请问738飞机在地面停留时TAT尾部为什么会有气流排出？作用

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Bleed Air
The TAT probe gets bleed air from the APU duct in the keel beam. See the bleed air chapter for more information.
(CHAPTER 36)
Bleed air into the probe makes a negative pressure inside the probe. The negative pressure pulls outside air across the sensing elements. This permits accurate temperature measurement when the airplane is on the ground or moving at low speed.