According to the FAA, three major factors affecting helicopter performance: weight, density altitude, and wind. Density altitude and wind can be affected by weather conditions, therefore weather plays an important role in helicopter performance and safety. It may also impact visibility and impact take-off and landing.
Though rain, snow, sleet, and fog may not have an effect on helicopter performance, they generally obstruct visibility, making make take-off and landing more difficult and hindering a pilot’s ability to see obstacles during flight. Rain, snow, and sleet may also make the tarmac or landing pad slippery, which can create problems.
During liftoff, helicopters generate thrust with their rotors, creating high air pressure below them and low air pressure above them due to Bernoulli’s principle. This pressure thrusts the helicopter upward, therefore, rain alone does not affect the ability of a helicopter to take off.
In general, though, light rainfall shouldn’t impact helicopter operation, also long as it’s not freezing rain.
“Wet snow and icing is a big issue. Ice can adhere to flight surfaces and change their aerodynamic properties, making them much less efficient in creating lift. Chunks of ice can shed from the rotor blade and get ingested in the engine, creating an engine stall situation or even hit the tail rotor,” says Samuel Evans, a research associate at Pennsylvania State University’s Aerospace Engineering department and retired U.S. Army Colonel/Aviator.
He adds that some helicopters “have thermal device capability, but most systems are difficult to maintain and rarely work with enough consistency to create enough confidence in pilots to attempt to fly in icing conditions.”
Two weeks ago, helicopters flew through the rain on North Carolina’s coast to rescue stranded residents. Coast Guard helicopters managed to reach many people in homes too remote for rescue boats.
“Yeah, just people that flag us down or really need help,” said one Guardsman. The National Guard, first responders, and volunteers rescued dozens of people living in low-lying areas of Onslow County, where flooding was chest-deep.
Coast guard helicopters also rescued residents in New Bern, an area flooded by a storm surge. One by one, the stranded residents were hoisted 40 feet into the sky. Thirteen people, adults, and young children had gathered in a home, wading through waist-deep water and crawling into the chopper’s rescue basket.
“We flew with the Coast Guard today as they responded to day two of flooding drama in eastern North Carolina over places like New Bern, recovering from 10 feet of storm surge,” said Pilot lieutenant Matt Delahunty.
Though the stormy weather had been too dangerous for most rescue helicopters to fly, once calmer conditions and lighter rains developed, Coast Guard air was able to rescue dozens of people from flood zones.