Engineer Aircraft key terms

The maximum load expected in service, used as the baseline for structural strength calculations. Ultimate load is a defined multiple of it, and exceeding it may cause permanent deformation without immediate failure.

The load the structure must withstand without failure, normally 1.5 times limit load. At this level permanent deformation is acceptable, but the structure must still carry the load for a short period.

The ratio between ultimate load and limit load, typically 1.5 in civil airworthiness codes. It covers uncertainties in loads, material properties and analysis methods rather than allowing routine overstress in service.

A design and inspection philosophy that assumes cracks or defects may exist and requires the structure to remain safe until they are found by scheduled inspection. It sets inspection intervals and allowable damage limits.

A design approach in which failure of a single element, such as a load path or fitting, does not cause loss of the aircraft. Redundant load paths and crack-arrest features are typical means of achieving it.

An alternative route for carrying structural load if the primary member fails. Multiple spars, straps or fittings share or back up the load so that a single crack or failure does not become catastrophic.

Crack growth caused by the combined effect of sustained tensile stress and a corrosive environment. It often starts at fastener holes or machined surfaces and can progress with little visible warning.

Progressive cracking caused by repeated cyclic loading at stress levels below the static strength of the material. Cracks usually initiate at stress concentrations such as holes, fillets or corrosion pits.

A local increase in stress caused by a change in geometry, such as a hole, notch, sharp corner or abrupt section change. It is a common initiation site for fatigue cracks and drives careful design and inspection.

Corrosion produced when two dissimilar metals are in electrical contact in the presence of an electrolyte. The less noble metal becomes anodic and corrodes, so isolation and correct fastener selection are essential.

Measures that protect metallic structures from environmental attack, including protective coatings, sealants, plating, corrosion-inhibiting compounds, drainage paths and regular inspection of known susceptible areas.

An electromagnetic non-destructive method that induces currents in a conductive part and detects changes caused by cracks, corrosion or thickness variation. It is widely used for fastener holes and surface-breaking defects.

A non-destructive method that sends high-frequency sound into a part and analyses reflections or transmission to find internal defects, delamination or thickness loss. It is effective for bonded structures and composite parts.

Inspection methods that reveal defects without damaging the part, including visual, penetrant, magnetic particle, eddy current, ultrasonic and radiographic techniques. The method chosen must suit the material and the expected defect type.

Separation between layers of a laminated composite structure, often caused by impact, manufacturing defects or overload. It may be invisible externally and is commonly detected by tap testing or ultrasonic inspection.

A mandatory directive issued by an authority to correct an unsafe condition in a type-certificated product. It specifies the affected aircraft, the required action, and compliance times that operators must meet.

A document from the type certificate holder recommending or requiring modification, inspection or repair of a product. Some bulletins are made mandatory by an airworthiness directive, others remain optional but affect continued compliance.

The set of processes and requirements that keep an aircraft in a condition meeting its type design and safe for operation throughout its life, covering maintenance, inspections, modifications, records and defect reporting.

A certification, signed by authorised personnel, that stated maintenance has been completed satisfactorily and the aircraft or component is fit for return to service within any stated limitations.

An organisation holding approval from the competent authority to perform specified maintenance on aircraft or components under defined scope, facilities, personnel and quality-system requirements.

A controlled document that describes a specific maintenance task, its references, required parts and tools, and the sign-off steps. It provides a traceable record that the task was performed as specified.

An aircraft document recording defects, deferred items, servicing and flight information. It links reported defects to maintenance action and supports the decision that the aircraft is fit for the next flight.

An approved document stating which instruments and equipment may be inoperative for dispatch, under specified conditions and procedures. It must be based on the master minimum equipment list for the type.

The study of how human capabilities and limitations interact with maintenance and flight tasks. It addresses error, fatigue, workload, communication and procedures to reduce the likelihood of unsafe acts.

A commonly taught set of twelve human-factor precursors to maintenance error, such as fatigue, distraction, pressure, complacency, lack of communication and inadequate resources. They are used to build awareness and mitigation.

A person's accurate perception of what is happening around them and what it means for the task. Loss of it in maintenance or flight can lead to missed steps, wrong aircraft or unsafe decisions.

A formal, organisation-wide framework for managing safety, including hazard identification, risk assessment, safety assurance, promotion and clear accountability. It moves beyond reacting to accidents toward proactive risk control.

The process of identifying hazards, estimating the severity and likelihood of resulting harm, and deciding whether risk is acceptable or needs mitigation. It supports prioritisation of safety actions.

The provision of duplicate or backup channels, computers or power supplies so that a single failure does not lose a critical function. It may be active, standby or dissimilar to reduce common-mode failure.

Failure of multiple redundant systems from the same cause, such as shared power, shared software, common connectors or a common environmental condition. It defeats redundancy and is a key design concern.

A widely used avionics digital data bus standard that transmits labelled parameters from one transmitter to multiple receivers over a twisted pair. It is simple and deterministic but limited in direction and bandwidth.

An avionics or system component designed to be removed and replaced quickly on the aircraft without extensive adjustment or calibration. Fault isolation usually leads to replacement of the LRU.

Self-test circuitry and software within a system that detects and isolates faults, often reporting results to maintenance. It may run continuously, at power-up, or on demand and can produce false or intermittent indications.

The maintenance process of narrowing a reported fault to a specific component, connector, wire or LRU using test equipment, built-in test results and system knowledge before replacement or repair.

Maintenance performed at defined intervals or thresholds, such as flight hours, cycles or calendar time, according to the approved maintenance programme. It includes inspections, servicing and component replacements.

The condition of an aircraft, engine or component meeting its approved type design and being safe for flight. It is established by certification and maintained through continuing airworthiness activities.

The ratio of mass airflow through the fan bypass duct to the mass airflow through the engine core. High bypass ratios improve propulsive efficiency and reduce noise and specific fuel consumption.

A gas turbine engine in which a fan accelerates air, part of which bypasses the core. The core drives the fan through a shaft, and the combination gives high thrust and good fuel efficiency at typical speeds.

The fuel flow required per unit of thrust or power produced, used to compare engine efficiency. Lower values indicate better efficiency, and it varies with operating conditions and engine design.

The pressure maintained in a hydraulic circuit to actuate components such as flight controls and landing gear. It is regulated by pumps and valves and monitored because loss of pressure degrades system performance.

A pressure vessel that stores hydraulic fluid under gas pressure to absorb pulses, supplement pump flow during peak demand and maintain pressure for a limited time after pump loss.

A system that allows fuel to be dumped overboard to reduce aircraft weight to a safe landing weight. It is used only under defined conditions and with strict operational precautions.

The system that measures and displays usable fuel on board, using probes, sensors and compensation for attitude and temperature. Accuracy matters for range planning and for detecting leaks or imbalance.

A flight control architecture in which pilot inputs are converted to electrical signals and processed by computers that command hydraulic or electro-mechanical actuators. It enables envelope protection and weight reduction.

The set of rules and algorithms in a fly-by-wire system that map pilot inputs and aircraft state to control surface commands. Different laws apply in normal, alternate and direct modes.

The sequence that raises the gear after take-off and locks it in the up position, usually driven by hydraulic pressure and controlled by sequencing valves and proximity sensors. Correct indication is essential before landing.

A non-contact device that detects the presence or position of a nearby target, such as a gear or door, and sends a signal to indication or control systems. It is common in landing gear and door monitoring.

The manufacturer's document defining approved repairs, allowable damage limits and procedures for structural components. It is a primary reference for assessing damage and deciding whether repair or replacement is required.