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Managing Rotorcraft Safety
DuringFrequently Performed Unique
MissionsSeptember 28, 2005
AHS International HelicopterSafety Symposium 2005
Philip G. PottsChief System Engineer – Safety / AirworthinessSikorsky Aircraft Corporation
2
Investigation Premise
Maintaining Fleet Airworthiness Presumes: Aircraft Operate within Certified Mission Profiles
Some Unique Missions Regularly Expect: Aircraft to Repeatedly Perform At or Beyond Certified
Limitations Frequent Short Distance Sorties at Maximum
Performance
ArgumentUnique Maximum Performance Missions,
Increase Potential toDegrade Fleet Safety and Compromise Structural
Fatigue Lives, Long TermAHS International HelicopterSafety Symposium 2005
3
Investigation Documents Overview Fixed Wing Aircraft and Rotorcraft Accident Reports
Arial Fire Fighting: Assessing Safety and Effectiveness
Harsh Mission Environment Dominates Mishap Causes
Fatigue of Primary Structure Caused Aircraft Mishaps
US Civil Rotorcraft Accidents – Twin Turbine Mechanical Failures Dominated Mishap Causes Fatigue of Primary Drive Structure is a Significant
Cause of Rotorcraft Mishaps
AHS International HelicopterSafety Symposium 2005
4
Aerial Fire Fighting Cause SummaryHeavy Transport Fleet
Mission Loads ‘The severity of the maneuver loads experienced
by airplanes involved in firefighting operations’ ‘exceeded both the maneuver limit and ultimate load factors’
Structural Fatigue ‘These repeated and high-magnitude maneuvers and the
repeated exposure to a turbulent environment hasten the initiation of fatigue cracking and increase the growth rate of cracking once it exists.’
Airworthiness Risk ‘…fatigue cracking and accelerated crack propagation can
and should be addressed through maintenance programs.’Conclusion: ‘…no effective mechanism currently exists to ensure the continuing airworthiness of these firefighting aircraft.’
AHS International HelicopterSafety Symposium 2005
5
US Civil Rotorcraft Cause Summary Twin Turbine Fleet
Structural Fatigue ‘Past design standards are
inadequate relative to the many new and varied activities’
Mission Loads ‘Pilots did exceed design limits’
Airworthiness Risk ‘required and timely maintenance was skipped’ ‘…less than thorough inspections were
performed,’
Conclusion: ‘The current fleet appears, broadly speaking, to be underdesigned in view of today's commercial usage’
AHS International HelicopterSafety Symposium 2005
6
Report Conclusions
Load Characterizations Helicopter: ‘Underdesigned’ Firefighting: ‘High magnitude maneuvers’
Responsibility to Airworthiness ‘Inadequate maintenance procedures to detect
fatigue cracking’ Operators ‘did not possess engineering expertise’ ‘to
predict the effects of those stresses on the operational life of the airplanes’
No Coordinated Management of Responsible Process Owners
Pilot, Operator, OEM, Maintenance, Regulator
AHS International HelicopterSafety Symposium 2005
7
Report Conclusions in Question Process Owners Limitations
Certification Standards Are Standards followed? Anticipate Mission Changes?
Maintenance Inspection Procedures Not Directly Tied to Unique
MissionsHow Does (Do):
Operators Achieve Operational Safety with Limited Technical
Awareness of Mission PerformanceOEM
Collect Technical Information for Unique MissionsPilot
Safely Operate to Unique Mission ParametersAHS International HelicopterSafety Symposium 2005
8
Operating StandardsNo Measurement Requirements
Airplane Firefighting Mission Measurements Certified Limit and Ultimate Loads were Regularly
Exceeded An Airframe Manufacturer Analyzed a ‘5 to 7 Time’
Usage Acceleration during Firefighting
Civil Helicopter Accidents Distribution Dynamic System Mechanical Failure Rate
OF 302 Total Twin Turbine Accidents (29%) 89 Dynamic System +(13%) 39 Engine
Fatigue is 42% of the ‘Cause’ Total This Rate is Equivalent to Single Turbine Accidents, by %
AHS International HelicopterSafety Symposium 2005
9
AirworthinessA Manual Method
Airframe Methodology …’visual inspections for cracks’ Pilots Enjoy Challenging Missions Operators Not Prepared to Analyze Unique Missions Effects
Rotorcraft Fatigue Lives Derive from Mission Loads, Frequencies, and
Material Strength Dynamic System Cracks are Very Small with Very Short
Inspection IntervalsProcess Gap:
Unique Missions Cause Significant Variations in Load Patterns and Frequency
Maintenance Intervals Not Tracked to Unique Missions Changes ‘Visual Crack Inspection Methodology’ is Impractical for
Rotorcraft
AHS International HelicopterSafety Symposium 2005
10
Investigation Findings Characteristics That Affect Safety
Unique Operations Are Rarely Standard Unique Missions Potentially Increase Loading or Frequencies Fatigue Lives Degraded by Unique Mission Cycles
Maintenance Inspections are Based on Certified Mission Maintenance Schedules not directly Linked to Mission Frequencies Airframe Visual Inspection Methodology Does Not Apply to
Rotorcraft System SafetyAirworthiness Requires an Automated Data Source
Technical Data Needed Mission Flight Loads Require Quantified Mission Inspection Intervals Crack Growth and Fatigue Lives Affected by Increased Loads
An Improved Way of Evaluating Missions is Required.
AHS International HelicopterSafety Symposium 2005
11
RecommendationsAutomate Mission Management
Monitor Mission Uncertainties for Rotorcraft Drive System
1. Significant Torque and Frequency Events Alert Operator & OEM of Potential System Life
Degradation Assess Pilot Techniques for Mission Adjust Maintenance and Inspection Intervals to Unique
Mission Conditions
Dynamic Monitoring of Tail Drive System: 2. Add Vibration and Thermal Monitoring to
Primary Drive Shaft Components AHS International HelicopterSafety Symposium 2005
12
Backup
13
Helicopter Accident DataCivil Twin Turbine Helicopter
Fatigue represents 42 % of Dynamic System Failures
Issue: ‘The current fleet appears …to be underdesigned in view of today’s commercial usage.’
AHS International HelicopterSafety Symposium 2005
14
Helicopter Accident DataCivil Twin Turbine Helicopter
Mechanical Failures Represent 42% of Fleet Accidents
(29%) 89 Dynamic System +(13%) 39 Engine
AHS International HelicopterSafety Symposium 2005
15
Fatigue – Elements of Life Analysis
1 1
STRESS (Flight Loads)
Mean Strength
Working Strength
10*5
Level Flight Load
Maneuver Flight Load
GAG Load
10*6 10*7 10*8CYCLES
Material Property Curve
Fatigue Test Data
[2] FatigueStrength Curve
[1] MeasuredFlight Loads
1
1. Flight loads 2. Material Strength of the Component 3. Frequency and duration of all maneuver loads
[3] Life in Load Cycles
Lower LoadsHigher Fatigue Life