23 April, 2011

Understanding Human Rating Considerations: Flight Test hours

Just picked up a copy of the Everett, Washington Herald. In it was an article noting that Boeing has just passed through 3,500 hundred hours of flight testing on their 787 Dreamliner.

Boeing's initial flight test program plan had estimated 3,000 hours to be required to complete the FAA certification process. Several problems that cropped up during testing of the six jet test fleet, including an electrical fire requiring an emergency landing, have pushed that number upwards of 3,500 and counting.

The point here is that this experience shouldn't be a surprise to anyone and it doesn't mean that Boeing doesn't know what they're doing. To the contrary, they seem committed to wringing out this new type with its new technology, construction materials and methods, and software very thoroughly and in an orderly fashion to facilitate the 787's eventual entry into revenue service. Surprises in flight test are usually not pleasant and are not unexpected.

In some cases, flight hours might be consumed searching for the right (usually bad) weather conditions required to address a test objective. I recall that a B-52 modification entailed a succession of flights over the Rockies in search of a 3-sigma wind shear at altitude (a so-called mountain wave effect). After weeks of frustration, their diligence and patience was rewarded when the test aircraft encountered what was more like a 6-sigma condition. A lot of the vertical stabilizer was torn off, but the B-52 proved it could sustain such damage and still return to a safe landing.

I'll contrast the 787 test program with that of the Space Shuttle program. The orbiter fleet has accumulated about 31,200 total flight hours in the course of 133 missions to date. That number is heavily skewed towards time in orbit (accounting for a bit more than 31,000 of the total hours). The dynamic (and most risk-prone) ascent and entry flight phases account for a bit more than 1 hour per mission, so the program's total flight test experience in this regime is roughly 140 hours.

There were a series of captive flight tests on the back of a 747 to help gain confidence in some of the flight control and other supporting systems. And there were five drop tests of the approach and landing behavior and capabilities. These drop tests accounted for a total of about 19 minutes of flying time, 4 of which were in a representative flight-like configuration (i.e., no aerodynamic tail cone mounted).

Even with the shuttle facing retirement, each mission typically still includes a set of "Detailed Test Objectives" (DTOs) for each entry flight phase designed to improve our understanding of the orbiter's aerodynamic performance capabilities. A DTO might call for a set of pitch up/down excursions at a specific mach number, for example.

According to NASA, the shuttle fleet was declared fully operational after the 4th orbital flight after accumulating a total of about 469 flight hours, about 5 of which were spent in dynamic ascent or entry flight regimes.

If NASA were not the ultimate customer for the Shuttle, or if the FAA applied its normal standards and treated it as an airliner, would it be fully operational (and commercially insurable) after a flight test program of less than 6 hours? Probably not.

What does this say for future commercial efforts? Do the commercial space contestants have a good understanding of what a realistic certification program should (and must)  look like, or are they assuming that they'll get the same pass as the shuttle? Are their cost models, projections, and business models based on a reasonable test program with sufficient margin to address the inevitable unknowns?

One issue I foresee is that a vehicle with any sort of flight certification "lien" against it will be anathema to commercial insurers. I'm not talking merely about insuring the value of cargo. Individual passengers might learn that their life insurance policies exclude losses due to spaceflight accidents and this reminder of the perceived risk might temper their exuberance for buying a very expensive ticket.

1 comment:

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