FORT RUCKER, Ala. (Nov. 19, 2013) - When you are downrange in an unmanned aircraft system platoon, everything gets repetitive. You launch your aircraft, fly your mission and land your aircraft several times every day. The platoon is usually isolated from the rest of your unit and everyone knows the rules of the flight line. It's when visitors arrive that problems arise.It was early in the afternoon when our vehicle mechanics came to the flight line from the motor pool to perform maintenance on one of our vehicles we use for flight operations. However, they did not follow the posted procedures for checking in at the hangar so they could be escorted to the taxiway we were using for UAS launch and recovery operations. The mechanics decided to drive unescorted all the way to the taxiway. One of my operators and the crew chief saw them heading toward the taxiway. The operator ran toward them, yelling for them to stop because I had just turned my aircraft onto final approach. Fortunately, they stopped in time and my operator stayed with them while I landed the aircraft.Once the aircraft was on the ground, I began my post-flight checks and waited for the crew chief to report that he had the aircraft secure. The call never came, so I called him for verification. He radioed back, saying, "Umm, aerial vehicle is on the ground, but you and the mission coordinator might want to come out here." From that, I knew I was about to go to the cache to "pee and bleed."I completed my post-flight checklist and met the crew chief, mission coordinator, my other operator and the mechanics at the end of the runway. That's when I saw my aircraft leaning in a hole with the arresting strap broken. All of them had the same dumbfounded look on their faces. The first thing I said was, "What happened out here? Everything looked good inside." Nobody could tell me anything. So, I looked around.The first thing I noticed was a large pickup truck sitting perpendicular to the taxiway. I couldn't see the Tactical Automated Landing System, which is what lands the aircraft and steers it down the runway until it stops. That's about the time I lost it. One of my operators and my crew chief allowed a truck to be parked so as to block the line of site between the TALS and the aircraft, preventing the TALS from steering the aircraft after landing. This resulted in the aircraft drifting to the right, missing the arresting strap and catching an assembly guide rope. The rope tensioned and broke, causing the aircraft to turn, just missing the arresting drum on the edge of the taxiway, which holds the braking system that stops the aircraft. The aircraft then ran off the taxiway and into the dirt.Luckily, the aircraft was not damaged and the broken rope was an easy fix. It could have been a lot worse, though. Not only could the aircraft have been damaged, but at that point in the recovery, the crew chief or those mechanics could have been struck while it was still traveling at 50-60 knots.In my opinion, this all happened because of complacency and lack of situational awareness by my crew and the failure of the mechanics to follow posted procedures. No matter how many times a team completes a task successfully, they must always maintain their vigilance.