N49746Bell 206B · 1976

On October 2, 2014, about 1255 central daylight time, a Bell 206B helicopter, N49746, had a hard landing following a loss of tail rotor effectiveness near Woodsboro, Texas. The airline transport rated pilot and two passengers received serious injuries. The helicopter sustained substantial damage during a subsequent roll over. The helicopter was registered to and operated by an individual doing business as Heartland Helicopters under the provisions of Code of Federal Regulations Part 91 as an aerial observation flight. Day visual flight rules conditions prevailed for the flight, which did not operate on a flight plan. The flight originated from the Alfred C 'Bubba' Thomas Airport (T69), near Sinton, Texas, about 1200, and was destined for the Beeville Municipal Airport, near Beeville, Texas.

According to the pilot's accident report, the purpose of the flight was a laser examination of a pipeline to detect methane gas. He indicated that he was flying the helicopter between 35 and 40 meters above ground level. The pilot reported that the helicopter suddenly yawed to the right without warning. He indicated that he did not recall "when a warning horn went off, but there was a warning horn." He recalled it as a low rotor rpm warning. The pilot stated that there were wires to his right side and building structures on his left. The pilot reported the helicopter yawed, he indicated he was sure he applied pressure to the left pedal, and a loss of tail rotor effectiveness occurred. The pilot said that the helicopter hesitated turning right for a "short" period of time and then continued the turn to the right. He intended to guide the helicopter away from the wires and structure. However, the helicopter continued one full revolution to the right. The pilot, in part, stated:

It was at this point I had two thoughts. I need to reduce collective in order to maintain sufficient rotor speed to cushion the landing. I also did not roll off the throttle because I did not want to shut down a good engine if I had one. I did my best to land the aircraft in a level and stable manner. The aircraft impacted the ground. I do not remember bouncing, but we impacted and rolled to the right and ended up with the aircraft positioned on the right side.

According to a passenger on the pipeline inspection flight, he heard another passenger guide the pilot to make a right turn as the helicopter approached a set of power lines. He felt the helicopter change "position" while making the turn. The passenger then "heard a change, decrease, in the engine RPM, followed almost immediately by a warning horn" and he "heard the pilot say that he had a low tail rotor warning." The helicopter started to spin and the he braced for impact. When asked to describe the sounds he heard, the passenger reported that the engine sounded "similar to that of a lawnmower, running at a normal speed and how it bogs down in tall weeds or grass. The engine bogging was immediately followed by the master warning horn."

The pilot, age 47, held a Federal Aviation Administration (FAA) airline transport pilot certificate with an airplane multi-engine land rating. He held commercial pilot airplane single-engine land, rotorcraft helicopter, and instrument helicopter privileges. He held a flight instructor certificate with airplane single-engine and rotorcraft helicopter ratings. He also held a flight engineer certificate with a turbojet powered airplane rating. He reported that he held a FAA second-class medical certificate dated August 15, 2014, with a limitation that he must wear corrective lenses. His most recent flight review was completed on May 20, 2014. The pilot's report indicated that he accumulated a total flight time of 13,916 hours, of which 2,376 hours were in rotorcraft and 35 hours were in the same make and model as the accident helicopter.

N49746 was a 1976 model Bell 206B helicopter with serial number 1928. A 420-shaft horsepower Rolls Royce 250-C20B engine with serial number CAE-834605 powered the helicopter. It was a five-place, single main rotor helicopter with a tail mounted anti-torque rotor. According to the pilot's accident report, the helicopter was maintained under a manufacturer's inspection program and its last inspection was a 100-hour inspection dated June 5, 2014. The helicopter accumulated 18,323.2 hours of total flight time at the time of the inspection. According to National Transportation Safety Board (NTSB) report FTW81DRA33, the helicopter was involved in a prior accident on May 19, 1981, when it sustained substantial damage during a takeoff accident in the Gulf of Mexico.

The helicopter was fitted with a Pergam Aerial Laser Methane Assessment (ALMA) device. The Pergam website indicated the system is a laser based natural gas detection system. ALMA consists of an on-board laptop that manages the data acquisition and data processing, an on-board electronic laser-control system, a GPS receiver, and a helicopter belly-mounted optical unit. The optical unit is a refraction-based system, which consists of a laser, a reference channel, and three digital video recording cameras. The cameras are integrated into the ALMA system and indicate the location that is scanned by the system's laser beam. According to the website, it is simultaneously used to help the pilot position the helicopter over the object that needs inspection.

A copy of a fuel receipt indicated the helicopter was serviced on October 2, 2014, with 60.1 gallons of Jet A fuel at T69.

At 1353, the recorded weather about 17 miles and 122 degrees from the accident site at the Aransas County Airport, near Rockport, Texas, was: Wind 170 degrees at 10 knots, gusting to 21 knots with winds varying from 130 degrees to 190 degrees; visibility 10 statute miles; sky condition few clouds at 1,600 feet, scattered clouds at 2,400 feet, broken clouds at 2,900 feet; temperature 32 degrees C; dew point 26 degrees C; altimeter 29.86 inches of mercury. The field elevation at the airport was about 24 feet above mean sea level (MSL).

N49746 was a 1976 model Bell 206B helicopter with serial number 1928. A 420-shaft horsepower Rolls Royce 250-C20B engine with serial number CAE-834605 powered the helicopter. It was a five-place, single main rotor helicopter with a tail mounted anti-torque rotor. According to the pilot's accident report, the helicopter was maintained under a manufacturer's inspection program and its last inspection was a 100-hour inspection dated June 5, 2014. The helicopter accumulated 18,323.2 hours of total flight time at the time of the inspection. According to National Transportation Safety Board (NTSB) report FTW81DRA33, the helicopter was involved in a prior accident on May 19, 1981, when it sustained substantial damage during a takeoff accident in the Gulf of Mexico.

The helicopter was fitted with a Pergam Aerial Laser Methane Assessment (ALMA) device. The Pergam website indicated the system is a laser based natural gas detection system. ALMA consists of an on-board laptop that manages the data acquisition and data processing, an on-board electronic laser-control system, a GPS receiver, and a helicopter belly-mounted optical unit. The optical unit is a refraction-based system, which consists of a laser, a reference channel, and three digital video recording cameras. The cameras are integrated into the ALMA system and indicate the location that is scanned by the system's laser beam. According to the website, it is simultaneously used to help the pilot position the helicopter over the object that needs inspection.

A copy of a fuel receipt indicated the helicopter was serviced on October 2, 2014, with 60.1 gallons of Jet A fuel at T69.

The helicopter impacted a field near Woodsboro, Texas, and was relocated to a local salvage yard. The NTSB investigator in charge (IIC) examined the wreckage at the salvage yard with the engine manufacturer. The observed damage and deformation was consistent with the pilot's report of the helicopter coming to rest on its right side. The helicopter's mast exhibited a separation, which was consistent with overload. The main rotor blades exhibited damage consistent with impact with terrain. The helicopter's transmission rotated when manipulated by hand. The tailboom was separated from the helicopter fuselage and the tail rotor driveshaft inside exhibited a separation consistent with being cut by recovery personnel. The tailboom separation exhibited deformation consistent with its contact with terrain during a helicopter rollover to the right. The tail rotor blades were intact on the aft portion of the tailboom. The skids exhibited deformation and separations consistent with overload. There was a tear observed in the lower portion of the fuel cell. Both of the cell's fuel boost pumps were intact and they pumped fuel when electrical power was applied to them. Airframe filters were opened and no anomalies were observed within them. The tail rotor and main rotor systems moved when the turbine blades inside the exhaust were rotated by hand. No airframe pre-impact anomalies were observed. The engine did not exhibit any exterior damages and it was separated from the airframe for a teardown examination.

The engine was examined under supervision of the IIC at the engine manufacturer. Disassembly did not reveal any anomalies that would have precluded engine operation. The engine's fuel control and governor were hand carried by the IIC for testing at their manufacturer.

FAA Advisory Circular 90-95 - Unanticipated Right Yaw in Helicopters and the Helicopter Flying Handbook describe the phenomenon of loss of tail rotor effectiveness (LTE). The handbook, in part, stated:

LTE or an unanticipated yaw is defined as an uncommanded, rapid yaw

towards the advancing blade which does not subside of its own accord.

It can result in the loss of the aircraft if left unchecked. It is very

important for pilots to understand that LTE is caused by an

aerodynamic interaction between the main rotor and tail rotor and not

caused from a mechanical failure. Some helicopter types are more likely

to encounter LTE due to the normal certification thrust produced by

having a tail rotor that, although meeting certification standards, is not

always able to produce the additional thrust demanded by the pilot.

...

LTE is an aerodynamic condition and is the result of a control margin

deficiency in the tail rotor. It can affect all single rotor helicopters that

utilize a tail rotor of some design. The design of main and tail rotor

blades and the tail boom assembly can affect the characteristics and

susceptibility of LTE but will not nullify the phenomenon entirely.

Translational lift is obtained by any amount of clean air through the

main rotor system. Chapter 3 discusses translational lift with respect to

the main rotor blade, explaining that the more clean air there is going

through the rotor system, the more efficient it becomes. The same

holds true for the tail rotor. As the tail rotor works in less turbulent air,

it reaches a point of translational thrust. At this point, the tail rotor

becomes aerodynamically efficient and the improved efficiency

produces more antitorque thrust. The pilot can determine when the tail

rotor has reached translational thrust. As more antitorque thrust is

produced, the nose of the helicopter yaws to the left (opposite

direction of the tail rotor thrust), forcing the pilot to correct with right

pedal application (actually decreasing the left pedal). This, in turn,

decreases the [angle of attack] AOA in the tail rotor blades. Pilots

should be aware of the characteristics of the helicopter they fly and be

particularly aware of the amount of tail rotor pedal typically required

for different flight conditions.

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. An

effective tail rotor relies on a stable and relatively undisturbed airflow in

order to provide a steady and constant antitorque reaction as discussed

in the previous paragraph. The pitch and angle of attack of the

individual blades will determine the thrust output of the tail rotor. A

change to any of these alters the amount of thrust generated. A pilot's

yaw pedal input affects a thrust reaction from the tail rotor. Altering

the amount of thrust delivered for the same yaw input creates an

imbalance. Taking this imbalance to the extreme will result in the loss

of effective control in the yawing plane, and LTE will occur.

This alteration of tail rotor thrust can be affected by numerous external

factors. The main factors contributing to LTE are:

1. Airflow and downdraft generated by the main rotor blades

interfering with the airflow entering the tail rotor assembly.

2. Main blade vortices developed at the main blade tips entering the

tail rotor.

3. Turbulence and other natural phenomena affecting the airflow

surrounding the tail rotor.

4. A high power setting, hence large main rotor pitch angle, induces

considerable main rotor blade downwash and hence more turbulence

than when the helicopter is in a low power condition.

5. A slow forward airspeed, typically at speeds where translational lift

and translational thrust are in the process of change and airflow

around the tail rotor will vary in direction and speed.

6. The airflow relative to the helicopter;

a. Worst case—relative wind within ±15 degrees of the 10 o'clock

position, generating vortices that can blow directly into the tail rotor.

This is dictated by the characteristics of the helicopters aerodynamics

of tailboom position, tailrotor size and position relative to the main

rotor and vertical stabilizer, size and shape.

b. Weathercock stability—tailwinds from 120 degrees to 240 degrees,

such as left crosswinds, causing high pilot workload.

c. Tail rotor vortex ring state (210 degrees to 330 degrees). Winds

within this region will result in the development of the vortex ring

state of the tail rotor.

7. Combinations (a, b, c) of these factors in a particular situation can

easily require more anti-torque than the helicopter can generate and in

a particular environment LTE can be the result.

Certain flight activities lend themselves to being more at high risk to

LTE than others. For example, power line and pipeline patrol sectors,

low speed aerial filming/photography as well as in the Police and

Helicopter Emergency Medical Services (EMS) environments can find

themselves in low and slow situations over geographical areas where

the exact wind speed and direction are hard to determine.

Unfortunately, the aerodynamic conditions that a helicopter is

susceptible to are not explainable in black and white terms. LTE is no

exception. There are a number of contributing factors but what is

more important to understanding LTE are taking the contributing

factors and couple them with situations that should be avoided.

Whenever possible, pilots should learn to avoid the following

combinations:

1. Low and slow flight outside of ground effect.

2. Winds from ±15 degrees of the 10 o'clock position and probably on

around to 5 o'clock position

3. Tailwinds that may alter the onset of translational lift and

translational thrust hence induce high power demands and demand

more anti-torque (left pedal) than the tail rotor can produce.

4. Low speed downwind turns.

5. Large changes of power at low airspeeds.

6. Low speed flight in the proximity of physical obstructions that may

alter a smooth airflow to both the main rotor and tail rotor.

Pilots who put themselves in situations where the combinations above

occur should know that they are likely to encounter LTE. The key is to

not put the helicopter in a compromising condition but if it does

happen being educated enough to recognize the onset of LTE and be

prepared to quickly react to it before the helicopter cannot be

controlled.

Early detection of LTE followed by the immediate flight control

application of corrective action; applying forward cyclic to regain

airspeed, applying right pedal not left as necessary to maintain rotor

RPM and reducing the collective thus reducing the high power demand

on the tail rotor is the key to a safe recovery. Pilots should always set

themselves up when conducting any maneuver to have enough height

and space available to recover in the event they encounter an

aerodynamic situation such as LTE.

Understanding the aerodynamic phenomenon of LTE is by far the most

important factor, and the ability and option to either go around if

making an approach or pull out of a maneuver safely and re-plan, is

always the safe option. Having the ability to fly away from a situation

and re-think the possible options should always be part of a pilot's

planning process in all phases of flight.

The helicopter was not equipped with a cockpit voice recorder or flight data recorder nor was it required to be. However, the ALMA system included a Lenovo ThinkPad, Hitachi Hard Drives, an Argosy PCMCIA Card, and a Garmin GPSMAP 296. These items were shipped to the NTSB Vehicle Recorder Laboratory for examination and downloading.

The GPS, hard drives, and PCMCIA card were examined by an NTSB Recorder Laboratory specialist. No useable data was able to be extracted from them that assisted in the investigation. However, the recorder specialist found that the laptop contained flight position data. This data was decoded and plotted in the specialist's factual report.

The data showed that during the time between 1249 and 1255, the helicopter flew south before executing a left 270-degree turn to the west on a path along a roadway. The helicopter then turned right 90 degrees and its path paralleled another roadway toward the north.

The helicopter then turned left 90 degrees and its path followed another roadway to the west. The helicopter's recorded groundspeed was varied between about 50 to 70 knots during this segment of the flight. The altitude flown began at about 200 feet MSL and trended down to about 70 feet before trending back up to about 120 feet. About 1255, the helicopter's ground track began to head to the north away from the road it had been flying along and its groundspeed decreased to about 15 knots. Simultaneously, the helicopter's altitude dropped from about 130 feet MSL to about 100 fee and then again to about 55 feet before it came back up to about 80 feet. The helicopter's groundspeed varied during its last 10 seconds of recorded data to a low of 9.4 knots, increasing to 25.2 knots one second later, and decreasing again to 12.3 knots. During this time period, the helicopter's altitude decreased to the 50-foot range. The last recorded point showed the helicopter was about 4.1 miles southeast of Woodsboro, Texas, on a track of 304 degrees true, an altitude of 53 feet MSL, and a groundspeed of 15.4 knots.

Both the fuel control and governor were operational during testing on test benches.