Loss of tail-rotor effectiveness

What LTE is — and isn't

The FAA and NTSB describe LTE as an uncommanded, rapid yaw that does not subside of its own accord and, uncorrected, can lead to loss of control. It is not a maintenance malfunction: the tail rotor is working, but the air reaching it isn't what it needs.

It can occur in varying degrees in any single-main-rotor helicopter below about 30 kt. The NTSB investigated 55 LTE accidents in the ten years from 2004 to 2014.

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AC 90-95's wind regions, mirrored

Main-rotor disc vortex

With wind of about 10–30 kt from about 45–75° off the nose on one side — for the Astar, the right, mirrored from the AC — the main rotor's tip vortex is blown into the tail rotor. The tail rotor works in disturbed air, its thrust varies, and the aircraft yaws unpredictably.

The pilot feels it as a sudden change in the pedal needed to hold heading. Left uncorrected, the yaw can build.

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Weathercock stability

With the wind from behind, the fin wants to turn the tail into the wind like a weathervane. Left alone, the aircraft yaws — and once the yaw starts, it can accelerate.

Downwind hovering and slow downwind turns are where it bites.

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Tail-rotor vortex ring

With the wind from the side the tail rotor blows towards, the tail rotor can enter its own vortex ring state — the same flow problem the main rotor has in a steep descent. Its thrust becomes erratic and pedal inputs give unexpected results.

For the Astar, mirrored from the AC, that's wind from the right side. The AC says rapid and continuous pedal work is needed to hold heading when hovering in it. The workload itself is routine; the danger is a late correction or over-controlling.

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Loss of translational lift

The fourth factor isn't a wind sector. When the aircraft slows below translational lift — on an approach, in a turn downwind, or when the wind drops — the rotor needs more power for the same lift. More power means more torque, and more right pedal in the Astar.

If the pilot doesn't anticipate it, the nose goes left. FAA material flags hovering in winds of about 8–12 kt as a typical case, where small changes in wind take translational lift away.

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What makes it worse

AC 90-95 lists the contributing factors:

• high power, high mass and high density altitude
• low airspeed, especially below 30 kt
• turns towards the torque yaw at low speed downwind — left turns in the Astar
• low rotor rpm
• loss of translational lift with the wind behind

The NTSB adds turbulent air near ridgelines and large buildings.

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Recognising it

LTE starts like an ordinary yaw — so the first sign is that it doesn't stop with the pedal you'd normally use, and the yaw rate may increase.

In the Astar the yaw is to the left, the same way the nose goes when you raise the collective. That makes it easy to read the first part as normal torque and be late with the pedal. The longer the rate builds, the harder it is to stop.

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Recovery, as the AC describes it

For American types, AC 90-95 describes applying full opposite pedal, moving the cyclic forward to gain speed, and — altitude permitting — reducing power. Mirrored for the Astar, the pedal is right.

The approved technique for your aircraft comes from your RFM and your instructor.

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The trade-offs in that recovery

Each part of the AC's recovery costs something. Reducing collective cuts the torque the tail rotor is fighting, so it helps stop the yaw — but it increases the rate of descent. The AC ties the amount to height above obstacles, gross weight and the atmosphere.

Forward cyclic gains speed so the fin and translational lift start to help, but it needs room ahead. Low over a confined area, neither comes free — which is why avoiding the set-up matters more than the recovery.

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Keeping out of it

FAA guidance, below about 30 kt:

• avoid tailwinds
• avoid out-of-ground-effect hovers and high-power demands such as slow downwind turns
• be alert to wind of about 8–12 kt in the hover
• keep rotor rpm in its normal range
• know the wind's direction and strength at all times, especially when busy, along ridgelines and around buildings

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Tailwind approaches

A tailwind approach stacks the LTE factors. The wind is in the weathercock region. Airspeed is low while groundspeed looks normal, so the aircraft reaches the bottom slower through the air than it seems. And it arrives at the point that needs the most power just as translational lift goes.

Gusts and downdrafts near terrain add sudden power demands on top. If the site can only be approached downwind, that is a planning question to settle before you start down.

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