Under a pressurized cabin is in the air and space travel a pressure-resistant design of the passenger cabin , cockpit or cargo space for higher-flying aircraft or spaceships understood that humans and animals makes it possible to stay under these debilitating to hostile conditions. This is achieved by maintaining the air pressure in the pressurized cabin, which is higher than that of the surroundings .
The air pressure in the cabin can be expressed in relation to a certain altitude. Instead of air pressure in the cabin and therefore will Cabin height (Engl. Cabin altitude ), or more precisely, the cabin altitude spoken. The cabin height is regulated in flight and should be in the range of -200 ft (-60 m) to +8,000 ft (+2,438 m). The pressure in the cabin should therefore correspond to an air pressure at this level.
If the air pressure in the cabin is increased, the cabin height drops , which is also referred to as the cabin descent . Conversely, the cabin height ( ascent of the cabin ) increases when the pressure in the cabin is reduced. The cabin height is to be seen in analogy to the flight height , which is related to the ambient pressure (pressure height , see density height ). In an aircraft without a pressurized cabin, the flight altitude and cabin height are identical. The fact that slight pressure deviations can occur in the cabin due to the airstream is neglected in this analysis.
The pressure conditions change considerably during a flight. At an altitude of 18,000 ft (approx. 5450 m) the air pressure of the outside air has halved, at 34,000 ft (approx. 10,300 m) it is only a quarter.
The cabin of an aircraft that is climbing expands due to the decreasing external pressure while the internal pressure remains the same . In the descent, the expansion of the cabin is reduced again. (Illustration 1)
The allowable difference between internal pressure and external pressure (differential pressure, delta P, .DELTA.P;. Maximum pressure difference or engl pressure differential and differential pressure ) is limited by the construction (weight) of the cabin but also by the behavior of the car in case of sudden decompression . Because of this design-related limitation, the interior cabin pressure must be reduced compared to the pressure at sea level when the aircraft is flying at high altitudes (Figures 2 and 3). With a cabin pressure after take-off of a constant cabin height of 0 m (= sea level), the pressure in the cabin would be greater than the external pressure, so that the aircraft envelope would have to be dimensioned more stable in order not to fail due to material fatigue.
With a pressure difference of 0.6 bar, the internal pressure on the inner wall of the pressure cabin is approximately six tons per square meter.
Depending on the flight phase , the pressure in the pressurized cabin is gradually changed or kept constant.
- Climb (engl. Climb ) should the cabin altitude increase (pressure decrease in the cabin). The outlet valve (see below ) moves to a more open position (compared to the stable state, for example on the ground or when cruising).
- In descent (Engl. Descent ) has the cabin altitude drop (air pressure in the cabin). The exhaust valve closes a little more.
- During cruise , the cabin height must be kept constant at a high value (“service ceiling of the cabin”, lower air pressure in the cabin compared to sea level). Under stabilized conditions with constant cabin height , the amount of air flowing through the exhaust valve is equal to the amount of air delivered by the air conditioning (pack) minus the air lost through leakage. The exhaust valve is partially open.
- While starting and landing (Engl. Take off and landing ), the cabin height lowered slightly under court level, which means that the cabin pressure is minimally increased. In this situation, in the case of a large angle of rotation (temporarily large angle of attack), the air flow can be partially directed towards the outlet valve in the rear area and the outflowing air. This causes a sudden increase in pressure inside the cabin. This effect can be avoided by briefly increasing the cabin pressure ( ground pressureurisation ) during take-off and landing by approx. 0.1 PSI (corresponding to a cabin height of 200 ft below the airport height ) (Figure 2). , Aircraft that do not bleed air (Engl. Bleed-air ) use need not pressurisation ground .
With some models, the standard start-up is “packless”, with the others only when required ( packs are the main units of the air conditioning system in the aircraft : packless = with the packs switched off) - this increases the effective engine thrust, because the engine power is not reduced by the bleed air from the compressor stage of the engine, which is required for the operation of the pack . This additional engine power (by switching off the packs ) is particularly required at high take-off weights, high outside temperatures or airports with low air pressure (e.g. mountains).
Technology of the pressurized cabins
A pressurized cabin requires a significantly higher structural effort. On the one hand, this leads to an increased weight load. On the other hand, the cabin pressure has to be regulated in a complex manner in order to avoid underpressure or overpressure in the cabin.
An aircraft with a pressurized cabin is not hermetically sealed ; fresh air is constantly supplied and some of the used air is vented from the aircraft. The engines and downstream packs ( air conditioning in the aircraft ) provide the necessary air. Additional turbo compressors support the engines of older machines until around the 1960s. What is new is the exclusive generation of compressed air with an electric compressor ( Boeing 787 ). Automatically working valves (see below ) then regulate the pressure in the pressurized cabin by backflow or escape - the supply air volume is unregulated.
Structural weaknesses in the sealing of the aircraft and for the stability under high internal pressure are:
- the aircraft doors (including cargo doors),
- the airplane windows and
- the rear pressure bulkhead (engl. rear pressure bulkhead ).
A relative overpressure is generated inside the cabin, mostly using the bleed air from the engines or an electric compressor ( Boeing 787 ). In order not to unnecessarily burden the construction of the pressure cabin, only a certain maximum pressure difference (differential pressure) is set. It is stipulated that the air pressure in the cabin must not fall below the value that prevails under normal conditions at an altitude of 8000 ft (2438 m) ( cabin height ).
The pressure regulation of the pressure cabin via the following valves (engl. Valves )
- positive pressure relief valve (normally outflow valve . called; dt exhaust ) valves (usually two or more) for the pressure regulation by the pressure release in normal operation,
- negative pressure relief valve (dt. vacuum emergency valve ): this function is normally performed by the door seals
- safety pressure relief valve (also called overpressure relief valve ; dt. emergency valve or safety valve ): these limit to the structural limit if the pressure control fails
Control of the cabin pressure
In modern aircraft, the pressure in the cabin (cabin pressure ) is regulated via outlet valves ( outflow valves ), which are controlled by the pressure controller , and also secured by emergency valves. These are present several times - due to redundancy - like most important (safety-relevant) parts in the aircraft.
Depending on the degree of automation of the control system for the cabin pressure, the pilot is more or less relieved of the automatic control. In the latest systems, the control of the cabin pressure is automatically taken over by the flight management computer (FMC). The controller for the cabin pressure receives information from the FMC about the altitude of the take-off and landing airfields, as well as the planned flight altitude. Even if the destination airport changes (new landing altitude) by the pilot during the flight, the FMC updates the information to the controller.
However, slightly older systems still require input from the pilot to the cabin pressure panel (dt. Panel for cabin pressure ) when the descent for landing begins, because the controller can not distinguish whether it is during a descent only a short segment in the cruise or the descent to the landing approach. In older systems, the controller does not yet evaluate the information relating to the horizontal position.
The outflow valves control the outflow of air from the cabin and thus maintain a specified overpressure (setpoint) in the cabin. When the doors are closed, the pressure controller (a computer) receives a signal to slightly increase the pressure in the cabin - that is, the cabin height is lowered by a few feet . This “inflates” the aircraft slightly. This increased internal pressure makes the aircraft more stable - it was designed and constructed precisely for this. All starting parameters (weights, speeds) are based on the strength of the aircraft construction, as it is generated by the slightly increased internal pressure. The same applies to landing - here, too, a slightly increased internal pressure is required for strength.
Even a balloon or a hermetically sealed tin can is more stable with increased internal pressure than with negative pressure. The aircraft doors are also designed in such a way that they remain tight in the event of increased internal pressure, even if the entire cell expands slightly.
Failure of the outflow valves
If the outflow valves fail, different scenarios can arise: The outflow valves no longer open (continue) or no longer close (continue).
Outflow valves closed
A possible failure of the outflow valves is that they can no longer open (further) and always remain closed (too far). A distinction is made here again as to whether this occurs while descending (or cruising) or on the ground.
- Variant 1
If the outflow valves fail during descent or cruise and remain closed (too far), the air conditioning systems continue to work at the same time and thus build up more and more pressure in the aircraft, then emergency valves open as soon as a differential pressure of around 9 PSI (approx. 0.6 bar ) between cabin pressure and external pressure is exceeded. Then they close again until the 9 PSI is reached again. This process is repeated over and over again at certain time intervals. This rough emergency control of the cabin pressure via the overpressure relief valve can, however, be felt clearly and unpleasantly due to the sawtooth-shaped pressure curve in the cabin. Switching off the air conditioning systems to stop the pressure build-up is also out of the question, as they are indispensable for temperature regulation and for maintaining a pressure in the cabin that is essential for survival (above 4 km flight altitude) - but you can reduce the number of active packs.
- Variant 2
If the outflow valves fail and remain closed (too far) while the aircraft is back on the ground (after landing), there is overpressure in the cabin (the cabin height is lower than the outside height), which must never be the case because Then the doors - especially in the event of a possibly necessary evacuation - could not be opened. For example, 0.1 bar differential pressure results in about 2 tons of force on a normal door surface.
Since manual opening of the “outflow valve” no longer works, the only option is to turn off the air supply, i.e. to switch off the air conditioning system that generates the air pressure (“Packs off”).
Special case: too fast descent
Normally the aircraft and cabin height sink at the same time, the aircraft mostly at 1000–2000 ft / min, the pressurized cabin at approx. 350 ft / min. That is enough for both to be “on the ground” at the same time. However, if the descent is carried out very quickly (for example due to ATC instructions or topographical requirements), the aircraft will reach the cabin height well before landing - the external pressure would be higher than the internal pressure (see Figures 4 and 5). For this case, there is a negative pressure relief valve , which for a B737 , for example, consists of a simple, A4-sized, spring-loaded flap which then opens inwards and relieves the negative pressure - with other models, this is done via the door seals. These procedures are clearly noticeable in the ears.
Outflow valves open
Another possible failure of the outflow valves is that they can no longer be closed (stay too wide open). Here, too, a distinction is made whether this occurs in descent (or cruise) or on the ground.
- Variant 1
If the outflow valves fail during cruise and do not close (sufficiently), a controlled descent is necessary to an altitude at which the passengers can be permanently secured 8,000 ft cabin height .
An immediate rapid descent , also called emergency descent (German: Notabstieg ), is only necessary in the event of major damage to the aircraft hull (significantly larger than an aircraft window). The oxygen masks only deliver oxygen for approximately 15 minutes at a cabin altitude of over 14,000 feet . The aircraft does not have any other limits for landing after decompression (landing weight, flap speed), but it is landed with greater caution after rapid decompression because of the assumed greater damage.
- Variant 2
If the outflow valves fail on the ground before take-off and no longer close, a maximum of one flight at low altitude (below 8,000 ft) that does not require a pressurized cabin - for example, a transfer flight to a hangar or an aeronautical facility Repair operation . In some aircraft there are reduced limits (weight reductions) for this case, since the stability of the aircraft is designed for the slightly increased pressure in the cabin for take-off. In the case of aircraft that always take off with the packs switched off, i.e. without putting the pressurized cabin into operation before take-off, this does not matter.
Civil cargo planes are equipped with a completely normal pressurized cabin, as modified versions of passenger planes are usually used. The cargo hold on passenger aircraft is part of the pressurized cabin. The outer skin of the aircraft fuselage practically represents the outer shell of the pressurized cabin. Passenger cabin, cockpit and cargo hold form a coherent pressurized cabin. Only the possibility of temperature regulation is limited for the cargo holds (in many aircraft), often only part of the cargo hold, which is intended for particularly sensitive cargo, is tempered. In some aircraft there is a better fresh air supply for the cockpit, but it is unfortunately also drier and can lead to corresponding complaints for the pilots (respiratory problems, conjunctivitis ).
Military transporters usually do not have a pressurized cabin because, among other things, they drop cargo from great heights via a loading ramp. Even light bombardment could result in decompression. Aircraft with loading ramps can only be made pressure-tight with a great deal of constructive effort. Sometimes a pressurized cabin is only used for the cockpit. The teams in the hold then have to wear oxygen masks at great heights.
Fighter aircraft, too, usually have a pressurized cabin because of the altitudes they can reach and the high climbing and descending performance. In order to keep the consequences of rapid decompression (for example if the cabin roof is lost or damaged after being shot at) to a minimum, the cabin pressure here, unlike in passenger aircraft, behaves analogously to the ambient pressure below 10,000 feet. Above this height, an internal pressure is generated in the cabin, which decreases more slowly than the external pressure. This procedure is maintained up to a maximum cabin height of 18,000 feet, then the pressure is stabilized. Oxygen masks are regularly put on to protect against a lack of oxygen. For flights at high altitudes, height protection suits are worn in addition to protection against the consequences of decompression sickness in the event of rapid decompression or in the event of an emergency exit .
The first pressurized cabin was built by Auguste Piccard . Hanging on a gas balloon, he reached a height of 15,781 m in it on May 27, 1931 and was able to surpass the 30-year-old height record of Arthur Berson and Reinhard Süring , who had climbed to 10,800 m in an open basket. As early as 1903, at the 232nd meeting of the Berlin Association for the Promotion of Airship Travel , Hermann von Schrötter proposed the use of a hermetically sealed basket with increased oxygen tension for balloon flights at altitudes above 10,000 m.
The Junkers Ju 49 , a special high-altitude aircraft that took off for the first time in 1931, had a pressurized cabin for two people. Auguste Piccard was involved by Alfred Renard in the development of a passenger aircraft with a pressurized cabin in the 1930s . The result was the Renard R-35 , which crashed on its maiden flight in 1938 and was not further developed.
The first series-produced passenger aircraft with a pressurized cabin was the Boeing 307 Stratoliner , which was built only 10 times and was used from mid-1940 by Transcontinental and Western Air (T&WA) in long-haul service between New York and Los Angeles, followed by Pan Am February 1946 by the four-engine Lockheed Constellation family, which was later manufactured in larger numbers . From July 1948, American Airlines began using the twin-engine Convair CV-240 for short-haul flights . As early as May 1944, the Boeing B-29 bomber, equipped with a two-part pressurized cabin, came to the United States Army Air Forces (USAAF) during the Pacific War .
In connection with the series of accidents involving the de Havilland Comet , which had a cabin height of 8000 ft at cruising altitude (35,000 to 40,000 ft) , the engineers had to determine after extensive investigations that the pressurized cabin was a previously unknown form of material fatigue can occur.
The crash of a Vickers 951 Vanguard on British European Airways Flight 706 in October 1971 was caused by the corrosion of the aft pressure bulkhead. After both horizontal stabilizer surfaces were torn down , the Vanguard was no longer controllable.
A Boeing 747 jumbo jet crashed on Japan Air Lines flight 123 in 1985 because the rear pressure bulkhead had not been repaired according to the manufacturer's instructions seven years earlier. The incorrectly repaired bulkhead burst at a height of 7,300 meters and the escaping cabin pressure blew off the rudder unit.
The accident on Aloha Airlines Flight 243 in Hawaii showed that under the influence of salty air, a high number of decompression cycles put additional stress on the pressurized cabin and this must be taken into account in the maintenance intervals for aircraft maintenance .
The traditional method, the pressure in the pressurized cabin with bleed air ( bleed air to produce), was in the Boeing 787 left. In this aircraft, bleed air was completely dispensed with in order not to impair the efficiency of the engines (power, fuel consumption) through the extraction of bleed air. Instead, the cabin pressure is generated by an electrically operated compressor.
In order to increase the comfort for the passengers, aircraft manufacturers are striving to adjust the cabin height closer to the pressure from the ground. With the Boeing 787 (at a maximum cruising altitude of 43,000 ft) it is a maximum of 6000 ft (1829 m), which corresponds to a relative difference to the standard pressure of approx. 20%. The cabin height of the Airbus A380 is raised to a maximum of 5000 ft (1525 m) (with the same maximum cruising altitude), which corresponds to a deviation of approx. 17% from the standard pressure.
Portable oxygen systems are a cost-effective alternative for smaller aircraft without a pressurized cabin, which, for example, only occasionally have to fly at great heights because of a mountain overflight.
Control of the cabin height using the B-747-400 as an example
With the Boeing 747, the pilot monitors and controls the cabin altitude on a switchboard ( cabin altitude control panel ) on the overhead panel . The position of the Outflow valve is also displayed here by the Outflow valve position indicator . There is a left and a right outflow valve (OP = Open, Cl = Closed).
The landing altitude selector ( LDG ALT ; bottom left; Landing Altitude Selector) can be set in the range of minus 1000 to plus 14000 feet. To adjust the landing height, it must be pulled out and can then be rotated. This selector switch overrides all other inputs of the landing altitude (from the FMC - Flight Management Computer ) to the cabin altitude controller (a small computer that controls the cabin pressure). When the selector switch is pulled, the landing height must be set manually on this switch. The set value is displayed on the EICAS (the screen in front of the pilot) and is then marked with the addition MAN (for manual). When the selector switch is pushed in again, the entry of the landing altitude is sent again to the FMC. In normal operation, the FMC forwards the landing altitude to the cabin altitude controller . In this case, the display of the landing altitude on the EICAS is provided with the addition AUTO . The landing altitude is transmitted either manually (selector switch pulled) or automatically from the FMC (selector switch pressed = normal operation) to the cabin altitude controller .
The outflow valves can also be controlled manually by the pilot in special situations. There is also a pressure switch on the cabin altitude control panel for the right and left valve ( outflow valve manual switch ). When this is pressed, the corresponding valve is controlled manually. The pressed button then lights up with the label ON (manual control switched on). In this position, the automatic controller is Outflow valve bypassed and also the cabin altitude limiter (dt. Limiters for the cabin altitude ) bridges. This activates the three-way switch (between the square pushbuttons; bottom center). A spring pulls this three-way switch into the middle position again and again after it is released. In the upper position ( OPEN ) the overridden outflow valves are slowly opened more and more (and the cabin height increases ); in the lower position ( CLOSE ) the overridden outflow valves are slowly closed more and more (and the cabin height decreases). Once you have reached the desired valve position, you simply let go and the switch jumps back to the middle position.
When the Outflow valve manual switch is switched off, the outflow valves are automatically controlled again.
At the bottom right is a rotary switch ( cabin altitude auto selector ), with which you can switch between the duplicate cabin altitude controllers (A and B). In the event of trouble-free operation, the automatic system is left with the choice of the controller. In the event of malfunctions, however, the pilot can switch to controller A or B in order to search for errors or to bridge detected errors. In normal operation the cabin altitude auto selector is set to NORM . He then automatically selects Cabin Altitude Controller A or B as the primary controller for each new flight. If the primary controller fails, the other (secondary) controller is automatically switched to. Even in manual mode, the selected controller is the primary and the other is the secondary controller.
The air requirement of a person is 8.5 l / min in the idle state at sea level approximately. With increasing altitude, the air pressure drops and the oxygen partial pressure is no longer sufficient to supply a person with enough oxygen . The oxygen partial pressure in the alveoli is then too low and too little oxygen passes into the blood. The lower amount of oxygen available in the blood ( lack of oxygen ) is initially compensated by faster breathing and after a few days (not relevant for flights) then by the increased production of red blood cells.
Lack of oxygen
Most individuals tolerate altitudes up to 8000 ft (2400 m) without any health problems or discomfort. However, some passengers - especially those with heart or lung conditions - may show symptoms as early as 5000 ft (1500 m). At 5000 ft, the body has 25% less oxygen available than at sea level. Fatigue and headaches can occur above 8000 ft. As the altitude increases, confusion, memory loss, muscle cramps, and loss of consciousness are possible. Longer flights over 10,000 ft (3050 m) require additional oxygen or a pressurized cabin. Staying over 13,000 ft (4000 m) can be used to altitude sickness with overconfidence, fatigue, disorientation and even unconsciousness lead. Staying over 20,000 ft (6100 m) can be fatal if you stay longer. Contrary to popular claims, however, a complete lack of pressure never leads to boiling blood or the like - the blood pressure in the circulatory system is sufficient to prevent the blood from boiling. However, since it is no longer possible to hold your breath under these circumstances, you will lose consciousness after about 15-20 seconds if blood without sufficient oxygen reaches the brain. Above the Armstrong limit , which is around 19,000 m on earth, however, blisters form in the blood, which can lead to ebullism and thus to serious health problems within a very short time.
Time of Useful Consciousness
The time of useful consciousness (for period of full consciousness , but not used as a term) is the time between the onset of decompression (the pressure in the aircraft falls to ambient pressure) and the onset of the inability to act. This is the maximum reaction time a pilot has in the event of decompression. The incapacity does not have to be unconsciousness or death, but the pilot is practically incapacitated. At 40,000 ft (12 km), the normal altitude of modern passenger aircraft, the time of useful consciousness is 15-20 seconds. During this time, the pilots must have their oxygen mask on.
Failure of the pressurized cabin can lead to unconsciousness of the pilots and plane crash.
Rapid changes in air pressure are perceived by the human ear as pressure on the eardrum, as the pressure equalization between the middle ear and the environment cannot take place quickly enough. Such fluctuations have a significant impact on wellbeing and health. Normally, you should therefore not exceed 500 ft / min when climbing and 350 ft / min when descending. To alleviate these symptoms, a candy used to be given at the start. When sucking, the soft palate and throat muscles move, which promotes the stretching and opening of the Eustachian tube , which is used to equalize pressure. If necessary, this can also be done via dry swallowing, but it only makes sense in a climb with decreasing external pressure. The Valsalva method (hold your nose and press air into your nose until your ears crackle) is also suitable for getting relief - but only during a descent when the external pressure increases again.
There are other effects on humans that can occur when the air pressure drops. For example, gases in the intestine expand with increasing cabin height (see section cabin height ) and can cause meteorism if the pressure in the cabin decreases . The air in the sinuses can also cause discomfort if it cannot escape because of inflammation. The same applies (theoretically) to trapped air in teeth (e.g. root inflammation, root canal treatment).
Sick passengers must clarify with their doctor before the flight whether they can tolerate a decrease in pressure that corresponds to a stay in the mountains at an altitude of 2500 m.
At a cruising altitude of up to 12,000 m, the air is so cold that it - absolutely - contains very little water vapor. Due to the warming of the outside air in the air conditioning system, the relative humidity drops so much that it can dry out people's airways. Therefore, humidification systems (humidifiers) can optionally be installed in modern travel aircraft. That costs energy for the evaporation of the water or heating energy. Drinking water is also served.
Pollutants in the cabin air
In modern aircraft, the cabin air is drawn from the engines. Under certain circumstances (in the event of a fault) oil vapors and with them pollutants such as the neurotoxin TCP can be proven to get into the cabin.
In 2010 hundreds of pilots and flight attendants reported serious and sometimes chronic illnesses, including occupational disability (“ aerotoxic syndrome ”). The Cockpit union , Ver.di and UFO then called for an impartial investigation of the possible causal chain of a contamination of the cabin air through to the clinical picture. A confidential paper from the Federation of German Airlines from the same year says that many airlines were concerned about their reputation.
The Boeing B787 ("Dreamliner") is one of the first modern passenger jets to stop taking cabin air from the engines.
The pressurized cabin serves as an artificial “atmosphere” for space travelers to survive in the total vacuum of space. Since the pressure difference is at a maximum in a vacuum, spacecraft must be particularly stable in order to avoid bursting. Also spacesuits are - held internal pressure - but less.
In order to increase the partial pressure of the oxygen in the lungs, the oxygen concentration in the cabin can alternatively be increased instead of increasing the pressure to a physiologically compatible level. This is exactly what pilots do in an airplane who fly at not too extreme altitudes (approx. 3000 to 5000 m) without a pressurized cabin and add some oxygen to their breathing air via a small oxygen probe. But this is only possible up to a certain decrease in the air pressure in the cabin.
In terms of design, oxygen enrichment of the cabin atmosphere with oxygen was not used in aircraft, but was used in the American Mercury , Gemini and Apollo spaceships . During the flight, your cabin atmosphere consisted of pure oxygen at a third of the pressure on earth (34% of the earth pressure at sea level = 344 hPa). The spaceships could be lighter due to the lower internal pressure. After the painful findings from the Apollo 1 disaster, 40% nitrogen was added (only in the start phase) . Since there was a nitrogen-oxygen atmosphere with pressure at sea level in the Soviet spaceships, there were some related problems to be solved in connection with the coupling between the two as part of the Apollo-Soyuz project (1975). In the Soyuz command capsule, the usual pressure of 100% of the earth's atmosphere (1013 hPa) for this mission was reduced to 68% (689 hPa). The Apollo spacecraft carried a docking adapter that served as a coupling module and air lock.
Increasing the oxygen (partial) pressure - and somewhat omitting nitrogen - increases the risk of fire. It was only with the space shuttle that NASA switched to a nitrogen-oxygen atmosphere at normal pressure.
Also trains such as the ICE , IC and almost all regional trains now use pressurized cabins to avoid incriminating eardrum air pressure surges when entering a tunnel or encounter with a return. This results from the higher driving speeds and increased comfort requirements.
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