After a helicopter accident, many airplane pilots are quick to conclude that the helicopter pilot violated one aviation regulation or another.  But the laws that airplane pilots know so well do not always apply to helicopters. Rather, helicopters operate largely under their own set of rules. Some of the differences between the rules applicable to airplanes and helicopters are:

Minimum Altitudes. Airplanes must stay 500 feet above any person and 1000 feet above any person or building in a populated area.  Flying any lower is a regulatory violation.  But helicopters can get as close to a building or person as they want, as long as the flight poses no "hazard to persons or property on the surface." 

Minimum Fuel. Airplanes must carry enough fuel to reach their planned landing site, then 30 minutes thereafter.  If it’s a night flight, airplanes must carry 45 minutes of extra fuel.  Helicopters are allowed to fly with a smaller fuel reserve — 20 minutes worth, day or night. 

Traffic Patterns. When approaching an airport, airplanes are supposed to fly in the designated traffic pattern.  But helicopters are expected to "avoid the flow of fixed wing aircraft."  That means helicopters are to take an alternate routing when there are airplanes in the area — a routing which would be unacceptable for airplanes.  

Visibility. Except when on an instrument flight plan, airplanes cannot fly unless the weather conditions allow a certain minimum visibility.  For example, in airspace outside of air traffic control jurisdiction, airplane pilots may not operate unless the weather is clear enough to see at least 1 mile ahead of them.  But in that situation, there is no minimum visibility requirement for helicopters.  Rather, helicopter pilots can fly in that same airspace as long as they stay out of any clouds.