the RVSM data package.
A.3.2 Non-Group Aircraft. If an airframe does not meet the conditions of paragraph A.3.1 to qualify as a member of a Group or is presented as an individual airframe for approval, then it must be considered as a Non-Group aircraft for the purposes of RVSM approval.
A.4 Aircraft Systems—Group and Non-Group Aircraft.
A.4.1 Equipment for RVSM Operations. The minimum equipment fit should be as presented below. Additional examples of aircraft systems found on older, “legacy” airframes are presented in paragraph A.6.
Appendix A A-7 A.4.1.1 Two Independent Altitude Measurement Systems. Each system should be comprised and configured with the following elements:
A.4.1.1.1 Static Sources. Cross-coupled static source/system, provided with ice protection if located in areas subject to ice accretion.
A.4.1.1.2 Altitude Display. Equipment for measuring static pressure sensed by the static source, converting it to pressure altitude and displaying the pressure altitude to the flightcrew.
A.4.1.1.3 Altitude Reporting. Equipment for providing a digitally coded signal corresponding to the displayed pressure altitude, for automatic altitude reporting purposes. The pressure altitude from which the signal is derived must meet the requirements of paragraphs A5.2.1 and A5.2.2, or A5.3.2, as appropriate.
A.4.1.1.4 Altimetry System Components. The altimetry system should comprise all those elements involved in the process of sampling free stream static pressure and converting it to a pressure altitude output. The elements of the altimetry system fall into two main groups:
• Airframe plus static sources (pitot-static probe/static port), including the area around the static sources in the system design that must be maintained.
• Avionics and/or instruments.
A.4.1.1.5 Altimetry System Accuracy. The total altimetry system accuracy should satisfy the requirements of paragraphs A.5.2.1 and A.5.2.2, or A.5.3.2, as appropriate.
A.4.1.1.6 Static Source Error Correction (SSEC). If the design and characteristics of the aircraft and altimetry system are such that the standards of paragraphs A.5.2.1 and A.5.2.2, or A.5.3.2, are not satisfied by the location and geometry of the static sources alone, then suitable SSEC should be applied automatically within the avionic part of the altimetry system. The design aim for SSEC, whether aerodynamic/geometric or avionic, should be to produce a minimum residual SSE, but in all cases it should lead to satisfaction of the standards of paragraphs A.5.2.1 and A.5.2.2, or A.5.3.2, as appropriate.
A.4.1.1.7 Output to the Automatic Altitude Control and Altitude Alert Systems. The altimetry system equipment fit should provide reference signals for automatic altitude control and alerting at selected altitude. These signals should be derived from an altitude measurement system meeting the full requirements of Appendix A. The output may be used either directly or combined with other sensor signals. If SSEC is necessary in order to satisfy the requirements of paragraphs A.5.2.1 and A.5.2.2, or A.5.3.2, then an
Appendix A A-8 equivalent SSEC must be applied to the altitude control output. The output may be an altitude deviation signal, relative to the selected altitude, or a suitable absolute altitude output. Whatever the system architecture and SSEC system, the difference between the output to the altitude control system and the altitude displayed must be minimal.
A.4.1.1.8 System Safety Analysis. During the RVSM approval process, it must be verified analytically that the predicted rate of occurrence of undetected altimetry system failures does not exceed 1 x 10-5 per flight hour. All failures and failure combinations whose occurrence would not be evident from cross-flight deck checks, and which would lead to altitude measurement/display errors outside the specified limits, need to be assessed against this budget. No other failures or failure combinations need to be considered.
A.4.1.1.9 Air Data Systems and Configurations with Multiple Static Source Inputs.
Many aircraft are produced with air data systems making use of three or more static source inputs, and/or three or more air data computers (ADC).
Such systems (often referred to as “triplex” systems or “voting” schemes) are designed with integrated algorithms that monitor and compare the pressures sensed at the static sources. Sources providing “good” pressure values are used in the calculation of corrected altitude. Such configurations are acceptable provided at least two air data systems meet the requirements of paragraphs A4.1.1.1 through A4.1.1.8. Upon failure of one air data system, a second system must remain fully functional in compliance with the requirements of paragraphs A4.1.1.1 through A4.1.1.8.
A.4.1.2 One Secondary Surveillance Radar (SSR) Altitude Reporting Transponder. Any transponder meeting or exceeding the requirements of Technical Standard Order (TSO) C74( ) or TSO C112( ), as applicable, in accordance with the operational regulations under which the airplane is approved.
• An aircraft may be equipped with one or more transponders. If only one is fitted, it should have the capability for switching to obtain input from either altitude measurement system.
A.4.1.3 An Altitude Alert System. The altitude alert system should be capable of operation from either of the two required independent altitude measurement systems. The altitude alert system may be comprised of one or more line replaceable units (LRU), or it may be an integral part of a flight management system or flight management computer.
Appendix A A-9
1. The altitude deviation warning system should signal an alert when the altitude displayed to the flightcrew deviates from selected altitude by more than a nominal value.
• For aircraft for which application for type certification or major change in type design is on or before April 9, 1997, the nominal value must not be greater than ±300 ft (±90 m).
• For aircraft for which application for type certification or major change in type design (e.g., STC) is made after April 9, 1997, the nominal value should not be greater than ±200 ft (±60 m). The overall equipment tolerance in implementing these nominal threshold values should not exceed ±50 ft (±15 m).
A.4.1.4 An Automatic Altitude Control System. The automatic altitude control system is generally comprised of an autopilot with altitude hold mode. The automatic altitude control system should be capable of operation from either of the two required independent altitude measurement systems. Paragraph 6 presents additional options for automatic altitude control configurations found on older, “legacy” aircraft.
A.4.1.4.1 As a minimum, a single automatic altitude control system should be installed which is capable of controlling aircraft height within a tolerance band of ±65 ft (±20 m) about the acquired altitude when the aircraft is operated in straight and level flight under nonturbulent, nongust conditions.
• Aircraft types for which application for type certification is on or before April 9, 1997, which are equipped with an automatic altitude control system with flight management system(FMS)/performance management system inputs allowing variations up to ±130 ft (±40 m) under nonturbulent, nongust conditions do not require retrofit or design alteration.
• If specific tuning is needed for a “legacy” autopilot to meet performance standards in RVSM airspace, this gain scheduling or tuning must not negatively impact the way the autopilot performs in other phases of flight and at non-RVSM altitudes. For example, it is common for older systems to be tuned to meet RVSM tolerance, only to realize they no longer have acceptable vertical performance on a coupled approach.
A.4.1.4.2 Where an altitude select/acquire function is provided, the altitude select/acquire control panel must be configured such that an error of no more than ±25 ft (±8 m) exists between the display selected by the flightcrew and the corresponding output to the control system.
Appendix A A-10 A.5 Altimetry System Performance.
A.5.1 General. The statistical performance statements of ICAO Doc. 9574, Manual on the Implementation of a 300 m (1,000 ft) Vertical Separation Minimum Between FL 290-FL 410 Inclusive, for a population of aircraft (see Appendix D) are translated into airworthiness standards by assessment of the characteristics of ASE and altitude control.
The following standards differ in some respects from that document, but they are consistent with the requirements of RVSM and in accordance with 14 CFR part 91 appendix G, section 2.
A.5.2 Group Approval.
A.5.2.1 The requirements in the basic RVSM envelope are as follows:
1. At the point in the basic RVSM envelope where ASEmean reaches its largest absolute value, the absolute value should not exceed 80 ft (25 m).
2. At the point in the basic RVSM envelope where ASEmean plus ASE3 SD reaches its largest absolute value, the absolute value should not exceed 200 ft (60 m).
A.5.2.2 The requirements in the full RVSM envelope are as follows:
1. At the point in the full RVSM envelope where ASEmean reaches its largest absolute value, the absolute value should not exceed 120 ft (37 m).
2. At the point in the full RVSM envelope where ASEmean plus ASE3 SD reaches its largest absolute value, the absolute value should not exceed 245 ft (75 m).
3. If necessary, for the purpose of achieving RVSM approval for an aircraft Group, an operating restriction may be established to restrict aircraft from conducting RVSM operations in areas of the full RVSM envelope where the absolute value of ASEmean exceeds 120 ft (37 m) and/or the absolute value of ASEmean plus ASE3 SD exceed 245 ft (75 m). When such a restriction is established, identify it in the data package and document it in appropriate aircraft operating manuals; however, visual or aural warning/indication systems should not be required to be installed on the aircraft.
A.5.2.3 Aircraft types for which application for type certification or major change in type design is made after April 9, 1997, should meet the criteria established for the basic envelope in the full RVSM envelope. The FAA will consider factors providing an equivalent level of safety in the application of this criteria as stated in part 21, § 21.21b(1).
Appendix A A-11 A.5.3 Non-Group Approval.
A.5.3.1 The standards of paragraphs A.5.2.1, A.5.2.2 and A.5.2.3, cannot be applied to Non-Group aircraft approval because there can be no Group data with which to develop airframe-to-airframe variability. Therefore, a single ASE value has been established that controls the simple sum of the ASEs. In order to control the overall population distribution, this limit has been set at a value less than that for Group approval.
A.5.3.2 The standard for aircraft submitted for approval as Non-Group aircraft, as defined in paragraph A.3.2 of this Appendix, is as follows:
1. For all conditions in the basic RVSM envelope:
( )m50ft160avionicscaseworstSSEsidualRe ≤+
2. For all conditions in the full RVSM envelope:
( )m60ft200avionicscaseworstSSEsidualRe ≤+
3. Worst-case avionics means that combination of tolerance values, specified by the manufacturer for the altimetry fit into the aircraft, which gives the largest combined absolute value of avionics errors.
For most systems, this may not be a fixed value over time.
A.5.3.3 An operating restriction may be established to restrict the Non-Group aircraft from conducting RVSM operations in areas of the full RVSM envelope where the requirements of paragraph A.5.3.2 cannot be met.
A.5.3.4 The ASE airworthiness standards in Appendix A, sections A.5.2 and A.5.3 should not be confused with the ASE values stated in the height keeping section 5.3. Appendix A sections A.5.2 and A.5.3 represent the ASE performance specification for RVSM airframe airworthiness certification, and is a key element of the RVSM airplane airworthiness certification process.
Section 5.3 presents performance criteria specified for the RVSM height monitoring program, which is an element of the operational quality assurance process. The monitoring program is independent of the airworthiness certification program.
A.6 Aircraft System Configurations: Older “Legacy” Airframes.
A.6.1 Background. This paragraph provides additional guidance regarding configurations found on older model airplanes (also known as “legacy” airplanes, e.g., B707, DC-8, older business jet, and turboprop aircraft, etc.) for which RVSM approval is sought.
A.6.2 Single Autopilot Installation. Paragraph A.4.1.1.7 states the air data system should provide reference signals for automatic control and alerting at selected altitude. These signals should preferably be derived from an altitude measurement system meeting the
Appendix A A-12 full requirements of this Appendix. In addition, paragraph A.4.1.1.7 states the altimetry system must provide an output which can be used by an automatic altitude control system to control the aircraft at a commanded altitude. The output may be used either directly or combined with other sensor signals. The altitude control output may be an altitude deviation signal, relative to the selected altitude, or a suitable absolute altitude output.
A.6.2.1 A distinction can be made between signals derived from an ADC and signals derived from an altitude measurement system. Paragraph A.4.1.1.7 does not mandate the need for dual ADC inputs to the automatic altitude control system.
A.6.2.2 Several airplane model types are equipped with a single autopilot installation.
In many cases, the autopilot is only capable of receiving altitude hold inputs from a single source. It has been further noted retrofitting of these autopilot installations to receive altitude hold input from additional sources
(e.g., another ADC) may yield one or more of the following problems:
1. The retrofit costs are a significant percentage of the total worth of the airframe.
2. The retrofit is not possible without replacement of the autopilot.
3. The retrofit increases air data system complexity, which in turn increases the scenarios and rates of failure.
4. Upgraded avionics (i.e., ADCs) are not available, or the vendors will not support retrofits.
A.6.2.3 There are two common avionics configurations that may meet RVSM requirements, but do not have dual ADC input to the autopilot. A general description and possible means of compliance are given below. They are:
1. Figure A.6-1, Example of Air Data System/Autopilot Configuration.
2. Figure A.6-2, Single Air Data Computer Configuration for Autopilot Input.
A.6.2.4 Independent Source for Altitude Hold Input to the autopilot.
Appendix A A-13 Figure A.6-1. Example of Air Data System/Autopilot Configuration
1. The air data sensor (ADS) is a single LRU activated in altitude hold mode when the pilot presses an ALT HOLD button, after reaching the desired cruise FL. It is not tied to either ADC or other component comprising the air data system. The ADS provides ∆H information to the autopilot so the airplane can maintain the desired altitude. In some configurations, the pilot further provides FL information to the autopilot by manually selecting the displayed altitude (either pilot’s or copilot’s).
2. Airplanes equipped with the avionics configuration shown in Figure A.6-1 may show compliance as follows:
• The airplane must maintain altitude to within ±65 ft of the acquired altitude as required by paragraph A.4.1.4.1. For RVSM compliance, the ∆H signal must be accurate enough such that the airplane maintains the required ±65 ft altitude deviation specification. This may be substantiated with flight test data and/or manufacturer’s specification data.
• The altitude alerter should function if the ADS/ADC fails. If the altitude alert function is not operational, altitude hold performance must be monitored manually.
• The ADS should be compensated such that an airspeed change at a cruise FL is not interpreted by the system as change in altitude, causing altitude hold deviation in excess of ±65 ft.
• The altimetry systems meet the RVSM accuracy requirements specified in paragraphs A.5.2.1 and A.5.2.2, or A.5.3.2, as appropriate.
• All other requirements set forth in this advisory circular (AC), as appropriate, are satisfied.
A.6.2.5 Single ADC Input to the Autopilot. On a large number of older airplane models, the avionics configuration is such that one ADC provides altitude hold information to the autopilot (Figure A.6-2). In most models, a second ADC is also present, or provisions exist so a second can be installed.
autopilot Cross-coupled static sources Cross-coupled static sources Alt Alt ADS 18001800 ADC 2 ADC 1
Appendix A A-14 Figure A.6-2. Single Air Data Computer Configuration for Autopilot Input
1. Airplanes equipped with the avionics configuration shown in Figure A.6-2 may show compliance as follows:
• The airplane must maintain altitude to within ±65 ft of the acquired altitude required by paragraph A.4.1.4.1. This may be substantiated with flight test data or manufacturer’s specification data.
• The altitude alerter should function if either air data system or ADC fails. If the altitude alert function is not operational, altitude hold performance must be monitored manually.
• If ADC1 fails, the airplane must be controlled manually until ATC contingency procedures are executed. Annunciation should be provided if the pilot deviates ±300 ft from desired altitude. This annunciation must be provided automatically by the altitude alert system. If the altitude alert system is not functioning, altitude hold performance must be monitored manually.
• The altimetry systems meet the RVSM accuracy requirements specified in paragraphs A.5.2.1 and A.5.2.2, or A.5.3.2, as appropriate.
• All other requirements in this AC, as appropriate, are satisfied.
A.6.3 Operational Restrictions. Applicants should be aware operational restrictions and/or changes may also be required for aircraft with avionics configurations shown in Figures A.6-1 and A.6-2, to meet all RVSM requirements.
A.7 Altimetry System Performance Substantiation.
A.7.1 Flight Testing: Group and Non-Group Aircraft.
A.7.1.1 Where precision flight calibrations are used to quantify or verify altimetry system performance they may be accomplished by any of the following methods. Flight calibrations should only be performed once appropriate ground checks have been completed, and the certifying authority should agree to the number of flight test conditions. Uncertainties in application of the method must be assessed and taken into account in the data package.
autopilot Cross-coupled static sources Cross-coupled static sources Alt 1 Alt 2 ADC 2 1800 1800 ADC 1
Appendix A A-15
1. Precision tracking radar in conjunction with pressure calibration of the atmosphere at test altitude.
2. Trailing cone.
3. Pacer aircraft. The pacer aircraft must have been directly calibrated to a known standard. It is not acceptable to calibrate a pacer aircraft by another pacer aircraft.
4. Any other method acceptable to the FAA or approving authority.
Note: Data acquired using elements from the RVSM monitoring program, such as a ground-based height monitoring unit (HMU) or Aircraft Geometric Height Measurement Element (AGHME), or a portable Global Positioning System (GPS)-based monitoring unit (GMU), is not acceptable for substantiating the ASE performance specified in paragraphs A.5.2 and A.5.3.
A.7.1.2 ASE will generally vary with flight condition. The data package should provide coverage of the RVSM envelope sufficient to define the largest errors in the basic and full RVSM envelopes. Note in the case of Group approval the worst flight condition may be different for each of the requirements of paragraphs A.5.2.1 and A.5.2.2, and each should be evaluated. Similarly, for Non-Group approval, the worst flight condition may be different for each of the requirements of paragraph A.5.3.2 and each should be evaluated.
A.7.2 Altimetry System Error (ASE) Variability.
A.7.2.1 In order to evaluate a system against the ASE performance statements established by the Review of the General Concept of Separation Panel (RGCSP) (see Appendix D), it is necessary to quantify the mean and three standard deviation (SD) values for ASE, expressed as ASEmean and ASE3 SD. In order to do this, it is necessary to account for the different ways in which variations in ASE can arise. The factors affecting ASE are as follows and should be considered in the ASE evaluation:
A.7.2.1.1 Unit to unit variability of avionics.
A.7.2.1.2 Effect of environmental operating conditions on avionics.
A.7.2.1.3 Effect of transducer and/or avionics component error drift over time.
A.7.2.1.4 Effect of flight operating condition on SSE.