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Grob spn Flutter Accident
Grob spn Flutter Accident
Grob spn Flutter Accident
European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 2
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 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
Grob spn Flutter Accident
European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 4
100 150 200 250 300 350 400 4500
0.5
1
Airspeed [kts EAS]
Grob spn specification
Short field and long range! Vmo 270 kts / Mmo M.70 Ceiling 41.000 feet Design and certification speeds:
Certification speeds
VcVmo270
Vd1.25Vc
338
Vflutter_min1.2Vd406
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
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 cmP1 = 559 cm
P1 = ~2.5kg
Elevator mass balance
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
Grob spn Flutter Accident
European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 8
100 150 200 250 300 350 400 4500
0.5
1
Airspeed [kts EAS]
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
VcVmo270
Vd1.25Vc
338
Vflutter_min1.2Vd406
P2Vmax297
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: 3-4 Hz @ 180-200 kts Maximum recorded speed 277 kts Plan for Nov 29th
Morning test flight Afternoon demo flight
Grob spn Flutter Accident
European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 10
Accident Flight Track
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: 240-270 kts
Grob spn Flutter Accident
European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 12
P2 GVT data received 12.12.2006
Asymmetric damping curveF = Curve of elevator tip vibration against trim tab.
Grob spn Flutter Accident
European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 13
min 20% flutter safety margin recommended by AC 23.629-1B
Only 5.4% safety margin between 297 and 313 kts
100 150 200 250 300 350 400 4500
0.5
1
Airspeed [kts EAS]
P2 Speed Envelope?
Certification speeds
VcVmo270
Vd1.25Vc
338
Vflutter_min1.2Vd406
297
P2 speeds 313261209
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...
Grob spn Flutter Accident
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P3 GVT animation
Iterative process; envelope expansions; structural modifications; elevator horn eventually removed
July 2008: Envelope expansion to 41.000 ft and M.72
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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
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
Grob spn Flutter Accident
European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK 18
Questions?