AS350 B3e course · Rotors and flight controls
Tail rotor and yaw
Holding the nose straight — and when it can't
The essentials
The tail rotor answers the main rotor's torque. On the Astar that torque swings the nose left, so right pedal comes in with power.
It is a two-bladed rotor driven by a shaft along the boom and a tail gearbox. Its pitch is hydraulically boosted, and a yaw load compensator helps hold the pedals.
Its two famous failure modes are a loss of effectiveness at low speed (LTE, a left yaw on this aircraft) and the loss of pedal boost — the Frisco accident.
The tail's one job
Torque from a clockwise main rotor tries to spin the fuselage anticlockwise — nose left. The tail rotor pushes the tail back the other way, and your pedals set how hard.
On the AS350 it's a two-bladed rotor about 1.86 m across.
How it's built
The tail rotor is a teetering spar-and-paddle design: one composite spar runs through both blades. The original AS350 B pivoted it on a self-lubricated plastic bearing; later aircraft use laminated elastomeric half-bearings — four of them — which wear and are inspected and replaced.
Alternatives exist. In 2025 Van Horn Aviation gained an FAA STC for a replacement with two individually replaceable composite blades on a steel hub with self-lubricating spherical bearings, aimed at the half-bearing maintenance. Know which one is on the aircraft you fly.
How it's driven
A two-section drive shaft runs along the top of the tail boom, supported on bearings, to the tail gearbox, which turns the drive through 90° to the tail rotor.
The tail rotor is driven from the rotor side of the freewheel. When the engine stops, the main rotor keeps the tail rotor turning — so you still have yaw control in autorotation.
The drive on the walkaround
The tail shaft is made of sections joined by flexible couplings, which take up the boom's bending and small misalignments, and held by bearings along the boom. The tail gearbox has its own oil.
Things worth a look: coupling condition, bearing mounts, the gearbox oil level and any leak, and the tail rotor's freedom to turn and teeter. Movement that feels gritty, or play that's new, is a report. The full drive chain is in Part 3.
Pitch change
The pedals move a linkage to a single tail-rotor servo, which drives the pitch-change mechanism at the tail rotor. Right pedal increases tail-rotor pitch and thrust; left pedal reduces it.
On dual-hydraulic aircraft only one circuit — the lower one — powers the tail servo and the yaw load compensator. Dual hydraulics doubles the main rotor's boost, not the tail's.
Translating tendency
Tail-rotor thrust doesn't just turn the aircraft — it pushes the whole helicopter sideways. On the Astar the tail rotor pushes left, so in the hover the aircraft drifts left unless held with a little right cyclic.
It's the mirror image of the American types the handbook describes, and why the hover attitude and lift-off feel different if you're converting.
Pedals in cruise
As speed builds, the fin takes over much of the anti-torque work, and less tail-rotor thrust is needed. Pedal positions in cruise are closer to neutral than in the hover.
It also explains why airspeed is your friend after a tail-rotor failure: near cruise speed, the handbook says, a fin may give enough directional control, depending on the design.
LTE, mirrored
Loss of tail-rotor effectiveness is an unanticipated yaw at low speed — the FAA says any single-main-rotor helicopter can meet it below about 30 kt.
In American helicopters it's a right yaw. In the AS350 it goes left, the way torque turns the fuselage.
Source 3
Where the wind hurts
Wind azimuths where loss of tail-rotor effectiveness is more likely. This diagram needs JavaScript.
Drag the wind arrow round the aircraft and see which sectors make the tail rotor's job harder.
The yaw load compensator
On the yaw channel a load compensator, with its own accumulator, keeps assisting the pedals if tail-rotor hydraulic pressure is lost.
In tests after the Frisco accident, holding heading took about 30–45 lb of pedal with the compensator's help — and about 161 lb with it depleted. (Test figures, not flight-manual limits.)
Why the pedals get heavy
A tail-rotor blade, like any spinning blade, tends to twist back towards flat pitch. On the Astar that return-to-flat load is large — Vertical linked it to the bigger tail-rotor blade area from the B1 onward — and the pedals feel it the moment boost is gone.
The yaw load compensator acts on the tail servo's output. Its accumulator holds a nitrogen-charged bladder at about 15 bar (218 psi, in the NTSB's figure). In the Frisco tests it cut the pedal force to hold heading from about 161 lb to 30–45 lb.
Frisco, 2015
On 3 July 2015 an AS350 B3e with dual hydraulics lifted off at Frisco, Colorado, after a pre-flight hydraulic check had left the tail-rotor circuit without pressure. Nothing in the cockpit made that obvious. Without boost the pedals were too heavy to hold, the aircraft spun on lift-off, and the pilot was killed.
FAA AD 2015-22-53 then moved the yaw load compensator check out of the pre-flight sequence and required the yaw switch on before take-off; a later modification added a flashing caution when it is off.
A check that can leave a switch in the wrong place is a trap. Before lift-off, look at the configuration — not at what the checklist said a minute ago.
G-ECUK, 2013
An AS350 B3e was decelerating to the hover at a private landing site near Oxford when the pilot lost yaw control. The helicopter was substantially damaged and the pilot was seriously injured.
Hydraulic fluid had leaked from two unions on the pipes to the tail-rotor servo, until the system lost pressure. The pilot's detailed pre-flight check had found no fluid on the skins or the hangar floor.
Leaks don't always leave puddles. A loss of yaw boost arrives at the worst moment — slowing into the hover, with high power and little fin — so know what heavy pedals feel like before you meet them.
Source 16
The B3e's tail-rotor bearings
The B3e was designed without the yaw load compensator, using tail-rotor balance weights instead. Tail-rotor laminated half-bearings then failed — three in-flight vibration events and one accident.
EASA imposed a 100 kt never-exceed speed by emergency AD in 2012, and in 2013 required a modification that removed the extra weights and put the yaw load compensator back.
Design changes have second-order effects — and when an AD limits your aircraft, it's usually because something already happened to someone else's.
The bearing AD, in context
The half-bearing failures applied to B3s built with modification 07 5601 — the B3e configuration. EASA's emergency ADs of October 2012 reduced Vne to cut the dynamic loads on the tail rotor, added placards and RFM changes, and required bearing inspections; the FAA followed within days. In 2013 EASA made modification 07 5606, which restored the earlier tail-rotor load levels, the terminating action.
For a pilot the lesson is variant precision: this was a B3e problem, not an Astar-wide one, and it's why the B3e's yaw load compensator came back.
In an AS350, an unanticipated yaw from LTE goes…
Left — the direction torque turns the fuselage of a clockwise rotor. It's the mirror of the 'right yaw' in FAA material.
Which pedal comes in as you raise the collective?
Right. More power, more torque, nose wants left — right pedal adds tail-rotor thrust to hold it.
In the hover, which way does an Astar drift if you don't correct it?
Left. Tail-rotor thrust pushes the whole aircraft left; a little right cyclic holds it.
The engine fails. Does the tail rotor keep turning?
Yes. The main rotor drives the tail rotor through the gearbox, so you keep yaw control in autorotation.
On a dual-hydraulic Astar, which circuit powers the tail servo?
Only the lower circuit, which also feeds the yaw load compensator. Dual hydraulics doubles the main-rotor boost only.
After a loss of yaw boost the pedals feel very heavy. What's most likely?
The tail rotor still works — the load is reaching your feet. Airbus, quoted by the NTSB, warns this can feel like a jammed control.
Sources
- Clockwise-rotating helicopter differences · Helicopter Ground
- Aircraft Accident Report AAR-17/01, Airbus AS350 B3e N390LG, Frisco, Colorado · NTSB · 3 July 2015
- AC 90-95, Unanticipated Right Yaw in Helicopters · FAA · 26 December 1995
- Aérospatiale AS 350 AStar · This Day in Aviation
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 4, Helicopter Components, Sections, and Systems
- Van Horn Aviation tail rotor for Airbus AS350/H125 gains FAA STC · HeliHub · 26 February 2025
- Aérospatiale AS 350B Écureuil — in depth · Heli Archive
- Airworthiness Directives; Eurocopter France Helicopters — AD 2012-25-04 (FR Doc. 2013-09420) · FAA / Federal Register · 24 April 2013; Discussion
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 8, Ground Procedures and Flight Preparations
- Lessons Learned: Airbus AS350 B3e (Frisco, 2015) · FAA
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 2, Aerodynamics of Flight
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 11, Helicopter Emergencies and Hazards
- Eurocopter falls back on load compensator as fix for AS350 B3e · Vertical
- NTSB final report CEN14FA193 (AS350 B3e N395P, Albuquerque, loss of yaw control) · NTSB (hosted by FAA) · 9 April 2014, pp. 4–5
- Airworthiness Directive 2015-22-53 · FAA · 80 FR 74982, 1 December 2015 (superseded in 2018, 83 FR 2039)
- AAIB Bulletin 3/2014, EW/G2013/06/17, AS350 B3e G-ECUK · UK Air Accidents Investigation Branch · 19 June 2013 (bulletin p. 42)
- Emergency Airworthiness Directive 2012-0217-E (tail rotor) · EASA
- Airworthiness Directive 2013-0029 (AS350 B3 with MOD 07 5601, tail-rotor laminated half-bearings) · EASA · 8 February 2013; effective 1 March 2013
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.