Appendix A A-19 A.8.2.2 SSE Components. The component parts of SSE are presented in Table A.8-2, with the factors controlling their magnitude.
A.8.2.2.1 The reference SSE is the best estimate of actual SSE, for a single aircraft or an aircraft Group, obtained from flight calibration measurements. It is variable with operating condition, characteristically reducing to a family of W/δ curves that are functions of Mach. It includes the effect of any aerodynamic compensation incorporated in the design, and once it has been determined, the reference SSE is fixed for the single aircraft or Group, although it may be revised if substantiated with subsequent data.
A.8.2.2.2 The test techniques used to derive the reference SSE will have some measurement uncertainty associated with them, even though known instrumentation errors will normally be eliminated from the data. For trailing-cone measurements, the uncertainty arises from limitations on pressure measurement accuracy, calibration of the trailing-cone installation, and variability in installations where more than one is used. Once the reference SSE has been determined, the actual measurement error is fixed, but as it is unknown, it can only be handled within the ASE budget as an estimated uncertainty.
A.8.2.2.3 The airframe variability and pitot-static probe/static port variability components arise from differences between the individual airframe and pitot-static probe/static port, and the example(s) of airframe and probe/port used to derive the reference SSE.
A.8.2.3 Residual SSE.
A.8.2.3.1 Figure A.8-1 presents the components and factors. Residual SSE consists of those error components that make actual SSE different from the reference value, components 2), 3), and 4) from Table A.8-2, plus the amount by which the actual SSEC differs from the value that would correct the reference value exactly, components 2a), 2b), and 2c) from Table A.8-3.
A.8.2.3.2 There will generally be a difference between the SSEC that would exactly compensate the reference SSE, and the SSEC that the avionics is designed to apply. This arises from practical avionics design limitations. The resulting Table A.8-3 error component 2a) will therefore be fixed, for a particular flight condition, for the single aircraft or Group. Additional variable errors 2b) and 2c) arise from those factors causing a particular set of avionics to apply an actual SSEC that differs from its design value.
A.8.2.3.3 The relationship between perfect SSEC, reference SSEC, design SSEC, and actual SSEC is illustrated in Figure A.8-4, Static Source Error/Static Source Error Correction Relationships for Altimetry System Error Where Static Line, Pressure Measurement, and Conversion Errors Are Zero, for the case
Appendix A A-20 where static line errors and pressure measurements and conversion errors are taken as zero.
A.8.2.3.4 Account for factors creating variability of SSE relative to the reference characteristic in two ways. First, as noted for the SSE itself in Table A.8-2, and second, for its effect on the application of SSEC as in factor 2a)i) of Table A.8-3. Similarly, account for the static pressure measurement error in two separate ways. The main effect will be via the “pressure measurement and conversion,” but a secondary effect will be via factor 2a)ii) of Table A.8-3.
Table A.8-2. Static Source Error (Cause: Aerodynamic Disturbance to Free-Stream Conditions) Factors Error Components Airframe Effects Operating Condition (M, Hp, ∝, β)* Geometry: Shape of airframe Location of static sources Variations of surface contour near the sources Variations in fit of nearby doors, skin panels, or other items
1) Reference SSE values from flight calibration measurements.
2) Uncertainty of flight calibration measurements.
3) Airframe to Airframe variability.
4) Probe/Port to Probe/Port variability.
Probe/Port Effects Operating Condition (M, Hp, ∝, β)* Geometry: Shape of probe/port Manufacturing variations Installation variations
*M Mach, speed;
Hp pressure altitude;
∝ angle of attack;
β yaw (sideslip).
Appendix A A-21 Table A.8-3. Residual Static Source Error (Aircraft with Avionic Static Source Error Correction) (Cause: Difference Between the Static Source Error Correction Actually Applied and the Actual Static Source Error) Factors Error Components
1) As for SSE.
PLUS
2) Source of input data for SSEC function.
a) Where SSEC is a function of Mach:
i) PS sensing: difference in SSEC from reference SSE.
ii) PS measurement: pressure transduction error.
iii) PT errors: mainly pressure transduction error.
b) Where SSEC is a function of Angle of Attack:
i) Geometric effects on alpha
• Sensor tolerances
• Installation tolerances
• Local surface variations ii) Measurement error
• Angle transducer accuracy
3) Implementation of SSEC function.
a) Calculation of SSEC from input data.
b) Combination of SSEC with uncorrected height.
1) Static Source Error Components 2),
3), and 4) from Table A.8-2.
PLUS 2a) Approximation in fitting design SSEC to flight calibration reference SSE.
2b) Effect of production variability (sensors and avionics) on achieving design SSEC.
2c) Effect of operating environment (sensors and avionics) on achieving design SSEC.
Appendix A A-22 Figure A.8-4. Static Source Error/Static Source Error Correction Relationships for Altimetry System Error Where Static Line, Pressure Measurement, and Conversion Errors are Zero
A.8.2.3.5 Static line errors arise from leaks and pneumatic lags. In level cruise, these can be made negligible for a system correctly designed and correctly installed.
A.8.2.3.6 Pressure Measurement and Conversion Error.
1. The functional elements are static pressure transduction, which may be mechanical, electromechanical, or solid-state, and the conversion of pressure signal to pressure altitude. The error components are:
• Calibration uncertainty;
• Nominal design performance;
• Unit to unit manufacturing variations; and
• Effect of operating environment.
SSE Perfect SSEC Reference SSEC Actual SSEC Design SSEC Actual Altitude Hp (Corrected) Residual Static Source Components 2(b) & 2(c) (see table A.8-3) Residual Static Source Error Component 2(a) (see table A.8-3) Static Source Error Components
(2), (3) and (4) (see table A.8-2) Hp (Uncorrected) Residual Static Source Error (Total)
Appendix A A-23
2. The equipment specification usually covers the combined effect of the error components. If the value of pressure measurements and conversion error used in the error budget is the worst-case specification value, then it is not necessary to assess the above components separately. However, calibration uncertainty, nominal design performance, and effect of operating environment can all contribute to bias errors within the equipment tolerance. Therefore, if it is desired to take statistical account of the likely spread of errors within the tolerance band, it will be necessary to assess their likely interaction for the particular hardware design under consideration.
3. It is particularly important to ensure the specified environmental performance is adequate for the intended application.
A.8.2.3.7 Baro-Setting Error is defined as the difference between the value displayed and the value applied within the system. For RVSM operation, the value displayed should always be International Standard Atmosphere (ISA) standard ground pressure, but setting mistakes, although part of TVE, are not components of ASE.
1. The components of the Baro-Setting Error are:
• Resolution of setting knob/display (“Setability”);
• Transduction of displayed value; and
• Application of transduced value.
2. The applicability of these factors and the way they combine depends on the particular system architecture.
3. For systems in which the display is remote from the pressure measurement function there may be elements of the transduction and/or application or transduced value error components arising from the need to transmit and receive the setting between the two locations.
A.8.2.3.8 Imperfect conversion from altitude signal to display causes display error.
The components are:
1. Conversion of display input signal;
2. Graticule/format accuracy; and
3. Readability.
Note: In self-sensing altimeters, the first of these would normally be separate from the pressure measurement and conversion error.
A.8.3 ASE Component Error Budget: Group Approval. Where approval is sought for an aircraft Group, the data package must be sufficient to show the requirements of paragraphs A.5.1 and A.5.2 are met. Because of the statistical nature of these requirements, the content of
Appendix A A-24 the data package may vary considerably from Group to Group. Paragraph A.8 should serve as a guide to properly account for ASE sources.
A.8.3.1 Establish the mean and airframe-to-airframe variability of ASE based on precision flight test calibration of a number of aircraft. Where analytical methods are available, it may be possible to enhance the flight test database and to track subsequent change in the mean and variability based on geometric inspections and bench test or any other method acceptable to the approving authority. In the case of derivative aircraft, it may be possible to utilize data from the parent as part of the database. (For example, a fuselage stretch where the only difference in ASEmean between Groups could be reliably accounted for by analytical means.)
A.8.3.2 All avionics equipment contributing to ASE must be identified by function and part number. The applicant must demonstrate the avionics equipment can meet the requirements established according to the error budget when operating the equipment in the environmental conditions expected to be met during RVSM operations.
A.8.3.3 Assess the aircraft-to-aircraft variability of each error source. The error assessment may take various forms as appropriate to the nature and magnitude of the source and the type of data available. For example, for some error sources (especially small ones) it may be acceptable to use specification values to represent 3 SD. For other error sources (especially larger ones) a more comprehensive assessment may be required; this is especially true for airframe error sources where “specification” values of ASE contribution may not have been previously established.
A.8.3.4 In many cases, one or more of the major ASE sources will be aerodynamic in nature (such as variations in the aircraft surface contour near the static pressure source). If evaluation of these errors is based on geometric measurements, substantiation should be provided that the methodology used is adequate to ensure compliance. (See paragraph A.9, Figure A.9-2, Compliance Demonstration Ground-To-Flight Test Correlation Process Example.)
A.8.3.5 In showing compliance with the overall requirements, combine the component error sources in an appropriate manner. In most cases, this will involve the algebraic summation of the mean components of the errors, root sum square (RSS) combination of the variable components of the errors, and summation of the RSS value with the absolute value of the overall mean. Be sure the RSS combines only variable component error sources independent of each other.
A.8.3.6 The methodology described above for Group approval is statistical in nature.
This is the result of the statistical nature of the risk analysis and the resulting statistical statements of Appendix D, paragraphs D.1.5.1 and D.1.5.2. In the context of a statistical method, the statements of Appendix D, paragraph
Appendix A A-25 D.1.5.3 required reassessment. This item states, “Each individual aircraft in the Group must be built to have ASE contained within ±200 ft.” This statement does not mean every airframe should be calibrated with a trailing cone or equivalent to demonstrate ASE is within 200 ft. Such an interpretation would be unduly onerous considering the risk analysis allows for a small proportion of aircraft to exceed 200 ft. However, if any aircraft is identified as having an error exceeding ±200 ft then it should receive corrective action.
A.8.4 ASE Component Error Budget: Non-Group Approval. Where an aircraft is submitted for approval as a Non-Group aircraft, the data should be sufficient to show the requirements of paragraph A.5.3.2 are met. The data package should specify how the ASE budget has been allocated between residual SSE and avionics error. The operator and the FAA should agree on what data will satisfy approval requirements. The following data should be acquired and presented:
1. Calibration of the avionics used in the flight test as required establishing actual avionics errors contributing to ASE. Since the purpose of the flight test is to determine the residual SSE, specially calibrated altimetry equipment may be used.
2. All avionics equipment contributing to ASE must be identified by function and part number. The applicant must demonstrate the avionics equipment can meet the requirements established according to the error budget when operating the equipment in the environmental conditions expected during RVSM operations.
3. Specifications for the installed altimetry avionics equipment indicating the largest allowable errors must be presented. The error sources shown in paragraph A.7.2.1.1 through A.7.2.1.5 are necessary elements of the altimetry system component error budget for a Non-Group aircraft.
A.9 Establishing and Monitoring SSEs.
A.9.1 General. Paragraph A.8.3.4 requires the methodology used to establish the SSE be substantiated. Further, maintenance procedures must be established to ensure conformity of both newly manufactured airplanes, and those with in-service history. There may be many ways of satisfying these requirements; two examples are included below.
A.9.1.1 Example 1. Group Aircraft.
A.9.1.1.1 One process for showing compliance with RVSM requirements is shown in Figure A.9-1, Process for Showing Initial and Continued Compliance of Airframe Static Pressure System. Figure A.9-1 illustrates flight test calibrations and geometric inspections will be performed on a given number of aircraft. The flight calibrations and inspections will continue until a correlation between the two is established. Geometric tolerances and SSEC will be established to satisfy RVSM requirements. For aircraft being manufactured, every Nth aircraft will be inspected in detail and every Mth
Appendix A A-26 aircraft will be flight test calibrated, where N and M are determined by the manufacturer and agreed to by the approving authority. The data generated by N inspections and M flight calibrations must be used to track the mean and 3 SD values to ensure continued compliance of the model with the requirements of paragraphs A.5.2.1 and A.5.2.2. As additional data are acquired, they should be reviewed to determine if it is appropriate to change the values of N and M as indicated by the quality of the results obtained.
A.9.1.1.2 There are various ways in which the flight test and inspection data might be used to establish the correlation. The example shown in Figure A.9-2 is a process in which each of the error sources for several airplanes is evaluated based on bench tests, inspections, and analysis. Correlation between these evaluations and the actual flight test results would be used to substantiate the method. A highly favorable correlation may be used to augment flight test data, and if appropriate, mitigate the need to conduct periodic flight tests (every Mth aircraft) as presented in paragraph A.9.1.1.1 above.
A.9.1.1.3 The method illustrated in Figures A.9-1 and A.9-2 is appropriate for new models since it does not rely on any preexisting database for the Group.
A.9.1.2 Example 2. Group Aircraft. Figure A.9-3, Process for Showing Initial and Continued Compliance of Airframe Static Pressure Systems for In-Service and New Model Aircraft, illustrates flight test calibrations should be performed on a given number of aircraft and consistency rules for air data information between all concerned systems verified. Geometric tolerances and SSEC should be established to satisfy the requirements. A correlation should be established between the design tolerances and the consistency rules. For aircraft being manufactured, air data information for all aircraft should be checked in terms of consistency in cruise conditions and every Mth aircraft should be calibrated, where M is determined by the manufacturer and agreed to by the approving authority. The data generated by the M flight calibrations should be used to track the mean and 3 SD values to ensure continued compliance of the Group with the requirements of paragraphs A.5.2.1 and A.5.2.2.
A.9.1.3 Non-Group Aircraft. Where airworthiness approval has been based on flight tests, the continuing integrity and accuracy of the altimetry system must be demonstrated by periodic ground and flight tests of the aircraft and its altimetry system at periods to be agreed with the approving authority.
However, exemption from flight test requirements may be granted if the applicant can adequately demonstrate the relationship between any subsequent airframe/system degradation and its effects on altimetry system accuracy is understood and adequately compensated/corrected for.
Appendix A Figure A.9-1. Process for Showing Initial and Continued Compliance of Airframe Static Pressure System Flight test calibration Geometric inspections of all aircraft Number of aircraft as required flight tested (or more as required) to to meet the objective below meet objective below
OBJECTIVE OF INITIAL CALIBRATIONS AND INSPECTIONS
1. Establish correlation between geometric inspections and flight calibrations.
2. Establish geometric tolerances and SSEC necessary to show compliance with RVSM requirements.
Inspect each aircraft until confidence of geometric compliance is established Geometric inspection of every Nth aircraft Flight test calibrate every Mth aircraft Figure A.9-2. Compliance Demonstration Ground-To-Flight Test Correlation Process Example Conduct ADC ground Measure fuselage Fuselage flight test calibration conformance geometric conformance geometric calibration using inspection with xx ? Yes tool No Perform an Remove ADC calibration Rework analysis to estimate error airplane position error Residual Position Error Correlation Ground Checks Combine AOA vane functional/ calibration estimated P/S probe installation/ alignment component Flush static port installation error Estimated A-27
Appendix A Figure A.9-3. Process for Showing Initial and Continued Compliance of Airframe Static Pressure Systems for In-Service and New Model Aircraft Corresponding Documents and Results Tests * Identification of Static Pressure Error.
With development aircraft Flight Test Calibration (see note 1) * Certification Cards. Demonstration * Establish the SSEC Laws for the of compliance with requirements. Air Data Computers. Airworthiness Assessment * Definition of Consistency Rules.
For Each New Aircraft * Use the pre-delivery flight(s) to check the coherence of the air data information * Record data from captain's side Airworthiness Authorities Results Geometrical Inspection and NOT Theoretical Analysis.
Satisfactory?
Repairs, if necessary.
Aircraft Cruise Calibrate Every TBD Aircraft in Flight Manufacturer Responsibility Note 1: The flight test installation chosen to get the calibration data shall have an accuracy compatible with the level of performance to be demonstrated and an analysis of this accuracy shall be provided. Any possible degradation of this accuracy shall be monitored and corrected during the flight test period.
A-28
Appendix A A-29 A.10 Maintenance Requirements.
A.10.1 General. The data package must include a definition of the procedures, inspections/tests, and limits used to ensure all aircraft approved against the data package “conform to type design.” All future approvals, whether of new build or in-service aircraft, must also meet the error budget allowances developed according to paragraph A.8. The tolerances will be established by the data package and include a methodology allowing for tracking the mean and SD for new build aircraft.
A.10.1.1 Define compliance requirements and test procedures for each potential source of ASE. Ensure the error sources remain as allocated in the ASE budget.
Provide guidance for corrective action in the event of equipment, test, and/or inspection failure. Typical RVSM-specific maintenance procedures include the following:
1. Verification of avionics component part numbers.
2. Air Data System Ground Test. This is a direct assessment of altimetry system component errors and correct application of the SSEC.
3. Assessment/measurement of the skin surrounding the static sources. Skin waviness, skin splices/joints, access panels, radome fit/fair, and damage.
4. Inspection of the pitot-static probe or static port. Erosion, corrosion, damage, static port orifice degradation, static port step-height, excessive or non-homogenous paint.
5. SmartProbe©. Inspection for corrosion, erosion, damage, degradation.
A.10.1.2 RVSM-specific maintenance requirements may be necessary to ensure the automatic altitude control and altitude alerting systems meet the requirements of paragraphs A.4.1.3 and A.4.1.4. The data package should provide data to substantiate these requirements, if needed.
A.10.1.3 Where an operating restriction has been adopted, paragraphs A.5.2.2 or A.5.3.3, as appropriate, the data package should contain the data and information necessary to document and establish that restriction. The AFM, pilot operating manual, or a RVSM-specific flight manual supplement must be revised/created as necessary to reflect this restriction.
A.10.1.4 Any variation/modification from the initial installation affecting RVSM approval should be approved by the airframe manufacturer or approved design organization and allowed by the FAA to show RVSM compliance has not been compromised.
Appendix A A-30
1. Air Data System Modifications. Changes to the components comprising an RVSM-compliant air data system cannot be effectively evaluated without the development of a revised ASE budget. Such modifications must be approved by the airframe manufacturer or approved design organization.
2. Automatic Altitude Control and Altitude Alert System Modifications. Changes to components comprising an RVSM-compliant automatic altitude control or altitude alert system should be evaluated by the airframe manufacturer or approved design organization.
3. Altitude Reporting. As stated in paragraph A.4.1.2, any transponder meeting or exceeding the requirements of Technical Standard Order (TSO) C74( ), TSO C112( ), as applicable, in accordance with the operational regulations under which the airplane is approved.
4. Airframe Modifications. Over time, a RVSM-approved aircraft may become a candidate for airframe modifications, such as installation of large antennas, radomes, fairings, equipment lockers, winglets, etc. Any modification changing the exterior contour of the aircraft, or potentially impacting the air data system static sources and or pneumatic configuration, aircraft weight, and/or performance in any manner, must be evaluated by the manufacturer or design organization to ascertain the RVSM compliance status.
A.10.2 Continued Airworthiness Documentation.
A.10.2.1 Aircraft Manufacturers. Review and update the following items as appropriate to include the effects of RVSM implementation:
1. The Structural Repair Manual with special attention to the areas around the static source, angle of attack sensors and doors if their rigging can affect airflow around the previously mentioned sensors.
2. The Master Minimum Equipment List (MMEL).
A.10.2.2 Design Organizations. The RVSM airworthiness approval will generally take the form of a RVSM-specific STC. The STC should contain the following:
1. RVSM-specific maintenance instructions for initial and continued airworthiness. These maintenance instructions should include procedures ensuring all sources of ASE and aircraft systems performance degradation can be assessed and controlled.
Paragraphs A.10.1.1, A.10.1.2, and A.10.1.4 summarize key elements of RVSM-specific maintenance procedures.