Airport FBO employee observed an aircraft lose altitude while in a turn after takeoff from a non-towered airport then observed the aircraft perform a hard landing and runway excursion. The aircraft stopped off the runway surface and the pilot evacuated safely.

Narrative

There had been an airplane at ZZZ; which in fact has not been flown in over 20 years; doing engine checks and taxiing ops for a few hours. There were 3 young guys working on it (not even sure if they are mechanics or pilots). My coworker and I were closing up at our job and we were driving the fuel truck back to its parking spot and we noticed that the plane started taking off. This is an unairworthy aircraft with no working radios; transponder; ELT... because it had been sitting for so long; it has grown mold and has corrosion all over the wings and fuselage. On departure leg; we noticed it was not gaining much lift. He started a low crosswind turn and we noticed that he was losing altitude at a rapid rate. Luckily; he got wings level but; he disappeared under the hangar line/trees so we no longer had a visual. A few second later; we saw that he managed to get enough power to make it back to runway XX at ZZZ. He had a really hard landing which would have set off the ELT; but it does not work. He veered off the runway about 200 feet or so and got out of the plane with zero injuries; thankfully.ZZZ is notorious for pilot stupidity/poor decisions. Had we had a tower; they would have been able to visually see that was not an airworthy aircraft and know it had not had any work done knowing it had been collecting mold on the tiedowns; and that they had no working equipment. That would make this field that much more safe. Many reports have bee made by many other people and myself; ye it seems like nothing is ever done about it. I can only do my part tho and try to make a difference by myself.