If more than 50 NM from TOD with VNAV engaged and the MCP ALT is below current altitude and below a descent constraint altitude, what is the outcome?

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Multiple Choice

If more than 50 NM from TOD with VNAV engaged and the MCP ALT is below current altitude and below a descent constraint altitude, what is the outcome?

When VNAV is engaged and you’re well out from TOD, the system will start an early descent if the current altitude is higher than the altitude required by the descent constraint and the MCP altitude is set lower than both. In this scenario, the MCP altitude is below the current altitude and below the descent constraint, so the aircraft will descend immediately using a fixed rate of about -1,000 feet per minute. This descent continues until the flight path is intercepted or the MCP altitude is reached, at which point VNAV resumes normal guidance to meet the constraint. This behavior prevents holding cruise altitude when a lower altitude is mandated and keeps the descent on schedule. Cruise-descent mode would typically be engaged closer to TOD, and staying at constant altitude would ignore the constraint, which isn’t appropriate here.

When VNAV is engaged and you’re well out from TOD, the system will start an early descent if the current altitude is higher than the altitude required by the descent constraint and the MCP altitude is set lower than both. In this scenario, the MCP altitude is below the current altitude and below the descent constraint, so the aircraft will descend immediately using a fixed rate of about -1,000 feet per minute. This descent continues until the flight path is intercepted or the MCP altitude is reached, at which point VNAV resumes normal guidance to meet the constraint.

This behavior prevents holding cruise altitude when a lower altitude is mandated and keeps the descent on schedule. Cruise-descent mode would typically be engaged closer to TOD, and staying at constant altitude would ignore the constraint, which isn’t appropriate here.

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