Archive for May, 2012

Tail rotor strikes

Friday, May 25th, 2012

On March 16, 2003, a Bell 430 helicopter landed at an accident site at night. After the medical crew exited the helicopter the pilot decided to reposition the aircraft to face west for departure. During this repositioning the tail rotor hit a roadway sign and the aircraft became airborne to around five or six feet. The pilot lowered the collective and rolled the throttles to idle to stop the aircraft rotation. The helicopter touched down on the left rear skid first and came to rest 180 degrees from its initial heading. A post-accident examination revealed the tail rotor and gearbox had departed the helicopter.

Darkness certainly makes objects harder to see. However, two years prior to this accident, during daylight conditions, a Bell 222UT was damaged when its tail rotor impacted a barrel while landing on a paved traffic turn-around area. The pilot said that while hovering, he decided to reorient the aircraft to help with loading the patient. During the right pedal turn, the tail rotor struck a 55-gallon trash barrel. The helicopter yawed to the right and the pilot brought the throttles to flight idle and landed the helicopter. The tail boom was twisted, the tail rotor blades were damaged and the tail rotor gearbox was nearly separated from the airframe.

Not visible from the cockpit, a helicopter’s tail rotor is perhaps the most vulnerable component to striking objects in a hover. EMS pilots are especially at risk, as their job involves routinely landing in obstacle-rich environments. Although a tail rotor strike in a hover can cause serious damage, the potential for personal injury is low compared to what can happen in flight.

On May 17, 1999, a Bell OH-58A, operated by a public service agency on a photo flight, was destroyed on impact with the terrain and the private pilot and passenger sustained fatal injuries.

A witness reported that he saw a helicopter flying southbound at an altitude of approximately 350-400 ft. He saw what was possibly a large bird hit the rear rotor of the helicopter, after which two objects approximately the size of grapefruits fell to the ground. He said that the objects were falling slowly as though they were light, not fast like something heavy. The helicopter made three to four rotations during its descent.

The NTSB examined the tail assembly and found an elastic material with navy blue yarns wrapped around the tail rotor. The material, along with a sample of a navy blue warm-up jacket found along the reported flight path, was sent to the NTSB’s Materials Laboratory for examination. The color, size and texture of the navy-blue yarns in the elastic material were consistent with those found in the navy blue warm-up jacket.

The NTSB determined the probable cause of this accident was the tail rotor’s impact with the blue warm-up jacket and the subsequent overload of the tail rotor drive shaft. A contributing factor was the absence of the helicopter’s doors.

Removing a helicopter’s doors places the tail rotor at increased risk. In 1993, an R22 helicopter flying with its left door removed crashed after an aluminum kneeboard exited the helicopter and hit the tail rotor. The pilot and passenger were killed.

There have been numerous cases where objects have come out of the cabin or an unsecured baggage compartment and struck the tail rotor. In some cases the pilots have been able to enter autorotation or otherwise land with minor damage or injury. However, as with the two preceding accidents, the tail strike inflicted enough damage to cause the tail rotor assembly to come apart. In these cases, the resulting center of gravity shift made recovery impossible. The importance of protecting the tail rotor cannot be emphasized enough.

Cessna Skyhook

Wednesday, May 9th, 2012

Cessna is well known for building a complete line of airplanes, from two-seat trainers to business jets. However, in the late 50s and early 60s the company also built a helicopter with a two-bladed main rotor and a reciprocating engine. The official model was the CH-1; however, it was also called the Cessna Skyhook.

Designed with a four place cabin the CH-1 was first certified in June, 1955 as a two-seat helicopter. After solving some longitudinal stability problems, Cessna sought and received four-seat certification in February, 1956. The helicopter had a supercharged Continental FSO-470 engine mounted in the nose and a tail boom that looked like the small airplanes they were building at the time. The forward engine design made for easy maintenance, but had other difficulties like where to route the exhaust. Initial designs ran the exhaust under the cabin, however high noise levels created a problem that Cessna was never able to completely fix.

The CH-1 had a fast never-exceed-speed of 122 mph, a maximum gross weight of 3,000 pounds and was considered a good performing helicopter. In fact, just three months after certification the CH-1 landed, hovered and took off from Pike’s Peak, Colorado, at an elevation of over 14,000 feet.  The flight was done as a demonstration for the US Army who eventually ordered 10 upgraded CH-1s for evaluations. The upgraded model used a 270 hp Continental FSO-526 engine and was designated the CH-1B by Cessna. A CH-1C model followed with a gross weight increase to 3,100 pounds and the US Government bought 15 for its Military Assistance Program. Cessna competed for additional military helicopter contracts, but ultimately lost.

In October, 1960 Cessna announced that the CH-1C would be put into production for civilian sales at a price of $79,960 with initial deliveries beginning in July 1961. Just three months before deliveries were to begin, a suspected tail rotor malfunction caused a crash in Texas killing Cessna’s marketing pilot. There were rumors of additional crashes including one in the Gulf of Mexico that resulted in four fatalities.

In January, 1962 Cessna announced they were terminating the helicopter program entirely citing poor sales and lost military opportunities. Cessna bought back all existing civil CH-1s in the field. It is estimated that a total of about 50 helicopters were built. Finally, 1989 Cessna cancelled the CH-1 FAA Type Certificate.