Airworthiness

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Airworthiness is a term used to dictate whether an aircraft is worthy of safe flight. It is illegal in most countries to fly an aircraft without first obtaining an airworthiness certificate from the responsible government agency. The airworthiness usually must be maintained by a program of inspections by an authorized Aircraft Maintenance Technician, typically performed annually, or after a fixed elapsed flight time, such as every 100 hours.

One notable example of an aircraft that was not legally airworthy is Larry Walters' "Lawn chair flight." on July 2, 1982 in a homemade "aircraft" he called Inspiration I.

In the U.S., Title 14, Code of Federal Regulations, Subchapter F, Part 91.7 states: "a) No person may operate an aircraft unless it is in an airworthy condition. b) The pilot in command of a civil aircraft is responsible for determining whether that aircraft is in condition for safe flight. The pilot in command shall discontinue the flight when unairworthy mechanical, electrical, or structural conditions occur."

[edit] Definition of airworthy

The USA Federal Aviation Regulations, Part 21, §21.183(d) has a procedural definition of airworthy:
"other aircraft An applicant for a standard airworthiness certificate for aircraft not covered by paragraphs (a) through (c) (a:"New aircraft manufactured under a production certificate." b:"New aircraft manufactured under type certificate only." c:"Import aircraft.") of this section is entitled to a standard airworthiness certificate if -- (1) He presents evidence to the Administrator that the aircraft conforms to a type design approved under a type certificate or a supplemental type certificate and to applicable Airworthiness Directives; (2) The aircraft (except an experimentally certificated aircraft that previously had been issued a different airworthiness certificate under this section) has been inspected in accordance with the performance rules for 100-hour inspections set forth in part 43.15 of this chapter and found airworthy by-- (i)The manufacturer; (ii)The holder of a repair station certificate as provided in part 145 of this chapter; (iii)The holder of a mechanic certificate as authorized in part 65 of this chapter; (v)The holder of a certificate issued under part 121 of this chapter, and having a maintenance and inspection organization appropriate to the aircraft type; and (3)The Administrator finds after inspection, that the aircraft conforms to the type design, and is in condition for safe operation."

A more generic and non-process orientated definition is required. Airworthiness is defined in JSP553 Military Airworthiness Regulations (2006) Edition 1 Change 5 as:

The ability of an aircraft or other airborne equipment or system to operate without significant hazard to aircrew, ground crew, passengers (where relevant) or to the general public over which such airborne systems are flown

This definition applies equally to civil and military aircraft.

This definition clearly addresses the safety of operators, passengers and the public as much as possible in an inherently dangerous activity. But how does the aviation industry reduce the hazard to below a ‘significant’ level? The military apply a risk reduction technique, widely used throughout engineering, known as ALARP (As Low As Reasonably Practicable). This is defined as:

‘The principal, used in the application of the Health and Safety at Work Act, that safety should be improved beyond the baseline criteria so far as is reasonably practicable. A risk is ALARP when it has been demonstrated that the cost of any further Risk reduction, where cost includes the loss of capability as well as financial or other resource costs, is grossly disproportionate to the benefit obtained from that Risk reduction.’