Introduction of Statistical Process Control (SPC)

22/03/2018 Blog

Challenges of decentralised aircraft manufacturing

In contrast to its main competitor, Boeing, which has opted for centralised manufacturing at its Everett plant in Washington State, Airbus makes use of a decentralised approach to its manufacturing, in which the individual sections are produced and assembled at different locations. In this manufacturing system, Airbus specifies a focus for production at each of its locations: for instance, the Hamburg plant is responsible for the producing the fuselage and cabin. This involves everything from development through to production for all of Airbus’s civil aircraft. For example, the Stade plant in northern Germany produces vertical stabilisers and CFRP components, Filton near Bristol in England makes wings, while high-lift systems for wings are made in Bremen. Final assembly of the A330, for instance, takes place in Toulouse, France.

To make sure that final assembly can be completed without any unnecessary effort, it is important to ensure that the individual sections are brought together within the applicable tolerances while achieving high standards of quality and reproducibility. Slight deviations may occur at the fuselage joints, which are the locations where sections are connected with each other. The dimensions of these joints may only vary by a matter of millimetres, but this can still amount to an excessive variance in the Airbus’s fuselage diameter, with slight geometric variations making it harder to fit the fuselage sections together. As a result, higher quality standards for Airbus require reduced tolerances.

Due to their size and complexity in particular, a number of corrections are required to the joints of the fuselage. Statistical process control (SPC) represents one option for optimising the complete process of structural assembly.

Fuselage sections of an Airbus A350 are combined on the factory floor under close supervision
© Airbus S.A.S. 2012 - photo by e*m company / P. Pigeyre

Statistical Process Control and Optimisation

Airbus is already making use of the statistical process control (SPC) process for its A350 and A380 aircraft. SPC is an internationally established quality management methodology, which was developed and described by Walter A. Shewhart during the early twentieth century.

The aim is to make use of statistical methods to establish a tightly controlled process, enhancing performance and ultimately to reduce deviations from the process and avoid wastage. If the stages in the process are understood, it is possible to intervene early on in the process and undertake appropriate control measures. The use of this process and the introduction of the SPC system at Airbus aims to narrow down the process of troubleshooting and problem management. By recording process and product data in the system, it is possible to obtain an all-encompassing overview of the relevant section and demonstrate contextual relationships between the product and the process and highlighting possible weaknesses. As a result, measures to combat these issues can be introduced in good time.

A better understanding of the difference between product and process data can be obtained by reference to this example from the specialist literature that can also be found at Airbus. Product data may include the diameter of a borehole. The process data for the borehole are the rotation speed and the feed rate to be used when drilling the hole.

This methodology can be applied to any sector. In the aviation industry, it is not yet widespread. One reason is that large sample sizes are generally required to generate statistical data. In the aviation sector, these are not common due to low production volumes: for instance, Airbus produces only seven A330 aircraft per month.

SPC in the structural assembly of the A330, Section 16 at Location 601

ARTS expert, Kevin Czibor, works at Airbus on the introduction of the SPC process for the structural assembly of the A330 in Hamburg. Location 601 served as a pilot project, in which the assembly of the side shells, the floor grid and the bilge of the lower half section 16 takes place. There is a joint at the middle fuselage section, to which the wings will be attached in a later step. Specific radii are defined for this joint, as are permitted tolerances. A total of thirty parameters are identified and recorded during the assembly process at the location. Photogrammetry is one of the processes used to evaluate the process data, whereby the joint for each section is measured by attaching small plates measuring around 10 x 15 cm at selected positions. The plates include positions to reflect light during the measuring process. Photogrammetry of the joints takes place at known distances and angles, with points for measurement being located on the outer skin, gangways, and seat rails. With the introduction of the SPC process, the focus is specifically on the measurement points on the gangways, which show a particularly wide range of tolerances compared to other locations. The data is automatically transferred into a tablet using the measurement equipment, with the tablet representing the user interface with the worker. Measuring tools may include Bluetooth-equipped callipers, laser distance meters or tape measures. Whenever data is stored after measurements have been taken, it is available for analysis at a later stage. This allows contextual relationships to be identified, which could lead to small changes in positioning during the assembly process with a large effect on the quality of the finished product – in this case, the joint. In addition to the monitoring of process data, the SPC process also serves as a way of generating alerts about non-standard product data.

Photogrammetry is used to measure joins in the context of a GTI (Ground Test Instruction). GTIs are mandatory tests which must be carried out on every single aircraft, with other GTIs including the ventilation system or hydraulics, for example.

Future perspectives for Statistical Process Control (SPC)

In the future it is possible that the following may occur: suppliers will deliver a “digital patient record” alongside individual components. The data and relationships previously identified from the recording of process data could allow target measurements for the location to be determined flexibly. Aircraft assembly employees would then know exactly which target measurements to aim for and what they should specifically seek to prevent during the current process stage. Target measurements would, therefore, be corrected at each assembly location to achieve the best possible overall quality. This would be aligned with a more modern concept of quality, in which measurements no longer vary within tolerances, but match up with the target instead.

Ahead with ARTS

Experts such as Kevin Czibor work for our customers to ensure that projects and processes are efficiently delivered. Each of ARTS’ customers requires individual solutions and tailored consulting. In this case, we provide this to Airbus in the context of our technology consulting and quality engineering solutions. Once again, we benefited from our close collaboration with universities and research institutes, which helped us to enable our customers to innovate.

Sources: Wikipedia Airbus A330 | Wikipedia Walter A. Shewhart

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