Transcript ABS Spar Web Investigation

ABS Spar Web
Investigation
Report to Wichita ACO/
FAA Small Airplane Directorate
Wichita, Kansas
July 23, 2009
ABS-ASF Proprietary Data
7/23/2009
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Mandatory Service Bulletins
• MSB 2269 issued August 1989
– Barons (except 58P and 58TC)
– Travel Airs
• MSB 2360 issued November 1990
– Bonanzas H35 and later
– All Model 33 Debonairs/Bonanzas
– All Model 36 Bonanzas
• Required inspection and repairs
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Airworthiness Directives
• AD 90-08-14
– Incorporates MSB 2269 (multiengine)
• AD 92-08-07
– Incorporates MSB 2360 (singe engine)
– One time inspection
– Required reporting to Beech
• No record of any data
• AD 95-04-03
– Repetitive inspections
– No reporting
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Current AD Inspection
• Initial inspection
@1500TT
• No cracks: every
500 hours
thereafter
• Small cracks:
every 200 hours
thereafter
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ABS Member Survey
Survey
Reported
%
Model
35 (O/IO470)
Responses
Cracks
Cracked
328
1
0.3%
Short body, <3300 MGW
35 (IO-520)
538
19
3.5%
Short body, 3300 MGW or greater
33 (IO-470)
163
4
2.5%
Short body, <3300 MGW
33 (IO-520)
182
8
4.4%
Short body, 3300 MGW or greater
36
25
4
16.0%
Long body, MGW 3600 lbs
A36
232
30
12.9%
Long body, MGW 3600 lbs (3650 lbs 1984 and later)
A36TC
21
0
0.0%
Long body, MGW 3600 lbs
B36TC
21
2
9.5%
Long body, MGW 3850 lbs
Baron 55/56
218
34
15.6%
Short body, twin-engine
Baron 58
41
14
34.1%
Long body, twin engine
95
9
1
11.1%
Short body, twin-engine
Notes
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Location of Cracks
140
120
117
100
80
58
60
42
40
20
12
5
0
Total
Front only
Front and rear
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Rear only
Not reported
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FAA No-Crack Policy
• The Old Policy
– For cracks found in structural components some ADs
have allowed continued operational flight if the cracks
are smaller than a certain size.
– Inspection requirements will be more stringent for
known cracks.
• The New Policy
– Continued operational flight with known cracks in
primary structural components should not be
allowed.
– Small Airplane Directorate will adhere to this basic
policy.
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ABS Position
• Known issue for two
decades
• No history of inflight failure
• Current inspection
process has ensured
ongoing safety
• ABS asked FAA for
time to investigate
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Summary of Analysis
Data Presented Nov 2008
• Preliminary fail-safe analysis
– Demonstrated >limit load capability with one web
completely failed
• Preliminary FEA of “medium hard” braking
condition
– Showed peak web bending stress of ~36 ksi at Huck
bolt collars
– Sufficient to cause early cracking with repeated
applications
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3 PT. BRAKED ROLL
µ = 0.30
PRINCIPAL STRESS – AFT SIDE OF FORWARD WEB
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Completion of Investigation
Focus:
• Ultimate load capability with cracks
• Most likely initial cracking causes
• Post-cracking web bending stress
alleviation
• Substantiation that existing AD
requirements adequately maintain safety
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The Critical Issue
• Is the existing AD sufficient to ensure
continued safe operation?
• Need credible substantiation
– Evidence from two decades of safe fleet
experience
• Impressive, but qualitative
– Detailed engineering analysis
• Will provide quantitative data
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Analytical Substantiation
Dependent on four quantitative elements:
1. That ultimate load is sustainable with
cracks equal to or greater than the
existing AD limits
2. That, in the event of complete failure of
one web, the remaining structure can
sustain at least limit load
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Analytical Substantiation
Dependent on four quantitative elements:
3. A credible post-cracking crack growth
assessment to validate the fleet-observed
crack-limiting behavior
4. Confirmation that the existing inspection
program will ensure timely detection of
any significant crack growth (regardless
of likelihood)
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Analytical Substantiation
• Items 1 & 2 are essentially unaffected by
the cause and mechanism of cracks
• Items 3 & 4 require rational insight into
likely causes, pre-crack stresses and
post-cracking behavior
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Post-Cracking Strength
Lower front spar carry-thru with failed web
•
Critical ultimate condition
– 6.6g positive maneuver
– 71% wing bending carried by front spar
– Net wing moment/side = 429,298 in-lb
– Couple load @ upper & lower spar caps =
39,027 lb
– Effective lower cap area ~1.45 sq in
•
•
•
Ft Ultimate = 26,915 psi
Ftu = 54,000 psi
MS >1.0
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Post-Cracking Strength
One web failed between upper &
lower spar caps
•
Critical ultimate condition
– 6.6g positive maneuver
– Net F/spar shear/side transferred to skin =
4557 lb
– Critical element, 13 AD4 rivets
– Load/rivet = 350.5 lb
– Shear critical at 375 lb
– Minimum MS = 0.07
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Summary: PostCracking Strength
• The analysis substantiates ultimate load
capability for cracks ranging from existing
AD limiting sizes through complete failure
of one web
• Obviously satisfies fail-safe limit load
requirements
• Existing requirements for repair if cracks
exist in both webs on one side should
continue in force
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Likely Causal Conditions
Primary mechanism producing web bending
• High fore/aft load transferred from wing
lower attach fitting to belly skin
• Web bending tendency increases with
belly in tension from fuselage bending
• Decreases as belly loading changes to
compression
• Unless local belly compression is high,
web bending almost totally a function of
the fore/aft load transfer
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Lower Fitting Load Transfer Schematic
P – Load transferred from wing fitting
LBT – Local belly tension due to fuselage bending
ABS-ASF
ABS-ASFProprietary
Report to FAA
Data
7/23/2009
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Likely Causal Conditions
• Normal operating spectrum investigated
as a possible cause
• Potential random conditions more severe
than typically encountered
– No individual known history
– Arbitrary but reasonable selection
• To provide insight into post-cracking
behavior
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Normal Operating Spectrum Analysis
• Conditions within normal operating flight
spectrum unlikely to cause cracking
• Most conservative negative
gust/maneuver condition investigated to
confirm this
– Selected -0.6g condition with cruise thrust
– Frequency: Once per 1500 flights
• Web bending stresses are low & unlikely
to cause early cracking
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-0.6g Up Gust Condition
3544.3
531
Net Lwr Ftg. Load: 63 lb
62
20
73.9
593
100
80
593
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-0.60 G GUST CONDITION
FEA RESULTS
WEB BENDING STRESSES
2 ksi
7.4 ksi
ABS-ASF
ABS-ASFProprietary
Report to FAA
Data
7/23/2009
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Investigation of Off-Normal
Conditions
• Considered several possible relatively
severe conditions to study forward and aft
web bending environments
– Arbitrary but rational
• Forward web cracking
– 3-point braked roll, mu = 0.30
– Engine run-up, 350 lb thrust
– 300 lb forward towing load reacted at main
gear
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Investigation of Off-Normal
Conditions
• Aft web cracking
– Tied down airplane in 50 kt crosswind
– 25 fps gust at 1g cruise
– 300 lb reverse towing condition
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Investigation of Off-Normal
Conditions
Approach:
• For each condition, the fore/aft wing fitting
load is calculated, together with the local
fuselage belly loading
• The 3pt braked roll condition is analyzed
by the FEA model to develop peak web
bending stresses at the crack locations
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7/23/2009
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Investigation of Off-Normal
Conditions
Approach:
• Using the relative F/A wing fitting loads
the bending stresses for the other
conditions are estimated
• Those conditions yielding high stresses
are considered to be the most likely to
initiate early cracking
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3-pt Braked Roll condition, mu=0.3
Peak Web Bending Stresses
Huck bolt collars
Web face
Stress 14,174 psi
at flange radius
Flange of web
ABS-ASF Proprietary Data
Stresses, Forward
Side, Forward Web
7/23/2009
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3-pt Braked Roll condition, mu=0.3
Peak Web Bending Stresses
Huck bolt collars
Web face
Plate element 45799
Stress 33,240 psi
Detail A
Flange of web
ABS-ASF Proprietary Data
Principal Stresses,7/23/2009
aft Side of Forward Web
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3 Point Braked Roll, mu=0.30
Fatigue Assessment
• Peak web bending stress = 33.24 ksi
(assumes no hard turns)
• For 16.6 + 16.6 ksi, predicted fatigue life
= 2400 cycles (typically 1560 hours)
• Potentially 3 or more hard baking
events/landing
• Assuming frequency of 1 event in 4
landings, 2400 cycle life reached in 3200
landings (~2000 hours)
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Off-Normal Conditions:
Comparative Results
Fwd or
Fwd Attach
Peak Web
Est. Fatigue
Aft Web
Fwd
Fitting Load (lb)
63
Bending Stress
7.4
Life (Cycles)
>500,000
3 PBR, mu=0.3
Fwd
249.5
33.24
2400
350 lb thrust
Fwd
222
29.58
3400
300 lb fwd towing
Fwd
208.5
27.8
5000
Tied down
Aft
159
21.2
14,000
25 fps fin gust
Aft
67
8.9
>500,000
300 lb reverse
Aft
208.5
27.8
5000
Condition
Neg. 0.6g gust/
maneuver
engine run-up
w 50kt crosswind
towing
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Off-Normal Conditions: Conclusions
• Flight conditions appear non-critical
• Ground conditions all demonstrate
significant web bending stresses,
potentially sufficient to initiate early
cracking
• Estimated cycles to failure range from
2400 to 14,000 cycles
• Results consistent with fleet observed
cracking at 3-4000 hours and greater
incidence of forward web cracking (4:1)
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Post-cracking Stress Alleviation, 3pt Braked Roll Condition, mu=0.3
Huck bolt
“B”
Huck bolt
“B”
33,240 psi
pre-crack
15,089 psi
post-crack
Web face pre-crack
Web face post-crack
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7/23/2009
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Post-Crack Damage Growth
• With AD allowable crack lengths
– Post-crack web bending stresses reduced >50%
– Normal operating conditions generate negligible
web bending (pre- or post-cracked)
– Worst-case (3 PT Braked Roll), peak stress –
15.1 ksi & estimated life to significant crack
growth is ~50,000 cycles
• Even assuming frequent repeated
aggressive operation, anticipated crack
extension rate will be very slow
– As evidenced by fleet experience
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Investigation Findings
• Ultimate loads sustainable with cracks
beyond AD limits—up to complete failure
of one web
• Web bending stresses are low for all
normal operating spectrum loads
• Several aggressive ground operating
conditions generate high stresses
– Potential for early cracking
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Investigation Findings
• Post-cracking stresses reduced by >50%
– Any subsequent crack growth will be very
slow
• These findings are consistent with 20
years of safe fleet experience
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Conclusions
• Cracks up to and beyond existing AD
limits pose no threat to continued safe
operation
• Existing inspection requirements will
continue to adequately monitor/maintain
on-going safety
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Recommendations
• Continue with existing AD requirements
• Additional emphasis to operators on
avoidance of aggressive ground
operations
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Questions?
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