Boeing 777X

Boeing 777X –Breaking-Test Blunder or Engineering Achievement?

Marco Theiss
03.12.2019
5 minutes

The failed breaking test of the static airframe of the 777X has publicly attracted very negative comments. But is this perspective really warranted?

Considering the complexity of the structure of an aircraft and of the overall development process behind it, it is only fair to scrutinise what the non-achievement of the target means exactly.

Known facts about the test of the 777X

Definitions

Load factor: The factor by which the safe load is multiplied.

The safe load is the maximal possible load that can occur during operation when all factors are taken into account.

The breaking load is the load above which structural failure is allowed to occur. It is legally stipulated to be 1.5 times the safe load, i.e. the maximal possible load in service.

The test airframe

Naturally, it is always the aim to pass a test the first time, not least because of the high direct costs of carrying out such a test as well as the ensuing costs arising from time lost in the development process of the aircraft.

For a test to be repeated, a new test airframe has to be constructed and elaborately prepared for the trial. A new test airframe is relatively simply available from the production line that is being prepared for serial production. However, this then has to be fitted out with a large number of sensors, which are what makes it possible at all to record and evaluate the test.

Before the airframe can be used, its construction has to be changed and reinforced in light of the lessons learnt from the test. The reinforcements have to be manufactured and installed in the new airframe. That must be done in a manner that corresponds to the solution that will later be produced in series. From this it can be deduced that the reinforcements need to be optimised not only for the loads but also for the production process. Just the efforts described here result in costs running into several millions, for a project of these dimensions, and lead to corresponding delays.

Model Boeing 777X
Model of the Boeing 777X / © WingMag

And from the technical viewpoint?

Despite all the technology that is available today, and likewise the modern methods of computation, at the beginning of the aircraft construction process only theoretical load hypotheses are available, which are based on a multitude of optimised models, including aerodynamic tests in the wind tunnel.

Every model and every assumption inherently include inaccuracies, which is completely normal. These inaccuracies are also treated conservatively, i.e., “on the safe side”, in aeronautics. In addition, the calculated loads have to be “transformed” into a test arrangement, which simulates the effective loads that will occur later in flight.

The aerodynamic flight loads are induced in the test airframe, as far as is practically possible, via substitute elements (a very complicated hydraulic load induction system). Here too, the conservative approach is used, i.e. in cases of doubt a higher rather than too low a load is used.

Unjustly criticised?

All in all the divergences can add up to give a distinctly higher load than occurs in practice. But this is deliberate and anchored in the philosophy of conservatism in aviation.

Ultimately the manufacturer, or rather the development team, of such an aircraft is under great pressure to keep the weight of the aircraft as low as possible. If the test clearly exceeds the required load factor of 1.5, there is immediately talk of excess weight in the structure, which reflects negatively on the developers.

If you now exclude polemics and consider the complexity of this test, as described above, then a difference of 1 or 2 percent more or less can be viewed as a precision landing and therefore a very good engineering performance.

There are many calculations in various branches where such a small difference would be welcomed. But according to the principle “a miss is as good as a mile”, the result is not what it should have been. Nevertheless, there is no solid basis for criticism of the Boeing engineers or Boeing as an aircraft manufacturer for this. On the contrary, the work was outstanding. The engineers from Boeing will use the results of this test to design the lightest and thus most efficient structure for the 777X.

What does this mean in terms of weight?

On the basis of the lessons learnt from the failed test, the engineers from Boeing have the opportunity to tailor the structural weight to the theoretical ideal value for the load factor of 1.5. Conversely, if the test had resulted in a load factor of 1.52, for example, the structure would have passed the test and the aircraft would needlessly consume 4 to 5 tonnes of kerosene on each long-haul flight. From an environmental point of view, Boeing has developed perhaps the lightest structure that they could have, and thus prevented unnecessary consumption of fuel.

References:

[1] Seattle Times (Article not available any more)

[2] CS 25 Amendment 23

[3] Boeing 777X Specifications

[4] Calculation of fuel consumption (German) per kg payload of a normal 777 (current production series)

Cover picture © Boeing

by Marco Theiss

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