021 01 00 00 SYSTEM DESIGN, LOADS, STRESSES, MAINTENANCE

Last Updated on 4 years by teboo

021 01 01 00 System design
021 01 01 01 Design concepts
(01) X Describe the following structural design philosophy:
— safe life;

Explicitly stated design life, no catastrophic failures may occur in normal operation. Or number of cycles allowed before replacement.

— fail-safe (multiple load paths);

No one part takes the whole load, multiple paths. Critical areas are at least doubled. Redundancies are required where a failure could result in the loss of an aircraft. Doublers can allow cracks to formed until replacement is practical.

— damage-tolerant.

Weight can be saved, ie by cutting down on redundancy, a part or section may have crack arresting features or surrounding parts may be design to take the load of a failed part.

(02) Explain the purpose of redundancy in aircraft design.

To prevent catastrophic or critical failure, quite obvious really.

021 01 01 02 Level of certification

(01) X Explain why some systems are duplicated or triplicated.

The more critical the part or system the more redundancy is required for certification. For example fly-by-wire control systems.

(02) X Explain that all aircraft are certified according to specifications determined by the competent authority, and that these certification specifications cover aspects such as design, material quality and build quality.

There we go, explained.

(03) X State that the certification specifications for aeroplanes issued by EASA are:

— CS-23 for Normal, Utility, Aerobatic and Commuter Aeroplanes;

Small – 9 or fewer seats. 5679kg (12,500lb) or less.

Commuter aeroplanes – turbine powered

— CS-25 for Large Aeroplanes.

Yep…

021 01 02 00 Loads and stresses
021 01 02 01 Stress, strain and loads

(01) Explain how stress and strain are always present in an aircraft structure both when parked and during manoeuvring.

We’re talking Hooke’s law, Stess = Force/Area in Pa N/m²,

we’re also talking Strain = Change/Original

Ductile materials having no defined yield point.

Plastic deformation is beyond 0.2% yield.

Remark: Stress is the internal force per unit area inside a structural part as a result of external loads. Strain is the deformation caused by the action of stress on a material.

(02) Describe the following types of loads that an aircraft may be subjected to, when they occur, and how a pilot may affect their magnitude:

— static loads;

Constant or slowly applied. When the aircraft is stationary because…

— dynamic loads;

Dynamic loads involver movement, resonances can make the forces much greater because the loads can be self perpetuating.

— cyclic loads.

Cycles of compression and tension, reversal of forces. Can lead to metal fatigue.

(03) Describe the areas typically prone to stress that should be given particular attention during a pre-flight inspection, and highlight the limited visual cues of any deformation that may be evident.

Wrinkling of the skin is a clue to buckling, positive g with show wrinkling on the upper surface leaving loose or sheared rivets on the bottom.

Delamination of composite structures.

Important to log any incident.

021 01 03 00 Fatigue and corrosion
021 01 03 01 Describe and explain fatigue and corrosion

(01) Describe the effects of corrosion and how it can be visually identified by a pilot during the pre-flight inspection.

Dissimilar metals breaking down chemically or by movement.

Corrosion can weaken the structure. and be the epicentre for fatigue, as the metal corrodes away there is less surface area to take the loads, increasing stress.

Pitting or discolouration, can be difficult to spot due to obstructions such as paint or cabin trim, corrosion can also be internal. High stress areas should have particular attention during the pre-flight.

(02) Describe the operating environments where the risk of corrosion is increased and how to minimise the effects of the environmental factors.

Salt laden and humid areas are particularly good for promoting corrosion. The salt and water help the electrolysis process.

(03) Explain that aircraft have highly corrosive fluids on board as part of their systems and equipment.

They do…

(04) Explain fatigue, how it affects the useful life of an aircraft, and the effect of the following factors on the development of fatigue:

— corrosion;

Enhances as the amount of metal is reduced by corrosion leaving the remaining material to take the shit…

— number of cycles;

More cycles is more stress.

— type of flight manoeuvres;

High g manoeuvres is higher stress.

— stress level;

More stress increases all loads and can get further to UTL

— level and quality of maintenance.

Good practice can extend the live of a component, stop drilling for example or replace a part before needing to replace many parts.

021 01 04 00 Intentionally left blank
021 01 05 00 Maintenance
021 01 05 01 Maintenance methods: hard-time and on-condition monitoring

(01) Explain the following terms:

— hard-time or fixed-time maintenance;

Definite time limit of a component

— on-condition maintenance;

Component inspected to see if it needs replacing.

— condition monitoring.

Vibration, temperature monitoring for example, changes may indicated problems. (ACMS) can be printed or sent via (ACARS)