On Oct. 8, 2024, a Leonardo A109SP, operated by IHC Health Services Inc. near Kamas, Utah, experienced a loss of tail rotor effectiveness (LTE) during a low-speed maneuver. The ensuing hard landing/accident caused substantial damage to the aircraft, but fortunately, the pilot and passengers were uninjured. LTE is not a well-known phenomenon for the A109, but the accident highlights this often poorly understood issue.
LTE can be a critical control issue that can happen almost without warning, perhaps leading to a loss of control and potentially an accident — or at a minimum, a seriously disorientated crew. It primarily presents itself as a significant reduction in the tail rotor’s ability to counteract the torque produced by the main rotor, and in essence, removing the pilot’s ability to successfully maintain directional control.
LTE occurs when the amount of thrust produced by the tail rotor is insufficient to counter the torque produced by the main rotor. In those cases where recovery seems to be impossible, it’s often described as a stall of the tail rotor blades — but that’s really simplifying the explanation.
There are also cases where the tail rotor system can get into its own vortex ring state, or perhaps the amount of force being exacted on the tail rotor system exceeds the capability of the yaw control actuator. Whatever the cause, as a pilot, the effect remains the same as pedal inputs become ineffective — or, in the worst case, you reach the pedal stop and the aircraft keeps yawing.
While aircraft design can try to eliminate this major issue by giving you more tail rotor power than you really need, it will always exist as a risk at the edges of the hover envelope and has the following aggravating factors:
- Weather – Adverse weather conditions, especially in relation to strong winds and gusty crosswinds, can both cause and increase the effects of LTE. Importantly for the pilot is the direction of the wind in relation to the aircraft, which is hopefully always on their mind.
- High power settings – Operating at high power, whether due to environmental factors or just close to maximum all up weight (MAUW), will increase your risk of LTE. It’s a basic rule of rotor performance and operating close to the aircraft limits that don´t just relate to engine torque.
- Hover and low airspeed – While the out-of-wind hover or spot turn are obvious risk areas, pilots also need to think about transitions and where they might be in relation to the low-speed envelope. Anyone who has experienced the rather unsettling feeling of an aircraft hitting the LTE limit when crosswind, as the pedal hits the stop and the aircraft yaws into wind, will understand that issue.
- Poor center of gravity (CG) management – Moving the CG substantially away from the ideal position can change the way the aircraft behaves in low-speed flight. What we need to understand is that due to the complexity of interactions between the rotor system and the fuselage, it also effects where you are sitting that in low-speed envelope.
The problem is that unless you fly a particularly susceptible helicopter type, your exposure to information and training on LTE is probably very limited. There is little information about it in most flight manuals, apart from a diagram showing a low-speed envelope, or a statement saying that effective heading control is maintainable up to 17 knots — a certification-derived number that is often meaningless in terms of actual aircraft capability.
As pilots, what should we be doing on a regular basis to mitigate the risks of LTE and, moreover, recover safely from it if it happens?
- Understand the risks – As pilots, we really should be better educated about LTE — both in general and specific to the aircraft we fly. While we seem to concentrate our understanding of those risks associated with system failures, LTE is seemingly off the radar, with few words in flight manuals outside of those concerning straightforward loss of tail rotor control or thrust.
- Monitor the wind – Think about how the wind is affecting you through every stage of flight and where the LTE risk points might be. Perhaps the out-of-wind approach should be amended, or when conducting an area navigation (RNAV) approach, plan for what you will do at the bottom. How many of us consider this in our pre-approach landing brief/checks?
- Think about performance – If you’re hot and high or low and heavy, don’t just think about the hover torque and how you might achieve that perfect Category A profile. You should think about what that performance means in terms of LTE. Landings on sites with limited approach paths really need more thought when close to the low-speed envelope boundary. And when you’re flying in difficult conditions and close to the edge, keep the control inputs smooth and gradual to prevent sudden demands on the tail rotor, and give yourself room to overshoot.
- Maintain adequate airspeed – We all know that airspeed is important and that the tail rotor works less during forward flight, but during transitions, how many go for the shallow and slow approach? It’s not only an issue of what happens if the engine fails but also one of minimizing time in the LTE critical area.
- CG management – Don’t just assume the CG you have is going to be OK. Make sure to properly distribute weight and adhere to the limits. Make note of what the low-speed envelope actually says about CG and understand the effects.
- Training and simulation – While I have done a lot of simulator rides dealing with tail rotor problems, I don’t think I have ever done one where LTE has been introduced. What the recovery technique is for each aircraft really demands more understanding from instructors. Perhaps the main licensing authorities should think more about it.
As evidenced by the accident in Utah, LTE continues to give pilots difficulty, leading to incidents and accidents on a relatively regular basis. Unlike the effects of a direct tail rotor or engine failure, understanding the dynamics of LTE and the baseline methods for recovery, which can be highly aircraft specific, are not well known or regularly practiced.
Pilots can reduce, but never entirely eliminate, the risk of LTE occurring. They can certainly become better informed with implementing proactive measures to keep themselves and their cargo or passengers safe. When things go wrong, LTE can be sudden, often close to the ground and distinctly unforgiving. It really pays to know what to do.
Simon Sparkes is a test pilot for the Norwegian Defense Materiel Agency who started his flying career with the Royal Navy at the end of the 1980s. With over 50 aircraft types in his logbook, his experience has ranged from anti-submarine warfare operations on the Sea King, to basic helicopter instruction on the Gazelle, to commercial light twin operations in both the EC135 and AS355. Previously the commanding officer of the Empire Test Pilots’ School, he currently works on a variety of projects with the AW101 SAR Queen in the challenging Norwegian environment.