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Aviation History
1987
1987 - 0099.PDF
related to same-direction, opposite- direction, and crossing traffic. Typical deviations were distinguished from blun ders, and those due to "rogue" extreme deviations. Whatever proposals are adopted, the two primary considerations are the collision-risk model and the target level of safety (TLS). Aircraft equipment and procedures will need to be compatible with new proposals, and provision will have to be made for closely monitoring the altitude-holding performance and separa tions achieved. An upper limit will need to be set to the depth of airspace over which any new separation rules would apply. The target level of safety is determined by extrapolating presently achievable safety levels to those more likely to be obtainable by the end of the century, and allocating an agreed share to the sepa ration question. Objectors to this approach say that the TLS will be unsatisfactory because it is based on global civil aviation performance, whereas it is likely that any revised system of sepa ration will be set up in specific geographi cal areas of high traffic density. The allotted share of the target risk is halved before comparison with the perceived actual risks determined from the model. The other half is set aside as an allowance for blunders, such as pilots accurately flying the wrong flight level. Plausible failure modes of crew and equip ment are assessed separately. Arguably, this share of risk is independent of sepa ration. In the American surveys, aircraft show ing large deviations were contacted by ATC, while in Europe operators were informed in writing. Two instances of deviations in excess of 300ft were found to be caused by the altimeter pressure- correction device of a particular aircraft being set to apply in the wrong sense. Participating airlines whose aircraft were fitted with equipment to record altitude returns from their secondary surveillance radar transponders also contributed studies of poor flight-level compliance by crews or aircraft. One factor can be the deliberately "soft" altitude hold of some flight-management systems (FMSs) in the cruise. These slowly trade small changes and recoveries in altitude for speed, to reduce throttle activity. Some operators claim that an allowance of 50ft for this factor should be satisfactory; a study of FMS algorithms by the US Federal Aviation Administration suggests that 150ft might be used. With aircraft of similar type, closely situated in the same meteorological environment, altitude variations under "soft" altitude hold may be in close anti-phase, and the deviations additive. Rogue aircraft were more likely to be general-aviation machines, older aircraft such as Caravelles, and those where the crew still use manual altimeter-correction cards. The main sources of error are "flight technical" rather than altimeter inaccuracy. No significant discernible height-change problems have resulted from slip in turns. Differences between the indications of captains' and first officers' altimeters have been examined by operators in France, the Netherlands, and the UK. A representative figure is probably less than 50ft on modern aircraft and, in practice, altimeters are already calibrated to a higher standard than the specified limits. Electronic flights instrumentation also removes some indicator error, but that implicit in basic pressure measurement remains. The vertical airspace required must also take into account the effective size of the aircraft in normal operation. The aircraft has a normal dimension in depth, but this can grow significantly as a large aircraft banks into a turn. Standing waves The behaviour of flight-control systems in strong, upper-altitude wave formations and turbulence may demand more study, and the FAA is evaluating data obtained from standing waves taken near the Rockies. It may be necessary to apply local geographical limitations to the application of reduced vertical separation in areas where such phenomena are common. Pilots accept that, in areas of high- density traffic, the blunder is a particular problem. Many routes have two-way flows of aircraft which keep accurately over the centreline of the route, and therefore over each other. To make matters worse, crew mistakes tend to involve errors in whole thousands of feet, rather than some hundreds. Risks might be increased in like manner if there were no longer any "dead" thousands above FL290. A problem separate from the cruising case is that of climbs or descents initiated in error, causing a conflict with nearby traffic. If the vertical interval is reduced, then so is time for error detection. Since this is already the situation below FL290, however, the argument is not easy to support. Pilots also say that the justification for reducing altitude separation must be assessed in the total context of benefit to the traffic system. Is the apparent advan tage of creating greater numbers of economic cruising levels capable of being achieved in the practical air-traffic - control environment? Critics of reduced vertical separation discount suggestions that the successful reduction of lateral and longitudinal route separations over the North Atlantic set precedents. They point out that, apart from eastern and western coastal areas, the amount of en-route crossing or opposing traffic is relatively small. They suggest that difficulties are experienced in parts of the world where crossing routes exist in the absence of good VHF commu nication between aircraft and ATC. Unless national ATC systems develop to cope with a major traffic increase on intersecting routes at higher levels, the capability of aircraft to remain separated accurately in altitude cannot be used. The US Air Line Pilots Association (Alpa) says: "Show us the ATC system first". In any case, such a system would have to be predicated on assumed performance of all participating aircraft. In a world where even a serviceable autopilot is not manda tory, this raises further questions. The capabilities of military aircraft must also be taken into account. It is possible that satellite-navigation systems may be able to give an extremely high level of accuracy in three dimensions, and so "tighten" vertical navigation, but this is likely to be only a long-term pros pect. It might then be possible to designate checkpoints along routes to highlight gross altimeter errors. In the long term it might even be pos sible to incorporate altitude holding performance more fully into airworthiness standards, while Mode S (data trans mission) developments in transponders may also be helpful. The shorter-term prospect is for the present separation interval of 1,000ft to be extended, perhaps up to 39,000ft, but for this to be applied only in specific regions where the standard of automatic flight control is already sufficiently high. The Atlantic and Pacific are likely areas of first application. The traffic is orderly and well equipped, with few crossing routes away from the extreme ends. There is minimal military VFR traffic, and the "tidal flow" of aircraft is advantageous. It is also possible that amendments to sepa ration standards may be made in selected areas of the United States. In any such early applications it will be important to provide some form of monitoring which can provide an element of quality control, because it is less than an absolute certainty that the target level of safety has been set correctly and is achievable. Operators have said that they want more usable airspace at the high-level economic cruising altitudes, and they want it within the state of the art. So far, the RGCS panel has said that it "has not encountered any significant difficulties which would prevent a reduction in verti cal separation above FL290 from being achieved". This could be as little as 1,000ft, subject to equipment specifica tions, calibration, and maintenance. Q FLIGHT INTERNATIONAL, II April 1987 35
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