AS350 B3e course · Astar hazards
Servo transparency
When the controls push back
The essentials
The main servos can only hold so much force. When the rotor is loaded hard enough — high speed, high mass, high g, high density altitude, turbulence — the blade loads can exceed what the servos resist, and the controls move by themselves.
In the Astar the cyclic moves right and aft and the collective down: the aircraft pitches up and rolls right. It is called servo transparency or jack stall.
It is self-limiting if you let the load come off. Fighting it keeps the load on.
When the controls push back
The main servos can only hold so much force. Load the rotor hard enough — high speed, high collective, high weight, high g, high density altitude — and blade loads can exceed what the servos resist.
The controls then move on their own. It's called servo transparency, or jack stall.
Load against capacity
Blade loads against servo capacity. This diagram needs JavaScript.
Push the sliders and watch the right-hand servo reach its limit first.
Source 1
Why right and aft
On a clockwise rotor the right servo carries the most load. When it's overpowered, the pilot feels uncommanded right-and-aft cyclic and the collective going down; the aircraft pitches up and rolls right.
The conditions in detail
EASA's 2022 bulletin lists the factors: high airspeed, high collective pitch, high gross weight, high g and high density altitude. It adds that a rapid change in wind can bring it on suddenly. Canada's TSB notes it can happen even inside the approved envelope.
Each one raises the force the blades feed back into the controls. The servo's capacity doesn't change — the hydraulic system works at a fixed pressure, which Flight Safety Australia gives as just under 600 psi on the single system.
So the factors stack. A heavy aircraft, hot and high, pulling hard at speed has far less margin than the same pull light on a cold day.
Where it happens
Servo transparency needs high rotor loading. In practice that means a hard, fast manoeuvre in a heavy aircraft: a steep turn at high speed, a pull-up from a dive, an abrupt manoeuvre in turbulence, or aggressive flying at high density altitude.
Low-level utility, heli-ski and filming work, where pilots manoeuvre hard close to the ground, is where it's most dangerous — there's little height to let it pass.
How it feels — and what doesn't warn you
Pilots describe the controls going heavy and momentarily frozen. The cyclic is hard to move left while the aircraft rolls right and the nose comes up; the collective tries to go down.
The Swiss and Norwegian authorities both note there is no indicator or warning on the type. Nothing has failed — the system is working at its limit. With a gradual load increase it may start as vibration in the cyclic; with a sudden one, the controls just go stiff.
That's why the defence is knowing which manoeuvres put you there, not waiting for a cue.
Why it passes on its own
When the controls move, the collective comes down and the cyclic eases the manoeuvre. Both reduce the blade loads — so the servos regain control. Flight Safety Australia puts it at about two seconds; the TSB cites the manufacturer's two to three.
Pulling against the controls holds the load on the rotor, keeps the servos overpowered and prolongs the event. That's the reasoning behind the guidance to reduce collective and follow the controls.
When the servos take over again
There is a second trap at the end. A pilot holding hard left cyclic against the uncommanded roll is pushing against a stick that won't move. When the load comes off and hydraulic assistance returns, that force is suddenly unopposed — and the cyclic can jump left.
The Swiss regulator's guidance asks pilots to counter the right-cyclic tendency smoothly for exactly this reason. The aim is to come out of the event without starting a new one.
Source 5
Not retreating blade stall
Both happen at high speed and high load, and both pitch the Astar up and roll it right. The difference: in servo transparency the controls stiffen and move by themselves; in retreating blade stall the controls stay where you put them and vibration comes first. (Servo transparency can start with some cyclic vibration too, so the stick is the better clue.)
The responses overlap — reduce the load — but knowing which you have helps you stay calm.
Single and dual hydraulics
Servo transparency is a characteristic of the single hydraulic system. EASA's 2022 bulletin, which accompanied a flight-manual update, applies to all AS350 models except B3s fitted with the dual hydraulic system, and also to the AS355 E and EC120 B.
On the dual system both circuits boost the main-rotor servos. Whether your airframe has it is a question for its records, not its model name. Its RFM says what applies.
Fort McMurray, 2007
An AS350 BA carrying a pilot and four firefighters was cruising at about 1,500 ft above ground in Alberta. The pilot began a rapid, high-speed descent to just above the trees and lost control on levelling off. The helicopter rolled right, nosed down and struck marshy ground; one passenger died.
The TSB found servo transparency on the level-off, and that the pilot's initial reaction was not the correct recovery — with too little height left to correct it.
The pull-out at the bottom of a dive is a high-g, high-speed manoeuvre. Close to the ground there's no room for the seconds the event needs.
Source 6
Ullensvang, 2011
A heavily loaded AS350 B3 was descending towards a mountain cabin site in Norway on a hot day. Witnesses saw it turn tightly to the right; control appeared to be lost at a steep bank, and it struck the ground at high speed. All five on board died.
The investigation found no technical defects. It believed the hydraulic system may have reached its limit — servo transparency — noting that mass, speed, torque and density altitude all precipitate it.
Every factor was present at once. The hazard is the combination, and the right turn made it worse.
Source 7
Smithers, 2016
An AS350 FX2 left the bottom of a heli-ski run with seven on board and descended into a ravine at low altitude. Airspeed built quickly; the helicopter rolled right, pitched up, and struck a steep snow slope. The pilot reported difficulty moving the cyclic left. Everyone got out uninjured.
The TSB found servo transparency from a high-power descent near Vne, a 1.5–1.8 g pull and a mass near maximum. The aircraft regained assistance by itself — too low to avoid the slope.
Self-limiting doesn't mean harmless. It needs height, and low-level work rarely has it.
Source 4
Which combination makes servo transparency more likely?
Anything that loads the blades — speed, g, weight, collective, density altitude — raises the force the servos must hold.
In an AS350, the cyclic moves by itself toward…
Right and aft, with the collective going down: the right servo, most loaded on a clockwise rotor, is the one overpowered.
Why does servo transparency usually pass within seconds?
The collective drops and the manoeuvre eases, so the load falls below what the servos can hold.
What warns you that servo transparency is close?
Nothing has failed, and the type has no servo-load warning. Knowing the conditions is the only early warning.
Why is a hard right turn a particular trap?
The roll goes right, the same way as the turn — steepening it while the cyclic resists your recovery.
Sources
- Servo transparency · SKYbrary
- Jack be nimble · Flight Safety Australia · August 2024
- Safety Information Bulletin 2022-05, Rotorcraft Flight Manual Update – Servo Control Transparency · EASA · 24 June 2022, corrected 1 July 2022
- Aviation Investigation Report A16P0045 (AS350 FX2 C-FBLW, near Smithers) · Transportation Safety Board of Canada · 16 March 2016
- SAND-2014-001, AS350 Types: Hydraulic Transparency – Information and Training Recommendation · Swiss Federal Office of Civil Aviation (FOCA) · 2014
- Aviation Investigation Report A07W0138 (AS350 BA C-FHLF, near Fort McMurray) · Transportation Safety Board of Canada · 23 July 2007
- Report concerning air accident at Dalamot in Ullensvang, Hordaland county in Norway on 04.07.2011, LN-OXC (Eurocopter AS 350 B3) · Accident Investigation Board Norway (now NSIA) · Report SL 2012/13, 1 November 2012; summary and safety recommendation SL 2012/09T
- 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
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.