Having flown the AS-350B2 at higher elevations (6000 to 10000 ft Pressure altitude) I was always careful to calculate the aircraft performance limitations before and during the flight. At altitudes above 6k we would often run up against the engine’s Ng limits. If you were not careful during high altitude takeoffs or landings the rotor speed would droop when the Ng limit was reached. In one instance I landed at the clinic in White River, Az (elevation 5250 ft.) but not able to takeoff with the addition of a patient onboard due to rotor droop at the engines Ng limit. Just saying that an engine limitation could have been a contributing factor in this incident.