During the 1920s, the first laws were passed in the USA to regulate civil aviation. Of particular significance was the Air Commerce Act 1926, which required pilots and aircraft to be examined and licensed, for accidents to be properly investigated, and for the establishment of safety rules and navigation aids, under the Aeronautics Branch of the Department of Commerce.
Despite this, in 1926 and 1927 there were a total of 24 fatal commercial airline crashes, a further 16 in 1928, and 51 in 1929 (killing 61 people), which remains the worst year on record at an accident rate of about 1 for every 1,000,000 miles flown. Based on the current numbers flying, this would equate to 7,000 fatal incidents per year.
Fortunately, the fatal incident rate has continued to decline steadily ever since, and since 1997, the number of fatal air accidents has been no more than 1 for every 2,000,000,000 miles flown, making it one of the safest modes of transport.
Safety improvements have resulted from a wide variety of factors, including improved aircraft design, engineering and maintenance, and the evolution of 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 lightning 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.
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  . 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.
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 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.
Stalling an aircraft (reducing the speed 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 hood s or misting system s 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 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.
Human factors including pilot error are another potential danger. 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 
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.
However, 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.
Accidents and incidents
- Air Accidents Investigation Branch (UK)
- National Transportation Safety Board (USA)
- Transportation Safety Board (Canada)
- Federal Aviation Administration
- United Kingdom Civil Aviation Authority
- Aviation Safety: Advancements Being Pursued to Improve Airliner Cabin Occupant Safety and Health, 2003
- International regulatory & government agencies
- Airline Safety.com
- Aviation Safety Reporting System (ASRS) run by NASA
- The Adolescence of Aviation Psychology