eight 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. WoW switches are important because they help to prevent accidents relating to the incorrect position of the landing gear, as well as protect the aircraft from damage.
How to Detect Aircraft Weight on Wheels
One way to detect whether an aircraft is on the ground or in the air is to use a sensor on the wheels to provide information about their position and weightbearing status. Another option is to compute the position of the aircraft landing gear from other state data. Some systems use a combination of these two approaches. The proximity sensors used to detect weight on the wheels can be active, with integrated electronics, or passive with external electronics.
Effects of Bad WoW Proximity Switches
Faulty weight-on-wheels proximity switches can send an intermittent electrical signal to the landing gear mechanism. This leads to intermittent hydraulic pressure in the landing gear hydraulics, which can cause uncommanded landing gear extension. When the landing gear deploys or retracts at the wrong time, the result can be a gear-up or belly landing. It can cause serious damage to the aircraft and even put people on board at risk of injury.
Dangers of Uncommanded Landing Gear Extension
During the early 2000s, two T-45 military jet trainer aircraft experienced uncommanded landing gear extension when the aircraft were traveling at high speed and in high conditions. The Navy had to limit these aircraft to flying only at low speeds until they were able to find the problem with the landing gear control system and resolved it.
Promixity Switches are Safety Critical Components
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.
Proximity switches and sensors should be considered a safety critical part of aircraft landing gear control systems. They provide a reliable information on the status of the landing gear. They can help to ensure that aircraft landing gear remains stowed while the aircraft is in the air and deploys only when it is needed for landing.