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Airworthiness may be defined as the fitness of an aircraft to fly when it meets the minimum conditions laid down in its type certificate. This includes the design and construction (in accordance with specific certification codes). An airworthy aircraft is one that is fit to fly. Additionally it must be operated within the limits laid down; an aircraft which exceeds any limit may be judged to become un-airworthy. In service, an aircraft must also be maintained to specific requirements for it to remain airworthy, the latter case is referred to as Continuing Airworthiness.
The connection between the condition of airworthiness and flight safety is an obvious but complex one. The design activity, besides meeting the applicable certification code, often seeks to improve the aircraft’s economics and cost benefit to both the maker and the operator. Therefore, certification authorities will examine all aspects of the design and construction of an aircraft, even when there is apparent improvement to minimum standards. When an aircraft type is first judged to meet all the certification requirements it will be issued with a Type Certificate (TC) A deficiency in the airworthiness of an aircraft can be a significant cause for an in-service incident or accident.
A defect may have a significant effect on safety but, if not rectified, or only partly rectified, may also be a cause for an accident at a later time. Inappropriate crew actions in response to a malfunction which arises in flight have often led to a much worse outcome than can be attributed to the underlying defect and if adequate flight crew procedures and training were provided, it would be wrong to attribute the outcome in such cases primarily to the airworthiness of the aircraft.
Wherever practicable, the original design will embody redundancy features; that is, an allowance for the failure of a system or component without any reduction in airworthiness. In some cases, the failure only becomes significant after an aircraft has landed but regulation requires rectification before further flight. In more extreme cases a major failure, such as an in-flight failure of a single engine on a multiengine aircraft, will not lead to an accident as the design combined with the training of the pilot will permit a safe landing to be made. The same criteria applies to flight in adverse weather, maintenance activity and even, in some cases, human fallibility. In addition, each aircraft must hold a certificate (universally called The Certificate of Airworthiness or C of A) to prove that it met the requirements at the date it was issued (or renewed). The regulator will also require that the operator has in place a system to ensure compliance with the following:
- Compliance with the maintenance programme.
- Embodiment of Mandatory Modifications and Inspections.
- Rectification of reported defects and investigation of adverse reliability matters.
A common phrase used to cover the above items is the Certificate of Maintenance Review. (CMR)
- An aircraft may be modified to include a new passenger entertainment system but the electrical load on the existing wiring may be misjudged leading to an in flight fire. The circuit breakers used were of a general type in use but not capable of electrical protection in some fault conditions.
- An uncontained failure of an engine causes secondary damage to the hydraulic system which was not anticipated when the original design was approved. The effect is that the flight crew can no longer maintain effective use of the flight controls.
- A complex repair to the rear pressure bulkhead of an aircraft is incorrectly carried out. During a subsequent flight much later, there is rapid cabin pressure loss causing structural damage which renders the pitch of the aircraft uncontrollable.
- During the climb to the cruise altitude, un-commanded asymmetric deployment of the thrust reversers occurred and the aircraft suddenly entered a steep descending turn from which it was impossible for the flight crew to regain control before ground impact occurred. Previous maintenance of the system had been ineffective but not related to the failure on the accident flight. One result from the investigation was a review of the certification of thrust reverser operation.
- A twin engined jet suffered an engine failure while climbing after take-off. The engine failure was misdiagnosed by the two pilots partly due to the type of engine instrumentation installed and led to the serviceable engine being shut down instead of the faulty one. On final approach to land, the aircraft impacted the ground and was destroyed. In this case, reduced airworthiness (the engine failure) was compounded by incorrect crew actions and their knowledge of the systems installed.
There are several factors which impinge on the airworthiness of an aircraft. These include:
- Inadequate or incomplete maintenance.
- Errors in maintenance which may result in a fault becoming obvious a long period after the error was originally made.
- Operation outside the certificated limits such as those laid down for flight in ice or snow conditions.
- Lack of adequate oversight of the operator, its practices and policies including training, operation and maintenance by the regulator.
- Deficiencies in the process which led to the issue of the original aircraft Type Certificate.
- A process of aircraft Type Certificate issue which allows the work which is carried out to achieve it to be documented in such a way that it can remain an accessible foundation for the continued airworthiness of the aircraft type thereafter
- A full understanding of the human factors issues involved in aircraft engineering and aircraft maintenance.
- The achievement of standards of flight crew training, proficiency and CRM which can minimise the number of instances where the effects of the onset of an in-flight reduction in airworthiness are worsened rather than effectively managed by the actions of a flight crew.
- Airworthiness - The System
- Continuing Airworthiness
- Accident and Serious Incident Reports: AW - a selection of reports concerning events where airworthiness was a causal or contributory factor.
- Airworthiness - The System provides a summary of the processes involved in airworthiness
- Continued Airworthiness - information leaflet prepared by the International Federation of Airworthiness (IFA) to assist the understanding of the basic Continuing Airworthiness requirements and control functions
- The FAA Website "Lessons Learned From Transport Airplane Accidents"