Power Supply Detector 0.3V = logic 0 13V = logic 1 Detector 28V DC Red Blue Red Blue Target Target near To logic card Target far Target Primary and secondary downlock sensors Rectangular proximity sensor Target Mounting bracket Side strut Lock strut Sensor leads UP INBD Proximity card Sensor Figure 13-41.  Proximity sensors are used instead of contact switches on many landing gear. Nose Wheel Centering Landing Gear Position Indicators Landing gear position indicators are located on the instrument  panel adjacent to the gear selector handle. They are used  to inform the pilot of gear position status. There are many  arrangements for gear indication. Usually, there is a dedicated  light for each gear. The most common display for the landing  gear being down and locked is an illuminated green light.  Three green lights means it is safe to land. All lights out  typically indicates that the gear is up and locked, or there  may be gear up indicator lights. Gear in transit lights are  used on some aircraft as are barber pole displays when a  gear is not up or down and locked. Blinking indicator lights  also indicate gear in transit. Some manufacturer’s use a  gear disagree annunciation when the landing gear is not  in the same position as the selector. Many aircraft monitor  gear door position in addition to the gear itself. Consult the  aircraft manufacturer’s maintenance and operating manuals  for a complete description of the landing gear indication  system.  [Figure 13-43] Since most aircraft have steerable nose wheel gear assemblies  for taxiing, a means for aligning the nose gear before  retraction is needed. Centering cams built into the shock strut  structure accomplish this. An upper cam is free to mate into a  lower cam recess when the gear is fully extended. This aligns  the gear for retraction. When weight returns to the wheels  after landing, the shock strut is compressed, and the centering  cams separate allowing the lower shock strut (piston) to  rotate in the upper strut cylinder. This rotation is controlled  to steer the aircraft.  [Figure 13-44]  Small aircraft sometimes  incorporate an external roller or guide pin on the strut. As  the strut is folded into the wheel well during retraction, the  roller or guide pin engages a ramp or track mounted to the  wheel well structure. The ramp/track guides the roller or  pin in such a manner that the nose wheel is straightened as  it enters the wheel well. 13-24