NTSB Factual Report

: **This report was modified on July 28, 2016. Please see the docket for this accident to view the original report.*

On July 12, 2015, about 1300 mountain daylight time, a Robinson R22 Beta II helicopter, N441MM, the helicopter was substantially damaged when it collided with rising terrain 10 miles west of Escalante, Utah. The pilot sustained serious injury and his passenger sustained minor injury. The helicopter was registered to Cornerstone Management in Newark, Delaware, and operated by the pilot as a day, visual flight rules, personal flight under 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed at the time of the accident and no flight plan was filed. The helicopter departed private property near Escalante, about 1230.

According to the pilot, he had departed a friend's property after refueling. He reported that, as the helicopter was traveling up a canyon and climbing in rising mountainous terrain, he noticed a substantial tailwind gust. He reported that the helicopter's airspeed had decreased below Effective Translational Lift (ETL) and that the helicopter had stopped climbing. The pilot reported that he immediately made a left turn with the intention of reversing course and subsequently turning into the wind. The pilot reported that after turning left approximately 90 degrees, he noticed a change in engine noise, he heard the Low Rotor RPM Warning Horn, and "dropped the collective" to regain rotor rpm as he continued to try to make the turn into the wind, but the helicopter was about 50 ft above ground level (AGL). The pilot said that he was unable to regain the decreased rotor rpm and the helicopter impacted the terrain. The helicopter sustained substantial damage to the fuselage and tailboom.

The terrain elevation at the accident site was 7,461 feet, and the temperature at the time of the accident was 75 degrees Fahrenheit (F). The Pressure Altitude (PA) was 7,011 ft. and the density altitude (DA) was 9,600 ft.

The calculated gross weight of the helicopter at the time of the accident was 1,400.42 pounds. Per the Robinson 22 Pilot Operating Handbook (R22 POH), maximum allowable gross weight was 1,370 pounds. Per the R22 POH, the maximum weight when operating at 7,011 ft. PA and 75 degrees F is 1,275 pounds. The pilot reported that he executed an immediate left turn after experiencing a tail wind gust.

According to the manufacturer, when the R22 main rotor RPM falls below 515 RPM, the low rotor RPM horn sounds. Subsequently the main and tail rotor inertia decreases rapidly and the helicopter descends, or airspeed is traded for altitude. According to the pilot, when he heard the low rotor RPM horn he lowered the collective and the helicopter impacted the ground and came to rest on its left side facing west.

ADDITIONAL INFORMATION

According to the Federal Aviation Administration (FAA) Helicopter Flying Handbook FAA-8083-21A (HFH) (pg. 2-4, para. 5), turns in a helicopter increase the load factor exponentially, ultimately increasing the power requirement that is necessary to maintain the helicopter's altitude. Left pedal turns increase the quantity of anti-torque produced by the tail rotor, by demanding additional power from the 124 brake horsepower engine. Pilot flight control inputs demanding more power than the engine is capable of producing, with respect to the atmospheric conditions, adversely affects the helicopters ability to sustain its altitude. Available engine power is directly correlated to main and tail rotor RPM. When the engine fails to produce the required power to sustain airspeed and altitude at high DA, the main rotor blades will exceed their critical angle of attack, consequently decreasing main rotor blade RPM.

According to the FAA Helicopter Flying Handbook FAA-8083-21A, Chapter 7 (pg. 7-2, para. 5) per the section entitled Factors Affecting Performance:

A helicopter's performance is dependent on the power output of the engine and the lift produced by the rotors, whether it is the main rotor(s) or tail rotor. Any factor that affects engine and rotor efficiency affects performance. The three major factors that affect performance are density altitude, weight, and wind.

DENSITY ALTITUDE

According to the PHAK, Chapter 3 (pg. 3-3, para. 1) per the section entitled Density Altitude (DA):

DA is the vertical distance above sea level in the standard atmosphere at which a given density is to be found. The density of air has significant effects on the aircraft's performance because as air becomes less dense, it reduces:

• Power because the engine takes in less air.

• Thrust because a propeller is less efficient in thin air.

• Lift because the thin air exerts less force on the airfoils.

Per the Robinson Helicopter Pilot Operating Handbook (Section 5 – Performance):

Relevant meteorological data was used to estimate the DA and PA at the time of the accident. The DA was 9,600 ft. and the PA was 7,011 ft. The PA combined with temperature and weight, were used to determine in ground effect (IGE) and out of ground effect (OGE) capability, per the POH. According to the R22 POH, Chapter 5 (pg. 5-6) the R22 Beta II was capable of conducting IGE flight at the maximum approved gross weight of 1370 pounds and a temperature of 75 degrees F, with the maximum IGE capability at 7,800 ft. PA. The maximum OGE capability under the same conditions was 3,500 ft. PA.

According to the United States Army Field Manual 1-203 (Task 1016), OGE maneuvers include, amongst others: terrain flight, terrain flight approach, terrain flight decelerations. The pilot reported that his airspeed decreased below that of ETL prior to executing the left turn. He further asserted that the helicopter was approximately 50 ft. AGL when the left turn was executed.

The Height/Velocity Diagram in the R22 POH, Chapter 5 identifies that the pilot shall avoid operating the helicopter at airspeeds below 24 KIAS at altitudes between 10 and 50 ft. above ground level (AGL). When flying above 7,000 ft. DA, weighing 1,300 pounds, the area that pilots shall avoid operating within increases from 10 to 510 ft. AGL.

WEIGHT

Per the FAA Aircraft Weight and Balance Handbook FAA-H-8083-1A (pg. 1-1, para. 2), per the section entitled Weight Control:

Improper loading cuts down the efficiency of an aircraft from the standpoint of altitude, maneuverability, rate of climb, and speed. It may even be the cause of failure to complete the flight, or for that matter, failure to start the flight. Because of abnormal stresses placed upon the structure of an improperly loaded aircraft, or because of changed flying characteristics of the aircraft, loss of life and destruction of valuable equipment may result. The responsibility for proper weight and balance control begins with the engineers and designers, and extends to the aircraft mechanics that maintain the aircraft and the pilots who operate them.

According to the FAA Helicopter Flying Handbook FAA-8083-21A (pg. 2-4, para. 5 and 6), per the section entitled Weight:

The weight of the helicopter can also be influenced by aerodynamic loads. When you bank a helicopter while maintaining a constant altitude, the "G" load or load factor increases. The load factor is the actual load on the rotor blades at any time, divided by the normal load or gross weight (weight of the helicopter and its contents). Any time a helicopter flies in a constant altitude curved flightpath, the load supported by the rotor blades is greater than the total weight of the helicopter. Above 30° of bank, the apparent increase in gross weight soars. At 30° of bank, or pitch, the apparent increase is only 16 percent, but at 60°, it is twice the load on the wings and rotor system. For example, if the weight of the helicopter is 1,600 pounds, the weight supported by the rotor disk in a 30° bank at a constant altitude would be 1,856 pounds [1,600 + 16 percent

(or 256)]. In a 60° bank, it would be 3,200 pounds; in an 80° bank, it would be almost six times as much, or 8,000 pounds. It is important to note that each rotor blade must support a percentage of the gross weight. In a two bladed system, each blade of the 1,600 pound helicopter as stated above would have to lift 50 percent or 800 pounds.

The FAA Helicopter Flying Handbook FAA-8083-21A (pg. 2-4, para. 6) entitled Weight, asserts:

To overcome this additional load factor, the helicopter must be able to produce more lift. If excess engine power is not available, the helicopter either descends or has to decelerate in order to maintain the same altitude. The load factor and, hence, apparent gross weight increase is relatively small in banks up to 30°. Even so, under the right set of adverse circumstances, such as high DA, turbulent air, high gross weight, and poor pilot technique, sufficient or excess power may not be available to maintain altitude and airspeed.

The FAA Helicopter Flying Handbook FAA-8083-21A (pg. 2-5, para. 1) entitled Weight, further states:

Regardless of how much weight one can carry or the engine power that it may have, they (helicopters) are all susceptible to aerodynamic overloading. Unfortunately, if the pilot attempts to push the performance envelope the consequence can be fatal. Aerodynamic forces effect every movement in a helicopter, whether it is increasing the collective or a steep bank angle. Anticipating results from a particular maneuver or adjustment of a flight control is not good piloting technique. Instead pilots need to truly understand the capabilities of the helicopter under any and all circumstances and plan to never exceed the flight envelope for any situation.

WIND

The FAA Helicopter Flying Handbook FAA-8083-21A (pg. 7-2, para. 5) per the section entitled Factors Affecting Performance:

The wind direction is also an important consideration. Headwinds are the most desirable as they contribute to the greatest increase in performance. Strong crosswinds and tailwinds may require the use of more tail rotor thrust to maintain directional control. This increased tail rotor thrust absorbs power from the engine, which means there is less power available to the main rotor for the production of lift.

According to the FAA Helicopter Flying Handbook FAA-8083-21A (pg. 11-18, para. 1):

Loss of Tail rotor Effectiveness (LTE) is a condition that occurs when the flow of air through a tail rotor is altered in some way, either by altering the angle or speed at which the air passes through the rotating blades of the tail rotor system. With respect to LTE, the pilot was operating the helicopter with a significant tailwind, in a region known as Weathercock Stability. As the pilot made the immediate left turn, the pilot was operating the helicopter in the region known as Tail Rotor Vortex Ring State.

Per the FAA Helicopter Flying Handbook FAA-8083-21A (pg. 11-20, para. 5), entitled LTE at Altitude:

At higher altitudes where the air is thinner, tail rotor thrust and efficiency are reduced. Because of the high DA, powerplants may be much slower to respond to power changes. When operating at high altitudes and high gross weights, especially while hovering, the tail rotor thrust may not be sufficient to maintain directional control, and LTE can occur. In this case, the hovering ceiling is limited by tail rotor thrust and not necessarily power available. In these conditions, gross weights need to be reduced and/or operations need to be limited to lower DA's.

In order to prevent the onset of LTE in either of the two aforementioned regions, The HFH prescribes the steps imperative to reducing the onset of LTE:

1. Maintain maximum power-on rotor rpm. If the main rotor rpm is allowed to decrease, the anti-torque thrust available is decreased proportionally.

2. Avoid tailwinds below airspeeds of 30 knots. If loss of translational lift occurs, it results in an increased power demand and additional anti-torque pressures.

3. Avoid OGE operations and high power demand situations below airspeeds of 30 knots at low altitudes.

4. Be especially aware of wind direction and velocity when hovering in winds of about 8–12 knots. A loss of translational lift results in an unexpected high power demand and an increased anti-torque requirement.

5. Be aware that if a considerable amount of left pedal is being maintained, a sufficient amount of left pedal may not be available to counteract an unanticipated right yaw.

6. Be alert to changing wind conditions, which may be experienced when flying along ridge lines and around buildings.

7. Execute slow turns to the right which would limit the effects of rotating inertia, and the loading on the tail rotor to control yawing would be decreased.

NTSB Probable Cause

The pilot’s inadequate preflight planning and his subsequent decision to attempt to climb over rising terrain with the helicopter over maximum gross weight while operating in high-density altitude conditions with a tailwind, which resulted in its inability to maintain a positive climb rate and a subsequent impact with terrain.