Structural Safety Measures for Airplanes

Passenger aircraft structural design is based on a safety factor of 1.5, and this safety factor alone is equivalent to a probability of failure of between 10^-2 and 10^-3. Yet airliners are much safer, with crashes caused by structural failure being extremely rare based on accident records. The probability of structural failure of transport aircraft is of the order of 10^-8 per flight segment. This paper looks at two additional contributions to safety-the use of conservative material properties and certification tests-using a simple model of structural failure. We find that the three safety measures together might be able to reduce the calculated probability of failure to about 10^-7. Additional measures, such as conservative load specifications, might be responsible for the higher safety encountered in practice, explaining why passenger aircraft are so structurally safe. In addition, the paper sheds light on the effectiveness of certification tests for improving safety. It is found that certification tests reduce the calculated failure probabilities by reducing the modeling error. We find that these tests are most effective when safety factors are low and when most of the uncertainty is caused by systemic errors rather than variability.