PA-31 Navajo/Chieftan/Mojave/T1040
Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Commercial
Situational Awareness; Communication Breakdown
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Took Evasive Action
Environment - Non Weather Related; Human Factors
Departing ZZZ I had 7 passengers; which meant all of the seats were [filled] up in the PA31. I had previously installed all of the seats on the right side of the cabin the day before (Seat Configuration 1) and during my pre-flight went through the cabin and installed all of the seats on the left side changing it into Configuration 2. As I was in the cabin I buckled all the seat belts on the seats and didn't note anything wrong with the seats.Upon arrival in ZZZ1 I was unloading bags out of the back and noticed that the back right seat was all the way forward against the seat in front of it. The passenger that was sitting in that seat had exited in ZZZ1; so I thought the seat in front may have slid back. So I asked the passenger that was still seated in that seat (2nd seat from the back on the right side) if their seat had moved at all and they said no. I grabbed the right rear seat and discovered it was moving freely on the seat tracks. I slid the seat back to attempt to lock it into place and discovered it would not lock into place and that it had been unsecured for the entire flight.Upon further investigation I discovered that floor anchors had been installed on the very aft securing point on the seat rails from the previous flight the night before. This flight had only mail on board going and I had helped load the flight and once the loading was done had gone inside and gotten my release and inspected how the load was secured when I left. The ramper that secured the load had moved the aft seat forward in order to install the floor anchors; but had not said anything to me. So essentially the seat was slid all the way back against these anchors that were attached to the seat rails; but it could not slide back far enough for the pins to secure into place. When looking at the seat it looked to be in its normal position; but was not secure. I had never put seat rail anchors in place with this rear seat in place and was not notified by ramp staff that the seat had been unsecured. Additionally ramp staff are not authorized to do seat removal and installation. Additionally we have a couple PA31's that have the capability to have aft seat rail anchors installed with the rear seat locked in place; which is why seeing those anchors there did not raise any flags. In the end I should have inspected the seat that morning. But I had previously installed the seat; inspected it and initialed on the flight log showing it was done correctly and the seat was secure. I had to reason to believe that the seat had been moved and when going through the cabin that morning I had buckled the seatbelt on that seat and didn't notice anything. I will be checking security of all seats even if previously installed and confirmed secure the day before as I go through and buckle belts in the cabin. Additionally ramp staff are not authorized to install or remove seats. They should not have unsecured a seat and if it needed removed they should have notified me to come out and take the seat out. I'd rather have a seat behind the net and not loose on a seat rail than what occurred.
PA-31 Captain reported operating a flight with a passenger seat not properly secured.