Managing Rotorcraft Safety

1
Managing Rotorcraft Safety
AHS International Helicopter Safety Symposium 2005

• During
• Frequently Performed Unique Missions
• September 28, 2005

Philip G. Potts Chief System Engineer Safety /
Airworthiness Sikorsky 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
• Argument
• Unique Maximum Performance Missions,
• Increase Potential to
• Degrade Fleet Safety and Compromise Structural
  Fatigue Lives, Long Term

AHS International Helicopter Safety 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

AHS International Helicopter Safety Symposium 2005
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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.

AHS International Helicopter Safety Symposium 2005
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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

AHS International Helicopter Safety Symposium 2005
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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

AHS International Helicopter Safety Symposium 2005
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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
  Missions
• How Does (Do)
• Operators
• Achieve Operational Safety with Limited Technical
  Awareness of Mission Performance
• OEM
• Collect Technical Information for Unique Missions
• Pilot
• Safely Operate to Unique Mission Parameters

AHS International Helicopter Safety 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

AHS International Helicopter Safety Symposium 2005
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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 Intervals
• Process 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 Helicopter Safety Symposium 2005
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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 Safety
• Airworthiness 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 Helicopter Safety Symposium 2005
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RecommendationsAutomate Mission Management

• Monitor Mission Uncertainties for
• Rotorcraft Drive System
• 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
• Add Vibration and Thermal Monitoring to
• Primary Drive Shaft Components

AHS International Helicopter Safety Symposium 2005
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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 todays commercial
usage.
AHS International Helicopter Safety Symposium 2005
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Helicopter Accident DataCivil Twin Turbine
Helicopter

• Mechanical Failures Represent 42 of Fleet
  Accidents
• (29) 89 Dynamic System (13) 39 Engine

AHS International Helicopter Safety Symposium 2005
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Fatigue Elements of Life Analysis

1. Flight loads
2. Material Strength of the Component
3. Frequency and duration of all maneuver loads

2 Fatigue Strength Curve
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1 Measured Flight Loads
Lower Loads Higher Fatigue Life
3 Life in Load Cycles