Electrical System Fault and Airworthiness Decision
During a routine pre-flight check, the AME notices that the left pitot heat fails to activate. The aircraft is a Boeing 737-800, and the outside air temperature is 5°C with visible moisture. The minimum equipment list (MEL) allows dispatch with one pitot heat inoperative under certain conditions, but the MEL requires that the aircraft must not be operated in known icing conditions. The flight is planned to a destination with forecast icing conditions. The operator wants to dispatch the aircraft because the flight is short and the icing forecast is for later in the day. The AME must decide whether to release the aircraft or not. The AME also considers that the pitot heat is a critical system for airspeed indication.
1. Evaluate the MEL relief for the pitot heat failure and determine whether the aircraft can be dispatched. What factors must be considered?
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The MEL allows dispatch with one pitot heat inoperative, but with conditions: the aircraft must not be operated in known icing conditions, and typically the affected pitot heat must be on the non-flying pilot's side or other restrictions. The AME must verify the exact MEL entry. The planned flight is to an area with forecast icing conditions, which would violate the MEL. The AME must consider that the forecast is for later in the day; if the flight can be completed before icing conditions are expected, it might be permissible, but the AME must be conservative. The AME should check the actual weather and NOTAMs. The safety risk is that without pitot heat, the pitot tube could ice over, causing erroneous airspeed indications, which is especially dangerous during takeoff and initial climb. The AME should not release the aircraft if there is any possibility of encountering icing. The operator's desire to dispatch should not override safety. The AME should recommend repairing the pitot heat before flight or delaying until conditions improve. If the MEL allows, the AME could defer the item with a placard and appropriate maintenance action, but only if the flight can be conducted safely without entering icing conditions. Given the forecast, the AME should refuse dispatch.
2. If the AME decides to release the aircraft under MEL, what documentation and operational procedures must be followed?
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If the AME releases under MEL, they must ensure that the MEL conditions are met: the flight must not operate in known icing conditions, and the aircraft must be placarded as required. The AME must complete a maintenance record entry referencing the MEL item, and ensure that the operator's dispatch office is aware of the restriction. The flight crew must be informed and the aircraft's flight manual limitations must be observed. The AME must also consider if the MEL requires an operational check of the remaining pitot heat and any associated systems. The release must be documented with a certificate of release to service, noting the MEL deferral. Additionally, the AME should monitor the weather and if conditions change, the aircraft may need to be grounded. The AME should also ensure that the defect is rectified within the MEL time limit. Finally, the AME should consider that the MEL is part of the approved maintenance program and must be followed strictly.