D.C. Crash Update: Helicopter crew may have been getting incorrect altitude readings - MSNBC Reports

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2. My National Guard unit had a bad accident due to this decades ago.

Military helicopters often set their altimeters to AGL (Above ground level) during ops as opposed to ASL (above sea level). In my unit's case it impacted a repel exercise which left the repeller 50' off the ground when he ran out of rope. He survived with extensive injuries. First thing I thought of when I heard about the accident.