N121MRRobinson Helicopter R22 ALPHA · 1985

On December 18, 2016, about 1100 Alaska standard time, a Robinson R-22 Alpha helicopter, N121MR, sustained substantial damage during a collision with mountainous, snow-covered terrain about 12 miles north of Palmer, Alaska. The two occupants aboard, the certificated flight instructor seated in the left seat, and the private helicopter pilot seated in the right seat, sustained minor injuries. The helicopter was registered to, and operated by, Castor Aviation Ltd. of Wasilla, Alaska, as a visual flight rules (VFR) instructional flight under the provision of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed at the time of the accident, and no flight plan was filed. The flight originated from the Wolf Lake Airport, Palmer, at 1002.

During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on December 18, the flight instructor stated that he was providing flight instruction to the private pilot who was working towards a commercial helicopter pilot certificate. He added that at the time of the accident, they were practicing pinnacle landings to an area of remote, snow-covered mountainous terrain in the Hatcher Pass Management Area.

The flight instructor said that just before the accident, the private pilot accomplished two successful pinnacle landings to sites situated about 4,200 and 4,600 feet mean sea level (msl). After a third pinnacle landing site was selected, a gravel-covered site on a mountain ridgeline situated about 4,300 feet msl, the private pilot circled the site several times for reconnaissance. He said that while circling, the pair discussed the maneuver, which included a preplanned escape route that was just to the right of the landing site.

The flight instructor said that during the accident approach, while the private pilot was manipulating the flight controls, he confirmed that all cockpit indications were "in the green," no warning lights were illuminated, the manifold pressure was between 20 to 21 inches, and the descent rate was at 150 feet per minute. He reported that as the helicopter neared the site, about 10 to 20 feet above the surface, he realized that it had a steep uphill grade making the site unsuitable for landing. He explained that, as he was getting ready to tell the private pilot to initiate a go-around, the low rotor revolutions per minute (RPM) warning light and horn activated. The private pilot reported that when the low rotor RPM warning light and horn activated, he observed the gauge indicated about 90 percent RPM.

The flight instructor then took control of the helicopter, attempting to maneuver it to the right and towards the predetermined escape route, but it descended and the skids subsequently struck the uneven terrain. He said that after the initial collision, he increased collective pitch and applied right cyclic, but it began to spin to the right, while descending. The helicopter continued to spin, while descending, and it subsequently struck an area of steep, snow-covered terrain. The helicopter then rolled downhill multiple times before coming to rest in an area of steep, snow-covered terrain. Both occupants egressed from the wreckage, a cellular phone was utilized to request rescue assets, and the occupants were extracted from the accident site via a helicopter from a separate operating company.

The helicopter sustained substantial damage to the main rotor system, fuselage, tail boom, and tail rotor system.

The flight instructor reported that there were no preimpact mechanical failures or malfunctions with the airframe or engine that would have precluded normal operation.

In the recommendation section of the NTSB Accident/Incident Reporting Form 6120.1, the flight instructor reported that to never execute a practice approach to an area you are not 100 percent sure you could land the helicopter to in the event of something happening in the last 25 to 50 feet. He further reported that if the landing surface would have been a bit more suitable, the helicopter might have been able to touch down and then come back up to take the planned escape route.

The closest official weather observation station is located at the Palmer Airport, Palmer about 12 miles south of the accident site. At 1053, an Aviation Routine Weather Report (METAR) was reporting, and stated in part: Wind, 20 degrees (true) at 18 knots, gusting 24 knots; visibility, 10 statute miles; clouds and sky condition, scattered clouds at 8000 feet, broken clouds at 14,000 feet; temperature, 34 degrees F; dew point, 14 degrees F; altimeter, 28.86 inHg.

The accident helicopter was not equipped, nor was it required to be equipped with an emergency locator transmitter. The pilot and passenger were not wearing flight helmets for the flight. The helicopter was equipped with 3-point restraint systems for the two seats.

Robinson Helicopter Company has published the R-22 Pilot's Operating Handbook (2016). This document discusses the low RPM light and horn system and states in part:

The low RPM light and horn indicate rotor RPM at 97 percent or below.

Robinson Helicopter Company has published Safety Notice SN-24 Low RPM Rotor Stall Can Be Fatal (1994). This document discusses main rotor stall and states in part:

Rotor stall due to low RPM causes a very high percentage of helicopter accidents, both fatal and non-fatal. Frequently misunderstood, rotor stall is not to be confused with retreating tip stall which occurs only at high forward speeds when stall occurs over a small portion of the retreating blade tip. Rotor stall, on the other hand, can occur at any airspeed and when it does, the rotor stops producing the lift required to support the helicopter and the aircraft literally falls out of the sky. Fortunately, rotor stall accidents most often occur close to the ground during takeoff or landing and the helicopter falls only four or five feet. The helicopter is wrecked but the occupants survive. However, rotor stall also occurs at higher altitudes and when it happens at heights above 40 or 50 feet above ground level it is most likely to be fatal.

The pilot flew to a practice area and landed the helicopter on what the pilot thought was firm snow. As he landed, the helicopter suddenly broke through the surface, and its tailrotor struck the snow. The pilot repositioned the helicopter, and the tailrotor chip light illuminated. An inspection showed that the tailrotor driveshaft had sheared.