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Grob spn Flutter Accident
Special circumstances which allow me to hold this presentation; company is bankrupt so I don't have to ask anyone permission.
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Gérard Guillaumaud 1961 - 2006 French Air Force NTPS graduate
Competent and respected Grob Chief Test Pilot Active SETP member In his memory: EFTSW award Gérard was well known to the SETP; presentation on spn development; 2 previous presentation on GA diesel engine development European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK
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Grob spn project Grob background 2004: spn light business jet launch
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 Successful despite the name has an ominous meaning in eastern Europe Composite specialist Prototype specialist – playground for Dr Grob European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK
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Grob spn specification
Short field and long range! Vmo 270 kts / Mmo M.70 Ceiling feet Design and certification speeds: Vc Vmo 270 Vd 1.25Vc 338 Vflutter_min 1.2Vd 406 Certification speeds Vmo = 270 kts Vd = Vmo + 25% = 338 kts Vflutter = Vd + 20% = 406 kts iaw. AC European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK
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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 Rapid development at the cost of an immature prototype “Flying wind tunnel model” We flew the aircraft 3 years on the standby instruments... Landing gear problems – brakes etc. Interesting PIO mode resembling a ground version of a dutch roll European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK
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Second protoype 2 (P2) Systems testbed 16% increased stabilizer span
No intention of making high risk tests Avionics + pressure cabin 16% increased stabilizer span P1 = ~2.5kg Elevator mass balance a) Concentrated mass balances Increased span – more elevator which must be balanced; Geometry: shorter arm means more mass; also slimmer elevator close to horn b) 4 hinges – increased distance between #1 (outboard) and #2 hinge = more sensitive to torsion. c) Single nut securing elevator – Very critical part; failure = catastrophic. Elevator hinges P1 = 559 cm P2 = 648 cm European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK
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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 450% increase; should always be careful with extrapolated data! Mass balance is critical; failure = catastrophic Ad-hoc modification of very a critical part European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK
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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 European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK
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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 29th Morning test flight Afternoon demo flight PEC test point reduced due lack of conformity European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK
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Accident Flight Track Uncontrolled airfield; I was traffic watch on the hangar roof Normal takeoff however low cloud base ~1000 ft Skimming the base of the clouds on downwind 4 Munich ATC radar contacts on downwind: 250 kts I saw him turning in; then looked the other way to watch for traffic Witnesses saw parts falling off the aircraft I looked back and saw the fireball rising behind the trees European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK
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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: kts European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK
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P2 GVT data received Also a symmetrical mode going unstable at 320 kts Asymmetric damping curve F = Curve of elevator tip vibration against trim tab. European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK
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P2 Speed Envelope? min 20% flutter safety margin recommended by AC B Only 5.4% safety margin between 297 and 313 kts Vc Vmo 270 Vd 1.25Vc 338 Vflutter_min 1.2Vd 406 Certification speeds 209 261 313 P2 speeds 297 It is to me obvious that the 297 kts limit was far too high. European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK
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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... To get good performance you need good handling; and to get good handling you need good systems! Systems => handling => performance European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK
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P3 GVT animation Note gap between hinges #1 and #2 Iterative process; envelope expansions; structural modifications; elevator horn eventually removed July 2008: Envelope expansion to ft and M.72 European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK
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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?... Doomed: Critical problem unknown to flight test Design not failsafe: Horn with single concentrated mass, elevator secured with 1 nut, no provision or calculation to allow mass balance adjustments Manufacture: Highly critical part modified in a casual manner GVT: Extrapolation of results; wrong limit Envelope expansion: Stick raps not sufficient above 7-8 Hz; the flutter problem was between Hz An accident waiting to happen; the aircraft would have crashed soon anyway. European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK
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Conclusion Classical error chain Main problem: Lack of COMMUNICATION:
Design/Manufacturing flaw(s) => Wrong limit => No envelope expansion Main problem: Lack of COMMUNICATION: The flutter problem was not communicated Why?... Quality Time pressure Doomed: Critical problem unknown to flight test Design not failsafe: Horn with single concentrated mass, elevator secured with 1 nut, no provision or calculation to allow mass balance adjustments Manufacture: Highly critical part modified in a casual manner GVT: Extrapolation of results; wrong limit Envelope expansion: Stick raps not sufficient above 7-8 Hz; the flutter problem was between Hz An accident waiting to happen; the aircraft would have crashed soon anyway. European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK
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Questions? European Flight Test Safety Workshop, Sept 28-29th 2010, London, UK
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