Leeham News and Analysis
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Leeham News and Analysis
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Bjorn’s Corner: New aircraft technologies. Part 36. Prototype manufacturing and Testing
By Bjorn Fehrm
October 27, 2023, ©. Leeham News: We are discussing the different design phases of an airliner development program. After covering Conceptual, Preliminary, and Detailed design (Figure 1), we now discuss prototype manufacturing and testing, where we today go deeper into structural testing.
Figure 1. The development plan for a new airliner. Source: Leeham Co.
Structural testing
An essential part of the testing for a new airliner program is the stress testing of the airplane structure. The manufacturing of prototype structures has the static load test structures as one of the first items produced.
The airplane structure must pass the Limit load test before it’s allowed to fly the first test flight. Limit load is the highest load the structure is foreseen to be subject to during its operational lifetime. During these tests, there can be no deformations or changes in the structure’s integrity.
The static test is done similarly to the most spectacular static load test, the Ultimate load test (Figure 2), where the structural parts are subject to 1.5 times the Limit load. Here, damage to the structure is allowed, such as wrinkles, etc., but there can be no structural failures before the limit load is reached.
Figure 2. The most famous of the structural tests, the wing Ultimate load test. Here, the Boeing 787 is starting its wing test. Source: Boeing.
There have been several cases of structural failures just before 150% of the Ultimate load is reached for the wing. If the failure mode is according to predictions but happens a few percent before 150%, the OEM can suggest structural reinforcement and, via simulations, convince the regulator that the final certified structure will meet the certification criteria for structural static load.
The Ultimate load test does not have to take place before the first flight but shall be completed satisfactorily before the aircraft can be certified.
Fatigue testing
The Limit and Ultimate load tests are static test cases; that is, it does not simulate long-time operation which subjects the aircraft to repeated stress cycles. This is done in the Fatigue tests, where the parts of the aircraft that see a continuously varying load are long-time tested.
The goal is to test the structure to high material fatigue cycles long before an operational aircraft reaches such cycle counts. For the A350, the fatigue tests simulated 86,000 flights, which was three times the planned cycle life of the aircraft.
The fatigue tests can be done separately on different aircraft parts, as in Figure 3, as long as the loads are representative of the loads in the complete airplane. Airbus gave iABG in Munich the contract to expose the fuselage middle section and wing to the 86,000 flight cycles.
Figure 3. Fatigue structural tests of the A350 middle section and wing at iABG Munich. Source: iABG.
An essential part for cycle tests is the pressurized section of the fuselage. It gets exposed to about 8.6PSI/0.6 Bar higher pressure than the surrounding air at cruise altitude. The pressure difference cycles the fuselage pressure tube, creating hoop stresses in the fuselage skins during flight.
To avoid fatigue problems, designers avoid non-round fuselage sections, as these create bending forces on top of the hoop stresses in the skins.
The cabin floor can be used as a stress-transferring member, which allows the fuselage to be designed with a “double-bubble” cross-section without causing troubling bending moments, Figure 4.
Figure 4. The Bombardier CRJ and Embraer ERJ have circular cross sections, whereas the CSeries (today A220) and E-Jets have double-bubble cross sections. Source: Leeham Co.
Metal fatigue from cabin pressurization was poorly understood when jet flights to high cruise altitudes started in the 1950s and caused the tragic De Haviland Comet accidents. The ensuing fatigue research involved complete Comet fuselages pressurized underwater in tanks (to dampen ruptures). The tests taught the aeronautical community a lot about metal fatigue when subjected to a high number of repeated stress cycles.
You are 2 steps back in certification if the failure mode during e.g. an ultimate load test is happening in a way / place you didn’t expect. Like on the 777-9.
https://simpleflying.com/boeings-777x-ripped-apart/
Apart from redesigning, testing and recertifying the structure you have to explain why the unexpected failure occurred, who was responsible, if the findings are relevant for similar structures, why not, and how you are going to secure this kind of miscalculations are not going to happen again.
On the specific 777-9 test Boeing and FAA were and remained tight lipped, the real failure leaking out months after the occurrence. I would not be surprised if the root cause investigation on the 777x certification strategy after this test and its conclusion are behind much of the long delay on the program.
Basically certifying an entirely new aircraft 777X (new wings, engines, systems, fuselage, landing gears, tail) as a minor modification of an existing aircraft (77W).
Everybody has been focused on the choices, power play, errors during 737MAX certification. But it pales compared to the 777X certification drama..
https://www.dailymail.co.uk/news/article-7732719/Boeing-fuselage-ruptured-777X-stress-test-blowing-passenger-door-damaging-wing.html
Bjorn covered why the rupture ‘doesnt matter’
‘ Just as the test approached its target stress level, an explosive depressurization tore through the fuselage….The relatively good news for Boeing is that because the test failed so explosively *at just 1% shy* of meeting federal requirements, it will almost certainly not have to do a retest. Regulators will likely allow it to prove by analysis that it’s enough to reinforce the fuselage in the localized area where it failed.”
https://www.seattletimes.com/business/boeing-aerospace/boeing-777xs-fuselage-split-dramatically-during-september-stress-test/
Isnt there a ‘rule’ about quoting the Daily mail ? ….Dont.
Its a feature not a bug to get within 1% of the ultimate load test when you have a very large plane.
I think Boeing is re-doing much of the 777x certification. Because the certification strategy approved in 2014 (raising eyebrows already then) didn’t pass the sniff test looking back. 737 MAX, Boeing, FAA, congress, crashes led to sweeping reforms in certificafion approach. Pushing back Boeing direct and indirect influence.
https://www.faa.gov/aircraft/air_cert/airworthiness_certification/certification_reform.
After the test there was a big effort to play down the test failure, focussing on the “99% ok” wing test.
If you do a test drive on a new car to check if it’s stable in sharp turns, and the steering wheel breaks off, the conclusing the car is stable in turns isn’t good enough for the police and potential buyers.
So when the facts dont agree with you , invent new claims that are less credible ?
As far as Im aware for 777X the new structure – stretch fuselage, wings, tail-horizontal tail arent the blocking issues. Its more the flight controls, its actuators and the complete new software system with some other issues with the new engines.
Fascinating stuff- thanks for this post.
As regards the DH Comet fatigue testing a lot was known about fatigue from pressurisation cycles and they followed very similar testing as today.
‘The entire forward fuselage section was tested for metal fatigue by repeatedly pressurising to 2.75 pounds per square inch (19.0 kPa) overpressure and depressurising through more than 16,000 cycles, equivalent to about 40,000 hours of airline service.”
What they wouldnt do nowadays was what DH did and apply a 2 x max operating P static test at the beginning before the cyclic pressurisation testing at 1.25 x P.
The square windows werent a basic error either as many US pressurised piston airliners had square windows too, starting with the Stratoliner 307 of the late 1930s and ending with DC-7 and L-1649 Starliner ( which could fly non stop LAX to London)
Duke:
I would add that the aircraft of the time did not fly at the altitudes the Comet did and that was an aspect of the pressure rupture.
Ie the higher differential involved.
DH knew that the Comet would cruise at up to 35,000 ft and the cabin interior was pressurised for 8000 ft, the 16,000 pressure cycles testing were done to reflect that pressure differential
And that was done in a similar way today, a 5hr flight was simulated in 5 min or less
Thanks for this accurate information.
As an unusual version of development testing Bombardier built a ‘wooden bird’ mock up for the Cseries:
“… full-scale fuselage mock-up used to evaluate the Environmental Control Systems. By September, the rig will be fitted with a full cabin interior and will test the pressurisation, air conditioning, heating, lighting and cabin management systems. ”
this was mentioned in Sept 2012 issue of Air International
https://www.key.aero/article/wooden-and-iron-mock-ups-testing-airbus-a220-revealed
The previous time a full size mockup in wood was done was the swing wing B2707 SST. However when the swing wing was dropped a second mockup
for the -300 version ,mostly in aluminium was made
Pipe is tested with pressurized water, if a rupture occurs the stored energy is less due to water not compressing much and you don’t have the explosive venting.
Will the FAA still allow the 777-9 be certrified under the same TC as the 777-300ER it will replace?
FAA approved that certification strategy a decade ago. Since then sweeping changes to the Boeing certification process were implemented. For a reason, and that reason is not Covid-19, the GE engines, the economic downturn, the supply chain or the FAA having been non-cooperative towards Boeing.
https://leehamnews.com/2020/01/16/certification-process-for-777x-is-another-hurdle-for-boeing/
The ongoing process resulted in 777x type certification delayed another 2 years, until 2025 recently.
For those noticing certification strategy (77W baseline for a basically a new aircraft) was never officially confirmed by Boeing as major reason for the years of delay, you are 100% correct..
Type Certification Basis
“Under the provisions of title 14, Code of Federal Regulations (14 CFR), § 21.101, Boeing must show that the Model 777-9 airplane meets the applicable provisions of the regulations listed in Type Certificate No. T00001SE[777-300ER], or the *applicable regulations in effect on the date of application* for the change, except for earlier amendments as agreed upon by the FAA.”
Regulations arent static either nor is the method of review by FAA
I think some commentary over does the issues around the airframe changes- which in this case was validated by full testing to applicable limit ( or within 1%) like a totally new plane would be . Normally a major modification to an existing type cert doesnt involve a repeat of the
*full* airframe testing
The harder issue is the complete change to the FBW system/cockpit displays and its computer hardware
https://aviationweek.com/aerospace/aircraft-propulsion/easa-boeing-compromise-positive-move-777x
“The 777X flight control system and its supporting systems architecture is an evolution of the heritage 777 system with new elements added from the 787.
Like the 787, but unlike the 777, the 777X fly-by-wire flight control system operates in all three axes (pitch, roll and yaw). The current 777 family uses the same pitch control law as the 787, but not the same roll control law—whereas the 777X will.”
From memory, the FAA found cut and paste 787 code in the 777X.
There is nothing wrong with using good code if it is a repeat, but for control there has to be changes as the forces involved as well a the actuators and those systems involved are different.
Its good the FAA is slow walking this, Boeing has built up some really bad habits.
thanks for info.