EASA AD US-2021-21-05
Engine Controls - Electronic Engine Control MN4 Microprocessor - Replacement / Minimum Equipment List - Amendment
Summary
The document titled "Engine Controls - Electronic Engine Control MN4 Microprocessor - Replacement / Minimum Equipment List - Amendment" is a Federal Aviation Administration (FAA) Airworthiness Directive (AD) addressing General Electric Company GEnx model turbofan engines, including GEnx-1B64, GEnx-1B67, GEnx-1B70, GEnx-1B74/75, GEnx-1B76, and GEnx-2B67 variants. This AD mandates revisions to operators' FAA-approved minimum equipment lists (MEL) and requires initial and repetitive replacement of the electronic engine control (EEC) MN4 microprocessor to prevent uncommanded high thrust events. The directive aims to mitigate an unsafe condition identified from in-service occurrences of loss of engine thrust control.
What Changed
This new FAA AD introduces requirements for revising the operator's MEL to include specific dispatch restrictions and mandates the initial and repetitive replacement of the EEC MN4 microprocessor on affected GE GEnx turbofan engines. The AD removes a previously proposed installation prohibition related to multiple microprocessor replacements and updates compliance times to allow replacement based on engine cycles or elapsed time since EEC entry into service. It also clarifies that the master minimum equipment list (MMEL) will not be revised as part of this AD.
Why It Matters
This AD is critical for aviation professionals because it addresses a safety risk involving uncommanded high thrust due to EEC microprocessor failure, which can impact flight safety and engine performance. Operators and maintenance organizations must comply to ensure continued airworthiness and regulatory compliance, avoiding potential dispatch restrictions or grounding. The directive also affects maintenance planning and parts replacement schedules, requiring careful tracking of microprocessor cycles or elapsed time.
What To Do
Affected operators must revise their FAA-approved MEL to incorporate the specified dispatch restrictions and perform initial and repetitive replacements of the EEC MN4 microprocessor according to the compliance intervals defined in the AD. If the number of cycles on the microprocessor cannot be determined, replacement must occur within 12 years for GEnx-1B engines or 14 years for GEnx-2B engines from EEC entry into service. Compliance with these requirements is mandatory by December 28, 2021.
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