Archive for July, 2012

Bird strikes

Tuesday, July 24th, 2012

On March 5, 2009, an Agusta A109E helicopter hit a bird on a medical evacuation flight while approaching Gainesville, Florida. The pilot received minor injuries, while the crew and trauma patient were not injured. It was a night VFR flight from an automobile accident site in Trenton, Florida, to a hospital helipad.

According to the pilot, the incident occurred when the helicopter was about 3 minutes from landing at the hospital’s rooftop helipad. The helicopter was descending at 145 knots through 800 feet, when the windshield exploded and the pilot was pelted with Plexiglas and other debris. The master caution warning light started flashing, but the pilot had difficulty reading the caution warning panel as the left lens to his eyeglasses was missing. The pilot was eventually able to determine that SAS number 1 had been disengaged, and after resetting the switches the master caution light extinguished. The pilot also noted that the instrument panel lights were off on the pilot’s side, so he reached up to the overhead panel and turned the lights back on. He then noticed that several circuit breakers and switches were broken off, and that several other switches had been moved aft, to the off position. The entire overhead panel was covered in blood. The pilot said that despite the wind noise, the helicopter was still operating normally and he then landed at its home base without any further problems.

Examination of the helicopter revealed that a 2 to 3 pound duck hit the helicopter and came to rest inside the cabin at the feet of one of the medical crewmembers. The pilot also stated that aside from electrical control switches, the power control levers were also located on the overhead panel and that if they had been hit and moved aft there would have been a reduction of engine power.

Just two months prior to this incident, that is exactly what happened to a Sikorsky S-76C++ that was en route to an offshore oil platform with two pilots and seven passengers. Data from the helicopter’s flight data recorder indicated that the helicopter was established in cruise flight at 850 feet and 135 knots. About 7 minutes after departure, the cockpit voice recorder recorded a loud bang  followed by sounds consistent with rushing wind, a power reduction on both engines and a decay of main rotor rpm. Due to the sudden power loss, the helicopter departed controlled flight and descended rapidly into marshy terrain – only one person survived.

Examination of the wreckage revealed that both the left and right sections of the cast acrylic windshield were shattered. Feathers and other bird remains were collected from the canopy and windshield at the initial point of impact and from other locations on the exterior of the helicopter. Laboratory analysis identified the remains as coming from a female red-tailed hawk; the females of that species have an average weight of 2.4 pounds. Based on main rotor speed decay information provided by Sikorsky, the accident flight crew had, at most, about 6 seconds to react to the decaying rotor speed condition. Had they quickly recognized the cause of the power reduction and reacted very rapidly, they would likely have had enough time to restore power to the engines by moving the overhead engine control levers back into position. However, the flight crewmembers were likely disoriented from the bird strike and the rush of air through the fractured windshield; thus, they did not have time to identify the cause of the power reduction and take action.

As can be seen with these two accidents, bird strikes are disorienting and can require quick action to recover. One of the reasons helicopter pilots wear helmets is to protect their face and vision in case of a bird penetrating the windscreen.

Ground resonance

Friday, July 13th, 2012

According to the NTSB a certificated flight instructor and student pilot were conducting a hover taxi in a Schweizer 269C helicopter from the hangar area to a fuel pump. The student was initially at the controls. The flight instructor took the controls from the student upon reaching the fuel pump, after the student stated he was uncomfortable landing on a raised platform in the confined area. The flight instructor landed the helicopter on the platform, where it then entered into ground resonance. The flight instructor rolled off the throttle immediately, but the ground resonance intensified, resulting in substantial damage to the helicopter.

Ground resonance happens in helicopters with fully-articulated rotor systems (rotor systems with three or more blades), or more specifically rotor systems with lead-lag hinges. These hinges permit the blades to independently move slightly forward and aft in the plane-of-rotation allowing them to speed up and slow down at different points as they spin around the mast. Known as a drag hinge they are necessary to relieve the stress that might otherwise damage the blades from the acceleration and deceleration of the rotor system. To prevent this back-and-forth movement from creating a serious vibration, hydraulic dampers are used to slow down the movement. Ground resonance cannot occur in a two bladed semi-rigid rotor system because the blades do not lead and lag.

Ground resonance typically occurs during a hard landing when the pilot sets the helicopter down on one corner of a skid or on one tire of a wheel equipped helicopter. The jolt transmits a shock through the fuselage to the main rotor system causing the blades to move out-of-phase with each other. In this condition the weight of the rotor system becomes concentrated on one side of the rotor disk causing the rotor system to become unbalanced. As long as the helicopter stays in contact with the ground the out-of-balance condition in the rotor system rapidly increases in frequency until the helicopter shakes itself apart.

If ground resonance starts, the best option is to lift the helicopter into the air allowing the blades to realign. If flight is not achievable then some improvement might be possible by reducing blade pitch and shutting down the engine. However, since the out-of-phase condition can cause major damage in a matter of seconds this approach is only sometimes successful. Helicopters with fully-articulated rotor systems can have shock-absorbing landing gear that will absorb the energy that feeds ground resonance. When ground resonance happens in these helicopters, it is usually because dampeners or shock absorbers have been improperly serviced.

Human-powered helicopter record

Monday, July 2nd, 2012

In 1980 the American Helicopter Society offered a $250,000 prize for the first human powered helicopter. Known as the Sikorsky Prize, it was named in honor of the late helicopter pioneer Igor Sikorsky. To win the prize, the aircraft must reach a height of three meters and remain airborne for 60 seconds while staying in a 10 meter square.

 Although some attempts have been made, so far 32 years later the prize has not been won. The first helicopter to try to win the contest was the Da Vinci III in 1989, designed and built by students at Cal Poly San Luis Obispo in California. It flew for 7.1 seconds and reached a height of 8 inches (20 cm). The second was the Yuri I in 1994, designed and built by students at Nihon University in Japan. It flew for 19.46 seconds and reached an altitude of 20 cm.

On June 21, 2012 a team of engineering students from the University of Maryland’s A. James Clark School of Engineering have gotten the closest to wining the prize and achieved an unofficial world record of 50 seconds with their Gamera II human-powered helicopter, far surpassing any previous records. It will not become official until validated by the National Aeronautic Association. The pilot was Kyle Gluesenkamp, a Ph.D candidate at the school’s mechanical engineering department.

The first version (the Gamera I) stayed airborne for 11.4 seconds. The aircraft was re-engineered with improved airfoils and a new structural design that reduced weight by 39 percent. As with the previous version, the pilot produced power by pedaling. However, the Gamera II has hand cranks that can increase power output by as much as 20 percent. However, due to additional exertion required by the pilot, the output gains drop off after about 60 seconds. The Gamera II is about 105 feet from tip to tip and weighs about 75 pounds without a pilot.