Aircraft Systems: Landing Gear

          Often times we tend to overlook something as simple as landing gear.  Most airlines that use larger commercial aircraft, tend to have retractable landing gear on their aircraft.  Even though retractable landing gear is beneficial to streamline the aircraft, if the system does not operate correctly, it could cause dangerous and sometimes fatal results.  Take for example of the accident in Aalborg Denmark. 
         A retractable landing gear works in a few ways; electronically, hydraulically, or a combination. 
"An electrical landing gear retraction system utilizes an electrically-driven motor for gear operation. The system is basically an electrically-driven jack for raising and lowering the gear. When a switch in the flightdeck is moved to the UP position, the electric motor operates. Through a system of shafts, gears, adapters, an actuator screw, and a torque tube, a force is transmitted to the drag strut linkages. Thus, the gear retracts and locks. Struts are also activated that open and close the gear doors. If the switch is moved to the DOWN position, the motor reverses and the gear moves down and locks. Once activated, the gear motor continues to operate until an up or down limit switch on the motor’s gearbox is tripped."(https://www.flightliteracy.com/retractable-landing-gear-part-one/)
"A hydraulic landing gear retraction system utilizes pressurized hydraulic fluid to actuate linkages to raise and lower the gear. When a switch in the flightdeck is moved to the UP position, hydraulic fluid is directed into the gear up line. The fluid flows through sequenced valves and down locks to the gear actuating cylinders. A similar process occurs during gear extension. The pump that pressurizes the fluid in the system can be either engine driven or electrically powered. If an electrically-powered pump is used to pressurize the fluid, the system is referred to as an electrohydraulic system. The system also incorporates a hydraulic reservoir to contain excess fluid and to provide a means of determining system fluid level."(https://www.flightliteracy.com/retractable-landing-gear-part-one/)
          The problem with the electrical landing gear system that could possibly arise is a short in the wiring system going to the landing gear, or one of the switch's malfunctioning showing an incorrect position.  Shorts can be caused by damage caused to the wiring harness, by fraying, pinching, or corrosion over time.  Switches themselves can go bad, but it tends to be the sensor associated with that switch that tend to cause the problem.  These sensors can go bad for a number of reasons such as, dirt, debris, corrosion, improper exposure to elements, etc.  To combat these faults there are rigorous inspections, also back up sensors as well as possible ways to manually extract the landing gear outward through emergency gear extension systems.
          The problem with hydraulic landing gear is that debris can get into the hydraulic system and cause an under pressure or over pressure of the hydraulic system.  Theses sensors will restrict the landing gear and depending on the aircraft will cause the aircraft landing gear to stay in one position or another.  Once again these faults can be combated by back up systems, continued thorough inspections and emergency gear extension systems.
          One more way that the landing gear may be monitored to ensure whether or not it is in the stated position from what the displays or switches are showing, would be to have cameras near the landing gear for observation.





Pilot Handbook of Aeronautical Knowledge Chapter 7, pg. 7-34

https://www.flightliteracy.com/retractable-landing-gear-part-one/

https://www.skybrary.aero/index.php/DH8D,_Aalborg_Denmark,_2007