On the morning of August 7, 1998, near Uvalde, Texas, a Hughes 269B helicopter was substantially damaged during a hard landing.
According to the NTSB, after flying about 30 min. 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 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 deg. 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.
It is important to remember that 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 in 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 can decrease exponentially.
Charts in the flight manual can be used to predict aircraft performance for a given density altitude. However, they are typically for dry air conditions. When temperature and humidity are high, it becomes extremely 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 day versus around 3 percent for a turbine.
Moreover, charts typically show in-ground-effect hover performance, out-of-ground effect hover performance and certain take-off criteria. When a pilot must maneuver in less dense air, the charts can be helpful but require more interpretation. Nevertheless, understanding the effects of high density altitude in all flight regimes is critical to safe operations. Consider the following accident:
On the afternoon of August 10, 2001, a Eurocopter AS350-B2 helicopter, on a sightseeing flight, collided with terrain at 4,041 ft. during an uncontrolled descent about 4 mi. east of Meadview, Ariz. Impact forces and a post-crash fire destroyed the helicopter. The pilot and five passengers were killed, and the remaining passenger sustained serious injuries.
According to the NTSB, company pilots who landed at the accident site within minutes of the accident were asked about the atmospheric conditions. One pilot noted that the winds were calm and the temperature was approximately 106 deg. F. (41 deg. C.) and that there was no turbulence. Another pilot said that the weather was clear, sunny, hot, and that there was no turbulence at all, especially when crossing the ridge.
At the time of the accident the helicopter's gross weight was calculated to be 4,515 lb., 446 lb. below its maximum allowable weight. The density altitude at the point of impact was over 8,000 ft.
The NTSB determined that the probable cause of this accident was the pilot's decision to maneuver the helicopter in a flight regime and in a high density altitude environment which significantly decreased the helicopter's performance capability, resulting in a high rate of descent from which recovery was not possible. Factors contributing to the accident were high density altitude and the pilot's decision to maneuver the helicopter in proximity to precipitous terrain, which effectively limited remedial options available.
Again, according to the NTSB, interviews with company pilots and supervisors revealed that they considered the accident pilot one of their very best. He was consistently praised for his knowledge of the helicopter and its systems. They stated that even the mechanics and other maintenance personnel within the company praised his knowledge, skills, and abilities.
When a highly skilled pilot under-estimates the effects of high density altitude, it underscores the importance of completely understanding the performance limitations imposed by the environment. Any pilot flying in these conditions would be wise to error on the side of caution to ensure an adequate margin of performance.