On January 28th of this year, a thrilling scene unfolded at Elson Air Force Base in Alaska, USA: an F-35A fighter jet belonging to the 354th Fighter Squadron lost control and crashed and exploded during a training mission, with thick smoke rising from the scene. Seven months later, the investigation report released by the US Pacific Air Force Command revealed the truth behind the accident, which was caused by about one-third of the water mixed into the hydraulic oil of the fighter jet. It was precisely these unwanted moisture that triggered a chain reaction in Alaska's extremely cold environment of -17 ℃, ultimately leading to the accident. In fighter jets, the hydraulic system is responsible for accurately transmitting energy to various key parts of the fuselage, thereby driving control surfaces such as the horizontal tail and flaps to achieve precise attitude control, driving the landing gear to complete retraction and retraction actions, and providing power for key operations such as wheel braking and front wheel turning. To ensure safety, fighter jets are often equipped with multiple sets of main hydraulic systems to increase safety redundancy, and emergency hydraulic systems are equipped just in case, forming a safety barrier for the fighter jet hydraulic system. The "fluid" in hydraulic systems refers to specialized hydraulic oil, not ubiquitous water. Although both are liquids, only the former can simultaneously meet various stringent requirements for transmitting kinetic energy, while water poses a fatal risk to hydraulic systems. Firstly, hydraulic oil can form a stable oil film on the surface of metal components, reducing the wear of precision parts such as actuator pistons and preventing metal corrosion. Water has no lubricating effect and can not only cause component wear, but also react with metal to rust. Secondly, hydraulic oil has good adhesion, which can enhance the sealing effect, reduce leakage, and prevent the formation of bubbles under high pressure, thus maintaining stable power transmission. Water has poor adhesion, is prone to leakage, and easily mixes with air to form bubbles, leading to pressure fluctuations. Again, hydraulic oil has a high boiling point and a low freezing point, allowing it to remain liquid in environments with a wider temperature range. Water will freeze and expand in volume at 0 ℃, and boil and vaporize at 100 ℃. The condensation point and boiling point of water will change in high-pressure environments, but it still cannot meet the requirements of hydraulic systems. Therefore, the water content standard for aviation hydraulic oil is usually controlled below 0.05%, and the water content requirement for some high-performance hydraulic oils does not exceed 0.02%. The F-35A that crashed had a hydraulic oil water content of one-third. In the extremely cold weather of Alaska, so much moisture can quickly freeze, directly freezing some key components such as the front landing gear buffer strut, making accidents inevitable. In theory, the aviation industry has strict requirements for oil pollution prevention. For example, hydraulic oil needs to be stored in sealed oil drums, and must be tested for qualification before refueling. It is forbidden to use unclean gloves or tools that are prone to fiber detachment to contact the refueling area. The removed hydraulic system pipeline interfaces and uninstalled parts must be immediately sealed with clean plugs or wrapped with film or even oil seals. It is puzzling that the 'water oil' still flowed into the hydraulic system of the F-35A. This accident has also sounded an alarm for the aviation industry of other countries: no matter how advanced the aircraft is, it cannot withstand the negligence in work. The safety of equipment cannot be achieved without both scientific system design and rigorous and meticulous basic maintenance work. (New Society)