Grob spn Flutter Accident Grob spn Flutter Accident

Grob spn Flutter Accident

Grob spn Flutter Accident Gérard Guillaumaud 1961 - 2006 French Air Force NTPS graduate Competent and respected Grob Chief Test Pilot Active SETP member In his memory: EFTSW award European Flight Test Safety Workshop, Sept 28 -29 th 2010, London, UK 2

Grob spn Flutter Accident Grob spn project Grob background Composite specialist since 1971 Gliders, trainers and special purpose aircraft 2004: spn light business jet launch 8 passengers All-composite Conventional flight controls First flight 2005 European Flight Test Safety Workshop, Sept 28 -29 th 2010, London, UK 3

Grob spn Flutter Accident Grob spn specification Short field and long range! Vmo 270 kts / Mmo M. 70 Ceiling 41. 000 feet Design and certification speeds: Certification speeds Vc Vmo 270 Vd 1. 25 Vc 338 Vflutter_min 1. 2 Vd 406 European Flight Test Safety Workshop, Sept 28 -29 th 2010, London, UK 4

Grob spn Flutter Accident First Prototype (P 1) Demonstrator; FF 2005 Limited/immature systems No initial envelope expansion Envelope expansion after 1 year: Eventually max ~280 kts CAS based on GVT data Full envelope expansion with no problems detected Limited by elevator authority – larger tail required European Flight Test Safety Workshop, Sept 28 -29 th 2010, London, UK 5

Grob spn Flutter Accident Second protoype 2 (P 2) Systems testbed No intention of making high risk tests Avionics + pressure cabin 16% increased stabilizer span P 1 = ~2. 5 kg Elevator mass balance Elevator hinges P 1 = 559 cm P 2 = 648 cm European Flight Test Safety Workshop, Sept 28 -29 th 2010, London, UK 6

Grob spn Flutter Accident P 2 Ground Vibration Test (GVT) External flutter expert GVT result: Tested: 1 kg elevator mass balances = max 135 kts Calculated: 4. 5 kg elevator mass balances Improvised retrofit for adjusting mass balance (1) Elevator horn was opened (2) Lead pellet/resin mixture poured into the horn (3) Horn sealed No new GVT after retrofit; no new structure stress/strength recalculation European Flight Test Safety Workshop, Sept 28 -29 th 2010, London, UK 7

Grob spn Flutter Accident P 2 envelope Data received per Telefax from flutter expert Limit 297 kts with 4. 5 kg elevator mass balances Similar situation to P 1 initial envelope – the limit was considered preliminary pending more data Data accepted with no further questions Certification speeds Vc Vmo 270 Vd 1. 25 Vc 338 Vflutter_min 1. 2 Vd 406 P 2 Vmax 297 European Flight Test Safety Workshop, Sept 28 -29 th 2010, London, UK 8

Grob spn Flutter Accident P 2 status before the accident Two months since first flight 31 flight hours Light nose boom vibration: 3 -4 Hz @ 180 -200 kts Maximum recorded speed 277 kts Plan for Nov 29 th Morning test flight Afternoon demo flight European Flight Test Safety Workshop, Sept 28 -29 th 2010, London, UK 9

Grob spn Flutter Accident Flight Track European Flight Test Safety Workshop, Sept 28 -29 th 2010, London, UK 10

Grob spn Flutter Accident P 2 accident conditions Impacted wings level and exactly on track Tail parts 400 meters before impact Elevator horns broken off elevator Several symmetric damages and cracks on left and right side of stab/elevator Unknown speed No FDR; all recording media destroyed No telemetry BFU estimate: 240 -270 kts European Flight Test Safety Workshop, Sept 28 -29 th 2010, London, UK 11

Grob spn Flutter Accident P 2 GVT data received 12. 2006 Asymmetric damping curve F = Curve of elevator tip vibration against trim tab. European Flight Test Safety Workshop, Sept 28 -29 th 2010, London, UK 12

Grob spn Flutter Accident P 2 Speed Envelope? min 20% flutter safety margin recommended by AC 23. 629 -1 B Only 5. 4% safety margin between 297 and 313 kts Certification speeds P 2 speeds 209 Vc Vmo 270 261 Vd 1. 25 Vc 338 Vflutter_min 1. 2 Vd 406 313 297 European Flight Test Safety Workshop, Sept 28 -29 th 2010, London, UK 13

Grob spn Flutter Accident P 1 and P 3 findings after accident P 1 flight ops restarted after 2 ½ months Loose mass balances discovered during a preflight inspection; repaired with bolts fixing mass balance to structure P 3 design changes Met the “short field - long range” performance specification Less balance mass in elevator horn; distributed mass balances along elevator leading edge However, there still was a flutter problem. . . European Flight Test Safety Workshop, Sept 28 -29 th 2010, London, UK 14

Grob spn Flutter Accident P 3 GVT animation Iterative process; envelope expansions; structural modifications; elevator horn eventually removed July 2008: Envelope expansion to 41. 000 ft and M. 72 European Flight Test Safety Workshop, Sept 28 -29 th 2010, London, UK 15

Grob spn Flutter Accident Technical Conclusion Flutter Possible flutter cause(s): Excessive speed? Loose mass balance(s)? Elevator/horn strength/stiffness? But a flutter problem should not cause an accident? . . . European Flight Test Safety Workshop, Sept 28 -29 th 2010, London, UK 16

Grob spn Flutter Accident Conclusion Classical error chain Design/Manufacturing flaw(s) => Wrong limit => No envelope expansion Main problem: Lack of COMMUNICATION: The flutter problem was not communicated Why? . . . Quality Time pressure European Flight Test Safety Workshop, Sept 28 -29 th 2010, London, UK 17

Grob spn Flutter Accident Questions? European Flight Test Safety Workshop, Sept 28 -29 th 2010, London, UK 18