Monday, November 1, 2004
Watch Your Tail
On March 16, 2003, a Bell 430 was substantially damaged when its tail rotor impacted a roadway sign during an aerial taxi. Prior to touchdown, the pilot reported he rotated the aircraft and landed on an easterly heading, at which point the medical crew departed the helicopter.
According to the National Transportation Safety Board, the pilot decided to reposition the aircraft to face west for departure. He stated, "I cleared the tail to move left when the tail rotor hit a steel reflector post.
The aircraft became airborne to around five or six feet. I lowered the collective and rolled the throttles to idle to stop the aircraft rotation." The aircraft touched down on the left rear skid first and came to rest 180 deg. from its initial heading. A post-accident examination revealed the tail rotor and gearbox had departed the helicopter.
The NTSB determined the probable cause of this accident was the pilot's failure to maintain clearance of the roadway sign. Contributing factors were the failure of the tail rotor gearbox, the roadway sign and night conditions.
Darkness certainly makes objects harder to see. However, two years prior to this accident, during daylight conditions, a Bell 222UT was substantially damaged when its tail rotor impacted a barrel while landing on a paved traffic turn-around area near Alcova, Wyo.
The pilot said that while hovering, he decided to reorient the aircraft to facilitate 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.
The NTSB determined the probable cause of this accident was the pilot's failure to maintain tail rotor clearance during a right pedal turn while hovering.
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 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.
Examination of the tail assembly revealed 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 entry 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 the pilot allowed an aluminum kneeboard to exit the helicopter and strike the tail rotor. The pilot and passenger were killed.
In response to this accident, the manufacturer issued a safety notice warning pilots of the dangers of not adequately protecting the tail rotor. Since the R22's tail rotor is on the left side, it is strongly recommended that pilots never fly with the left door removed.
Not removing the door that is on the same side as the tail rotor is good advice for any model helicopter.
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.