pressure points

航空发表于 2010-6-28 18:25:30

pressure points
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航空发表于 2010-6-28 18:25:52

36 FLIGHT SAFETY AUSTRALIA MARCH–APRIL 2006 FLYING OPERATIONS Loss of pressure is a big killer. The internationally respected Air Safety Network lists six major regular public transport (RPT) accidents involving depressurisation since 1970, with a combined death toll of 719. The most recent – and infamous – was the crash of Helios Airways Flight 522 on August 14 last year, when all 121 on board perished. The Boeing 737-31S ran out of fuel and slammed into a hill 40 km north of Athens after it had failed to pressurise. Most likely, the crew lost consciousness as the aircraft climbed above 10,000 ft. In Australia, in September 2000, a Beech Super King Air, registered VH-SKC, claimed the lives of seven miners and their pilot when it crashed on a property in Burketown in remote north Queensland. The men were travelling to Western Australia’s Goldfields from Perth., a distance of some 600 km. After flying five hours and 3,000 km across Western Australia and the Northern Territory, the aircraft ran out of fuel. Although the cause of the accident may never be known, the ATSB found that the aircraft was, “… probably unpressurised for a significant part of its climb and cruise for undetermined reasons. The pilot and passengers were incapacitated, probably due to hypobaric hypoxia because of the high cabin altitude”. No-one is immune. In 1981 I also nearly joined the long list of pressurisation victims. I was taking a Cessna 425 (twin PT6 powered cabin-class Cessna) from Bankstown to Essendon for some maintenance. I felt very comfortable in the aircraft, because I was part of the crew that flew it from the US to Sydney. I lodged a flight plan, filled the aircraft’s fuel tanks and departed for Essendon. I was by myself, so early in the climb I engaged the autopilot, preset the cruise altitude for the cleared FL250, and dealt with ATC and the busy traffic out of the Sydney area. During the climb I became aware that something was not right. I was a little flushed and slow, but at first I could not work out what the problem was. Then, remembering the symptoms of hypoxia from my military decompression chamber training, I checked the cabin altitude. I was shocked to see the cabin at 16,000 ft, and climbing. Straight away I slapped on an oxygen mask. Once I felt my senses restored, I contacted ATC and asked for descent to a lower level. Troubleshooting on descent I found the “pressurise/depressurise” switch (hard to see under the cabin dump knob) in the POINTS Hundreds have lost their lives as a result of pressurisation problems. John McGhie explains the basics of how the pressure system works and what to do when there is a problem. Out of it: As soon as there is an indication of a pressurisation problem you need to don an oxygen mask. Otherwise you risk loss of judgement and other even more serious effects due to hypoxia. Rob Fox MARCH–APRIL 2006 FLIGHT SAFETY AUSTRALIA 37 FLYING OPERATIONS “depressurise” position. My mistake – I had missed it on the pre-flight! I reset the switch, reset the cabin altitude, and climbed back to my planned cruise level. I was lucky. If I had not felt uneasy and remembered my experience in the decompression chamber the aircraft would have continued the climb. I would have lapsed into unconsciousness and the aircraft would have continued on its way until it ran out of fuel, probably somewhere south of Tasmania. If you want to avoid becoming a pressurisation statistic, particularly if you are new to flying pressurised aircraft, it is vital that you understand the basics of why and how the system works, and the importance of correct handling of pressurisation problems. Pump up the pressure: Let’s start with the basics. Luckily for us, the ratio of oxygen to the other gasses remains constant at 21 per cent, even as we climb and the atmospheric pressure drops from its sea level value. However, as the atmospheric pressure drops the pressure of oxygen also drops – until your lungs struggle to transfer enough oxygen to the bloodstream. That’s when you suffer the effects of lack of oxygen – the dreaded hypoxia. There are several key altitudes involved in staving off hypoxic effects: above 10,000 ft you need supplemental oxygen to maintain bodily functions, above 30,000 ft this supplemental oxygen needs to be 100 per cent of the gas you breathe, and above 40,000 ft the oxygen needs to be delivered to your lungs under pressure. Because the ratio of oxygen remains the same at different altitudes in the atmosphere, a supply of compressed air will allow you to continue to function. This is done by pressurising the cockpit and cabin areas of the aircraft. You can then function without the impediment of oxygen masks. A back-up supply of oxygen is provided should there be problems with the pressurisation system. For passenger comfort the aircraft cabin should remain no higher than around 8,000 ft, and for safety not above 10,000 ft. Cabin temperature also needs to be maintained at a comfortable level of about +25º C. For pressurised flight you need two things: an enclosed cabin, in the form of a pressure vessel, and a way to compress air to raise the pressure inside the chamber, in effect lowering the altitude of the cabin to a safe level. In most cases the outer walls of the cabin area (the aircraft skin) can form the side walls of the pressure vessel. The front and back of the pressure vessel usually take the form of bulkheads as the extremities of the aircraft are not needed for passenger use. Often these unpressurised areas will contain services such as hydraulic and electrical systems. Some smaller aircraft may have The forces on a normal entry door at maximum differential can be over five tonnes. Helios tragedy: On August 14 last year, Helios Flight 522 enroute from Cyprus to Prague, crashed into a hilly area near Athens after losing pressure. The accident serves as a stark reminder of the dangers of hypoxia. Courtesy: Quinn Savit AAP 38 FLIGHT SAFETY AUSTRALIA MARCH–APRIL 2006 FLYING OPERATIONS baggage holds in these unpressurised areas, leading to some interesting effects with passenger personal items such as shampoo bottles! To maintain a cabin altitude of 8,000 ft at an aircraft altitude of 35,000 ft requires a differential in pressure between the inside and outside of about 7.0 psi or 50 kPa (usually measured on a cabin pressure differential gauge). To contain this pressure differential, the aircraft structure must be strong enough to withstand high outward loads from the internal pressure. Additionally, the cycle of pressure – in which the aircraft moves from being unpressurised on the ground to pressurised in the climb and cruise – will impose cyclic fatigue loading on the structure. It was this cyclic load that led to serious problems in the early days of jet air transport. In 1953 the problem surfaced when a De Havilland Comet crashed in mysterious circumstances shortly after takeoff. Two similar accidents followed soon afterwards, causing British authorities to ground the entire fleet of the world’s first jet airliner. The cause was eventually traced to fatigue cracks around the square windows following repeated pressurisation and depressurisation. The problem is now well understood. Several methods have been used to “pump up” the cabin pressure. In the early days, dedicated compressors – or blowers – were used. In jet and turbo prop aircraft, the engines provide a source of air suitable for pressurisation by tapping the compressor sections of the engines. Similarly, piston engines using turbo chargers now provide a source of air suitable for pressurisation of the cabin, and aircraft such as the Cessna 421 use this method very successfully. Air from jet-engine compressors, or turbo chargers, is normally very hot and needs to be cooled before use in the cabin. Air entering the cabin must also be regulated and conditioned; passengers do not appreciate raw engine bleed air in their faces! This can be done by combinations of air-to-air intercoolers or, as used on most jet aircraft, by passing the air through air-cycle machines, which can also provide a source of cold air for cooling in hot conditions. Freon cooling systems, similar to car air conditioning, are also used in some aircraft. Control: Once a source of regulated and conditioned air entering the cabin is assured, you need to control the pressure in the cabin to safe and comfortable levels. This is the job of the pressurisation controller and associated outflow valves. Using a relatively constant inward flow – that is, from the bleed-air source – the pressure differential is controlled by allowing excess air pressure to escape outside the cabin through outflow valve(s). Preset safety features of these valves, or a dedicated safety valve, prevent excess pressure building up in the cabin. There are many variations of controllers – you should check your flight manual for details. The simpler controllers use air pressure and a source of vacuum to operate the valves to maintain a preset cabin altitude or cabin rate of climb. The actual cabin altitude, pressure differential and cabin rate of climb and descent are shown on dedicated gauges. Theoretically, you would want a perfectly sealed pressure vessel to ease the amount of air required from the engines for pressurisation. Practically you must have access holes in the structure such as doors, windows, cargo hatches, control cable runs, electrical cables and hydraulic system pipes. Various methods are used to seal these access areas, such as inflatable seals around doors, or seals around control cables. Ideally doors and hatches will be of a “plug” type where the increase in pressure as you climb will tend to force the door onto the structure to maintain a good seal. In smaller aircraft, doors will normally open outwards and so need to be held in place with pins that seat in the door surround. The forces on a normal entry door at maximum differential can be over five tonnes, so some form of safety mechanism has to be used to prevent the door from opening in flight. Failure to correctly set the pressurisation value for landing can lead to problems resulting from pressure retained in the cabin, such as a sudden release of pressure when the safety valve opens on landing or the possibility of a sudden and violent opening of a door. Pressurisation systems are simple to set and use provided the flight manual procedures are followed. You set the pressurisation controller for the desired cruising level before takeoff and normally you select “ON” for the air source (bleed air). Before descent you set the destination altitude, usually plus a small margin to ensure a depressurised cabin. You then monitor the cabin rate of descent and altitude to ensure passenger comfort during descent and arrival. Care must be taken when you depart from – and arrive at – aerodromes with different altitudes. When you depart a high aerodrome for a low one, the aircraft’s pressure will climb and the cabin pressure will The most critical action is to maintain the level of oxygen in the body. You must don a working oxygen mask as soon as a pressurisation problem is suspected Altitude (ft) Approximate time of useful consciousness 18,000 20-30 minutes 22,000 10 minutes 25,000 3-5 minutes 28,000 2-3 minutes 30,000 1-2 minutes 35,000 30 seconds to 1 minute 40,000 15-20 seconds 43,000 9-12 seconds 50,000 9-12 seconds 21% 1% 78% Black-out: Without sufficient oxygen the brain shuts down quickly. At 35-40,000 ft you have onlyseconds of useful consciousness. Note that reactions to hypoxia vary widely, and time are indicative only. USEFUL CONSCIOUSNESS THE AIR YOU BREATHE Constant: The ratio of oxygen to other gasses in the atmosphere remains roughly constant at 21 per cent, regardless of altitude. MARCH–APRIL 2006 FLIGHT SAFETY AUSTRALIA 39 FLYING OPERATIONS go down – a unique situation. Before opening any door after landing at your destination, you must make sure that the differential pressure is at zero. A simple check is to try to open the pilots DV window – if it opens with no problem, the pressure must be at zero. Problem management: As with any aircraft system, problems can be encountered with pressurisation systems. In all cases, the aircraft manufacturer’s emergency and abnormal procedures must be followed explicitly, promptly and correctly. This is the only way to ensure a safe outcome to any pressurisation problem. The most critical action is to maintain the level of oxygen in the body. You must don a working oxygen mask as soon as a pressurisation problem is suspected. This is essential because your time of useful consciousness is limited (see chart). Problems can be broadly categorised into air leaks and air source problems. Leaks can occur slowly, such as when door seals do not work correctly, or suddenly such as a major component failure. Often door and hatch failures result from incorrect closing procedures. You should treat door and hatch closing as an important part of the aircraft preflight checklist. A small leak can be handled by the system and its only effect may be the noise created by the escaping air. Large or sudden leaks will cause a rise in cabin altitude, with the rate of the rise depending on the size of the leak. A major leak can lead the cabin altitude to rise quickly to the same value as the aircraft altitude (the term “explosive decompression” is used to describe the extreme case of this). This causes considerable distress to crew and passengers, and the effects on ears and sinuses can be painful. Aircraft capable of flight above 25,000 ft provide passengers with automatically presented oxygen masks. Problems can also occur due to the source of the bleed air. Internal engine oil leaks can contaminate the bleed air leading to fumes and even smoke in the cabin. Again, the first action when you notice any smoke or fumes should be to don an oxygen mask and select 100 per cent oxygen to ensure you have an uncontaminated breathing source. You should then follow the manufacturer’s instructions to ensure a safe outcome. This will normally require the shutting off of the source(s) of bleed air to shut off the smoke or fumes. You need to be careful to correctly operate the pressurisation system. Failure to turn on the pressurisation air source can result in the onset of subtle hypoxia during climb. It is critical to check that the cabin is pressurised before a climb above 10,000 ft. You should check the pressurisation every 5,000 ft during climb, checking that the cabin pressure gauge shows an appropriate differential pressure and that the cabin altitude is less than the aircraft altitude. Pressurised aircraft regularly operate in the rarefied air above 10,000 ft. Understanding the environment in which we operate, and the aircraft systems that allow us to operate in this area, can lead to many thousands of hours of safe and comfortable flying. John McGhie is an authorised testing officer and former CASA flying operations inspector. For further information check out, “Oxygen first”, a video about hypoxia. It’s available from CASA’s online store at <A href="http://www.casa.jsmcmillan.com.au">www.casa.jsmcmillan.com.au</A>. 150 200 250 300 0 5 10 15 x 104 0 .5 1.0 1.5 200 250 300 350 K (temp) 0 10 20 30 40 50 60 70 80 90 100 Geometric altitude in kilometres Temperature Speed of sound Density Pressure N/m2(pressure) kg/m3(density) m/sec(speed of sound) Mesopause Stratopause Tropopause Troposphere Mesophere Stratosphere Thermosphere Struggle: As atmospheric pressure drops the pressure of oxygen also drops until your lungs struggle to transfer enough oxygen to the bloodstream. Above 10,000 ft you need supplemental oxygen to maintain bodily functions, above 30,000 ft this supplemental oxygen will need to be 100 per cent of the gas you breathe, and above 40,000 ft the oxygen will need to enter your lungs under pressure. ALTITUDE, DENSITY AND PRESSURE

小勺儿 发表于 2010-7-1 18:36:21

太好了感谢分享

胖子发表于 2010-8-31 20:35:39

:) :) 感谢

bocome 发表于 2011-7-31 11:29:29

pressure points

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pressure points