Publication: Magyar Közlöny
Issue: MK-2009-104 (Year: 2009, Number: 104)
Era: 2004-2010
Section: 
Paragraph Index: 2595

c) the other aircraft is equipped with an ACAS having a collision avoidance logic identical to that of own ACAS. 6.2.7.2 The first circumstance a) ensures that the logic performs satisfactorily in encounters with an unequipped intruder. The other two circumstances both test the collision avoidance logic when the other aircraft is equipped but do so from different perspectives. Circumstance b) ensures that the logic performs satisfactorily under the constraints of the coordination process, while circumstance c) ensures that the benefits to be expected when both aircraft are equipped are realized. 6.2.7.3 The conditions applying in circumstance b) are intended to allow own ACAS to select its initial RA but to then apply the most pessimistic reasonable assumptions about the effect of the need for coordination on the performance of the own ACAS logic. When own aircraft has the lower aircraft address, the conditions of the test imply that the sense of the RA cannot be reversed. Furthermore, the intruder does not generate an RA and an RAC until the own ACAS RA is announced because an early design included an initial coordination delay (the purpose of which was to allow the coordination to complete and avoid the pilot seeing rapid changes in the RA); the intention of the requirement is to ensure that performance is satisfactory in spite of the deleterious effects of any such delay. 6.2.7.4 Circumstance c) requires that the behaviour of the two aircraft be fully cooperative, but the fact that both ACAS are using the subject logic ensures that the performance measure relates to the subject logic and that the subject logic is effective. 6.2.7.5 As discussed above, the performance specifications are intended to ensure satisfactory operation of the logic and not the system as a whole. To the extent that they are capable of wider interpretation in terms of the benefits of the system as a whole in an operational environment, circumstance c) might be thought to provide the more credible performance measure for ACAS-ACAS encounters. The specified performance of the logic in circumstance b) is worse than that where the intruder is not equipped, because circumstance b) invokes only the constraints imposed by coordination. However, the fact that the cooperation of an intruder cannot be guaranteed and that some pilots will fail to respond to RAs on occasion means that all three measures have operational relevance. Annex 10 — Aeronautical Communications Volume IV 22/11/07 ATT-62 6.3 Reduction in the risk of collision 6.3.1 STATUS OF THE LOGIC RISK RATIO 6.3.1.1 The risk ratio calculated for the purposes of Chapter 4, 4.4.3 is a measure of the performance of the logic and not the ACAS as a whole. For example, ACAS can prevent a collision by prompting the pilot to carry out a successful visual search for the intruder and it can fail because a track is not established or the pilot ignores the RA; these are aspects of the total system that are not reflected in the calculations required for Chapter 4, 4.4.3. 6.3.1.2 When considering the relevance of the “logic risk ratio” figures calculated for Chapter 4, 4.4.3 to operations or policy decisions, it might be helpful to regard them as solely the reliability that can be attached to RAs. They express the effect that following an RA will have on the immediate risk of collision when, at the time it is issued, the pilot has no information other than the RA on which to base a decision whether to follow the RA or ignore it. As a rough guide, the collision risk created by ACAS arises from following the RA so the logic risk ratio overstates this “induced risk ratio”; on the other hand, it also overstates the capability of ACAS to prevent collisions because of the many other failure modes in the total system. 6.3.1.3 The figures calculated for the purposes of Chapter 4, 4.4.3 are unsuitable as guidance concerning the effect of ACAS on the overall risk of collision in an airspace or faced by an airline. 6.3.2 CALCULATION OF THE LOGIC RISK RATIO 6.3.2.1 The risk ratio R can be written: probability of a collision with ACAS R probability of a collision without ACAS ∑ = ∑ where the summation is over all encounters, or, more practically, all encounters that contribute to the total risk of collision with or without ACAS. The need for the characteristics and statistics of the encounters to be representative of operational realities is standardized in Chapter 4, 4.4.2.6 and discussed in 6.2.6. 6.3.2.2 The estimated risk of collision depends on the interpretation of the word “collision”. While this problem is largely avoided by expressing the requirement in terms of the ratio between the risks of collision with and without ACAS, it is important that realistic allowance is made for the size of the largest aircraft. It would be reasonable to treat a vertical separation of less than 100 ft between the centre points of the two aircraft as if it were small enough to allow a collision. It would not be advisable to use significantly larger miss distances as approximations to collisions because it has been found that the calculated risk ratio is sensitive to the definition of “collision” even though it is a ratio. 6.3.2.3 If the approximation is made that a collision occurs when |d| < 100 ft, where d is the actual vertical separation Then prob ( ) prob ( ) d < 100 ft with ACAS R d < 100 ft without ACAS ∑ = ∑ where now the summation is over all encounters with zero or extremely small horizontal miss distance. 6.3.2.4 Now introduce e, the altimeter error and a, the apparent vertical separation and note that Attachment Annex 10 — Aeronautical Communications ATT-63 22/11/07 a = d + e a is conceptually the altitude separation as measured by altimeters. It should not be necessary to consider quantization errors because the modelled altimeter readings can be known with arbitrary precision in the computer simulations. They are quantized before they are provided to ACAS as modelled Mode C reports, which ACAS tracks. This is why the standard Chapter 4, 4.4.2 excludes quantization effects. 6.3.2.5 Define awith to be the apparent vertical separation with ACAS and awithout to be the apparent vertical separation without ACAS. Then |d| < 100 ft with ACAS if and only if |awith –e| <100 ft i.e. awith –100 ft < e < awith + 100 ft and similarly |d| <100 ft without ACAS if and only if |awithout –100 ft < e < awithout + 100 ft 6.3.2.6 Risk ratio is thus given by prob( ) prob( ) with with without without a ft e a ft R a ft e a ft − < < + ∑ = − < < + ∑ In order to use this formula to calculate risk ratio, the values of awith and awithout must be determined for a collection of encounters that is fully representative of all the potential actual encounters in which there is both a risk of collision without ACAS and a risk that ACAS will induce a collision. When these values of hypothetically measured altitude separation are known, knowledge of the errors in altitude measurement completes the calculation. 6.3.3 INDUCED AND UNRESOLVED RISK 6.3.3.1 It is not sufficient to demonstrate that ACAS will prevent collisions that might occur in its absence. The risk that ACAS logic could cause collisions in otherwise safe circumstances must be fully considered, not least because in managed airspace the number of encounters potentially facing an induced risk greatly exceeds the number of near collisions. 6.3.3.2 The upper limit on the logic risk ratio standardized at Chapter 4, 4.4.3 effectively places an approximate upper limit on the ACAS induced risk of collision. Although some other failures could cause ACAS to induce a collision, e.g. pilots manoeuvring on a TA or an RA directing the aircraft into the trajectory of an unseen third party, the induced risk is largely attributable to following RAs. In operational conditions, failure to raise or follow an RA will reduce the risk of an induced collision (even though it increases the absolute risk). 6.3.3.3 The requirement is that the logic is designed to reduce the risk of collision and no distinction is drawn between risk induced by the logic and risk that it is unable to resolve. It is possible to draw such a distinction and even to subdivide the risk into that due to altimeter error and that due to inappropriate operation of the logic but it is considered that this exercise has little value for the design of the logic. 6.3.4 USE OF GROUND RADAR DATA TO CALCULATE RISK RATIO It is possible to use encounters observed in ground radar data as the basis of the safety calculations described in 6.3.2. However, Annex 10 — Aeronautical Communications Volume IV 22/11/07 ATT-64 it is difficult to interpret the results because the calculation concerns extremely rare events and, even when many months of data are used, trajectories have to be modified to insert a risk of collision that was absent in the actual encounters. It is more practicable to use the radar data to inform the choice of the weights to be ascribed to the various encounter classes in the encounter model and thus produce a version of the idealized encounter model that is more representative of the airspace in question than the standard model presented here. 6.4 Compatibility with ATM 6.4.1 NUISANCE ALERT RATE 6.4.1.1 ACAS is required to diagnose a risk of imminent collision on the basis of incomplete information. Furthermore, this information has to be independent of that providing the primary basis for aircraft separation. It follows that there will be alerts in encounters where, from an operational perspective, there would seem to be no risk of collision. Standard Chapter 4, 4.4.4.1 requires that these nuisance alerts be as infrequent as possible. 6.4.1.2 The specification of a nuisance RA given in Chapter 4, 4.4.4.1.2 is made with the view that an RA is a nuisance if normal standard separation is not clearly lost. Additionally, it is intended that the horizontal separation threshold is sufficiently stringent to require the use of a horizontal miss distance filter. The horizontal separation threshold has been set at 40 per cent of normal separation, and the vertical separation threshold has been set at a figure based on an ATC tolerance of deviations of 200 ft from altitude clearance. 6.4.2 COMPATIBLE SENSE SELECTION The requirement at Chapter 4, 4.4.4.2 is not intended to constrain the manner in which dangerous encounters are resolved, but rather is based on an appreciation that the majority of RAs are likely to be generated in encounters where there is no danger of collision. It places a statistical limit on the frequency with which ACAS disrupts ATC or the normal operation of the aircraft by inverting the vertical separation of two aircraft. 6.4.3 DEVIATIONS CAUSED BY ACAS The restrictions on the deviations that may be caused by following RAs, Chapter 4, 4.4.4.3, limit the disruption to normal aircraft operation as well as to ATC. While deviations from altitude clearances are the most obviously disruptive to ATC, other deviations, such as that caused by an RA to climb when the aircraft is descending, could be viewed equally seriously by ATC. 6.4.4 USE OF GROUND RADAR DATA OR THE STANDARD ENCOUNTER MODEL 6.4.4.1 Conformance with the requirement for compatibility with ATM can be tested most convincingly using simulations based on reconstructions of actual operational encounters occurring within the coverage of ATC ground radars, provided that only a small proportion of the aircraft thus observed are equipped with ACAS. However, the results of such simulations based on actual data will reflect the particular properties of the airspace (or airspaces) in which the data were collected as much as those of the collision avoidance logic used. Thus, there are considerable practical difficulties in using real encounter data to validate collision avoidance logic, and the provisions of Chapter 4, 4.4.4 assume the use of artificial encounters based on the standard encounter model specified in Chapter 4, 4.4.2.6. 6.4.4.2 The use of the standard encounter model to obtain performance measures describing the operation of the collision avoidance logic will provide only indirect evidence concerning its operation in any particular airspace. Authorities that have access to ground radar data and wish to understand the interaction of ACAS with local ATC practices are advised to use Attachment Annex 10 — Aeronautical Communications ATT-65 22/11/07 simulations based on their ground radar data rather than the standard encounter model. In doing so, they need to note that the results can be subverted if the aircraft observed are already equipped with ACAS. They will also need to collect sufficient data to ensure that the simulated RAs derived from the data are statistically representative; for example, data collected over 100 days in one State contained very few examples of some types of RAs. 6.5 Relative value of conflicting objectives The design of the collision avoidance logic for ACAS must strike an operationally acceptable balance between the reduction in the risk of collision and the disruption caused by ACAS alerts. The requirements relating to the risk of collision (Chapter 4, 4.4.3) and the disruption to ATC (Chapter 4, 4.4.4) are minimum standards that are known to be achievable from work with a prototype system. Other designs are only acceptable when it can be demonstrated that the risk of collision and the disruption to ATC have both been minimized as much as practicable in the context of a need to minimize the other. Annex 10 — Aeronautical Communications Volume IV 22/11/07 ATT-66 TABLES Table A-1 Nominal altitude band SLC command code Altitude threshold at which sensitivity level value changes Hysteresis values 0 to 1 000 ft AGL 1 000 ft AGL ±100 ft 1 000 ft to 2 350 ft AGL 2 350 ft AGL ±200 ft 2 350 ft AGL to FL 50 FL 50 ±500 ft FL 50 to FL 100 FL 100 ±500 ft FL 100 to FL 200 FL 200 ±500 ft above FL 200 Table A-2. RA strength options Constraint Type Żg Upward sense RA Increased climb Positive >Żclm Climb Positive Żclm Do not descend VSL Do not descend faster than 2.5 m/s VSL –2.5 m/s (–500 ft/min) Do not descend faster than 5.1 m/s VSL –5.1 m/s (–1 000 ft/min) Do not descend faster than 10 m/s VSL –10 m/s (–2 000 ft/min) Downward sense RA Increased climb Positive <Żdes Descend Positive Żdes Do not climb VSL Do not climb faster than 2.5 m/s VSL +2.5 m/s (+500 ft/min) Do not climb faster than 5.1 m/s VSL +5.1 m/s (+1 000 ft/min) Do not climb faster than 10 m/s VSL +10 m/s (+2 000 ft/min) Attachment Annex 10 — Aeronautical Communications ATT-67 22/11/07 FIGURES Figure A-1. Illustration of ACAS functions Other aircraft tracking Altitude test Altitude test Ground stations Evaluation and selection of advisory Own aircraft tracking Range test Range test Other ACAS aircraft Surveillance Traffic advisory Threat detection Resolution advisory Coordination and communication Annex 10 — Aeronautical Communications Volume IV 22/11/07 ATT-68 Figure A-2a. Example of high-density whisper-shout sequence FORWARD DIRECTION S S S S S I I I I I I TOP ANTENNA EFFECTIVE RADIATED POWER (dBm) NOTES.— “I” indicates ERP of P , P , and P interrogation pulses. “S” indicates ERP of S suppression pulse. “S I” means that the S ERP is 2 dB less than the interrogation ERP. “S I” means that the S ERP is 3 dB less than the interrogation ERP. In steps 24, 63, 64, 79 and 83, no S pulses are transmitted. STEP NUMBER MTL (-dBm) INTERFERENCE LIMITING PRIORITY MINIMUM EFFECTIVE RADIATED INTERROGATION POWER (dBm) .. . S S I I... S S I I... S S I I. .. S S I I... S S I I... S S I I... S S I I... S S S S I I I I .. . .. . ... . .. Attachment Annex 10 — Aeronautical Communications ATT-69 22/11/07 Figure A-2a. Example of high-density whisper-shout sequence (cont) LEFT & RIGHT DIRECTIONS 25, 26 27, 28 29, 30 31, 32 33, 34 35, 36 37, 38 39, 40 41, 42 43, 44 45, 46 47, 48 49, 50 51, 52 53, 54 55, 56 57, 58 59, 60 61, 62 63, 64 S I I TOP ANTENNA TOP ANTENNA EFFECTIVE RADIATED POWER (dBm) NOTES.— “I” indicates ERP of P , P , and P interrogation pulses. “S” indicates ERP of S suppression pulse. “S I” means that the S ERP is 2 dB less than the interrogation ERP. “S I” means that the S ERP is 3 dB less than the interrogation ERP. In steps 24, 63, 64, 79 , no S pulses are transmitted. and 83 STEP NUMBER MTL (-dBm) INTERFERENCE LIMITING PRIORITY MINIMUM EFFECTIVE RADIATED INTERROGATION POWER (dBm) .. S S I I... S S I I. .. S S I I... S S I I... S S I I. .. S S I I... S S I I... S S S S I I I I .. . .. . AFT DIRECTION S I I.. S S I I... S S I I. .. S S I I... S S I I... S S I I. .. S S I I... S I. . .. 2, 3 6, 7 10, 11 14, 15 18, 19 22, 23 26, 27 30, 31 34, 35 38, 39 42, 43 46, 47 50, 51 54, 55 58, 59 62, 63 65, 66 68, 69 71, 72 74, 75 BOTTOM OMNI ANTENNA S I I. S S I I. . .. Annex 10 — Aeronautical Communications Volume IV 22/11/07 ATT-70 Figure A-2b. Example of low-density whisper-shout sequence S S S S S I I I I I I Forward Direction Top Antenna Note.— Each 1 dB reduction in the sequence follows the priority for the forward beam in Figure A-2a. 7, 8 9, 10 11, 12 13, 14 15, 16 S S S S S S S I I I I I I I I I I I I I Top Antenna Top Antenna Left & Right Direction Note.— Each 1 db reduction in the sequence follows the priority for the right/left beam in Figure A-2a. Note.— Each 1 db reduction in the sequence follows the priority for the rear beam in Figure A-2a. Note.— Each 1 db reduction in the sequence follows the priority for the bottom beam in Figure A-2a. MIN EFFECTIVE RADIATED POWER (dBm) NOTES.— “I” indicates ERP of P , P , and P interrogation pulses. “S” indicates ERP of S suppression pulse. “S I” means that the S ERP is 3 dB less than the interrogation ERP. “S I” means that the S ERP is 10 dB less than the interrogation ERP. In the last steps of each quadrant, no S pulses are transmitted. STEP NUMBER MTL (-dBm) INTERFERENCE LIMITING PRIORITY MINIMUM EFFECTIVE RADIATED INTERROGATION POWER (dBm) Rear Direction Bottom Omni S S S . Attachment Annex 10 — Aeronautical Communications ATT-71 22/11/07 Figure A-3. Timing for lowest power steps in omnidirectional whisper-shout sequence for top antenna 26 dBm dBm dBm 27 dBm 28 dBm 2 s µ 2 s µ 1ms 1ms S23 I24 I23 I22 S22 Annex 10 — Aeronautical Communications Volume IV 22/11/07 ATT-72 Figure A-4. Interference limiting flow diagram FOR MODE S REDUCE POWER 1 dB INCREASE MTL 1dB FOR MODE S INCREASE POWER 1 dB REDUCE MTL 1 dB SET 8 S FREEZE ON OTHER CHANGES RETURN DOES MODE S RANGE EXCEED MODE C RANGE? CAN MODE S RANGE BE INCREASED? WILL ADDING A W-S STEP VIOLATE INEQUALITY

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