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    CAP 693

    ACCEPTABLE MEANS OF COMPLIANCE

    HELICOPTER HEALTH MONITORING

    CAA AAD 001-05-99

    CIVIL AVIATION AUTHORITY, LONDON

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    CAP 693

    ACCEPTABLE MEANS OF COMPLIANCE

    HELICOPTER HEALTH MONITORING

    CAA AAD 001-05-99

    CIVIL AVIATION AUTHORITY, LONDON, MAY 1999

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    iii

    CONTENTS

    Page

    1 Introduction 1

    2 Scope 3

    3 Purpose 4

    4 Background 4

    5 Means of Compliance 6

    6 Compliance by Method A 6

    7 Compliance by Method B 9

    Appendices

    A Additional Guidance for compliance by Method A 11

    B References 17

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    1

    1 INTRODUCTION

    The Civil Aviation Authority has issued AAD 001-05-99, which became effective on

    7 June 1999. The AAD makes the installation and use of health monitoring systems

    (HMS) mandatory for United Kingdom registered helicopters issued with a Certificateof Airworthiness in the Transport Category (Passenger), which have a maximum

    approved seating configuration of more than 9 passengers. However this Directive is

    not applicable to helicopters certificated to BCAR 29 or JAR-29.

    This CAP provides operators with the basis for an acceptable means of compliance

    with the Directive.

    Figure 1 shows the operator options and timescales for compliance with this AAD.

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    2

    CAA Health Monitoring

    AAD

    Applicability

    Certificate of Airworthiness in the

    Transport Category (Passenger)

    with seating Configuration of more

    than nine

    Not Applicable to:

    Type Approvals

    including

    BCAR 29 or JAR-29

    Design Assessment

    Requirements

    Is an approved system

    currently fitted?

    Comply by Agreed

    Procedure/Practices

    Comply by fitting an

    approved system

    YES NO

    1 year

    YES

    Comply with

    procedures and

    practices

    2 years

    Certificate and fit

    new system and

    comply with

    procedures

    and practices.

    2 years

    NO

    FIGURE 1

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    3

    2 SCOPE

    AAD 001-05-99 when referring to HMS or HUMS means a system utilising on

    board equipment for monitoring the health of helicopter rotor and rotor drive systemcomponents. Helicopters affected by this AAD will need to demonstrate an effective

    vibration health monitoring capability. Consequently much of this CAP relates

    specifically to vibration monitoring systems. However, it is accepted that many

    methods of health monitoring, such as transmission magnetic plugs, chip detectors, oil

    analysis, will already be adequately controlled by the helicopter constructors

    maintenance instructions. The monitoring techniques to be considered are;

    Vibration Monitoring System (VMS) which should monitor :-

    Engine to main gearbox input drive shafts Main gearbox shafts, gears and bearings

    Accessory gears, shafts and bearings

    Tail rotor drive shafts and hanger bearings

    Intermediate and tail gearbox gears, shafts and bearings

    Oil cooler drive

    Main and tail rotor track and balance

    Plus existing established techniques :-

    Pressure

    Temperature

    Torque

    Physical Displacement (Rotor blade and tracking is a practical

    example)

    Debris generation via real time analysis in fluid streams (Oil and

    exhaust gases)

    Magnetic Plugs

    Spectrographic Oil Analysis

    Absence of any of the above techniques will need to be justified.

    It is acknowledged that existing health and vibration monitoring systems are complex,

    and that there may be many different methods to monitor a particular failure mode.

    For this reason this CAP is not prescriptive with respect to items such as the position

    of accelerometers, algorithms and threshold settings. The object of this CAP is to

    describe the level of monitoring required and give advice on ensuring that the

    monitoring is effective and reliable.

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    3 PURPOSE

    The purpose of this CAP is to provide guidance to industry on what the CAA consider

    to be an acceptable means of compliance with the subject AAD.

    The AAD has been raised in response to numerous AAIB rotorcraft accident/incidentreports which have highlighted the positive role that vibration monitoring systems

    could play in reducing the accident rate in the helicopter community.

    The benefits of vibration monitoring in the UK have so far been limited to North Sea

    operations of Transport Category (Passenger) helicopters, where the operators have

    fitted these systems in recognition of the benefits together with their desire to satisfy

    their customers safety objectives.

    The CAA has been proactive in promoting the positive aspects of health monitoring

    and has adopted requirements for the certification of large helicopters which include arotor and transmission design assessment, in which health monitoring is an acceptable

    compensating provision. The decision to mandate the installation and use of health

    monitoring systems has been taken in response to the recent AAIB recommendations

    relating to incidents and accidents.

    4 BACKGROUND

    In the early 1980s in response to a growing unease with the safety record of large civil

    transport helicopters, the Chairman of the CAA requested that the Airworthiness

    Requirements Board (ARB) establish a panel to review existing helicopterairworthiness requirements. This panel, the Helicopter Airworthiness Review Panel

    (HARP), reported in 1984 and confirmed that the fatal airworthiness accident rate for

    large twin engine helicopters was significantly higher than for comparable aeroplanes.

    From their review of failures, HARP recognised that the helicopter is different. So

    much of its critical mechanism, the rotors and rotor drive system, involved single load

    paths without duplication or redundancy. They identified a fundamental difference

    between the helicopter and fixed wing aircraft, the inability to guard against a failure

    by duplication.

    However, experience showed that although this machinery employed safe life rather

    than fail safe design, often defect propagation would occur for a period of time before

    failure occurred. This, coupled with the recognition that there had been important

    developments in health monitoring system (HMS) technology, encouraged the

    members of HARP to recommend the philosophy that where full redundancy has not

    been possible by design then warning of likely failure in a suitable time scale could

    provide an acceptable level of safety.

    The types in todays UK fleet are likely to remain in service for the foreseeable future.

    Therefore, all the really significant improvements to the airworthiness codes that the

    CAA/JAA has introduced over the past few years are effective for new helicopters,but are currently of no benefit to the existing fleet. Therefore, there is a need to

    improve the airworthiness of the current fleet.

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    5

    Operational trials of health monitoring systems were carried out between 1987 and

    1991 over the North Sea. These trials demonstrated the technical feasibility of first

    generation HMS, including vibration monitoring, in what must be considered as a

    very testing environment. Today all large UK registered helicopters employed in

    North Sea operations have HMS fitted, as a customer requirement, rather than amandatory requirement, except for the S61, where a CAA mandatory AAD

    necessitates HMS for a specific component.

    The CAA has recognised that helicopter rotor and transmission systems are

    susceptible to potentially hazardous and catastrophic failure effects, due to the very

    nature of their design (single load path) and has for many years realised the benefit of

    the installation of HMS as a compensating provision, for new certifications.

    The CAA has sponsored, along with the UK government, a comprehensive safety

    research programme which culminated in successful operational trials of HMSequipment on four North Sea oil/gas support helicopters. The Authority has also

    managed extensive research programmes exploring the technical feasibility of HMS,

    including in-service trials and seeded fault tests.

    It is considered that the first generation HMS, which added comprehensive vibration

    monitoring to existing health monitoring techniques, has already demonstrated the

    ability to identify potentially hazardous and catastrophic failure modes, and has

    already reduced fatal accident statistics.

    First generation HMS (including vibration monitoring) has been shown to be both

    technically feasible and economically justifiable for existing operational helicopters.The CAA believes that it is technically feasible to extend the benefits from HMS to

    helicopters currently in service (helicopters issued with a Certificate of Airworthiness

    in the Transport Category (Passenger), which have a maximum approve