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Grob spn Flutter Accident. European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 2 Gérard Guillaumaud 1961 - 2006 French Air Force NTPS.

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Presentation on theme: "Grob spn Flutter Accident. European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 2 Gérard Guillaumaud 1961 - 2006 French Air Force NTPS."— Presentation transcript:

1 Grob spn Flutter Accident

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

3 Grob spn Flutter Accident European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 3 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

4 Grob spn Flutter Accident European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 4 Grob spn specification Short field and long range! Vmo 270 kts / Mmo M.70 Ceiling feet Design and certification speeds: Certification speeds Vc Vmo 270 Vd 1.25Vc 338 Vflutter_min 1.2Vd 406

5 Grob spn Flutter Accident European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 5 First Prototype (P1) Demonstrator; FF 2005 Limited/immature systems No initial envelope expansion Eventually max ~280 kts CAS based on GVT data Envelope expansion after 1 year: Full envelope expansion with no problems detected Limited by elevator authority – larger tail required

6 Grob spn Flutter Accident European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 6 Second protoype 2 (P2) Systems testbed No intention of making high risk tests Avionics + pressure cabin 16% increased stabilizer span Elevator hinges P2 = 648 cm P1 = 559 cm P1 = ~2.5kg Elevator mass balance

7 Grob spn Flutter Accident European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 7 P2 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

8 Grob spn Flutter Accident European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 8 P2 envelope Data received per Telefax from flutter expert Limit 297 kts with 4.5 kg elevator mass balances Similar situation to P1 initial envelope – the limit was considered preliminary pending more data Data accepted with no further questions Certification speeds Vc Vmo 270 Vd 1.25Vc 338 Vflutter_min 1.2Vd 406 P2 Vmax 297

9 Grob spn Flutter Accident European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 9 P2 status before the accident Two months since first flight 31 flight hours Light nose boom vibration: kts Maximum recorded speed 277 kts Plan for Nov 29 th Morning test flight Afternoon demo flight

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

11 Grob spn Flutter Accident European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 11 P2 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: kts

12 Grob spn Flutter Accident European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 12 P2 GVT data received Asymmetric damping curve F = Curve of elevator tip vibration against trim tab.

13 Grob spn Flutter Accident European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 13 min 20% flutter safety margin recommended by AC B Only 5.4% safety margin between 297 and 313 kts P2 Speed Envelope? Certification speeds Vc Vmo 270 Vd 1.25Vc 338 Vflutter_min 1.2Vd P2 speeds

14 Grob spn Flutter Accident European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 14 P1 and P3 findings after accident P1 flight ops restarted after 2 ½ months Loose mass balances discovered during a preflight inspection; repaired with bolts fixing mass balance to structure P3 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...

15 Grob spn Flutter Accident European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 15 P3 GVT animation Iterative process; envelope expansions; structural modifications; elevator horn eventually removed July 2008: Envelope expansion to ft and M.72

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

17 Grob spn Flutter Accident European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 17 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 Conclusion

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


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