One of my duties as an Air Force photographer was to photograph air craft accidents for the investigation boards. Thankfully most of the ones I photographed did not involve the loss of life, but two of them did. A CH53C helicopter crashed at Sembach Air Base in Germany in 1976. Two of my photographers were photographing the training exercise and got to the crash before the fire trucks, they pulled two of the crewman out, but all four died. The accident board determined that it was caused by loss of control of the tail rotor and the result of the wrong type of cotter pin being used. Four men lost their lives for a part that cost less then $5.
The second one involved an Air Force F111 flying from England crashed in Germany and both crew members died. Accident investigation determined it was the failure of the terrain following system caused by a maintenance error.
There was (s) a tendency in the military and civilian aviation to blame the accidents on pilot error. My experience was that most of the ones I witnessed were caused by mechanical failure and many caused by poor maintenance.