November 17, 2023, ©. Leeham News: We are discussing the different design phases of an airliner development program. After covering Conceptual, Preliminary, and Detailed design and the manufacturing of prototypes and their roles, we now look deeper at the flight test phase.
We looked at the overall flight test period, the needed flight test aircraft, and what area they covered last week, Figure 1.
Now we discuss the actual flight testing, its organization, and what it must cover to get the aircraft certified.
The flight tests and all the activities that go along with the tests on the ground are divided into two phases;
The initial flight tests of a new aircraft are done to clear the flight envelope of the aircraft, as discussed in Part 36. It’s the most risky part of the flight test campaign, containing the described risk of flutter but also other risks.
Early jet airliner high-speed flight tests found dangerous transonic effects, such as Mach tuck (the aircraft dived deeper and deeper, ignoring Yoke back pulls), when testing maximum dive speed at Machs where supersonic flow moved the aerodynamic center back on the wing, and transonic effects rendered the horizontal tailplane less effective.
We also have the famous T-tail accidents with the BAC 1-11, where the wing wake shadowed the horizontal tailplane during stall tests, causing a deep stall where the crew and aircraft perished.
Another risky moment is the verification of One Engine Inoperative (OEI) characteristics, where the aircraft operates close to stall speeds with a highly loaded vertical tail, controlling the missing engine yaw moment. Straight stalls are less risky, but stalls with uneven flow on the airframe and highly loaded flight surfaces can result in dangerous discoveries.
The stall tests at various speeds, altitudes, and configurations used to be risky. Today, the detailed knowledge of flow separation behavior and flight control effectiveness from performed low-speed wind tunnel tests has made it less risky. But we have recent examples where the ever larger engine nacelles for wing-mounted engines create new pitch-up tendencies at stall that must be handled.
An essential part of the OEM flight tests is the performance flight tests. The projected field performance at normal, and hot-and-high conditions must be verified, and the fuel consumption of the aircraft must be documented at different flight conditions and speeds/altitudes. The tests are called NAMS flying (Nautical Air Miles (NAMS) fuel consumption test flights).
If there is a problem with the NAMS results, extra CFD runs and flight tests must investigate the root cause of any non-foreseen drag. It can be airframe problems but also a powerplant not performing to specification. It doesn’t have to be an engine or nacelle problem in the latter case. There are numerous examples of airframe/powerplant installations where interference drag between the airframe and nacelle/pylon has caused NAMS performance problems.
Any problems detected during OEM flight tests mean an extended test period, extra investigations and tests on the ground, and a delay in the certification flight tests.
When the OEM deems it’s ready for Certification flight tests and the Regulator agrees, the flying for Certification points can start.
The certification flights are done with a team of Regulator and OEM pilots and test engineers. All data is analyzed during and after the flights to verify that the test cases have covered the intended test points and that the agreed means of compliance have been fulfilled.
It takes a better part of a year to do these tests involving hundreds of carefully planned and conducted flights. All the flight test data must be documented and handed over to the regulator in a format that can support the validation of the applicable certification criteria.
An OEM flight test typically takes a year for a reasonably trouble-free project. When certain aspects of the aircraft need a redesign, this extends the time frame, and we have current examples of multiyear extensions.
The Certification flight tests then typically add another year, where the last months are dominated by report and documentation work to ensure all required documents are in the form and revision required so the Regulator can issue a Type Certificate.