If you wish to contribute or participate in the discussions about articles you are invited to join SKYbrary as a registered user
Airworthiness - The System
Beginning any flight in an airworthy aircraft is an important part of the achievement of acceptable levels of safety. The regulations require nothing less than this.
The maintenance effort which goes into delivering this stems from a huge, complex and highly regulated industry. Like the rest of aviation, it involves human interventions and is subject to human traits: see An Overview of Human Factors in Aviation Maintenance for a summary of the specific approach.
Each National Aviation Authority (NAA) (or in the case of the EU - EASA) is the Airworthiness Authority for that State and is responsible for developing the regulatory regime which will apply. An Airworthiness Authority will usually pre-announce an intention to issue new or revised regulations, including mandatory tasks, through a Notice of Proposed Amendment (NPA) or Notice of Proposed Rulemaking (NPRM) which allows for industry consultation before changes are made. In a few cases, the nature of the change is too important and/or to urgent to permit this, so regulators retain the authority to introduce a requirement without consulting industry.
In the case of EASA, the setting of policy and rulemaking is by the EASA organisation, while implementation is overseen by the relevant NAA. This includes oversight of operating standards. The NAA’s also carry out most of the work involved in ensuring that organisations in their territory meet requirements before approving them. Close cooperation exists between the world’s main regulators in attempting to harmonise, so far as is practical, their individual standards.
Newly manufactured aircraft are delivered to customers having been built according to the applicable Type Certificate. This certificate, also referred to in the article, Airworthiness, is issued in respect of the defined build standard of the first aircraft of the specific type. Thereafter all subsequent new aircraft of this type must meet the same build standard for the issue of an individual a Certificate of Airworthiness. The Type Certificate is valid throughout the life of a specific type and only varies in the event of a major change e.g. installation of a freight door. It also can confer what are commonly referred to as “grandfather rights” upon subsequent newer versions of the original aircraft model. For example, Airbus first produced their A320 aircraft and then subsequently added the A321, A319 and A318 to the same Type Certificate as they were deemed sufficiently similar. A similar process was applied to some versions of the Boeing 737.
When an aircraft type enters service for the first time with a new operator, that operator must have a Maintenance Programme in existence and approved by his NAA before he begins operation. This is usually and most simply achieved by the submission of the manufacturer’s Maintenance Planning Document (MPD) In the case of a new type never before in service, a process described in MSG-3 is followed.
It is also a requirement that the organisation which will carry out the work (which may be the airline’s own maintenance company or a contracted one – often termed a Maintenance and Repair Organisation or MRO) be approved. In addition to being a certificated Part-145 company, it will now need to add the new type to its Certificate Rating. A Part 145 organisation has to meet specific requirements particularly in its staffing, tooling, premises and capacity. In particular, certain management posts and the organisation’s Exposition are carefully scrutinised by the NAA prior to acceptance.
Aircraft in Service
From its very first flight, an aircraft progressively accumulates flying hours, flight cycles (a takeoff to landing is one flight cycle) and naturally elapsed calendar periods. Most, if not all, of these variables are tracked to ensure that the requirements of the Maintenance Programme are met.
Daily inspections are carried out covering a small number of important tasks as well as replenishment of lubricants and other fluids. At a greater interval (for example after 300 flight hours), an A Check would be carried out. This involves more extensive checks and would be performed by Line Maintenance personnel.
The next check interval can be a B Check or, more commonly a partial C Check. By carefully dividing the requirements of a C Check into a small number of packages, the aircraft will avoid lengthy time out of service. This practice is termed Equalised Maintenance. A typical B plus a portion of the C Check will be carried out by Base Maintenance personnel using proper accommodation buildings and access equipment. This check could take up to 24 hours elapsed time.
The final check in the “letter series” of maintenance is the D Check. This is a major activity when a very detailed inspection of the whole aircraft is performed. Often such items as landing gear and control surfaces are removed for service and the interior equipment such as seats and galleys are also removed for refurbishment. The D Check is both costly and time consuming. Many operators sub contract this work to avoid utilising their hangar space which is always needed for smaller work programmes. Equally, some operators use the onset of a D Check programme to change their fleet to new aircraft thus providing a new owner with a lower cost aircraft after the necessary D check is carried out. A typical D check takes between 15,000 and 35,000 man-hours of labour, and can put an aircraft out of service for 15 to 30 days, or more. The total cost averages between $1 million and $2 million.” A typical D check cost is 70% labour and 30% materiel”,
Planning and Control
The control of in service aircraft is both complex and vital. While day to day flying continues, maintenance technicians rectify any problem reported by the pilot as well as those found by inspection. In addition, other technical staff are monitoring every aspect of the aircraft, from its flight hours, through the performance of its engines and systems down to less obvious things such as planning the cleaning of the exterior and interior to preserve condition and appearance. An airline with more than a small number of aircraft will operate a maintenance control centre on a 24 hour basis. This technical centre (often called “Maintrol”) has much of the data relevant to each aircraft at its fingertips and also keeps in constant touch with aircraft via radio and data downlink. It is their job to minimise operational technical disruption as well as ensuring that day to day maintenance is performed at, or prior to, the period set in the Maintenance Programme. Yet more staff are involved in providing support to front line maintenance technicians in solving complex problems and acting as liaison with airframe and powerplant manufacturers.
A further level of oversight is provided in many countries by the regular scrutiny of maintenance history compared to the requirements of the Maintenance Programme including Mandatory tasks. This process is termed maintenance review and periodically will need the issue of a Certificate of Maintenance Review (CMR). The performance of this task needs a technician of great experience both of the aircraft involved and of the organisation which is doing the work. Much of what the regulator requires in terms of continuing airworthiness is dealt with by the foregoing activity which will include its oversight by a Quality Assurance system.
Aircraft, like any piece of complex machinery, require day to day care. It may involve a straight forward task such as changing a worn tyre or a complex investigation of a digital electronic system fault. This work is dealt with by Maintenance Technicians invariably qualified by holding an appropriate licence issued by the NAA. Faults reported by the pilot are entered into a log book (the technical log) and, after rectifying the fault, the technician signs for doing the work and appending his authorisation number. This certification is termed a Certificate of Release to Service (CRS.) Modern systems even permit the pilot and technician to record the fault and the fix using technology such as tablet computers. This is part of a modern on-board system called The Electronic Flight Bag or EFB.
On a few occasions, the reported defect can not be immediately fixed and, provided that the fault is listed as “Acceptable” in an approved manual known as the Minimum Equipment List or MEL, the aircraft may return to its base where the fault can be rectified. This MEL process is subject to the same degree of logic analysis as the MSG-3 and must be followed with care by both the pilot and the maintenance technician.
Aircraft in Storage or Out of Service
When aircraft are out of service and stored for any reason it becomes necessary to apply a preventative maintenance regime. This is laid down in the Aircraft Maintenance Manual (AMM) not the normal Maintenance Programme. Specific requirements will vary for such reasons as:
- Length of time that the aircraft is not flown.
- Environment in which it is stored e.g. in a dry desert environment compared to a moist sea air one.
- The degree to which the aircraft is prepared before it is put into storage. For example if engines are removed, this will negate the need to operate them periodically. If the passenger seats and other furnishings are removed that too will reduce the amount of preventative work.
- The installation of humidity reduction equipment will likewise have a very positive effect on the aircraft’s internal structure.
Notwithstanding the preparations, the need to provide proper care during storage can not be overstated. The cost and attendant down time to restore the aircraft to service will be increased considerably if less than rigorous storage procedures are permitted.
Service Bulletins and Modifications
Every type of aircraft, through its service life, will be subject to changes which come about from in-service problems as well as improvements sought by the owner or operator. These changes have varying titles but by far the most common is the Service Bulletin or SB. SB’s are issued by the manufacturer (which can be by the aircraft constructor, the engine constructor and even by equipment manufacturers.) In addition, SB’s can fall into more than one category. Some, the less vital ones, are optional in character but some are important to safety and/or reliability. In the latter case, and after consultation between the manufacturer and NAA of the Type Certificate issuing country, the SB will become mandatory. This method of disseminating and controlling the requirement is termed an Airworthiness Directive (AD). It is then incumbent on the operator to carry out the requirements of the AD within the time span permitted by the regulator. Modifications can also be variable in scope; for example the range of work from a change in the seating configuration to a change in the role of an aircraft from passenger to cargo work. To accomplish this safely, approved design organisations are necessary to engineer and produce the Service or Modification Bulletin for any modifications not underwritten by the Type Certificate holder i.e. the manufacturer.
- Continuing Airworthiness
- Aircraft Certification and Production Standards
- Accident and Serious Incident Reports: AW - a selection of reports concerning events where airworthiness was a causal or contributory factor.
- 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
- AAIB Safety Study - 1/2016: Airworthiness of Aircraft Registered Overseas and Resident in the UK, published January 2016.