According to the NTSB, on the morning of August 7, 1998, after flying a Hughes 269B helicopter about 30 minutes herding cattle, the pilot located three cattle that were in a gated adjoining pasture. He landed the helicopter in a large mesquite flat near the gate so his passenger could get out and open it. After positioning the helicopter, he attempted a confined area takeoff. He stated that he had to lift straight up to clear trees. Upon reaching a hover just above the treetops, the pilot felt power was bleeding off so he lowered the nose trying to get airspeed. Unable to reach effective translational lift he turned toward a narrow clearing using right pedal and reduced collective to make a run-on landing. Upon ground contact, the right skid dug into the rain soaked ground, and the helicopter rolled onto its side. The commercial pilot and passenger were not injured.
After the accident, the pilot reported to an FAA inspector that it had been raining for a day and a half prior to the accident and that the weather was hot and muggy. He estimated the temperature to be about 95 degrees with high humidity and no wind. He also stated that he did not believe he had any type of mechanical failure and that the engine seemed to be performing normally. He felt that the density altitude, gross weight and out-of-ground effect operation all contributed to the accident.
Helicopter performance is a function of the density of the surrounding air. Density altitude is the reference standard used to measure performance and is determined by correcting pressure altitude for temperature. What is normally not factored into performance charts is the amount of water vapor present. Known as relative humidity, it is the amount of water vapor present (expressed as a percentage) versus the amount of water vapor the air can hold for a given temperature. Water is comprised of hydrogen and oxygen, which is less dense than the oxygen and nitrogen that make up dry air. As the humidity rises, the water vapor displaces the air molecules and lowers the density. Cooler air cannot hold a significant volume of water vapor, however hot air can hold a large amount, so as temperature and humidity rise aircraft performance will decrease.
Charts in the flight manual can be used to predict aircraft performance for a given density altitude. Since they are typically for dry air conditions, when temperature and humidity are high it becomes important to reduce expected performance levels. It is not just airfoils that are affected by humidity, but engine performance as well. A combustion engine can lose as much as 12 percent of its power on hot and humid days versus around 3 percent for a turbine.