Every Aircraft that moves from the designed and manufactured state into an accepted and authorized safe flying state requires certification. Any new aircraft designed by a manufacturer will need to be submitted to appropriate regulatory authorities for testing for a type certificate. In the US this is the FAA (Federal Aviation Administration), and in Europe EASA (European Aviation Safety Agency).
Airworthiness has a number of aspects that relate to the legal and physical state of an aircraft. According to the U.S. Federal Aviation Administration (FAA) (1998), the term airworthy “is when an aircraft or one of its component parts meets its type design and is in a condition for safe operation.” A definition used by the U.K. Ministry of Defence includes a wider definition, which includes people on the ground (third parties) – “Airworthiness is the ability of an aircraft or other airborne equipment or system to be operated in flight and on the ground without significant hazard to aircrew, ground crew, passengers or to third parties; it is a technical attribute of materiel throughout its lifecycle.” (Ref: MAA 02 Glossary)
Additionally, an aircraft must be operated within the limits laid down in the Flight Manual; an aircraft that exceeds any limit may compromise its airworthiness. In service, an aircraft must also be maintained according to its Approved Maintenance Schedule for it to remain airworthy; through-life maintenance would be included in the term Continuing Airworthiness.
Type Certificate and Certificate of Airworthiness
Once approved and certified, the aircraft is cleared for service. The “Type Certificate” – issued to signify the airworthiness of an aircraft manufacturing design – is followed by the “Airworthiness Certificate,” which authorizes aircraft operations in a certain countries or regions.
Having established that the type of aircraft is acceptable, it is then necessary to certify that the specific serial number aircraft produced by the manufacturer or imported from a non-EU country actually meets the certification requirements. Each produced aircraft then needs an airworthiness certificate, which will involve specific production and flight tests for each aircraft of that type. Only then is the aircraft ready to fly.
The competent authorities in each geographical jurisdiction control the certification process. Today, the two main aircraft certification systems are:
- For the United States, FAR 25 regulations,
- For the European Union, JAR 25 regulations.
These Federal Aviation Regulations, called FARs, are part of Title 14 of the Code of Federal Regulations (CFR). As an operator, you must adhere to airworthiness standards as dictated by your part number under 14 CFR. Test guidelines are published, and followed, by each regulator. For example, the FAA uses AC25-7D – “Flight Test Guide for Certification of Transport Category Airplanes.” Compliance with these specifications or standards is approached in one of two ways depending on the requirement.
The airworthiness standards outlined in CFR Part 25 apply to aircraft in the transport category. The term “transport,” widely used by aviation regulatory bodies in the U.S., Canada, and Europe, is typically used to describe large civil airplanes or helicopters. Under the FAA’s regulations, transport category aircraft can fall into one of two categories: jets with at least 10 seats or a maximum takeoff weight (MTOW) above 12,500 pounds; or propeller aircraft with either more than 19 seats or an MTOW above 19,000 pounds. Transport category aircraft are designed and certified under CFR Part 25 and Part 26, while transport category helicopters fall under Part 29.
For structures typically the approach is known as Deterministic whereas for systems, a Probabilistic approach is taken. One example of each approach would be:
For structure – No detrimental deformation of the airframe under the loads produced by a given magnitude of manoeuvre.
For systems – Any catastrophic failure condition must (i) be extremely improbable [1 in 109 flight hours], and (ii) must not result from a single failure.
The process for this involves extensive testing – first in simulators based on aircraft design, then on the airframe structure, and finally in the air. This is a lengthy process and often very expensive. It forms a major part of the cost of development and often has a significant impact on the commercial success of the new model. The certification process usually involves building several prototype models. These are often used just for testing and will never see active service with an airline.
Airbus reported that it’s 21st century flagship A380 was certified by the two major international governing bodies – the European Union Aviation Safety Agency (EASA) and the U.S. Federal Aviation Administration (FAA) – in December 2006, following a programme that began more than five years earlier and ultimately comprised more than 2,600 flight hours with a fleet of five test aircraft.
The certification process covers the complete development process of a new aircraft. It includes various phases:
- Detailed design review,
- Test review and participation in laboratory,
- Test review and participation in flight (designed to take into account modifications in light of the results),
- Aircraft operators (closely involved in design definition, development and service introduction).
The EASA’s 4 steps of the type-certification process :
1. Technical Familiarisation and Certification Basis
The aircraft manufacturer presents the project to EASA when it is considered to have reached a sufficient degree of maturity. The EASA certification team and the set of rules that will apply for the certification of this specific aircraft type are being established (Certification Basis).
2. Establishment of the Certification Programme
EASA and the manufacturer need to define and agree on the means to demonstrate compliance of the aircraft type with each requirement of the Certification Basis. This goes hand in hand with the identification of EASA’s “level of involvement” during the certification process.
3. Compliance demonstration
The aircraft manufacturer must demonstrate compliance of its product with regulatory requirements: the structure, engines, control systems, electrical systems and flight performance are analysed against the Certification Basis. This compliance demonstration is done by analysis during ground testing (such as tests on the structure to withstand bird strikes, fatigue tests and tests in simulators) but also by means of tests during flight.
EASA experts perform a detailed examination of this compliance demonstration, by means of document reviews in their offices in Cologne and by attending some of these compliance demonstrations (test witnessing). This is the longest phase of the type-certification process. In the case of large aircraft, the period to complete the compliance demonstration is set at five years and may be extended, if necessary.
4. Technical closure and issue of approval
If technically satisfied with the compliance demonstration by the manufacturer, EASA closes the investigation and issues the certificate. EASA delivers the primary certification for European aircraft models which are also being validated in parallel by foreign authorities for operation in their airspaces, e.g. the FAA for the US or TCCA for Canada. Conversely, EASA will validate the FAA certification of US aircraft models (or TCCA certification of Canadian models) according to applicable Bilateral Aviation Safety Agreements between the EU and the concerned Third Country.
Design and Construction: Control surfaces, control systems, landing gear, floats and hulls, personnel and cargo accommodations, emergency provisions, ventilation and heating, pressurization, fire protection, miscellaneous
Powerplant: Fuel system, fuel system components, oil system, cooling, induction system, exhaust system, powerplant controls and accessories, powerplant fire protection
Equipment: Instruments installation, electrical systems and equipment, lights, safety equipment, miscellaneous equipment
Operating Limitations: Operating limitations, markings and placards, airplane flight manual
Electrical Wiring Interconnection Systems
Airworthiness requirements in FAR 25 which include raw materials, manufacturing processes, structure static strength, fatigue property and damage tolerance. Structural performances between composite and metal are different, with exhibitions of environment (temperature and humidity) sensitivities in envelop of usage, sensitivities of low energy impact and outof-plane loads.
Airworthiness Requirements of Composite Aircraft Structure for Transport Category Aircraft in FAA
Airworthiness certification of composite materials and manufacturing processes for transport category aircraft should be implemented per §25.603，§25.605，§25.609，§25.613 and §25.619 which require that all composite materials and processes used in structures are qualified through enough fabrication trials and tests to demonstrate a reproducible and reliable design. The depth and detail of certification are related with function of components and degree of importance.
The following aspects of airworthiness certification about material performance should be considered：
Thermal analysis and moisture absorption analysis including environmental top design temperature; thermal parameters during analysis, such as heat absorption rate, reflectivity and heat conduction coefficient of coating; parameters used in moisture absorption analysis, such as wet spread rate and so on; the selecting of adjustable parameter in accelerating environment; confirming materials including material and processes specifications which depend on mechanics, physical tests, chemical tests and so on; the requirement for candidate material suppliers; relater manuals should be used in handing tests data;
ASTM or equivalent methods; detail design features and so on. Material Specifications covering processing procedures that should be developed to ensure that repeatable and reliable structure is being manufactured are required to ensure consistent material is being procured. Once the fabrication processes have been established, changes should not occur unless additional qualification, including testing of
differences is completed. Environmental effects on critical properties of the material systems and associated processes should be established.
Airworthiness certification of static strength of composite aircraft structure for transport category aircraft should be implemented per §25.305 and§25.307 which should consider all critical load cases and associated failure modes and be demonstrated through a program of component ultimate load tests in the appropriate environment. The strength of the composite structure should be reliably established,
incrementally, through a program which referred to in industry as the “building block approach” of analysis and a series of tests conducted using specimens of varying levels of complexity.
At the lowest Building Block level, small specimen and element tests are most widely used to characterize basic unnotched static material properties, generic notch sensitivity, environmental factors, material operational limit and laminate fatigue response. Analysis in the second level uses the basic information obtained at the first level to calculate internal loads, identify critical areas, and predict critical failure modes. At subsequent levels, even more complex static and fatigue loadings are analyzed and verified, with particular attention directed toward assessing out-of-plane loads and identifying unanticipated failure modes. Variabilities introduced by scale-up and response of the structure as a whole are also addressed. The final building Block level involves full scale static and fatigue testing (as required). This testing validates predicted internal loads, deflections, and failure modes of the entire structure. It also serves to verify that no significant unpredicted secondary loads have appeared.
Some of the main areas of testing include tests to the airframe and flights tested.
Structural airframe tests
Before an aircraft even takes to the air, it is subjected to extensive structural tests. These stress the airframe and wings, in most cases significantly exceeding expected maximum loads that will be experienced in service. Tests include wing loading and deflection, aileron and spoiler functionality during wing loading, fuselage pressure tests, fatigue tests, and flight cycle simulations.
Fatigue testing examines how the aircraft structure responds to stress over a long period of time and during different stages of its operations, such as taxiing on the runway, take-off, cruising and landing. To re-create these conditions, a combination of loads is placed on the airframe and activated by computer-operated hydraulic jacks.
These tests will push the airframe more than it will experience in service. For example, Airbus quote that during A380 testing 47,500 full flight cycles were made. This is 2.5 times the number of flights it would make in over 25 years of operation. These, of course, are not full flights, but instead, loads are repeatedly placed on the airframe to simulate flight.
Flight test campaign
An aircraft’s flight test campaign is designed to assess general handling qualities, operational performance, airfield noise emission and systems operation in normal mode, failure scenarios and extreme conditions – culminating with certification by airworthiness authorities.
Flight tests are carried out to assess the aircraft’s general handling and performance, and also to test operations in extreme conditions. This includes operations in extreme heat, cold and at altitude. For this, aircraft are often flown to other locations. Northern Canada, for example, is common for extreme cold weather tests, and the Middle East for hot weather tests. La Paz, Bolivia is often used for high altitude and Iceland for strong wind testing.
Further certification flight testing is dedicated to water ingestion trials, low speed take-off tests, flutter and rejected takeoff and landing. In addition to the wake vortex trials – air turbulence created behind the aircraft at takeoff – required for certification, Airbus continues to perform an extensive series of tests and measurements in this area.
The various operational tests an aircraft must undergo include:
- Operation of aircraft systems, including autopilots.
- Water ingestion tests, to ensure water won’t enter aircraft systems.
- Flutter testing, where vibrations are measured to ensure they won’t cause structural damage.
- Low speed take off.
- Rejected takeoff (including testing at full aircraft load, with worn brakes).
- Assessment of the aircraft’s environmental footprint, including fuel burn.