Grob spn flutter accident
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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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Grob spn flutter accident

Grob spn Flutter Accident


G rard guillaumaud 1961 2006

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


Grob spn project

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


Grob spn specification

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


First prototype p1

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


Second protoype 2 p2

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


P2 ground vibration test gvt

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


P2 envelope

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


P2 status before the accident

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


Accident flight track

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

Accident Flight Track


P2 accident conditions

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


P2 gvt data received 12 12 2006

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.


P2 speed envelope

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


P1 and p3 findings after accident

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...


P3 gvt animation

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


Technical conclusion

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?...


Conclusion

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


Questions

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

Questions?


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