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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. Grob spn project. Grob background Composite specialist since 1971

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Grob spn Flutter Accident

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  1. Grob spn Flutter Accident

  2. European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK Gérard Guillaumaud1961 - 2006 • French Air Force • NTPS graduate • Competent and respected • Grob Chief Test Pilot • Active SETP member • In his memory: EFTSW award

  3. European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 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. European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK Grob spn specification • Short field and long range! • Vmo 270 kts / Mmo M.70 • Ceiling 41.000 feet • Design and certification speeds: Vc Vmo 270 Vd 1.25Vc 338 Vflutter_min 1.2Vd 406 Certification speeds

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

  7. European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 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. European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 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 Vc Vmo 270 Vd 1.25Vc 338 Vflutter_min 1.2Vd 406 Certification speeds P2 Vmax 297

  9. European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK P2 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 29th • Morning test flight • Afternoon demo flight

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

  11. European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 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: 240-270 kts

  12. European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK P2 GVT data received 12.12.2006 Asymmetric damping curve F = Curve of elevator tip vibration against trim tab.

  13. European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK P2 Speed Envelope? • min 20% flutter safety margin recommended by AC 23.629-1B • Only 5.4% safety margin between 297 and 313 kts Vc Vmo 270 Vd 1.25Vc 338 Vflutter_min 1.2Vd 406 Certification speeds 313 209 261 P2 speeds 297

  14. European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 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. European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK P3 GVT animation • Iterative process; envelope expansions; structural modifications; elevator horn eventually removed • July 2008: Envelope expansion to 41.000 ft and M.72

  16. European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 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?...

  17. European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 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

  18. European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK Questions?

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