It is traditional in looking to the future to analyse past experience. In this paper the processes of development of 25 UK combat (or fighter-bomber) types covering a period of 45 years have been analysed and compared with available data on 20 US types.

In this analysis:

1. (i) Date of the project has been taken as date of Instruction to Proceed rather than the more usual first flight or entry into service dates, as it is felt that this more accurately represents technological projections involved.

2. (ii) Of necessity aircraft empty weight has been used as a basic parameter. While AMPR weight is more conventional in modern estimating, it is impossible to obtain such data for older types, but in a broad review such as this empty weight would seem to be an acceptable parameter.

3. (iii) Costs have been corrected to 1978 monetary values using curves deduced from cost of living and aerospace product data for both UK and USA.

Many aircraft in service today have already achieved lives considerably greater than those originally anticipated by their designers. One thinks of aircraft like the Douglas DC-3, still going strong in many parts of the world after 40 years. Even aircraft like the Vickers Viscount, the world’s first successful turbo-prop air liner, are much ‘longer in the tooth’ than one might at first think; the oldest airframe still in regular service left its makers 27 years ago. Geriatric is the word which has been coined to describe such aircraft and, based on its dictionary definition of ‘care of the old’, the description is probably apt. However, this term is often wrongly thought of as synonymous with senile which, from its dictionary definition of ‘feebleness of old age’, is most inappropriate to such aircraft. It is interesting to examine first the longevity of current aircraft, secondly the application of the science of geriatrics to them and finally the means by which the expectation of life of new types of aircraft may be increased.

The keynote address given by Mr. Chouffot to this convention summarises the reasons for the growing concern of the airworthiness authorities and industry alike for the structural integrity of ageing civil transport aircraft and the measures taken to ensure their continued safety.

This paper explains in greater depth the background for these concerns and the actions taken.

I would like first to briefly review the airworthiness philosophy underlying the structural development of presentday aircraft.

During the past decade the UK Accidents Investigation Branch has been involved in the investigations into three major accidents to public transport aircraft, each caused by catastrophic structural failure in flight. During the same period other accidents involving structural failure with less structural damage, but with potentially equally serious consequences, have been investigated.

Boeing has recently completed a structural integrity reassessment of 707 aircraft models, and released a Supplemental Inspection Document (SID). Similar activities are now progressing for model 727. Structural reassessments are continuous over the operational life of all Boeing aircraft models and used for developing inspection information directed toward detecting structural damage before it becomes a serious problem in the fleet.

The fail-safe design concept requires interaction between the designer /manufacturer and the airline operators. Within this concept, maintenance and inspection are equally important as the basic design. It is important for operators to provide continuous vigilance of aircraft structure to detect damage and perform timely maintenance. Knowledge of damage that is occurring in the fleet is essential to the manufacturer to provide timely advice through service bulletins or other publications. This interchange is depicted in Fig. 1, and is the foundation of the fail-safe design concept.

Recent research into the fluid mechanics of hitting objects with air jets to eject unacceptable ones from a line of such particles in flight has shown the need for a better method of predicting the drag force involved since this is also the deflecting force in such an application. In most cases, the transverse dimensions of the jet are similar to, or smaller than, those of the target body so a drag force estimate based on infinite flow is likely to be seriously in error. Corrections are therefore needed to allow for jet diameters ranging from a fraction of the object’s size to many times that size. Achenbach investigated the blockage corrections for spheres in a closed circular wind tunnel but to the best of the authors’ knowledge no such results have been available for spheres in open jets, where the correction will in any case be of opposite sign.

A modification sometimes used to increase the effectiveness of a control surface is thickening of the trailing-edge by attaching small spanwise strips, normal to the upper and lower surface of the control, along the trailing-edge (see Fig. 1). While the effect of such a modification is fairly well known for steady flow, little evidence is available of the effect in unsteady flow. There are no theoretical methods of predicting the unsteady aerodynamic forces acting on an oscillating control surface which has trailing-edge strips (T-strips) attached and so flutter calculations are based on classical thin-liftingsurface theories. This practice is very dubious and two instances where flutter has occurred but has not been predicted by use of the classical unsteady aerodynamic forces, are described in Refs. 2 and 3.