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Well known is the last long endurance test flight by a Boeing B787-8, which drew its shape in the air for 18 hours while on a long endurance test flight: https://www.wired.com/story/boeing-787-8-drawing-test-flight/.

What exactly is the purpose of these long endurance test flights? What is tested, what shall be proven? - surely not that the aircraft can actually stay in the air that long, because that test flight happened long after the introduction of this type into the market...

On a side not, does every commercial airline type have to undergo such an endurance test flight?

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  • $\begingroup$ Bragging rights? $\endgroup$ – Sean Apr 26 '19 at 3:30
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This particular test flight tested the engines, not the aircraft:

At 15:38 local time N7874, the fourth 787 built, departed Boeing Field in Seattle for an 18-hour, 22 state test flight. The crew spent the overnight hours above the United States performing ETOPS testing on the new Rolls Royce Trent 1000 TEN engine, which will power the 787-10.

https://www.flightradar24.com/blog/boeing-uses-a-787-to-draw-a-787-dreamliner-during-etops-test/

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    $\begingroup$ So why not test these engines on the ground then? What's the benefit of using a test flight? $\endgroup$ – Stefan Mar 14 '18 at 9:50
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    $\begingroup$ @Stefan You can then ask the following questions: Why use engine testbed aircraft (search this term on Google) at all if you can test engines on the ground. Heck, why even have test flights at all? we can test all aircraft in a wind tunnel. The answer to both questions is that you can't test everything on the ground, especially not engines. Specifically, ETOPS test flights presumably involve flying on a single engine due to the nature of ETOPS. See en.wikipedia.org/wiki/ETOPS#Approval $\endgroup$ – DeepSpace Mar 14 '18 at 9:55
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    $\begingroup$ @Stefan an engine on the ground has a different atmospheric makeup, pressure, temperature, and doesn't have air being forced into it at 560mph. $\endgroup$ – JPhi1618 Mar 14 '18 at 19:19
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    $\begingroup$ @stefan, for smaller engines, like those for business jets or small fighters, testing like this can be conducted on the ground in special facilities (e.g. arnold.af.mil/About-Us/Fact-Sheets/Display/Article/409293/…). However, the mass flow rate of an engine like the Trent 1000 is around 1000 kg per second. Building a wind tunnel of that size would be orders of magnitude more expensive than just flying the plane around for a few hours (especially if you already own a plane) $\endgroup$ – Daniel K Mar 15 '18 at 2:13

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