Weight on wheels (WoW) switches indicate whether the weight of an aircraft is resting on its wheels. This information reveals whether the aircraft is airborne or on the ground.
Air/ground sensing prevents various systems from operating inappropriately on the ground or in flight. It also ensures systems are enabled or disabled as befits the aircraft’s situation.
Most aircraft utilize some type of WoW sensor or switch that activates when the aircraft is on the ground. They come in many different sizes shapes and technologies and can be in various positions in the aircraft and landing gear. The one thing they all have in common is they complete the circuitry required to do many other things on the aircraft. A faulty or incorrectly adjusted switch/sensor may cause vital systems to not function or function intermittently. Systems that utilize the WoW system include but are not limited to, thrust reversers, nose wheel steering, trim and autopilot.
The WoW System
There two basic types: mechanical switches and proximity sensors. Mechanical switches are easier to test; however, they fail more often because they rely on mechanical contacts to create the circuit. Proximity sensors do not utilize a direct mechanical contact but instead use circuitry to decipher when the magnetic field is interrupted. The proximity sensors are more reliable, but more difficult to troubleshoot. Most mechanical switches are either open or closed. Yet due to the circuitry of a proximity sensor it can be partially open and partially closed at the same time. So a proximity sensor that is known to be good but set to an incorrect clearance can cause very erratic behavior of other systems.
Systems that are run through the WOW system are exclusively used either in the air or on the ground, not both.