Uninhabited Air Vehicle (UAV) Certification

Uninhabited Air Vehicle(UAV)

Certification

Tim JennewineASC/ENFS(937) [email protected]

ASIP 2002 Special Session on Certification

December 11, 2002


Outline

FocusCertification ExpectationsUAV Development/Certification ChallengesSummary


Focus

What are the challenges to reducing the timeand cost to certify an aircraft structure?

Issues and Opportunities


Outline



Certification Expectations

• Certification requirements are a function of– Safety– Economic Value– Mission Criticality– System Longevity

• Developmental challenges (time/cost) vary directly with risk - known or unknown

• Expectations are baselined to prior experience– Developer– Program Management– User– Maintainer


Outline



UAV Certification Challenges

• UAV Acquisition Strategy• Technology Transition Drag• Overselling Analytical Capability• Developmental Design Data• Full-Scale Testing• Other Considerations


Spiral Acquisition Strategy

• Characteristics of Spiral Systems– Derived from technology demonstrators – System requirements/capabilities continuously evolving– Conventional design criteria not initially applied to demonstrators– Airframe structure treated as a candidate for evolving capability– Demonstrators are deployed in advance of developmental certification

?


Spiral Development Strategy

• Focused on rapid delivery of capability to the warfighter“minimum schedule - minimum cost”

– Can not tolerate out-of-control processes that drive component rework, inspections and compromise vehicle performance

– Design assumptions, criteria and concepts must accommodate design/analysis errors, producibility issues and external loads and environmental uncertainty, operational variability

• Strategy depends on low/manageable technical risk - Capability demands structural flexibility - “Robustness”– “Robust” design - proven design criteria, established materials/systems,

demonstrated production processes, conventional analyses and efficient certification

Structural methods and concepts that have been proven


Spiral Development Strategy

• Airframe structural certification cannot efficiently be spiraled– Airframe baseline design should accommodate reasonable

projections for payload and system growth • payload• weapons• avionics/electronics

– Airframe baseline design must be robust– Airframe development and certification should be

accomplished in one pass • Cost/schedule constraints preclude multiple airframe structural

redesigns and recertification activities

• Program’s are not budgeting ($, time) for ASIP


• Limited program funding for technology transition• Technology transition “factors” are not generally a

component of demonstrator development (ATDs)– Stabilized material and material processes– Verification of producibility through scale-up– Fully characterized mechanical properties – Ensuring predictability of structural performance – Designing for supportability

• Bonded structure design criteria viewed as an impediment• Hybrid structural designs do not lend themselves to

efficient certification analysis and testing

Technology Transition Drag

unreasonable


Addressing Technology Transition Drag

• Concurrent and fully funded technology transition– Technology on a smaller scale - details vs components -

components vs assemblies

– Technologist participating in the development process - early

• Technology transition “factors” must be better defined– Specific objectives, criteria and methods must be established

– Methodology/procedures for characterizing process variability must be developed/employed - measurable


Addressing Technology Transition Drag

• Baseline experience on structural bonding must change – Methods for verification of bondline integrity– Better design concepts promoting inspection and sustainment

• Unitized replaceable vs unitized repairable

– Standardized repair concepts • Room temperature adhesive systems • Soft tooling • Field capable processes with verifiable integrity

• Improved test methods/concepts– In-production vs dedicated test assets - fleet proof tests vs

single unit average ultimate– Accommodate environmental knockdowns, thermal loads


Bonded Structure Criteria

• Static strength.

– Sufficient static strength shall be provided in the airframe structure for reacting all loading conditions loads without degrading the structural performance capability of the airframe. Sufficient strength shall be provided for operations, maintenance functions, and any tests that simulate load conditions, such that:

• Detrimental deformations, including delaminations, shall not occur at or below 115 percent of limit loads, or during the tests required in 4.10.5.3 and 4.10.5.4. The deformation requirements of 3.2.13 apply.

• Rupture or collapsing failures shall not occur at or below ultimate loads.

• All structure shall be designed to nominal dimensional values or 110 percent of minimum values, whichever is less.

• Bonded structure shall be capable of sustaining the residual strength loads .......... without a safety of flight failure, with a complete bond line failure or disbond.


Bonded Primary Structure

• Static strength.

USAF structural criteria guidance for bonded primary structure identifies the following options for asserting structural integrity:– (1) Bonded structural joints shall demonstrate adequate

environmentally compensated "B" basis statistical strength and continued design strength with equal or higher statistical confidence throughout production (as demonstrated by representative test - proof test every unit) or

– (2) Bonded structure shall be capable of sustaining the residualstrength loads of 3.12.2 without a safety of flight failure with a complete bond line failure or disbond.


Overselling Analytical Capability

• Analytical capability is cited as the rationale for a reduced global factor of safety (from 1.50 to 1.25) – Enhanced modeling, simulation and computational capability

..... more reliable and affordable than testing ....

• Greater computing capability does not equate to greater accuracy– Test failures typically traceable to analysis/analysts errors,

material-process issues, production anomalies– USAF test failure database shows direct relationship between

increased computing capability and early test failures


USAF Static Test Experience

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170

Test Failure Percent Design Limit Load

Per

cen

t F

ailu

res

1940 - 1976 Aircraft

Modern Aircraft Designed with Finite Element ModelsLimit Load Ultimate Load

* Wing, Fuselage,Vertical Tail, Horizontal Tail, Landing Gear, Unique Major Components

Source: William Johnson



• Computing capability ROI is delivered with the design– Lower weight designs– Complex, efficient structural arrangements– Advanced product forms

• JSSG does not advocate carte blanche application of 1.25 factor for uninhabited air vehicles - function of– Material selection– Manufacturing controls and producibility– Economic value and mission criticality– Conduct of conventional developmental activities



• USAF experience suggests that deterministic factor of safety of 1.5 has contributed to acceptable operational performance - 1x10-7 losses per fleet flight hour– Perhaps more effective means of conveying structural failure

risk is required - mission capable, $ repair ....?

• JSSG Criteria, ASIP processes and USAF experience – Bulk of certification activity is presently by analyses

• Developmental - testing used to baseline analysis tools/methods• Legacy systems - major modifications/repairs are analytically

baselined to developmental testing and operational experience

– Test experience does not support reduction in testing


Risk Assessment of a Failure

Projected Failures Per Force Life

02468

1012141618

125 130 135 140 145 150 155

Mean Strength (% DLL)

Nu

mb

er o

f F

ailu

res

Total Fleet

Total USAF

Source: Risk AssessmentCornog D.O. & Lincoln J.W.


Augmenting Analytical Capability

• Development of probabilistic methods – Provides management an indication of risk of failure

associated with use of unconventional factor of safety– Depends on a well characterized stress and strength

characteristics

• Fundamental changes to the design processes– More efficient utilization of analytical codes - i.e. Adaptive

Modelling Language– Streamlining of the design process– Effective decoupling of flight control law changes and loads– Post processors for handling extremely large number of

external load cases



• UAV Acquisition Strategy• Technology Transition• Overselling Analytical Capability• Developmental Design Data• Full-Scale Testing• Other Considerations


Developmental Testing Challenges

• Pressure exists to reduce cost by reducing (even eliminating) design development testing – Relying on supplier allowables– Reliance on evolving modeling/simulation methods– Operational demonstrators perceived to be testimonials– Significant amount of coupon data generated, not used

• Funding of subcomponent and component tests rarely sustained in development programs– Present in early EMD program planning - typically

reduced/eliminated during cost reduction phase of EMD


Building Block Development

Verification of Analysis

Demonstration ofIntegrity

•Material Properties•Repair•Physical/ChemicalProcessing

•Environmental Effects•Mechanical Properties•Knockdown Values•Fatigue Scatter•Effects of Defects

Process Characterization

•Design Details•Failure Modes•Effects of Defects•Repair•Analysis Methods•Structural Performance•Process Verification•NDT Verification

•Configuration Details•Damage Tolerance•Structural Performance•Repair •Validation of AnalysisMethodology

•Process Verification•NDT Verification

•Flight Tests•Ground Tests

•Static•Fatigue•Drop•Dynamics

Subcomponents

Components

Full-scaleCertification Tests

PreproductionVerification

Risk Reduction

Coupons

•Design Details•Failure Modes•Analysis Methods•Process Development•NDT Methods

Elements

Manufacturing Processes

Material & Process Selection

Allowables

Failure Modes

Certification Testing

NDT


Enhancements in Developmental Testing

• “Building block” test programs should be tailored with emphasis on optimizing developmental blocks– More judicious allocation of coupon testing

• Initially for material selection• Subsequent to process verification testing (elements &

subcomponents) for allowables

• “Feature” testing is an essential component of the building block approach– Element, subcomponent and component testing required to

establish process dependent allowables– Identifying failure modes associated with scale-up – Useful in hybrid certification testing



• UAV Acquisition Strategy• Technology Transition• Perceptions About Analytical Capability• Design Development Testing• Full-Scale Testing• Other Considerations


Full-Scale Testing

• Perceptions/conditions that promote elimination of static & fatigue test programs– Flying demonstrator perceived to be a testimonial – Computational capability provides risk free alternative

• Presumes computational capability exists• More affordable• Particularly appealing for uninhabited - no safety issues

– Benign, predictable design environment• Gust• Benign maneuver

– Operational service life not defined• Requirements continuing to evolve

– Reduced or overextended flight test program• No calibration testing for loads bridges• Limited strain/acoustic and acceleration measurement


Full-Scale Testing

• Full-scale static and fatigue testing provide– Static - Strength demonstration– Fatigue - Identify durability hot spots and inspection

requirements

– Verification of manufacturing/process capability

– Demonstration of growth capability– Analytical baseline

• Calibrated flight test aircraft provide– Operational and maneuver performance data– Verified loads/environments– Functional proofing opportunity


Full-Scale Testing

• Potential Impact of No-Full-Scale Testing– ~60% Probability of a significant structural event during

development flight testing which would likely entail:• Test fleet operational restrictions• Structural redesign/repair/retrofit

– ~20% Probability of a catastrophic structural failure in fleet even if flight testing successful

• 1 in 5 test failures occur between limit and ultimate• Design margins not validated without ultimate testing

– Multiple durability failures anticipated• System A had 121 minor and 8 major failures• System B had 98 minor and 4 major failures• Commercial aircraft had 253 minor and 0 major failures

USAF Fatigue Test ExperienceTypical Failure Distribution 100 minor failures4 major failures


USAF Structural Development & Certification

0

50

100

150

200

250

300

0.00 0.50 1.00 1.50 2.00 2.50 3.00

Test Lifetimes

Cum

ulat

ive

Num

ber o

f Dis

crep

anci

es

System A

System B

System C

Commercial Transport

Source: Chuck Babish



• UAV Acquisition Strategy• Technology Transition• Perceptions About Analytical Capability• Design Development Testing• Full-Scale Testing• Other Considerations


Other Considerations

• Design– Design service life and design

usage– Material allowables– Service loading spectra– Chemical/thermal environment

spectra– Stress analysis

• Buckling and crippling criteria• Factor of safety

– Damage tolerance approach– Damage tolerance analysis– Durability analysis

• Development Testing– Environment

• DaDT testing

• Surface Preparation– Cleaning– Environment

• Bonding Techniques– Controls on process– Environmental controls

• Nondestructive Testing Procedures– Inspection prior to bonding– Inspection for bondline voids– In-service inspections

• Training and Certification of Technicians

Dr. Lincoln’s thoughts on certification of bonded structures

ASIP 2002 Special Session on CertificationOther Considerations

• Analyses are validated by test• Documented analyses used to

certify system safety/performance• Data from test/usage establishes

force management approach• Force management is the basis for

sustainment - utility• Force management depends on

documented analyses

Building Blocks of Certification

Development Testing

GroundTesting

FlightTesting

ForceManagementAnalyses

Certification

SAFETy

UTILITY

Undercut any of the foundational blocks and certification is unstable


Outline

FocusReviewing USAF Certification Policy & ProcessesUSAF Structural Development & CertificationUAV Development/Certification ChallengesSummary


Summary

• Robust design and design processes– Design cycle streamlining required

• Concurrent technology development is essential– Smaller technology packages– Technologist participating in development environment - earlier

• The concept of “Technology Transition Factors” must be developed - EN has the action

• Bonded applications must buy their way on– Criteria for the developmental engineer is experience– Alternative testing concepts have a role - require development


Summary

• Avoid overselling/mischaracterizing analytical capability– Computational capability has already yielded a positive ROI

– Analytical methods can be improved• Pre/post processors for data handling

• Streamlined design processes

• Knowledge based engineering software (i.e. AML)

– Probabilistic methods have potential - require development

– Documentation, testing and reliable materials data are the underpinning for analysis


Summary

• Optimization of developmental “building block” testing needs to occur – Emphasis on features for processes

• The utility of full-scale testing speaks for itself– Improved test concepts for hybrid structure are needed

– Improved control surface load calibration methods instrumentation are required

– Production concepts for testing should be studied


Backup


USAF Structural Development & Certification

Aircraft Structural Integrity Program - The Process

JSSG 2006 Aircraft StructuresStructural Performance &

Verification Requirements

Design InformationCriteria, Plans, Usage

M&P Screening

Design Analysis & Development Tests

Structural AnalysesM&P TestingManufacturing

Inspection Methods

Full-Scale TestingFlight Test AC

Ground Test ACResults Evaluation/Application

Force Mgmt Data PackageForce Structural Maintenance Doc

Aircraft Tracking ProcessesInspections/Modifications

Technical Data

Fleet ManagementField Problem Solution

Updated Operational SpectraUpdated Inspection/Modification

Technology Transition FactorsStabilized material and material processes.ProducibilityCharacterized mechanical propertiesPredictability of structural performanceSupportability

Verification of AnalysesVerification of Operations& Operational Impact Safety and Utility

Uninhabited Air Vehicle (UAV) Certification

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