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Aviation History
1968
1968 - 0009.PDF
FLIGHT International, 4 January 1968 11 SUPERSONIC PILOTS—3 BALPA's Symposium Reviewed SEVERAL OF THE PAPERS at the end of the BALPA SSTsymposium concentrated on the effect of the environmentat high altitudes on both the aircraft and the crew. DR G. BENNETT, chief medical officer at the Board of Trade, spoke about some of the medical aspects. Pressurisation "At SST altitudes," Dr Bennett said, "pressurisa-tion failure clearly presents a major hazard. Consciousness is very rapidly lost on exposure to 40,000ft and the body fluids boil at63,000ft. The UK and US airworthiness regulations therefore require that the cabin altitude shall only exceed 25,000ft in the event of an'extremely remote' failure or combination of failures, i.e less than 1 in 10 million probability. This degree of prote'ction has beenachieved by such features as multiple high-capacity air supplies, small windows, and multiple discharge valves. Because of the timerequired to carry out emergency descent to low altitude, it is unfortunately still necessary to carry protective oxygen equipmentfor passengers as well as crew." Ozone "An additional complication of the pressurisation systemarises because the ambient air at high altitude contains toxic quanti- ties of ozone. Its concentration in the stratosphere varies withlatitude and season as well as altitude, and may occasionally reach 12 parts per million by volume at SST cruising levels. Fortunatelyit dissociates rapidly at the high temperatures produced during compression and though provision has been made in the Concordefor a catalytic filter, it is not expected that this will be necessary. Without it, the concentration in the cabin is calculated to be lessthan 0.1 parts per million except under occasional transient con- ditions when 0.2-0.5 p.pjn. may be reached for a few minutes.A careful check will be kept on this point during flight trials." Radiation "A further effect of flying at high altitudes is thatprotection against cosmic radiation by the 'atmospheric umbrella' is reduced. Continuous background cosmic radiation dosage varieswith magnetic latitude and with the 11-year sunspot cycle as well as altitude. Measurements show that the maximum total bodydosage at 65,000ft is about 1.3 millirem/hr, so that the total exposure from this source in a transatlantic crossing is less thanl/30th of that in a chest X-ray. Because of the shorter journey time, the dosage per mile travelled is about the same as in currentsubsonic jets, where about 0.5 millirem/hr is being recorded in northern latitudes. On southern routes, dosage rates will beconsiderably lower. "A complicating factor is that streams of high-energy particlesare periodically ejected from the Sun in association with solar flares. Very occasionally these produce dosage rates of 2-4 rem/hrat SST cruising levels and the prediction methods are not at present sufficiently effective to give more than a short period of warning.Research is continuing but until forecasting becomes reliable, Concorde will carry its own radiation monitoring and warningequipment. This gives several minutes' warning of an impending significant rise in radiation. An amber 'Alert' warning is given ifthe level reaches 10 millirem/hr, followed by a red 'Action' warning at 100 millirem/hr. If the aircraft then descends below 50,000ft therewill be sufficient protection from the upper atmosphere. The equip- ment also measures the total dosage accumulated during each flightso that a check can be kept on the yearly total received by individual crew members. With the likely crew utilisation in termsof annual flying hours and routes, it is unlikely that the annual dosage will reach the level of 1.5 rem at which it is necessary totreat crews as radiation workers. "At 60,000ft only very small numbers of the biologically destruc-tive heavy nuclear particles are present. Numbers do not increase very greatly between 60,000 and 75,000ft, and allowance has beenmade for their effects when calculating the permissible overall radiation dosage. Radioactive debris from nuclear tests will not behazardous unless further contamination of the atmosphere occurs." One of the first questions to follow Dr Bennett's paper was whether the glare at high altitude would necessitate continuous use of dark glasses by the crew. He said that they would only be needed by people who were normally susceptible to glare. The next questioner wanted to know whether present flight- time limitations would apply to the SST. Dr Bennett hoped that flight-time limitations would be different in any case by the time the Concorde was in service, so that the more demanding operations had a shorter maximum permissible duty period. A pilot wondered whether radiation would have any permanent genetic effect on the crews. This started off an exchange as to whether the radiation meter which will be fitted to Concorde would be sufficient safeguard, in the long term, for crews. Several pilots thought that film badges ought to be worn, but Dr Bennett said that these were not as accurate an indication as the meter. A KLM pilot asked what aeromedical participation there would be in the Concorde test programme, and whether the increasing frequency of heart attacks was causing concern. Dr Bennett thought that it was well worth trying to measure the amount of effort put into a task as well as how well the task was performed. He explained that techniques for doing this were experimental at the moment, but that they may be established practice in the next few years. The "met man," MR M. H. FREEMAN, then spoke: — "Wind will be less of a problem," he said, "than it is for sub-sonic aircraft. Wind speeds in the lower stratosphere (between 100 and 50mb where Concorde will be cruising) are in general lowerthan those found in the jet streams of the upper troposphere. Above 50mb there is a tendency for higher wind speeds, and the Mach 3aircraft may sometimes have to contend with a winter stratospheric jet. The much higher cruising speeds and lighter winds will meanthat it is less profitable to deviate from the Great Circle in order to find a least-time track."The accuracy now obtainable in forecasting tropospheric winds should be adequate for the climb and descent stages of flight bySST aircraft. The day-to-day variability of wind between 100 and 30mb is much less than that between 300 and 200mb and forecastingwill be correspondingly easier. In fact the most recent actual chart may well provide the best 12 or 18hr forecast, and should easilymeet the stated requirement for root mean square vector error of less than 15kt over 500 n.m. segments. At 50mb the standarddeviation of 24hr vector change in wind has been quoted as about lOkt." Temperature "The provision of accurate temperature forecastswill be of much greater importance to SSTs than it now is for subsonic aircraft. It is worth remembering, however, that in theearly days of jet engines, forecasters were asked to provide much greater precision in temperature forecasts than is now needed, andas SSTs develop we may see the same thing happening again. "Meanwhile the performance of SST engines appears rathersensitive to changes in temperature of the environment, and fore- casters will be called on to improve the accuracy of their tempera-ture forecasts. Nowadays upper air forecasts are normally produced by computer. In the UK we are using a baroclinic model whichuses and forecasts directly data at three levels. For other levels forecasts are obtained by regression techniques. A recent check ofspot temperatures produced in this way showed that 68 per cent of the errors in 24hr forecasts at 300mb were 3°C or less and thestandard (root mean square) error was about 2^°C. Results at 250 and 200mb were less good. There is little doubt these figurescan be improved on. Improved regression techniques taking account of 100mb data will be introduced shortly and streamlining of datahandling will mean that 12 or 18hr forecasts will be available in time for operational use. In USA a six-level model for numericalforecasting has recently been introduced and this has enabled greater accuracy in temperature forecasts to be obtained. Work isprogressing in UK on developing a ten-level model with a smaller grid length, but its operational use awaits a larger, much fastercomputer. "The computer forecasting I have been discussing is currentlydealing with levels up to 200 or 100mb. Above this, in the strato- sphere, the day-to-day variability of temperature is small for muchof the year. At 50mb over the Atlantic between April and October the use of monthly mean temperatures as forecasts would givestandard errors of less than 3°C, and elementary persistence fore- casting would improve on that. During the winter months quitelarge temperature changes, the so-called 'sudden warmings,' can occur. These are not, however, so sudden as to be unforecastable.The effect is often first observed at high levels (30mb or above) and propagates downwards, and a warm area once found movescomparatively slowly from chart to chart, so that extrapolation methods will produce quite good forecasts for much of tie time.If nothing better than a 24hr persistence forecast were attempted a standard error of 2.7°C would be achieved at 50mb, taking the yearas a whole. "Summing up the position on temperature forecasting I wouldsay that current methods can be made to produce forecasts having standard errors of about 3°C (the stated requirement) and thatforeseeable improvements must bring this down towards a standard
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