Master minimum equipment list (MMEL) / engine time limited dispatch (TLD) on helicopter

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Hatzak, M.
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The certification regulations published by Joint Airworthiness Authorities (JAA), European Aviation Safety Agency (EASA) or Federal Aviation Administration (FAA) require that all equipment installed on a helicopter must be operative in compliance with the airworthiness standards and the operating rules. However, the rules (e.g. in JAR-MMEL/MEL or CS-MMEL) permit the publication of a Minimum Equipment List (MEL) where compliance with certain equipment requirements is not necessary in the interests of safety under all operating conditions. Experience has shown that with the various levels of redundancy designed into helicopter, operation of every system or installed component may not be necessary when the remaining operative equipment can provide an acceptable level of safety. Hence helicopter utilization is improved and more convenient and economic air transportation for the public is provided thereby. In order to enable the aircraft operators to establish their individual MELs, the Master Minimum Equipment List (MMEL) is developed as a basis for the MEL by the type certificate holder of the respective aircraft as part of the Operational Suitability Data (OSD) and approved by the competent authority. The MMEL includes those items of equipment related to airworthiness and operating regulations and other items of equipment which the competent authority finds may be inoperative and yet maintain an acceptable level of safety by appropriate conditions and limitations. Special attention has to be paid if engine related items shall be implemented in the MMEL. As engines have their own type certificate (TC), engine related parts cannot be directly implemented in the aircraft MMEL by the aircraft manufacturer. To implement these items, a �permission� given by the engine manufacturer is needed. To have this permission officialised, the competent airworthiness authority has to approve it in the TC. To obtain the approval, a �Time Limited Dispatch� (TLD) approach needs to be performed for failures leading to redundancy failures in the engine control system. For failures not leading to redundancy failures, the �classical� MMEL approach can be conducted. The compliance demonstration is based on the list of relevant failures leading to redundancy failures in the engine control system. These failures have to be justified by appropriate means, e.g. fault tree analyses, taking into account that one failure has already occurred. For the TLD approach also new analysis methods need to be applied taking into account the requirements of the certification specification for engines (CS-E). Airbus Helicopters performed this approach as the first helicopter manufacturer in the world successfully in a joined approach together with the engine manufacturer.