Airworthiness may be defined as the fitness of an aircraft for flight in all the environments and circumstances for which it has been designed and to which it may therefore be exposed. An Airworthy aeroplane is one which is fit to fly.

Description

The connection between this condition of airworthiness and flight safety is an obvious but complex one. When the airworthiness of an aircraft is compromised by an error in design, production or maintenance, the prospects of an accident outcome should be almost eliminated by the existence and application of a comprehensive and effective range of Regulatory processes which have been developed for that purpose. However, this is not always the case and the desired condition of continued airworthiness for operating aircraft is therefore not always achieved. A deficiency in the airworthiness of an aircraft is a significant cause of loss of control accidents both directly and indirectly.

Effects

Flight Safety may be affected in various ways. An aircraft may unknowingly depart in a significantly un-airworthy condition which then manifests itself during the flight, possibly suddenly. This fault condition might be directly traceable to a prior maintenance error or may have arisen from a less obvious history. It is equally possible that a sudden and significant technical malfunction may affect a previously airworthy aeroplane.

It is also sometimes the case that a relatively minor airworthiness deficiency may manifest itself during a flight and be followed by a flight crew response which is not in accordance with their training, the applicable procedures or the principles of airmanship. This can lead to a far more serious problem than can reasonably be attributed to the onset of an airworthiness defect which is periodically expected and for which the correct application of procedures produces a safe outcome.

Defences

(1) The process by which a new aircraft designs is given a Type Certificate

(2) The process by which a Maintenance Schedule for an aircraft is developed and approved

(3) The effective communication to those persons carrying out aircraft maintenance including work on their engines or component parts on or off the aircraft

(4) The diligent execution of aircraft maintenance tasks

(5) The process of certifying satisfactory completion of all aircraft engineering and aircraft maintenance work carried out on aircraft

(6) The requirement for effective systems which identify, record and investigate maintenance error and its context

(7) The process of issuing and validating a Certificate of Airworthiness for all aircraft

(8) The process by which a Certificate of Maintenance Review is periodically issued for aircraft which hold a Certificate of Airworthiness and operate commercially

Typical Scenarios

(1) Work has been carried out to modify an aircraft which has involved complex work on the electrical wiring installation. A fault in that work in a hidden area leads to a fire in flight which cannot be controlled and the aircraft crashes.

(2) An uncontained failure of an engine causes secondary damage to the hydraulic system which was not anticipated when the design was approved. The effect is that the flight crew can no longer maintain effective use of the flight controls and the aircraft crashes.

(3) A complex repair to the rear pressure bulkhead of an aircraft is incorrectly carried out. In a subsequent flight, there is an explosive decompression which renders the pitch of the aircraft uncontrollable and it cashes

(4) Pitch control is lost during flight shortly after major maintenance input on critical stabiliser control components found that in service wear was within the prescribed limits for continued safety. Control of the aircraft is lost with ground impact.

(5) Shortly after take off in a relatively old aircraft, the wing main spar breaks and control of the aircraft is lost with ground impact. The subsequent investigation finds that serious airframe corrosion had gone undetected because of inadequacies in the Maintenance Schedule.

(6) During the cruise, un-commanded asymmetric deployment of the thrust reversers occurs as a result of a pre-flight maintenance error and the aircraft suddenly enters a steep descending turn from which it is impossible for the flight crew to regain control before ground impact occurs.

(7) A heavily loaded twin-engine light aircraft suffers an engine failure shortly after take off. An attempt is made to return to the departure aerodrome using a poorly executed turn during which airspeed decrease to the stall speed is not noticed and control of the aircraft is lost at low level with ground impact.

(8) Because of poor flight crew familiarity with flight deck engine instrumentation differences between aircraft type variants and poor CRM, the sudden onset of engine vibration (indicative of a degree of engine distress is incorrectly attributed to the serviceable engine which is then shut down in accordance with procedures. When the remaining unserviceable engine us called upon to produce thrust after the flight idle descent carried out after the shutdown fails to do so and control of the aircraft is lost with ground impact

(9) An aircraft has been under maintenance and is returned to operations with the pressurisation switch inadvertently left in the ground position. This condition is not noticed despite it being impossible not to become aware it by routine Flight Crew Checklist use both before and after take off. When the unpressurised aircraft cabin reaches 10000ft altitude, the correct warning occurs but no action is taken by the crew who assume that the warning is false and from another condition. The flight crew subsequently loose consciousness due to hypoxia and the aircraft crashes.

(10) Shortly after take off whilst flying in instruments, the pilot’s EADI develops a fault in which bank angle is indicated incorrectly. Although there are two other correctly functioning equivalent instruments visible to the pilots, a cross check is not done quickly enough and control of the aircraft is lost at low level with subsequent ground impact

Contributory Factors

These include: (1) Inadvertent maintenance error within a complex human factors context

(2) Ineffective regulatory oversight by a National Aviation Authority of maintenance organisations or aircraft operators

(3) An ineffective Quality System at an aircraft maintenance organisation or an aircraft operator.

(4) Deficiencies in the process which led to the issue of the original aircraft Type Certificate which mean that, as aircraft remain in service, the safety assessment process originally carried out for the new aircraft cannot be easily re-applied to the older in-service aircraft and its deteriorating systems.

(5) Absence of effective safety assessment in the case of some issues of a Supplementary Type Certificate especially where the subject is an optional modification involving a system, such as IFE, assessed as non-critical

(6) To the extent that unexpected airworthiness defects are ‘planned for’ and usually survivable if the prescribed flight crew response follows, the absence of this in respect of either decision making and/or aircraft handling

Solutions

(1) 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 the continued airworthiness of the aircraft type thereafter

(2) A full understanding of the human factors issues involved in aircraft engineering and aircraft maintenance.

(3) 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.