AMC1 ACNS.E.RVSM.035 Altimetry system accuracy

• ED Decision 2013/031/R found in: GM Remote Aerodrome Air Traffic Services (ED 2019/004/R) Issue 2 (Jul 2020)

AMC1 ACNS.E.RVSM.035 Altimetry system accuracy ED Decision 2013/031/R To demonstrate the compliance with ASE performances the following steps should be performed: (a) Group determination: (1) Aircraft should have been constructed to a nominally identical design and be approved on the same Type Certificate (TC). Aircraft modified to a TC amendment, or by a Supplemental TC may be considered as part of the same group providing that all height keeping performance characteristics as described in the following paragraphs remain the same. (2) The static system of each aircraft should be nominally identical. The Static Source Error and any applied SSE Corrections should be the same for all aircraft of the group. Differences affecting factors that contribute to the Static Source Error (see Appendix A, Table 1), that effect RVSM performances and accuracy should be demonstrated as negligible. (3) The operational flight envelope should be the same. (4) The avionics units installed on each aircraft to meet the minimum RVSM performance requirements should demonstrate equivalent height keeping system performance in relation to; altitude control, altitude reporting and the interface to the altimetry system sensors. Altimetry system integrity should be the same with equivalent reliability, degradation and failure rates. If an airframe does not meet the conditions above to qualify as a member of a Group, or is presented as an individual airframe for approval, then it will be considered as a non-group aircraft for the purposes of RVSM approval. (b) RVSM Flight envelopes boundaries (Full and Basic) The RVSM full flight envelope boundaries should be defined based on the RVSM airspace and aircraft or group aircraft characteristics as summarised in Table 1. The RVSM basic envelope boundaries are similar to the ones of the full flight envelope, however, the upper Mach boundary may be lower than the one of the full flight envelope but not be less than the Long Range Cruise Mach Number plus 0.04 Mach, unless limited by available cruise thrust, buffet or other flight limitations. This reduction in upper Mach value would typically apply to cases where airspeeds could be limited to the range of airspeeds over which the aircraft can reasonably be expected to operate most frequently. <table border="1" cellpadding="0" cellspacing="0" width="605"><tr><td valign="top" width="124"><p>Condition</p></td><td valign="top" width="244"><p>Lower Boundary is defined by</p></td><td valign="top" width="236"><p>Upper Boundary is defined by</p></td></tr><tr><td valign="top" width="124"><p>Flight Level</p></td><td valign="top" width="244"><p>FL 290</p></td><td valign="top" width="236"><p>The lower of :</p><p>· FL 410</p><p>· Aircraft maximum certified altitude</p><p>· Altitude limited by: cruise thrust; buffet; other aircraft flight limitations</p></td></tr><tr><td valign="top" width="124"><p>Mach or Speed</p></td><td valign="top" width="244"><p>The lower of:</p><p>· Maximum endurance (holding speed)</p><p>· Manoeuvre speed</p></td><td valign="top" width="236"><p>The lower of :</p><p>· MMO/VMO</p><p>· Speed limited by cruise thrust; buffet; other aircraft flight limitations</p></td></tr><tr><td valign="top" width="124"><p>Gross Weight</p></td><td valign="top" width="244"><p>· The lowest gross weight compatible with operations in RVSM airspace</p></td><td valign="top" width="236"><p>· The highest gross weight compatible with operations in RVSM airspace</p></td></tr></table> TABLE 1 - Full RVSM envelope boundaries (c) Test performance results presentation: The test performance results may be presented on a single chart if the RVSM flight envelope is plotted using W/δ (weight divided by atmospheric pressure ratio) versus Mach number. Note: This is due to the relationship between W/δ and the fundamental aerodynamic variables M and lift coefficient as shown below. W/δ = 1481.4CLM2 SRef, where: δ = ambient pressure at flight altitude divided by sea level standard pressure of 1013.25 hPa W/δ = Weight over Atmospheric Pressure Ratio CL = Lift Coefficient M = Mach number SRef = Reference Wing Area 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. Furthermore, over the RVSM altitude range, it is a good approximation to assume that position error is uniquely related to Mach number and W/δ for a given aircraft. (d) Error budget The demonstration of compliance with the RVSM performance criteria should include a justification of the contribution of all significant errors to the ASE (Error Budget). Appendix A provides guidance supporting the development of such justification. *Note: A trade-off may be made between the various error sources which contribute to ASE (e.g.: in the case of an aircraft group approval, the smaller the mean of the group and the more stringent the avionics standard, the larger the available allowance for the SSE variations). The ASE performance demonstration should consider this ASE trade off.* (e) ASE Flight Calibration Methods Where flight calibrations are used to quantify or verify altimetry system performance they should be accomplished by any of the following methods. Flight calibrations should be performed only when appropriate ground checks have been completed. Uncertainties in application of the method will need to be assessed and taken into account in the data package. (1) Precision tracking radar in conjunction with pressure calibration of atmosphere at test altitude. (2) Trailing cone. (3) Pacer aircraft. (4) Any other method acceptable to the competent authority Note: When using pacer aircraft, the pacer aircraft will need to be calibrated directly to a known standard. It is not acceptable to calibrate a pacer aircraft by another pacer aircraft. (f) Compliance Demonstration for Groups of Aircraft. Because of the statistical nature of the performance requirements, the demonstration of the compliance may vary considerably from group to group and therefore for a group aircraft the following process should be applied: (1) The mean and airframe-to-airframe variability of ASE should be established, based on flight test calibration of the accuracy for a number of aircraft. Where analytical methods are available, it may be possible to enhance the flight test data base and to track subsequent changes in the mean and variability based on geometric inspections and bench test, or any other method acceptable to the responsible authority. In the case of derivative aircraft it may be possible to use data from the parent as part of the data base, providing adequate provision is made for the changes that may contribute to difference in ASE characteristics. Note: This is particularly important when a derivative involves changes to the airframe structure that may alter the SSE characteristics. (2) An assessment of the aircraft-to-aircraft variability of each error source should be made. The error assessment may take various forms as appropriate to the nature and magnitude of the source and the type of data available. It may be acceptable to use specification values to represent three standard deviations for smaller error sources; however a more comprehensive assessment may be required for those sources that contribute a greater proportion of the overall error. Note: This assessment is particularly important for airframe error sources where specification values of ASE contribution may not have been previously established. (3) In many cases, one or more of the major ASE error sources will be aerodynamic in nature, such as variations in the airframe surface contour in the vicinity of 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. (4) An error budget should be established to ensure that the RVSM performance criteria are met. Note: the worst condition experienced in flight may differ for each criterion and therefore the component error values may also differ. (5) In showing compliance with the overall criteria, the component error sources should be combined appropriately. 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. Care should be taken that only variable component error sources that are independent of each other are combined by rss. (6) A statistical study based on a representative sample of measured data should provide sufficient confidence that each individual aircraft in the group would have an ASE contained within ±60m (±200 ft). Note: It is accepted that if any aircraft is identified as having an error exceeding ±60m (±200 ft) then it should receive corrective action. (g) Compliance Demonstration for a Non Groups Aircraft. For non-group aircraft, the following data should be established: (1) Flight test calibration of the aircraft to establish its ASE or SSE over the RVSM envelope should be conducted. The flight test calibration should be performed at points in the flight envelope(s) as agreed by the responsible authority using one of the methods identified in (e) above. (2) Calibration of the avionics used in the flight test as required may be conducted for establishing residual SSE. The number of test points should be agreed by the responsible authority. Since the purpose of the flight test is to determine the residual SSE, specially calibrated altimetry equipment may be used. (3) The installed altimetry avionics equipment specification should identify the largest allowable errors.