Archive for March, 2012

High DA

Wednesday, March 28th, 2012

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.

Loss of tail rotor effectiveness

Wednesday, March 14th, 2012

At Baltimore-Washington International Airport a helicopter approached a construction site on the airport property with a quartering left tailwind. The helicopter turned right, and slowed to a stationary hover at about 250 ft with a direct tailwind. Once in a hover, it made a right rapid 180 degree pedal turn. Stopped momentarily and then initiated another rapid pedal turn to the right. The helicopter turned at a faster rate than the initial turn and continued into a spinning vertical descent and collided with Alpha taxiway abeam Runway 15 Right. The FAA’s examination of the helicopter found no mechanical anomalies.

The NTSB determined the probable cause was the pilot’s improper decision to maneuver in an environment conducive to a loss of tail rotor effectiveness and his inadequate recovery from the resulting unanticipated right yaw.

According to FAA Advisory Circular AC90-95, any maneuver which requires the pilot to operate in a high-power, low-airspeed environment with a left crosswind or tailwind creates an environment where unanticipated right yaw may occur. It also advises of greater susceptibility for loss of tail-rotor effectiveness in right turns and states the phenomena may occur in varying degrees in all single main-rotor helicopters at airspeeds less than 30 kt.

Bell’s Operations Safety Notice OSN 206-83-10, regarding loss of tail-rotor effectiveness in the 206B and similar airframes describes the phenomenon as an unanticipated right yaw. It contains the following warnings when maneuvering between a hover and 30 mph:

“Be aware that a tail wind will reduce relative wind speed if a downwind translation occurs. If loss of translational lift occurs, it can result in a high power demand and an additional anti-torque requirement. Be alert during hover (especially OGE) and high-power-demand situations. Be alert during hover in winds of about 8-12 kt (especially OGE), since there are no strong indications to the pilot [of] the possibility of a reduction of translational lift… Be aware that if a considerable amount of left pedal is being maintained, that a sufficient amount of left pedal may not be available to counteract an unanticipated right yaw.”

The pilot at BWI was performing an aerial photography mission, the nature of which requires maneuvering at low altitudes and slow speeds. Add to that the distraction of trying to work with a photographer to line up the desired shot and the mission can become very demanding. Any pilot performing a similar mission needs to understand the aerodynamics and limitations of maneuvering at slow speeds.