起落架平常是收起，稍微突出来有很好的流线型，在受到压力（机腹着地），应自动用力弹出。pressure switch

In a hydraulic system, the pressure switch continuously monitors the process pressure and is activated when a specified pressure is reached. The pressure switch is responsible for activating or deactivating the hydraulic pump when the pressure threshold is met.

https://www.flowtech.co.uk/news/2022/september/how-does-a-pressure-switch-work/?