managing rotorcraft safety during frequently performed unique missions september 28, 2005 ahs...

Managing Rotorcraft Safety

DuringFrequently Performed Unique

MissionsSeptember 28, 2005

AHS International HelicopterSafety Symposium 2005

Philip G. PottsChief System Engineer – Safety / AirworthinessSikorsky Aircraft Corporation

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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

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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


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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.’


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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’


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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


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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

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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 %


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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


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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.


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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

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Backup

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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.’


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Helicopter Accident DataCivil Twin Turbine Helicopter

Mechanical Failures Represent 42% of Fleet Accidents

(29%) 89 Dynamic System +(13%) 39 Engine


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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