The Landing gear in a retractable aircraft, is the movable undercarriage of an aircraft that is used for either takeoff or landing the aircraft. The landing gear when retracted creates a smother airflow and less wind resistance among other benefits, allowing the aircraft more efficiency when flying through the air. When extended the landing gear creates drag slowing the aircraft down.
When an aircraft landing gear is extended (put down), the aircraft should not show signs of changes in pitch, yaw, or roll after the gear has transitioned to the full down position and locked into place. Locked into position also refers to the gear being safe for landing the plane. In this configuration, the pilot will be given 3 green lights showing that the gear is in fact down, locked into position and safe to land. If the plane at this point has pitch changes or yaw it requires inspection from a certified aircraft mechanic.
During a few recent flights in our PA-28R-201T (Piper Turbo Arrow III) doing instrument approaches, we noticed a consistent yaw after the landing gear was extended and verified by the 3 green status indicators. There did not seem to be any change on this yaw based off attitude of flight (level, climb, descent) nor speeds (top of gear speed or top of flap speed). This was both confusing and challenging to diagnose.
What we saw: Take off, retract the gear, fly the plane as normal. In the course of the flight you are working the rudder pedals normally and they in turn are moving the steering arms compressing the rod into the springs. However, one of the spring assemblies is sticky and it sticks in place causing the other steering arm to extend to compensate. No anomalies of flight are present as the steering arms are not connected to the steering cam thus not causing any change to the rudder. Step on the rudder to correct the yaw and the steering rod moves back to it’s normal position allowing the pressure of the rudder pedal to be released and the yaw is gone.
More research: In our efforts we mounted a go-pro style camera (Garmin Virb) to the tail and secured it with safety wire. This would allow us to take a flight, cycle the gear multiple times and record what was going on during cruise. The reason for taking video in flight is because in some situations the gear extension or retract issue may only present itself in cruise vs. on jacks in a hangar.
After mounting the camera, taking the flight and proceeding back to the hangar, we watched the replay and found nothing abnormal. You can see the video below. This also validated our thoughts that the yaw introduced by extending the gear had nothing to do with gear itself but of secondary components impacted by the gear extension. In fact the gear was extending fast, strong and solid in every aspect.
Next Step: Our minds were now wondering what would cause the yaw during the gear extension and at the locked position phase. The seeming obvious answer…..the rudder. But, why it was the rudder would be the discovery we had yet to make. To understand this we would have to dive into the IPC (Illustrated Parts Catalog) and validate how all the parts work together. We short cut this with the help of a very experienced A&P / IA. He quickly determined that this was for sure connected somehow to the steering bungees on the plane.
What is a steering bungee you might ask? I had the very same question. Refer to the inline IPC depiction with highlights of the bungee location. I think another term for this set of parts would be spring loaded steering arms.
On the Piper arrows the steering bungee (spring loaded steering arms) are connected through the firewall to the rudder pedals then to the steering cam which is made active once the gear is extended.
How could this possible make the plane yaw? Great questions, let me try and answer that. Inside each steering rod (bungee) there is an actuator an spring. Given enough wear and time and you end up with loads of grime in these springs, causing them to lock up and stick. This can cause the rudder to be in a deflected stated when the gear comes out and once the cam makes contact it puts you into a yaw state.
More Parts: As we disassembled the steering arms we found that the cam bushings and bearings were also worn as well as one of the tie-rod ends. We also found the source of a long and frustrating heat leak into the cabin that had been happening all summer due to a torn boot on the firewall (pictured above). In the end we replaced 4 bushings, 4 bearing, a boot and 1 tie-rod end allowing us to clean up all the tolerances.
Here is another picture of the IPC with more parts identified that we replaced.. We found many of the bushings and internal bearing worn so we also replaced them.