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

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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

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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

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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

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

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?