Cabin pressurization [EN]

Cabin pressurization is the active pumping of compressed air into an aircraft cabin when flying at altitude to maintain a safe and comfortable environment for crew and passengers in the low outside atmospheric pressure. Pressurization is essential over 3,000 metres (9,800 ft) above sea level to protect crew and passengers from the risk of hypoxia and a number of other physiological problems that occur in the thin air above that altitude; it also serves to generally increase passenger comfort. At a cruising altitude of 39,000 feet, a Boeing 767's cabin will be pressurized to an altitude of 6,900 feet.

The need for cabin pressurization

Flights above 3,000 metres (9,800 ft) in unpressurized aircraft put crew and passengers at risk of four illnesses: hypoxia, altitude sickness, decompression sickness and barotrauma.
Hypoxia. The low partial pressure of oxygen at altitude reduces the alveolar oxygen tension in the lungs and subsequently in the brain leading to sluggish thinking, dimmed vision, loss of consciousness and ultimately death. In some individuals, particularly those with heart or lung disease, symptoms may begin as low as 1,500 metres (4,900 ft) although most passengers can tolerate altitudes of 2,500 metres (8,200 ft) without ill effect. At this altitude, there is about 25% less oxygen than there is at sea level. Hypoxia may be addressed by the administration of supplemental oxygen, usually through an oxygen mask sometimes through a nasal cannula.
Altitude sickness. The low local partial pressure of carbon dioxide (CO₂) causes CO₂ to out-gas from the blood raising the blood pH and inducing alkalosis. Passengers may experience fatigue, nausea, headaches, sleeplessness and on extended flights even pulmonary oedema. These are the same symptoms that mountain climbers experience but the limited duration of powered flight makes the development of pulmonary oedema unlikely. Altitude sickness may be controlled by a full pressure suit with helmet and faceplate, which completely envelopes the body in a pressurized environment; this is clearly impractical for commercial passengers.
Decompression sickness. The low local partial pressure of gases, principally nitrogen (N₂) but including all other gases, may cause dissolved gases in the bloodstream to precipitate out resulting in gas embolism or bubbles in the bloodstream. The mechanism is the same as for compressed air divers on ascent from depth. Symptoms may include the early symptoms of the diver's bends: tiredness, forgetfulness, headache, stroke, thrombosis subcutaneous itching but rarely the full symptoms of the bends. Decompression sickness may also be controlled by a full pressure suit as for altitude sickness.
Barotrauma. As the aircraft climbs or descends passengers may experience discomfort or acute pain as gases trapped within their bodies expand or contract. The most common problems occur with air trapped in the middle ear (aerotitus) or paranasal sinuses by a blocked Eustachian tube or sinuses. Pain may also be experienced in the gastrointestinal tract or even the teeth (barodontalgia). Usually these are not severe enough to cause actual trauma but can result in soreness in the ear that persists after the flight and can exacerbate or precipitate pre-existing medical conditions such as pneumothorax.

Pressurized flight

Maintaining the cabin pressure altitude to below 3,000 metres (9,800 ft) generally avoids significant hypoxia, altitude sickness, decompression sickness and barotrauma. Emergency oxygen systems are installed, both for passengers and cockpit crew, to prevent loss of consciousness in the event that cabin pressure rapidly falls below the level equivalent to 10,000 feet with respect to mean sea level. Those systems contain more than enough oxygen for all on board, to give the pilot adequate time to descend the plane to a safe altitude, where supplemental oxygen is not needed. Federal Aviation Administration (FAA) regulations in the U.S. mandate that the cabin altitude may not exceed 8,000 feet at the maximum operating altitude of the airplane under normal operating conditions. Prior to 1996, approximately 6,000 large commercial transport airplanes were type certificated to fly up to 45,000 feet, without being required to meet high altitude special conditions. In 1996, the FAA adopted Amendment 25-87, which imposed additional high altitude cabin pressure specifications, for new type aircraft designs. For aircraft certified to operate above 25,000 feet, it "must be designed so that occupants will not be exposed to cabin pressure altitudes in excess of 15,000 feet after any probable failure condition in the pressurization system." In the event of a decompression which results from "any failure condition not shown to be extremely improbable," the plane must be designed so that occupants will not be exposed to a cabin altitude exceeding 25,000 feet for more than 2 minutes, nor exceeding an altitude of 40,000 feet at any time. In practice, that new Federal Aviation Regulations amendment imposes an operational ceiling of 40,000 feet on the majority of newly designed commercial aircraft.
However, companies that build the newer designed aircraft can apply for exemption from that more restrictive rule. In 2004, Airbus successfully petitioned the FAA to allow cabin pressure of the A380 to reach 43,000 feet in the event of a decompression incident, and to exceed 40,000 feet for one minute. This special exemption allows the A380 to operate at a higher altitude than other newly designed civilian aircraft, which have not yet been granted a similar exemption. The pressure maintained within the cabin is referred to as the equivalent effective cabin altitude or more normally, the "cabin altitude". Cabin altitude is not normally maintained at average mean sea level (MSL) pressure (1013.25 hPa, or 29.921 inches of mercury) throughout the flight, because doing so would cause the designed differential pressure limits of the fuselage to be exceeded. An aircraft planning to cruise at 40,000 ft (12,000 m) is programmed to rise gradually from take-off to around 8,000 ft (2,400 m) in cabin pressure altitude, and to then reduce gently to match the ambient air pressure of the destination. Pressurization is achieved by the design of an airtight fuselage engineered to be pressurized with a source of compressed air and controlled by an environmental control system (ECS). The most common source of compressed air for pressurization is bleed air extracted from the compressor stage of a gas turbine engine, from a "low" or "intermediate" stage and also from an additional "high" stage. "The exact stage can vary, depending on engine type." By the time the cold outside air has reached the bleed air valves, it has been heated to around 200 °C (392 °F) and is at a very high pressure. The control and selection of high or low bleed sources is fully automatic and is governed by the needs of various pneumatic systems at various stages of flight. The part of the bleed air that is directed to the ECS, is then expanded and cooled to a suitable temperature by passing it through a heat exchanger and air cycle machine ('the packs system'). In some of the larger airliners, hot trim air can be added downstream of air conditioned air coming from the packs, if it is needed to warm a section of the cabin that is colder than other sections.
At least two engines provide compressed bleed air for all of the plane's pneumatic systems, to provide full redundancy. Compressed air is also obtained from the auxiliary power unit (APU), if fitted, in the event of an emergency and for cabin air supply on the ground before the main engines are started. Most modern commercial aircraft today have fully redundant, duplicated electronic controllers for maintaining pressurization, along with a manual back-up control system.

Outflow and pressure relief valve on a Boeing 737-800

 

All exhaust air is dumped to atmosphere via an outflow valve, usually at the rear of the fuselage. This valve controls the cabin pressure and also acts as a safety relief valve, in addition to other safety relief valves. In the event that the automatic pressure controllers fail, the pilot can manually control the cabin pressure valve, according to the backup emergency procedure checklist. The automatic controller normally maintains the proper cabin pressure altitude by constantly adjusting the outflow valve position, so that the cabin pressure is as near to sea level pressure as practical, without exceeding the maximum differential limit of 8.60 psi. At 39,000 feet, the cabin pressure would be automatically maintained at about 6,900 feet (450 feet lower than Mexico City), which is about 11.5 psi of atmosphere pressure (76 kPa). Some aircraft, such as the Boeing 787, have re-introduced the use of electric compressors previously used on piston-engined airliners to provide pressurization. The use of electric compressors increases the electrical generation load on the engines and introduces a number of stages of energy transfer, therefore it is unclear whether this increases the overall efficiency of the aircraft air handling system. It does, however, remove the danger of chemical contamination of the cabin, simplifies engine design, avoids the need to run high pressure pipework around the aircraft and provides greater design flexibility. Cabin altitudes are maintained at up to 2,500 metres (8,200 ft), so pressurization does not eliminate all physiological problems. Passengers with conditions such as pneumothorax are advised not to fly until fully healed; pain may still be experienced in the ears and sinuses by people suffering from a cold or other infection; SCUBA divers flying within the 'no fly' period after a dive may risk decompression sickness, because accumulated nitrogen in their bodies can form bubbles when exposed to reduced cabin pressure.

History

The aircraft that pioneered pressurized cabin systems include:

• Junkers Ju 49 (1931 - a German experimental aircraft purpose built to test the concept of cabin pressurization)
• Lockheed XC-35 (1937 - the first American pressurized aircraft also purpose built to test the concept)
• Boeing 307 (1938 - the first pressurized piston airliner)
• Lockheed Constellation (1943 - the first pressurized airliner in wide service)
• Avro Tudor (1946 - first British pressurized airliner)
• de Havilland Comet (British, Comet 1 1949 - the first jetliner, Comet 4 1958 - resolving the Comet 1 problems)
• Tupolev Tu-144 and Concorde (1968 USSR and 1969 Anglo-French respectively - first to operate at very high altitude)

The first airliner with a pressurized cabin was the Boeing 307 Stratoliner, built 1938, prior to World War II, though only ten were produced. The 307's "pressure compartment was from the nose of the aircraft to a pressure bulkhead in the aft just forward of the horizontal stabilizer."
World War II was a catalyst for aircraft development. Initially the piston aircraft of World War II, though they often flew at very high altitudes were not pressurized and relied on oxygen masks. This became impractical with the development of larger bombers where crew were required to move about the cabin and this led to the first bomber with cabin pressurization (though restricted to crew areas), the B-29 Superfortress. The control system for this was designed by Garrett AiResearch Manufacturing Company, drawing in part on licensing of patents held by Boeing for the Stratoliner.
Post-war piston airliners such as the Lockheed Constellation (1943) extended the technology to civilian service. The piston engined airliners generally relied on electrical compressors to provide air and operated below 20,000 ft where the piston engine is more efficient. Designing a pressurized fuselage to cope with this altitude was within the engineering and metallurgical knowledge of the time. The introduction of jet airliners required a large increase in cruise altitude to 30,000 ft where the jet engine is more efficient. This increase in altitude required far more rigorous engineering of the fuselage and in the beginning not all the engineering problems were understood.
The world’s first commercial jet airliner was the British de Havilland Comet (1949) designed with a service ceiling of 36,000 ft (11,000 m). It was the first time that a large diameter, pressurized fuselage with windows had been built and flown at this altitude. Initially the design was very successful but two catastrophic airframe failures in 1954 resulting in the total loss of the aircraft, passengers and crew grounded what was then the entire world jet airliner fleet. Extensive investigation and groundbreaking engineering analysis of the wreckage led to a number of very significant engineering advances that solved the basic problems of pressurized fuselage design at altitude. The critical problem proved to be a combination of an inadequate understanding of the effect of progressive metal fatigue as the fuselage undergoes repeated stress cycles coupled with a misunderstanding of how aircraft skin stresses are redistributed around openings in the fuselage such as windows and rivet holes.
The critical engineering principles learned from the Comet 1 program were applied directly to the design of the Boeing 707 (1957) and all subsequent jet airliners. One immediately noticeable legacy of the Comet disasters is the oval windows on every jet airliner; the metal fatigue cracks that destroyed the Comets were initiated by the small radius corners on the Comet 1’s almost square windows. The Comet fuselage was redesigned and the Comet 4 (1958) went on to become a successful airliner, pioneering the first transatlantic jet service, but the program never really recovered from these disasters and was overtaken by the Boeing 707.
Concorde had to deal with unusually high pressure differentials, as of necessity it flew at unusually high altitude (up to 60,000 ft) while the cabin altitude was maintained at 6000 ft. This made the vehicle significantly heavier and contributed to the high cost of a flight. Concorde also had to have smaller than normal cabin windows to limit decompression speed in the event of window failure. The designed operating cabin altitude for new aircraft is falling and this is expected to reduce any remaining physiological problems. The lowest cabin altitude of any airliner either already flying or in current development is the A380, designed to maintain a cabin altitude of 1,520 m (5,000 ft).

Loss of pressurization

Passenger oxygen mask deployment

 

Unplanned loss of cabin pressure at altitude is rare but has resulted in a number of fatal accidents. Failures range from sudden, catastrophic loss of airframe integrity (explosive decompression) to slow leaks or equipment malfunctions that allow cabin pressure to drop undetected to levels that can lead to unconsciousness or severe performance degradation of the aircrew. Any failure of cabin pressurization above 3,000 metres (9,800 ft) requires an emergency descent to 3,000 metres (9,800 ft) or the minimum altitude that maintains terrain clearance (MSA), whichever is higher, and the deployment of an oxygen mask for each seat. Without emergency oxygen hypoxia may lead to loss of consciousness and a subsequent loss of control of the aircraft, the Time of Useful Consciousness varying according to altitude. The air temperature will also plummet to the ambient outside temperature with a danger of hypothermia or frostbite.

Uncontrolled decompression

 Uncontrolled decompression refers to an unexpected drop in the pressure of a sealed system, such as an aircraft cabin. Where the speed of the decompression occurs faster than air can escape from the lungs, this is known as explosive decompression (ED), and is associated with explosive violence. Where decompression is still rapid, but not faster than the lungs can decompress, this is known as rapid decompression. Lastly, slow decompression or gradual decompression occurs so slowly that humans may not detect it before hypoxia sets in.
Generally uncontrolled decompression results from human error, material fatigue, engineering failure or impact, that causes a pressure vessel either not to pressurize, or to vent into lower-pressure surroundings.
The term uncontrolled decompression here refers to the unplanned depressurisation of vessels that are occupied by people, for example an aircraft cabin at high altitude, a spacecraft, or a hyperbaric chamber. For the catastrophic failure of other pressure vessels used to contain gas, liquids, or reactants under pressure, the term explosion is more commonly used, or other specialised terms such as BLEVE may apply to particular situations.
Decompression can occur due to structural failure of the pressure vessel, or failure of the compression system itself. The speed and violence of the decompression is affected by the size of the pressure vessel, the differential pressure between the inside and outside of the vessel and the size of the leak hole.
The Federal Aviation Administration recognizes three distinct types of decompression events in aircraft:

• Explosive decompression
• Rapid decompression
• Gradual decompression

Explosive decompression

Explosive decompression occurs at a rate swifter than that at which air can escape from the lungs, typically in less than 0.1 to 0.5 seconds. The risk of lung trauma is very high, as is the danger from any unsecured objects that can become projectiles because of the explosive force, which may be likened to a bomb detonation.
After an explosive decompression within an aircraft, a heavy fog may immediately fill the interior as the cabin air very rapidly cools and condenses. Military pilots with oxygen masks have to pressure-breathe, whereby the lungs fill with air when relaxed, and effort has to be exerted to expel the air again.

Rapid decompression

Rapid decompression typically takes more than 0.1 to 0.5 seconds, allowing the lungs to decompress more quickly than the cabin. The risk of lung damage is still present, but significantly reduced compared with explosive decompression.

Slow decompression

Slow, or gradual, decompression occurs slowly enough to go unnoticed and might only be detected by instruments. This type of decompression may also come about from a failure to pressurize as an aircraft climbs to altitude. An example of this is the Helios Airways Flight 522 crash, in which the pilots failed to check the aircraft was pressurising automatically and then react to the warnings that the aircraft was depressurising.

Fallacies

Exposure to a vacuum causes the body to explode

This persistent myth is based on a misunderstanding of explosive decompression possibly fuelled by many misrepresentations in popular media such as the film Licence to Kill; in this film one character's head explodes after his hyperbaric chamber is rapidly depressurized. Research and experience in space exploration and high-altitude aviation have shown that while exposure to vacuum causes swelling, human skin is tough enough to withstand the drop of one atmosphere although the resulting hypoxia will cause unconsciousness after a few seconds. Misconceptions may also arise from confusion with diving accidents where the pressure differentials are much higher than those in space. In the Byford Dolphin accident a catastrophic pressure drop of eight atmospheres caused massive, lethal, barotrauma but this does not apply to a loss of only one atmosphere although some pulmonary barotrauma is possible if the breath is forcibly held.

Bullets cause explosive decompression

Aircraft fuselages are designed with ribs to prevent tearing; the size of the hole is one of the factors that determines the speed of decompression, and a bullet hole is too small to cause rapid or explosive decompression.

A small hole will blow people out of a fuselage

The television program Mythbusters examined this belief informally using a pressurised aircraft and several scale tests. The Mythbusters approximations suggested that fuselage design does not allow this to happen. There has been no scientific verification of their results: conclusions obtained by Mythbusters should not be taken as scientifically conclusive. An air hostess was blown from Aloha Airlines Flight 243 when a large section of cabin roof (about 18' x 25') detached; the report states she was swept overboard rather than sucked through the hole.

Decompression injuries

The following physical injuries may be associated with decompression incidents:

• Hypoxia is the most serious risk associated with decompression, especially as it may go undetected or incapacitate the aircrew.
• Barotrauma: an inability to equalize pressure in internal air spaces such as the middle ear or gastrointestinal tract, or more serious injury such as a burst lung.
• Decompression sickness.
• Physical trauma caused by the violence of explosive decompression, which can turn people and loose objects into projectiles.
• Altitude sickness
• Frostbite or hypothermia from exposure to freezing cold air at high altitude.

Implications for aircraft design

Modern aircraft are specifically designed with longitudinal and circumferential reinforcing ribs in order to prevent localised damage from tearing the whole fuselage open during a decompression incident. However, decompression events have nevertheless proved fatal for aircraft in other ways. In 1974, explosive decompression onboard Turkish Airlines Flight 981 caused the floor to collapse, severing vital flight control cables in the process. The FAA issued an Airworthiness Directive the following year requiring manufacturers of wide-body aircraft to strengthen floors so that they could withstand the effects of in-flight decompression caused by an opening of up to 20 square feet (1.9 m²) in the lower deck cargo compartment. Manufacturers were able to comply with the Directive either by strengthening the floors and/or installing relief vents called "dado panels" between the passenger cabin and the cargo compartment.
Cabin doors are designed to make it almost impossible to lose pressurization through opening a cabin door in flight, either accidentally or intentionally. The plug door design ensures that when the pressure inside the cabin exceeds the pressure outside the doors are forced shut and will not open until the pressure is equalised. Cabin doors, including the emergency exits, but not all cargo doors, open inwards, or must first be pulled inwards and then rotated before they can be pushed out through the door frame because at least one dimension of the door is larger than the door frame.
Prior to 1996, approximately 6,000 large commercial transport airplanes were type certificated to fly up to 45,000 feet, without being required to meet special conditions related to flight at high altitude. In 1996, the FAA adopted Amendment 25-87, which imposed additional high-altitude cabin-pressure specifications, for new designs of aircraft types. For aircraft certificated to operate above 25,000 feet (FL 250), it "must be designed so that occupants will not be exposed to cabin pressure altitudes in excess of 15,000 feet after any probable failure condition in the pressurization system." In the event of a decompression which results from "any failure condition not shown to be extremely improbable," the aircraft must be designed so that occupants will not be exposed to a cabin altitude exceeding 25,000 feet for more than 2 minutes, nor exceeding an altitude of 40,000 feet at any time. In practice, that new FAR amendment imposes an operational ceiling of 40,000 feet on the majority of newly designed commercial aircraft.
In 2004, Airbus successfully petitioned the FAA to allow cabin pressure of the A380 to reach 43,000 feet in the event of a decompression incident, and to exceed 40,000 feet for one minute. This special exemption allows that new aircraft to operate at a higher altitude than other newly-designed civilian aircraft, which have not yet been granted a similar exemption.

Fly-by-wire [EN]

A fly-by-wire (FBW) system replaces manual flight control of an aircraft with an electronic interface. The movements of flight controls are converted to electronic signals transmitted by wires (hence the fly-by-wire term), and flight control computers determine how to move the actuators at each control surface to provide the ordered response. The fly-by-wire system also allows automatic signals sent by the aircraft's computers to perform functions without the pilot's input, as in systems that automatically help stabilize the aircraft.

Development

Mechanical and hydro-mechanical flight control systems are relatively heavy and require careful routing of flight control cables through the aircraft by systems of pulleys, cranks, tension cables and hydraulic pipes. Both systems often require redundant backup to deal with failures, which again increases weight. Furthermore, both have limited ability to compensate for changing aerodynamic conditions. Dangerous characteristics such as stalling, spinning and pilot-induced oscillation (PIO), which depend mainly on the stability and structure of the aircraft concerned rather than the control system itself, can still occur with these systems.
The term "fly-by-wire" implies a purely electrically-signaled control system. However, it is used in the general sense of computer-configured controls, where a computer system is interposed between the operator and the final control actuators or surfaces. This modifies the manual inputs of the pilot in accordance with control parameters.
Side-sticks, center sticks, or conventional flight control yokes can be used to fly FBW aircraft. While the side-stick offers the advantages of being lighter, mechanically simpler, and unobtrusive, The Boeing Company's aerospace engineers decided that the lack of visual feedback (none given by side-sticks) is a significant problem, and so they designed conventional control yokes in the Boeing 777 and the brand-new Boeing 787, which is undergoing flight tests as of June 2010^[update]. This same approach has been used for the Embraer 170/190 jets. Most Airbus airliners are operated with side-sticks.

Basic operation

Command

Simple feedback loop

Fly-by wire systems are by their nature quite complex however their operation can be explained in relatively simple terms. When a pilot moves the control column (or sidestick) a signal is sent to a computer, this is analogous to moving a game controller, the signal is sent through multiple wires (channels) to ensure that the signal reaches the computer. When there are three channels being used this is known as 'Triplex'. The computer receives the signals, performs a calculation (adds the signal voltages and divides by the number of signals received to find the mean average voltage) and adds another channel. These four 'Quadruplex' signals are then sent to the control surface actuator and the surface begins to move. Potentiometers in the actuator send a signal back to the computer (usually a negative voltage) reporting the position of the actuator. When the actuator reaches the desired position the two signals (incoming and outgoing) cancel each other out and the actuator stops moving (completing a feedback loop).

Automatic Stability Systems

Fly-by-wire control systems allow aircraft computers to perform tasks without pilot input. Automatic stability systems operate in this way. Gyroscopes fitted with sensors are mounted in an aircraft to sense movement changes in the pitch, roll and yaw axes. Any movement (from straight and level flight for example) results in signals to the computer, which automatically moves control actuators to stabilize the aircraft.

Safety and redundancy

Aircraft systems may be quadruplexed (four independent channels) to prevent loss of signals in the case of failure of one or even two channels. High performance aircraft that have FBW controls (also called CCVs or Control-Configured Vehicles) may be deliberately designed to have low or even negative aerodynamic stability in some flight regimes, the rapid-reacting CCV controls compensating for the lack of natural stability.
Pre-flight safety checks of a fly-by-wire system are often performed using Built-In Test Equipment (BITE). On programming the system, either by the pilot or groundcrew, a number of control movement steps are automatically performed. Any failure will be indicated to the crews.
Some aircraft, the Panavia Tornado for example, retain a very basic hydro-mechanical backup system for limited flight control capability on losing electrical power, in the case of the Tornado this allows rudimentary control of the stabilators only for pitch and roll axis movements.

Weight Saving

A FBW aircraft can be lighter than a similar design with conventional controls. Partly due to the lower overall weight of the system components; and partly because the natural aerodynamic stability of the aircraft can be relaxed, slightly for a transport aircraft and more for a maneuverable fighter, which means that the stability surfaces that are part of the aircraft structure can therefore be made smaller. These include the vertical and horizontal stabilizers (fin and tailplane) that are (normally) at the rear of the fuselage. If these structures can be reduced in size, airframe weight is reduced. The advantages of FBW controls were first exploited by the military and then in the commercial airline market. The Airbus series of airliners used full-authority FBW controls beginning with their A320 series, see A320 flight control (though some limited FBW functions existed on A310). Boeing followed with their 777 and later designs.
Electronic fly-by-wire systems can respond flexibly to changing aerodynamic conditions, by tailoring flight control surface movements so that aircraft response to control inputs is appropriate to flight conditions. Electronic systems require less maintenance, whereas mechanical and hydraulic systems require lubrication, tension adjustments, leak checks, fluid changes, etc. Furthermore, putting circuitry between pilot and aircraft can enhance safety; for example the control system can try to prevent a stall, or it can stop the pilot from over stressing the airframe.
The main concern with fly-by-wire systems is reliability. While traditional mechanical or hydraulic control systems usually fail gradually, the loss of all flight control computers could immediately render the aircraft uncontrollable. For this reason, most fly-by-wire systems incorporate either redundant computers (triplex, quadruplex etc.), some kind of mechanical or hydraulic backup or a combination of both. A "mixed" control system such as the latter is not desirable and modern FBW aircraft normally avoid it by having more independent FBW channels, thereby reducing the possibility of overall failure to minuscule levels that are acceptable to the independent regulatory and safety authority responsible for aircraft design, testing and certification before operational service.

History

F-8C Crusader digital fly-by-wire testbed

Electronic signalling of the control surfaces was tested in the 1950s. This replaced long runs of mechanical and hydraulic connections with electrical ones.
The first non-experimental aircraft that was designed and flown (in 1958) with a fly-by-wire flight control system was the Avro Canada CF-105 Arrow.A feat not repeated with a production aircraft until Concorde in 1969. This system also included solid-state components and system redundancy, was designed to be integrated with a computerised navigation and automatic search and track radar, was flyable from ground control with data uplink and downlink, and provided artificial feel (feedback) to the pilot.
The first digital fly-by-wire aircraft to take to the air (in 1972) was an F-8 Crusader, which had been modified electronically by the National Aeronautics and Space Administration of the United States as a test aircraft, a feat mirrored in the USSR by the Sukhoi T-4. At about the same time in the United Kingdom a trainer variant of the British Hawker Hunter fighter was modified at the British Royal Aircraft Establishment with fly-by-wire flight controls for the right-seat pilot. This was test-flown, with the left-seat pilot having conventional flight controls for safety reasons, and with the capability for him to override and turn off the fly-by-wire system.

Analog systems

All "fly-by-wire" flight control systems eliminate the complexity, the fragility, and the weight of the mechanical circuit of the hydromechanical or electromechanical flight control systems. Fly-by-wire replace those with electronic circuits. The control mechanisms in the cockpit now operate signal transducers, which in turn generate the appropriate electronic commands. These are next processed by an electronic controller, either an analog one, or more modernly, a digital one. Aircraft and spacecraft autopilots are now part of the electronic controller.The hydraulic circuits are similar except that mechanical servo valves are replaced with electrically-controlled servo valves, operated by the electronic controller. This is the simplest and earliest configuration of an analog fly-by-wire flight control system. In this configuration, the flight control systems must simulate "feel". The electronic controller controls electrical feel devices that provide the appropriate "feel" forces on the manual controls. This was used in Concorde, the first production fly-by-wire airliner.
In more sophisticated versions, analog computers replaced the electronic controller. The canceled 1950s Canadian supersonic intercepter, the Avro Canada CF-105 Arrow, employed this type of system. Analog computers also allowed some customization of flight control characteristics, including relaxed stability. This was exploited by the early versions of F-16, giving it impressive maneuverability.

Digital systems

 



The Airbus A320, first airliner with digital fly-by-wire controls



A digital fly-by-wire flight control system is similar to its analog counterpart. However, the signal processing is done by digital computers and the pilot literally can "fly-via-computer". This also increases the flexibility of the flight control system, since the digital computers can receive input from any aircraft sensor (such as the altimeters and the pitot tubes. This also increases the electronic stability, because the system is less dependent on the values of critical electrical components in an analog controller.
The computers sense position and force inputs from pilot controls and aircraft sensors. They solve differential equations to determine the appropriate command signals that move the flight controls to execute the intentions of the pilot.
The programming of the digital computers enable flight envelope protection. In this aircraft designers precisely tailor an aircraft's handling characteristics, to stay within the overall limits of what is possible given the aerodynamics and structure of the aircraft. For example, the computer in flight envelope protection mode can try to prevent the aircraft from being handled dangerously by preventing pilots from exceeding preset limits on the aircraft's flight-control envelope, such as those that prevent stalls and spins, and which limit airspeeds and g forces on the airplane. Software can also be included that stabilize the flight-control inputs to avoid pilot-induced oscillations. Since the flight-control computers continuously "fly" the aircraft, pilot's workloads can be reduced. Also, in military and naval applications, it is now possible to fly military aircraft that have relaxed stability. The primary benefit for such aircraft is more maneuverability during combat and training flights, and the so-called "carefree handling" because stalling, spinning. and other undesirable performances are prevented automatically by the computers.
Digital flight control systems enable inherently unstable combat aircraft, such as the F-117 Nighthawk and the B-2 Spirit flying wing to fly in usable and safe manners.

Applications

• The Space Shuttle Orbiter has an all-digital fly-by-wire control system. This system was first exercised (as the only flight control system) during the glider unpowered-flight "Approach and Landing Tests" that began on the Space Shuttle Enterprise during 1977.
• During 1984, the Airbus Industries Airbus A320 became the first airliner to fly with an all-digital fly-by-wire control system.
• During 2005, the Dassault Falcon 7X (see the picture) became the first business jet with fly-by-wire controls.

Legislation

The Federal Aviation Administration (FAA) of the United States has adopted the RTCA/DO-178B, titled "Software Considerations in Airborne Systems and Equipment Certification", as the certification standard for aviation software. Any safety-critical component in a digital fly-by-wire system including applications of the laws of aeronautics and computer operating systems will need to be certified to DO-178B Level A, which is applicable for preventing potential catastrophic failures. Nevertheless, the top concern for computerized, digital, fly-by-wire systems is reliability, even more so than for analog electronic control systems. This is because the digital computers that are running software are often the only control path between the pilot and aircraft's flight control surfaces. If the computer software crashes for any reason, the pilot may be unable to control an aircraft. Hence virtually all fly-by-wire flight control systems are either triply or quadruply redundant in their computers and electronics. These have three or four flight-control computers operating in parallel, and three or four separate data buses connecting them with each control surface.

Redundancy

If one of the flight-control computers crashes, or is damaged in combat, or suffers from "insanity" caused by electromagnetic pulses, the others overrule the faulty one (or even two of them), they continue flying the aircraft safely, and they can either turn off or re-boot the faulty computers. Any flight-control computer whose results disagree with the others is ruled to be faulty, and it is either ignored or re-booted. (In other words, it is voted-out of control by the others.)
In addition, most of the early digital fly-by-wire aircraft also had an analog electrical, a mechanical, or a hydraulic back-up flight control system. The Space Shuttle has, in addition to its redundant set of four digital computer running its primary flight-control software, a fifth back-up computer running a separately developed, reduced-function, software flight-control system - one that can be commanded to take over in the event that a fault ever affects all of the computers in the other four. This back-up system serves to reduce the risk of total flight-control-system failure ever happening because of a general-purpose flight software fault has escaped notice in the other four computers.
For airliners, flight-control redundancy improves their safety, but fly-by-wire control systems also improve economy in flight because they are lighter, and they eliminate the need for many mechanical, and heavy, flight-control mechanisms. Furthermore, most modern airliners have computerized systems that control their jet engine throttles, air inlets, fuel storage and distribution system, in such a way to minimize their consumption of jet fuel. Thus, digital control systems do their best to reduce the cost of flights.

Airbus/Boeing

Airbus and Boeing commercial airplanes differ in their approaches in using fly-by-wire systems. In Airbus airliners, the flight-envelope control system always retains ultimate flight control, and it will not permit the pilots to fly outside these performance limits. However, in the event of multiple failures of redundant computers, the A320 does have mechanical back-up system for its pitch trim and its rudder. The A340-600 has a purely electrical (not electronic) back-up rudder control system, and beginning with the new A380 airliner, all flight-control systems have back-up systems that are purely electrical through the use of a so-called "three-axis Backup Control Module" (BCM)
With the Boeing 777 model airliners, the two pilots can completely override the computerized flight-control system to permit the aircraft to be flown beyond its usual flight-control envelope during emergencies. Airbus's strategy, which began with the Airbus A320, has been continued on subsequent Airbus airliners.

Engine digital control

The advent of FADEC (Full Authority Digital Engine Control) engines permits operation of the flight control systems and autothrottles for the engines to be fully integrated. On modern military aircraft other systems such as autostabilization, navigation, radar and weapons system are all integrated with the flight control systems. FADEC allows maximum performance to be extracted from the aircraft without fear of engine misoperation, aircraft damage or high pilot workloads.
In the civil field, the integration increases flight safety and economy. The Airbus A320 and its fly-by-wire brethren are protected from dangerous situations such as low-speed stall or overstressing by flight envelope protection. As a result, in such conditions, the flight control systems commands the engines to increase thrust without pilot intervention. In economy cruise modes, the flight control systems adjust the throttles and fuel tank selections more precisely than all but the most skillful pilots. FADEC reduces rudder drag needed to compensate for sideways flight from unbalanced engine thrust. On the A330/A340 family, fuel is transferred between the main (wing and center fuselage) tanks and a fuel tank in the horizontal stabilizer, to optimize the aircraft's center of gravity during cruise flight. The fuel management controls keep the aircraft's center of gravity accurately trimmed with fuel weight, rather than drag-inducing aerodynamic trims in the elevators.

Further developments

Fly-by-optics

Fly-by-optics is sometimes used instead of fly-by-wire because it can transfer data at higher speeds, and it is immune to electromagnetic interference. In most cases, the cables are just changed from electrical to optical fiber cables. Sometimes it is referred to as "fly-by-light" due to its use of fiber optics. The data generated by the software and interpreted by the controller remain the same.

Power-by-wire

Having eliminated the mechanical transmission circuits in fly-by-wire flight control systems, the next step is to eliminate the bulky and heavy hydraulic circuits. The hydraulic circuit is replaced by an electrical power circuit. The power circuits power electrical or self-contained electrohydraulic actuators that are controlled by the digital flight control computers. All benefits of digital fly-by-wire are retained.
The biggest benefits are weight savings, the possibility of redundant power circuits and tighter integration between the aircraft flight control systems and its avionics systems. The absence of hydraulics greatly reduces maintenance costs. This system is used in the Lockheed Martin F-35 Lightning II and in Airbus A380 backup flight controls. The Boeing 787 will also incorporate some electrically operated flight controls (spoilers and horizontal stabilizer), which will remain operational with either a total hydraulics failure and/or flight control computer failure.

Fly-by-wireless

Wiring adds a considerable amount of weight to an aircraft; therefore, researchers are exploring implementing fly-by-wireless solutions. Fly-by-wireless systems are very similar to fly-by-wire systems, however, instead of using a wired protocol for the physical layer a wireless protocol is employed.
In addition to reducing weight, implementing a wireless solution has the potential to reduce costs throughout an aircraft's life cycle. For example, many key failure points associated with wire and connectors will be eliminated thus hours spent troubleshooting wires and connectors will be reduced. Furthermore, engineering costs could potentially decrease because less time would be spent on designing wiring installations, late changes in an aircraft's design would be easier to manage, etc.

Intelligent Flight Control System

A newer flight control system, called Intelligent Flight Control System (IFCS), is an extension of modern digital fly-by-wire flight control systems. The aim is to intelligently compensate for aircraft damage and failure during flight, such as automatically using engine thrust and other avionics to compensate for severe failures such as loss of hydraulics, loss of rudder, loss of ailerons, loss of an engine, etc. Several demonstrations were made on a flight simulator where a Cessna-trained small-aircraft pilot successfully landed a heavily-damaged full-size concept jet, without prior experience with large-body jet aircraft. This development is being spearheaded by NASA Dryden Flight Research Center. It is reported that enhancements are mostly software upgrades to existing fully computerized digital fly-by-wire flight control systems.

The World’s Greatest Aviation Innovations [EN]

It seems all news is bad news when it comes to aviation these days, and it’s too bad because it overshadows just how wonderful it is that we can fly. Think about it — 100 years ago, few could imagine it. Today we take it for granted.

It’s an amazing accomplishment, and too often people lose sight of that. We came up with our top 12 because 10 wasn’t enough.

1. Cabin pressurization — The average passenger doesn’t think about cabin pressurization until their yellow safety masks fall from the ceiling, but the reality is that if the technology hadn’t been developed during WWII, we wouldn’t be able to fly much above 10,000 feet.
2. Black Box — Morbid but essential, the black box was invented in the mid-1950s, and not only helps investigators learn why a plane crashed, but how that information can be applied to other aircraft to prevent a repeat.
3. The Concorde — It never delivered on its commercial promise, and it was an environmental bad boy, but who can deny that breaking the sound barrier aboard a commercial aircraft is cool. And have you ever seen a more beautiful plane?
4. Radar — Sure, the airlines are dying to replace it with GPS technology, but for decades it’s been radar that helps air traffic controllers locate and track planes up to 200 miles away. Would our modern air traffic infrastructure exist without it? Probably not.
5. The jumbo jet — Whether you think they’re graceful or ungainly, you can’t deny that jumbo jets have changed the face of commercial aviation. The economies of scale provided by a 400-seat airliner meant airlines could offer cheap tickets that made it possible for the masses to fly.
6. The hub and spoke system — People hate, hate, hate having to make stopovers at jam packed airports controlled by a single airline. Yeah, they’re expensive to fly into and delay prone, but hub airports are a big part of the reason that you have 20 flights a day to choose from when flying between most large American cities.
7. The Very Light Jet (VLJ) — It’s been a tough road for the VLJ, with manufacturers suffering production problems and customers going out of business, but that doesn’t diminish the allure of a 37 foot, 3,500 pound plane designed to carry four to six passengers on short hops that would otherwise require a car ride.
8. Winglets — Here’s another one that most of us don’t think about. The small upward-pointing extensions at the tips of aircraft wings reduce drag, improve climb performance, increase range, and make flight more fuel efficient. With oil at over $100 a barrel, no wonder most airlines have added winglets across their fleets.
9. The flying wing — Yves Rossy keeps breaking records and defying expectations with his 8-foot-diameter, carbon composite flying wing. Last week he made a successful 13 minute, 125 mph trip across the English Channel.
10. Stealth aircraft — What’s cooler than a plane that can outsmart radar? Because the surfaces of a stealth are designed to absorb radio waves or reflect them away from the receiver, stealth planes can sneak in and sneak out undetected. Too bad they’re so expensive: The 21 plane B-2
program cost over $45 billion.
11. Jetway — Another one most of us don’t think about is the long covered walkway the connects our departure gate with our plane. It means we don’t have to wait outside on the tarmac in sleet and rain, or contend with the shriek of jet engines. The A380 is served by three jet bridges, one of them leading directly to the first class lounge. 
12. Deicing — Ice buildup is the cause of many fatal aircraft accidents, which is why applying monopropylene de-icing fluid to wings pre-flight has become standard operating procedure. Without it, air traffic would ground to a halt every time things got a little stormy.
That’s it. That’s our top 12. 

Aviation Biofuels: More Hype Than Hope? [EN]

The commercial aviation industry, hammered by rising fuel costs and increased pressure to curb emissions, is racing to find cleaner alternatives based on everything from coconut oil to pond scum. But a British aviation watchdog group warns that biofuels do not necessarily offer the salvation airlines seek.
Jeff Gazzard of the Aviation Environment Federation raises serious questions about the safety and viability of aviation biofuels touted by the likes of Virgin Atlantic and, just this week, Continental. He says the push to develop them is at least in part corporate greenwashing by an industry desperate to appease its critics.
"For us, the jury is still well and truly out as to whether either synthetic or biofuels are yet capable of being either entirely fail-safe for aviation use or environmentally sustainable in the longer term," Gazzard writes in his report, Bio-Fueled or Bio-Fooled (.pdf). 
The critical examination of aviation biofuel development comes as a growing number of airlines and aircraft manufacturers join energy firms in the hunt for alternatives to kerosene. The International Air Transport Association (IATA), which represents 93 percent of the world’s carriers, has set a goal of drawing 10 percent of its fuel from renewable sources by 2017.

But Gazzard says a lack of comprehensive safety standards poses a serious threat as increasing amounts of biofuel and other alternatives flow through an infrastructure designed for petroleum. He points to a British airport where conventional aviation fuel was contaminated by biofuel that had moved through the same pipeline. That’s problematic because unlike petroleum, untreated biofuels can freeze at low temperatures and damage seals in aircraft fuel systems.
The IATA says the example cited in Gazzard’s report isn’t relevant. "The AEF article quotes widely from Petroleum Review and faithfully recycles the view of big oil on the biofuels subject," IATA spokesman Steve Lott told Wired.com. "Their examples of contamination in the jet fuel supply are of first-generation biodiesels. Aviation is not even looking at these types of fuels."
Instead, Lott says, the IATA and its members are concentrating on second generation biofuels that are almost chemically identical to conventional jet fuel (Jet-A1), and therefore can be "dropped in" to existing fuel supplies, eliminating contamination concerns.
"The concept of biofuels in aviation has moved on very rapidly from even two years ago," Lott says. "Test results so far are very encouraging."
Encouraging or not, Gazzard is underwhelmed by the high-profile alt-fuel tests we’ve seen to far. Like others, he dismisses as a publicity stunt Virgin’s much-ballyhooed test flight of a Boeing 747 that flew from London to Amsterdam with one of its four fuel tanks carrying a 20 percent mix of biofuel. The plane, which used a mixture of coconut and babassu oils, would have needed some 3 million coconuts had it made the flight entirely on biofuel, he says.
That’s not stopping the industry from squeezing as much good PR out of its biofuel activities as possible. On Wednesday, Continental made a two-hour test flight of a Boeing 737 fueled by a 50-50 mix of jet fuel and a biofuel blended from algae and jatropha. It was the first test by an American carrier, and it came one week after Air New Zealand made a test flight with a plane fueled partly by a jatropha-based fuel. Later this month, Japan Airlines plans a test flight using fuel refined from camelina, a flowering plant grown in the high plains.
"This demonstration flight represents another step in Continental’s ongoing commitment to fuel efficiency and environmental responsibility," Larry Kellner, the airline’s chairman and CEO, said in a statement. "The technical knowledge we gain today will contribute to a wider understanding of the future for transportation fuels."
Boeing says aviation biofuel could be fueling commercial flights within three years. "We’ve been pleasantly surprised by how smoothly these tests are going,"  Billy Glover, Boeing’s managing director of environmental strategy, says, according to the Los Angeles Times.
Gazzard argues the aviation industry and governments are more interested in appeasing critics than finding alternatives to oil.
"Much of this (activity) we feel is political positioning by industry and friendly governments to manufacture consent to keep expanding aviation in the face of growing demands for environmental limits," he writes in the report, "particularly in respect of the worrying increase in aviation’s greenhouse gas and wholly negative climate change impacts."
The report also poses a question others have raised with regard to using biofuels in the transportation sector: Is there enough plant material to meet demand? Although the aviation industry is betting heavily on algal fuels and non-food crops like jatropha, thereby largely avoiding the food-for-fuel debate associated with ethanol and other biofuels, its an open question whether it’s possible to produce enough of it. The aviation industry currently burns about 240 million tons of kerosene a year, a figure that will only rise as countries like India and
China ramp up their aviation operations. China alone is expected to buy 2,230 new planes between now and 2025.
Petroleum Week estimates that producing that much fuel from jatropha, one of the more promising bio-fuel prospects, would require planting 1.4 million square kilometers of it, an area twice the size of
France. And that’s bad news for governments who see alternative fuels as a way to tell Middle Eastern oil barons to shove off. Because as oil supplies dwindle, you can bet your bottom petrol-dollar that the Saudi Arabias of the world ill invest heavily in helophytes and other alt-fuel plants that thrive in the deserts of the Middle East and Africa. Gazzard’s report says that raises the possibility of an OPEC-style biofuel cartel.
He also wonders which of the bio-fuel partnerships announced when oil was going for $150 a barrel will remain now that prices are below $50 a barrel. With the world locked in a global recession, it’s going to be cheaper for airlines to burn jet fuel than invest in alternatives.