Tail-rotor failures

Drive, control and boost — three different problems

The essentials

"Tail-rotor failure" covers three different problems:

drive failure — the tail rotor stops providing thrust at all
control failure — the pitch is stuck where it was
loss of boost — the tail rotor works, but the pedals become heavy

Each looks different and each has its own procedure. Common to all: airspeed helps, because the fin does more of the work as speed builds.

Sources 1, 2

How each one presents

Drive failure: often a bang or a change in noise, then a yaw left that the pedals can't stop. The FAA handbook notes the yaw is most violent at high power and low airspeed.
Stuck pitch: the pedals won't move, or move without effect, and the heading changes whenever power changes.
Loss of boost: the pedals stiffen, with a hydraulic warning; the tail rotor itself still works.

The first job is the same: work out which you have before choosing what to do.

Sources 1, 2

Drive failure

With no tail-rotor thrust, torque yaws the nose left, hard, in the Astar. In the hover the only way to stop the yaw is to remove the torque — by cutting power and landing from the hover.

In forward flight, close to cruise speed, the fin may keep the aircraft roughly straight long enough to reach a suitable area. The FAA handbook's general answer ends in an autorotation; your RFM sets out the Astar's.

Sources 1, 3

In-depth

Why autorotation helps

The yaw in a drive failure comes from engine torque. Reduce the power and the yaw reduces; enter autorotation and virtually all of it goes, because an autorotating rotor produces almost no torque.

That is the logic behind the forward-flight procedures: the fin keeps the nose roughly in line while there is speed, and taking the power away removes what the tail rotor was there to fight. The details — and when power may be used — are in the RFM.

Source 1

In-depth

Stuck pitch

If the tail rotor keeps turning but its pitch is fixed, its thrust stops responding to the pedals — so the collective becomes your yaw control. More power yaws the nose left; less power yaws it right.

The handbook's principle is to find a power setting and airspeed at which the nose lines up, and to fly the approach so that it still lines up at touchdown — for some cases with a run-on landing. Which way it's stuck changes the details.

Sources 1, 3

In-depth

Which way it's stuck

Stuck with a lot of right pedal in — a climb or hover setting — the tail rotor pushes hard. Reduce power for an approach and the nose swings right; it lines up only with power on.

Stuck with little right pedal — a cruise or descent setting — the opposite: adding power swings the nose left, and it lines up at low power.

This is the clockwise-rotor mirror of what most handbooks describe; check every direction against your RFM.

Sources 1, 3

In-depth

Loss of tail-rotor boost

If hydraulic boost to the tail rotor is lost, the tail rotor still works but the pedal forces rise. The yaw load compensator keeps them manageable for a time.

For the Frisco investigation, Airbus Helicopters calculated the pedal force to hold heading on a B3e: about 30–45 lb with the compensator charged, about 161 lb with it depleted and the yaw servo switch off. Those are calculated figures, not limits — but they show why the hover is the worst place to discover it.

Sources 2, 4

In-depth

Boost across the variants

The yaw channel's protection depends on the aircraft. The B3e was first built without the yaw load compensator; after tail-rotor bearing failures, a 2013 modification required by EASA put it back. On dual-hydraulic aircraft, both circuits boost the main rotor but only one boosts the tail.

Know your aircraft's modification state and which circuit drives the pedals.

Sources 2, 5, 6

In-depth

Tail-rotor strikes

Tail rotors hit things: a bush in a confined area, the ground in a tail-low flare, a load or line. The signs are a thump, a new vibration — often felt in the pedals — and possibly a change in yaw control.

A tail rotor that has struck something may keep working for a while and then fail. The sound principle is to land and have it inspected, not to test it.

Sources 1, 7

In-depth

Not a failure: LTE

Loss of tail-rotor effectiveness is an aerodynamic yaw — a left yaw in the Astar — with the tail rotor intact. It happens at low speed and high power with the wind in certain sectors, and it can feel exactly like a failure.

The difference matters because the responses differ. A yaw that starts in a low, slow, high-power, downwind moment is LTE until proven otherwise; the LTE chapter covers it.

Sources 1, 8

Tail-rotor drive fails in the hover. Which way does an Astar yaw?

In-depth

Tail-rotor pitch is stuck. What controls yaw now?

Why do tail-rotor procedures favour run-on landings?

In-depth

Pitch is stuck with a lot of right pedal in. You reduce power to approach. Which way does the nose go?

In-depth

When is a tail-rotor drive failure most violent?

Sources

Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 11, Helicopter Emergencies and Hazards
Aircraft Accident Report AAR-17/01, Airbus AS350 B3e N390LG, Frisco, Colorado · NTSB · 3 July 2015
Clockwise-rotating helicopter differences · Helicopter Ground
Airworthiness Directive 2015-22-53 · FAA · 80 FR 74982, 1 December 2015 (superseded in 2018, 83 FR 2039)
Emergency Airworthiness Directive 2012-0217-E (tail rotor) · EASA
Eurocopter falls back on load compensator as fix for AS350 B3e · Vertical
Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 8, Ground Procedures and Flight Preparations
AC 90-95, Unanticipated Right Yaw in Helicopters · FAA · 26 December 1995

Educational only. Not approved training material, and never a substitute for the Rotorcraft Flight Manual, an instructor, or your operator's procedures. Figures marked as manufacturer data or test results are not limitations. This content is a draft and has not yet been reviewed by an instructor.