Gate 41

Things that look scary but have an explanation

Why do the engines change sound?

The engines just went quiet after takeoff. Did something go wrong?

Takeoff uses the most thrust of the whole flight (often less than the maximum, when the runway and weight allow a "reduced thrust" takeoff that is kinder to the engines). Once the aircraft is safely climbing, the pilots select a lower climb thrust. You hear and feel the drop. It is a standard, planned step.

In the cruise the engines settle at a steady, moderate setting. In descent they spend long stretches near idle while the aircraft glides down a planned path, then spool up briefly on landing when reversers are used.

A modern turbofan makes most of its thrust by moving a huge amount of bypass air around the hot core, which is why it is far quieter and more efficient than old jets.

A little deeper

Thrust is simply air pushed backward (Newton's third law). The fan on a typical large turbofan moves roughly 8 to 12 times more air around the core than through it.

Where thrust is cut back after takeoff depends on airline and airport noise-abatement procedures. Whatever the setting, the aircraft is certified to take off and climb away safely on one engine, and the pilots can always select more thrust.

Pitch and tone also shift with altitude, temperature and cabin air demands. Autothrottle makes small adjustments you can hear as a slow "breathing" in the note.

See it in Gate 41, in the outside view

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