Wednesday, January 24, 2007

Air safety

Navigation aids
One of the first navigation aids to be introduced was the introduction of airfield lighting to assist pilots to make landings in poor weather or after dark, introduced in the USA in the late 1920s. The concept of approach lighting was developed from this in the 1930s, indicating to the pilot the angle of descent to the airfield, which later became adopted internationally through the standards of the International Civil Aviation Organization (ICAO).

With the spread of radio technology, several experimental radio based navigation aids were developed from the late 20s onwards. These were most successfully used in conjunction with instruments in the cockpit in the form of Instrument Landing Systems (ILS), first used by a scheduled flight to make a landing in a snowstorm at Pittsburgh in 1938. A form of ILS was adopted by the ICAO for international use in 1949.

Following the development of radar in World War II, it was deployed as a landing aid for civil aviation in the form of Ground Control Approach (GCA) systems, joined in 1948 by Distance Measuring Equipment (DME), and in the 1950s by airport surveillance radar as an aid to air traffic control.

All of the ground-based navigation aids are rapidly being displaced by satellite-based aids like GPS, which make it possible for aircrews to know their position with great precision anywhere in the world. With the arrival of Wide Area Augmentation System (WAAS), GPS navigation has become accurate enough for vertical (altitude) as well as horizontal use, and is being used increasingly for instrument approaches as well as en-route navigation. However, since the GPS constellation is a single-point of failure that can be switched off by the U.S. military in time of crisis, ground-based navigation aids are still required for backup.

Air safety topics


While aircraft are able to withstand normal lightning strikes, the dangers of more powerful positive lightning were not understood until the destruction of a glider in 1999 [1]. It has since been suggested that it may have been positive lightning that caused the crash of Pan Am Flight 214 in 1963. At the present time aircraft are not designed to withstand such strikes, since their existence was unknown at the time standards were set.

Engine failure
Although aircraft are now designed to fly even after the failure of one or more aircraft engines, the failure of the second engine on one side for example is obviously serious or even more when it's all of them, as illustrated by the 1970 Dominicana DC-9 air disaster, when fuel contamination caused the failure of both engines. To have an emergency landing place is then very important.

Metal fatigue can also have similar consequences (see below).

A very unusual class of "engine failure" occurred in 1979 when a complete engine detached from American Airlines Flight 191, causing damage to the aircraft from which the pilots were unable to recover.

Metal fatigue
Metal fatigue has occasionally caused failure either of the engine (for example in the 1989 Kegworth Air Disaster), or even of the aircraft body, for example of the De Havilland Comets in 1953 and 1954. Now that the subject is better understood, rigorous inspection and nondestructive testing procedures are in place to attempt to identify potential problems.

Composite materials consist of layers of fibers embedded in a resin matrix. In some cases, especially when subjected to cyclic stress, the fibers may tear off the matrix, the layers of the material then separate from each other - a process called delamination, and form a mica-like structure which then falls apart. As the failure insidiously develops inside the material, nothing much is shown on the surface; instrument methods (often ultrasound-based) have to be used.

On November 12, 2001, American Airlines Flight 587 crashed shortly after a takeoff, killing all 260 persons aboard and 5 more on the ground. Both the engines of the Airbus A300-600, the rudder and the tail fin separated from the plane before impact. Numerous more modern aircrafts developed related problems, but most were discovered before they caused a catastrophic failure.

Delamination risk is as old as composite material. Even in 1940s, several Yak-9s experienced delamination of plywood in their construction.

Stalling an aircraft (increasing the angle of attack to a point at which the wings fail to produce enough lift) is a potential danger, but is normally recoverable. Certain devices have been developed to warn the pilot as stall approaches. These include stall warning horns (now standard on virtually all powered aircraft) stick shakers and voice warnings. The best known stall-related airline accident was the Staines air disaster in 1972.

Safety regulations control aircraft materials and the requirements for automated fire safety systems. Usually these requirements take the form of required tests. The tests measure flammability and the toxicity of smoke. When the tests fail, they fail on a prototype in an engineering laboratory, rather than in an aircraft.

Occasionally these measures have failed. Fire on board the aircraft, especially the toxic smoke generated, have been the cause of several incidents. An electrical fire on Air Canada Flight 797 in 1983 caused the deaths of 23 of the 46 passengers, resulting in the introduction of floor level lighting to assist people to evacuate a smoke filled aircraft. Two years later a fire on the runway caused the loss of 53 lives, 48 from the effects of smoke, in the 1985 Manchester air disaster. This incident raised serious concerns over the standard aircraft emergency evacuation time of ninety seconds, and calls for the introduction of smoke hoods or misting systems although both were rejected. It did result in the introduction of revised overwing emergency exit doors on certain new aircraft, and a small increase in the spacing between seats next to the emergency exit.

Bird Strike
Bird strike is an aviation term for when there is a collision between a bird and an aircraft. It is a common threat to aircraft safety and has caused a number of fatal accidents. In 1988 an Ethiopian Airlines Boeing 737 sucked pigeons into both engines during take-off and then crashed in an attempt to return to the Bahir Dar airport; of the 104 people aboard, 35 died and 21 were injured. In another incident in 1995, a Dassault Falcon 20 crashed at a Paris airport during an emergency landing attempt after sucking lapwings into its rear engine, which caused an engine failure and a fire in the airplane fuselage; all 10 people on-board were killed. [2]

Modern jet engines have certain limited capability of surviving an ingestion of a bird. Small fast planes, like eg. military jet fighters, are in higher risk than big heavy multi-engine ones.

The highest risk of the bird strike is during the takeoff and landing, in low altitudes, which is in the vicinity of the airports. Some airports use active countermeasures, ranging from a man with a shotgun through recorded sounds of predators to employing falconers. There are also exotic solutions, like eg. planting poisonous grass along the runways that is not palatable to birds, nor to insect that attracts insectivorous birds. Passive countermeasures involve sensible land-use management, avoiding conditions attracting flocks of birds to the area (eg. landfills).

Volcanic ash
Plumes of volcanic ash in the vicinity of active volcanoes present a risk especially for night flights. It is hard and abrasive and can quickly cause significant wear on the propellers and turbocompressor blades, and scratch the cabin windows, impairing visibility. It contaminates fuel and water systems, can jam gears, and can cause a flameout of the engines. Its particles have low melting point, so they melt in the combustion chamber and the glass mass then sticks on the turbine blades, fuel nozzles, and the combustors, which can lead to a total engine failure. It can get inside the cabin and contaminate everything there, and can damage the airplane electronics. [3]

There are many instances of damage to a jet aircraft in an ash encounter. In one of them in 1982, a British Airways Boeing 747 flew through an ash cloud, lost all four engines, and descended from 36,000 feet to only 12,000 feet before the flight crew managed to restart the engines.

With the growing density of air traffic, encounters like this are becoming commonplace. In 1991 the aviation industry decided to set up Volcanic Ash Advisory Centers (VAACs), one for each of 9 regions of the world, acting as liaisons between meteorologists, volcanologists, and the aviation industry. [4]

Human factors
Human factors including pilot error are another potential danger, and currently the most common factor of aviation crashes. Much progress in applying human factors to improving aviation safety was made around the time of World War II by people such as Paul Fitts and Alphonse Chapanis. However, there has been progress in safety throughout the history of aviation, such as the development of the pilot's checklist in 1937. [5] The ability of the flight crew to continually maintain situation awareness is a critical human factor in air safety.

Failure of the pilots to properly monitor the flight instruments resulted in the crash of Eastern Airlines Flight 401 in 1972, and error during take-off and landing can have catastrophic consequences, for example cause the crash of Prinair Flight 191 on landing (also in 1972), and the 1958 Munich air disaster on take-off during a blizzard. As in this latter case, other factors such as the weather often contribute to pilot error incidents.

The collision of aircraft can take place in the air (1978 PSA Flight 182) and on the ground (1977 Tenerife disaster), both of which involved pilot error.

Very rarely, flight crew members are arrested or subject to disciplinary action for being intoxicated on the job. In 1990, three Northwest Airlines crew members were sentenced to jail time for flying from Fargo, North Dakota to Minneapolis-St. Paul International Airport while drunk. In 2001, Northwest fired a pilot who failed a breathalyzer test after flying from San Antonio, Texas to Minneapolis-St.Paul. In July 2002, two America West pilots were arrested just before they were scheduled to fly from Miami, Florida to Phoenix, Arizona because they had been drinking alcohol before the flight. The pilots have been fired from America West and the FAA revoked their pilot's licenses. As of 2005 they await trial in a Florida court [6]. The incident created a public relations problem and America West has become the object of many jokes about drunk pilots. While these drunk-flying incidents did not result in crashes, they underscore the role that poor human choices can play in air accidents.

Human factors incidents are not limited to errors by the pilots. The failure to correctly close a cargo door on Turkish Airlines Flight 981 in 1974 resulted in the loss of the aircraft - however the design of the cargo door latch was also a major factor in the incident.

Controlled flight into terrain (CFIT) is a class of accident in which a perfectly good aircraft is flown, under control, into terrain. CFIT accidents typically are a result of pilot error or of navigational system error. Some pilots, convinced that advanced electronic navigation systems such as GPS and INS coupled with Flight Management System computers are partially responsible for these accidents, have called CFIT accidents "computerized flight into terrain". Failure to protect Instrument Landing System critical areas can also cause controlled flight into terrain. Crew awareness and monitoring of navigational systems can prevent or eliminate CFIT accidents. Crew resource management is a modern method now widely used to improve the human factors of air safety. The Aviation Safety Reporting System, or ASRS is another.