Retractable gear planes fly faster and save fuel because they have less drag. The downside is that there is a risk of landing with the wheels retracted and causing significant damage to the plane. For this reason, some planes have been equipped with an automatic landing gear extension system.
This system will lower the landing gear before a landing if the pilot forgets.
How does the plane know you are landing?
The plane does not know when you are landing. Instead, it relies on conditions being met that are typical for landing.
I frequently fly a Piper Arrow with one of these systems and it detects the airspeed through its own pitot tube on the side of the fuselage. This placement ensures that it is reading the speed of the aircraft PLUS the propeller slipstream because it is placed behind the propeller blades.
According to the POH, the gear should extend between 85 and 105 miles per hour, depending on the power setting. There is no computer involved in this decision and the airspeed is not displayed anywhere.
Instead, think of it as an airspeed indicator that moves a landing gear handle instead of a needle.
Overriding the automatic landing gear extension system
This system can cause problems in flight training. For example, if you are practicing stalls
Your gear selector can be in the up position and the automatic landing gear extension will still occur. This can really mess up a power on stall. In order to deal with this issue, Piper provided a way to override the automatic landing gear extension.
This is done with a switch on the floor between the seats. The switch controls emergency landing gear extension when pushed down, but works as an override to the automatic system when pulled up.
The switch is spring loaded so if you pull up on it and hold it the automatic system will be overridden, but if you let go it will spring back down and the automatic system will be active again. Since you need to be able to leave this engaged for maneuvers Piper added a small metal rod to the side of the housing around the switch. This rod can be pushed in sideways when the gear override is pulled up. It will slide under the override handle and hold it up for you.
When the override is engaged a yellow light is illuminated below the gear handle. This is to remind you that the gear will not auto-extend.
How to use this system
I like this system because it provides an extra layer of safety for this rental airplane. However, there is a danger that it might create a sense of complacency, especially in new pilots.
If you fly with a system like this please do not count on it to lower the gear. If you landed with the gear handle up, even if the wheels are down, you have still performed a gear up landing in my opinion!