Keeping the nose straight
Earlier this month our Director of Operations and fellow DPE of Sweet Helicopters, Jeff Schorsch, conducted a recurrent training course for our pilot staff. One of his topics was a review of Loss of Tail Rotor Effectiveness (LTE). So, this month I selected a past accident report, onboard the helicopter was a pilot and passenger. The Part 91 flight was on an approach to the landing area and the pilot could not see a windsock and based his estimation of the wind by watching the leaves on the trees. As he approached the landing area at a “normal to steep angle,” and while stabilized at approximately 40 knots indicated airspeed (IAS), about 100 feet above the ground, the helicopter began a slow, un-commanded yaw to the right. The pilot responded by applying left anti-torque pedal; however, the helicopter continued to yaw to the right. The pilot observed that the engine and rotor RPM were “out of the green,” but felt that he could not enter an autorotation due to a power line that was below. He instead increased the throttle, and the helicopter continued to yaw to the right. About 10 feet above the ground, and after about 270 degrees of yaw, the helicopter descended rapidly and impacted the ground, seriously injuring the passenger. Examination of the engine and accessories showed no evidence of mechanical failure or malfunction while under power.
The report ends with the NTSB referencing FAA Advisory Circular 90-95. Loss of tail rotor effectiveness, (LTE) events typically occur in a low airspeed flight regime while maneuvering, such as on final approach to landing. Any maneuver that requires the pilot to operate at high-power, at low airspeed, and with a left crosswind or tailwind, creates an environment where unanticipated right yaw may occur. Additional factors that can influence the severity of the onset of LTE include increases in gross weight and density altitude, low indicated airspeeds, and power droop. Recovery from LTE should include the application of full left pedal, while simultaneously moving the cyclic forward to increase speed, and altitude permitting, a reduction in power. The NTSB determined the probably(s) of this accident to be: the pilot’s inadequate remedial action following a loss of control during the landing approach.
The FAA Flying Handbook defines LTE in Chapter 11 titled, Helicopter Emergencies and Hazards. Loss of tail rotor effectiveness (LTE) or an unanticipated yaw is an un-commanded, 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 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. Unlike the Enstrom helicopter, there are certain types that are more prone to encounter LTE due to 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.
As with any aerodynamic condition, it is important for pilots to not only to understand the definition of LTE, but more importantly, how, and why it happens, how to avoid it, and lastly, what is the pilot’s corrective action once it is encountered. It is best defined as a condition that occurs when the flow of air through a tail rotor is altered in some way, by altering the angle or speed at which the air passes through the rotating blades of the tail rotors disk. Remember when your flight instructor talked about the three different types of LTE? Main rotor disk vortex interference, weathercock stability and tail rotor vortex ring state. Student pilots need to be prepared to explain all aspects of LTE including how to reduce the onset of LTE when taking the practical test.
Back to our accident report, the FAA inspector wrote, “the pilot approached the landing area at a normal to steep angle from 100 feet above the ground, indicated airspeed (IAS) of 40 knots, and with the engine and rotor RPM out of the green. It is at this point the helicopter’s nose began an un-commanded yaw to the right. The pilot responded by applying left antitorque pedal and at the same time increased the power. At 10 feet above the ground the helicopter increased the yaw rate and subsequently descended rapidly and impacted the ground.
How could have this accident be avoided? First, we must recognize what is happening. If the nose is yawing to the right, it is probably related to the tail rotor. If the nose is yawing to the left it is likely engine related. I used to teach students to imagine a large “E” in the left windscreen and a large “T” in the right windscreen. (Some of us remember the movie E.T. the Extra-Terrestrial). If the aircraft’s nose yaws to the left it is the (E) ngine. Likewise, to the right – (T) ail rotor. So, for the purpose of this conversation, when the nose yaws right and starts spinning, apply full left pedal. Simultaneously, apply forward cyclic control to increase airspeed. If altitude permits, reduce power. If altitude does not permit “flying away” it is imperative to reduce power by moving the twist grip towards idle and be prepared to conduct a hovering autorotation. I can assure you when the torque is removed from the rotor system the aircraft will quit spinning immediately.
Ideally, we avoid finding ourselves in this scenario. For pilots operating Enstrom piston models, it is imperative to always keep the engine and rotor RPM in the green. If conditions are conducive for the onset of LTE I would recommend taking the RPM to the “high side” of the green thereby providing slightly more tail rotor authority. Secondly, avoid the winds off of the tail and left side of the helicopter. It is not a precise science; however, it is good practice. Keeping your nose straight will keep you out of trouble.
Randy is a dual rated Airline Transport Pilot with 13,000 flight hours in airplanes and helicopters. He has type ratings in the BE400 and CE500. Randy has been a rotorcraft Designated Pilot Examiner representing the Grand Rapids FSDO since 2014. Currently he is the Chief Pilot for Sweet Helicopters, a northern Indiana Part 135 air carrier operator and serves as the Airport Manager of the Goshen Municipal Airport.
About Enstrom HelicopterFrom Rudy Enstrom’s early designs in 1943 to initial testing in a Michigan Quarry in 1957 to aircraft operating on six continents, Enstrom Helicopter Corporation has maintained a reputation for safety, value and performance. Based in Menominee, Michigan and proudly made in the United States, Enstrom has a rich history for design innovation. The goal is to provide helicopters to the customer’s exact specification and deliver support and maintenance worldwide.