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More important, within the buffet zone as the airplane approached the stall, the control forces — the necessary backpressure on the control column — did not increase in a conventional linear manner as they had in previous 737s and as certification standards required.

That resistance creates a backpressure in the airways, dilating the muscles that would otherwise collapse in the middle of the night.

Another possible cause is built-up backpressure from a plugged exhaust system.

Worn fuel injectors or under-loaded gensets can cause a buildup of carbon in the exhaust that�s sufficient enough to increase backpressure.

Depending on the positioning of the engine, power loss due to exhaust backpressure may be less than what can be achieved in a conventional installation.