Aircraft structures design

Civil aircraft are designed for an approximate lifespan of 20 to 25 years and up to 90,000 flights. There are cases of operators of jets and turboprops exceeding this.

Future aircraft types will be designed for at least the same lifecycle, but aircraft structures will have higher fatigue life (endurance), higher damage tolerance and higher corrosion resistance, to minimize the maintenance costs and to comply with the requirements of the operator and the enhanced airworthiness regulations.

Non destructive testing will be used to monitor requirements. Widespread fatigue damage (WFD) and the assessment of existing repairs require the application of newly developed and available NDT methods.

Airworthiness requirements and compliance...

Structural damage which occurs during service and under airworthiness regulations for civil aircraft have been developed significantly in the past 45 years.

The objective: 'An evaluation of the strength, detailed design, and fabrication must show that a catastrophic failure due to fatigue, corrosion, or accidental damage, will be avoided throughout the operational life of the airplane.' and 'The ultimate purpose of the damage tolerance evaluation is the development of a recommended structural inspection program considering probable damage locations, crack initiation mechanisms, crack growth time histories and crack detectability.'

The requirements of the damage tolerance evaluation:

Widespread fatigue damage assessment

Identification of possible damage locations and extent of damage

Damage tolerance analyses and test

Determination of inspection threshold and intervals

This sets the bar for aircraft manufacturers:

Aircraft structural design must be in accordance with the fatigue and damage tolerance requirements.

A fatigue resistant design structure is based on fatigue life calculations for all structural elements during the design phase and is justified by full scale fatigue testing on the complete safe structure.

The design criteria to be met are static strength, residual strength, durability, crack growth, sonic fatigue strength and what is called the two-bay-crack criterion. It has to be shown, that a longitudinal crack in the skin of the pressurized fuselage with a length of two frame bays above a broken centre frame does not lead to a complete failure of the structure.

The structure of a pressurized fuselage which fulfills this criterion has to guarantee that neither the crack in the skin becomes unstable nor that the stiffeners perpendicular to the crack (i.e. the frames) fail statically.

The two-bay-crack criterion is the designing criterion for large areas in the upper and side shells of the pressurized fuselage of medium and long range aircraft. These aircraft types have lower design service goals in flights compared with short range aircraft with the result that the fatigue and damage tolerance criteria have less influence on the design.

To limit the implications on the weight due to the compliance with the two-bay-crack requirement, the following precautions are possible:

selection of skin material with high residual strength

selection of frame material with high static strength

limitation of the allowable frame pitch

adaptation of the stress level to the two-bay-crack criterion.

Material selection

During the initial design phase of the latest Airbus, the application of new materials and production methods reduces the production costs and the weight of the aircraft.

The development of a new production technique such as laser beam welding requires a comprehensive use of sophisticated inspection methods.

Fatigue damage and aircraft repair.

The main issue of the aging aircraft fleet is the occurrence of multiple damages at adjacent locations which influence each other.  Fourteen areas were identified as potentially susceptible:

Fuselages:

Longitudinal skin joints, frames and tear straps.

Circumferential joints and stringers.

Fuselage frames.

Aft pressure dome outer ring and dome web splices.

Other pressure bulkhead attachments to skin, to stiffener and pressure decks. 

Stringer to frame attachment.

Window surround structure.

Over wing fuselage attachments.

Latches and hinges of nonplug doors.

Skin at runout of large doubler.

Wings and empennage:

Skin at runout of large doubler.

Chordwise splices.

Rib to skin attachments.

Stringer runout at tank end ribs.

The next generation of aircraft has to comply with the forthcoming improved regulations regarding widespread fatigue damage The general aviation standard with respect to the two-bay-crack criterion should be reached without special design precautions, such as crack stoppers, and without disadvantages in weight.

Airlines' requirements for reducing maintenance costs have to be considered, i.e. the inspection intervals have to be increased by decreasing the crack growth. This may be achieved for fuselage structures by application of new materials. The development and application of new material is under constant investigation to reach the optimum of material and production costs, weight and maintenance costs.

During the development and certification of an aircraft, NDT plays a major role.

The Repair Assessment Guidelines which were developed by Airbus also rely on NDT for determination of the repair parameters and the inspection of repairs.