Chapter 1

Why We Have to do Maintenance

Why Maintenance• Provides assurance

• Flight Safety• Reliability• Airworthiness

Engineer’s Role• Designer

– Design best possible product– May be limited by

• Money• Ability• Technique

– Man-made entropy• Final design may not be optimum• Reduced tolerances, cheaper materials• Requires additional maintenance

Mechanic Role• Technician• Repairer• Maintainer• Goal: Combat natural entropy in

systems/components– Deterioration– Misuse– Remove/replace parts– Troubleshoot– Restore systems to intended use

Maintenance• Two types

– Preventive maintenance (scheduled)• Prevent deterioration of system to unusable

level• Corrective actions• Reduce entropy to original level• Regular intervals

– Various intervals• Daily• Number of flights• Flight hours• Cycles

Maintenance• Two types

– Unscheduled maintenance• System breaks rapidly• System breaks completely

– Definitive, extensive troubleshooting

Maintenance Reliability• Level of perfection

– Inherent reliability– Maintenance cannot increase level of

reliability above this reliability

Redesign• Level of perfection increase

– Material change• New materials• New techniques• New components

– Man-made change• Tighter tolerances• Improved design skills• Design philosophy

• Natural entropy is reduced– Maintenance schedule will adjust– May increase or decrease schedule

Failure Rate• Different patterns of failure

– Curve A• Early failures

– Poor design– Improper parts– Incorrect usage

• Failure rate levels off– Physical limit/wears out

Failure Rate• Different patterns of failure

– Curve B• Slightly rising rate• Definite wear-out period at end

– Physical limit/wears out

Failure Rate• Different patterns of failure

– Curve C• Slightly rising rate

– No infant mortality or wear-out period– Becomes unusable

Failure Rate• Different patterns of failure

– Curve D• Slow failure rate when new• Rises quickly after break-in period• Steady level through rest of life

Failure Rate• Different patterns of failure

– Curve E• Steady or slightly rising failure rate• No infant mortality or wear-out period

Failure Rate• Different patterns of failure

– Curve F• Infant mortality• Level• No wear-out period

Maintenance• Systemic approach

– Reduce peak periods of unscheduled maintenance

• Management techniques– Equipment techniques– Line replaceable units– Minimum aircraft dispatch requirements

Redundancy• Higher desired reliability

– Primaries and backups– Primary fails – backup takes over function

• Radios

– Various maintenance techniques possible

Line Replaceable Unit (LPU)• Quick replacement

– No undue maintenance delays– Failed part

• Discarded• Repaired

– No further delay to flight

Minimum Equipment List (MEL)• Loss of functionality

– Does not effect safety or operation of flight

– Manufacturer determined– Regulatory sanctioned– Deferral of maintenance without upsetting

mission requirements• Maintenance performed within specific time

guidelines

Dispatch Deviation Guide(DDG)• Instructions for line maintenance

• Capping connectors from removed units• Circuit breakers• Prevent inadvertent power-up of specific

equipment

Configuration Deviation Guide(CDG)

• Certain panels missing• Configuration differences not affecting

safety