AS350 B3e course · Power and systems
FADEC and engine control
The computer between your hand and the fuel
The essentials
FADEC — Full Authority Digital Engine Control — meters fuel to hold the free turbine, and so the rotor, at constant rpm as the load changes. It anticipates by reading collective position, protects engine limits, and runs the start.
The Arriel 2D's FADEC has two channels and an independent backup that takes over automatically if both fail.
Earlier B3s are different: the 2B had one channel with a manual backup through the twist grip; the 2B1 two channels with an automatic backup. Know which one you have.
Three generations on one airframe
- Arriel 2B (early B3): one digital channel. If it failed, the pilot controlled fuel manually through the twist grip.
- Arriel 2B1 (later B3): two digital channels and an automatic backup.
- Arriel 2D (B3e / H125): two digital channels, an independent backup channel that takes over automatically, and engine data recording.
Start procedures, twist-grip positions and failure handling differ between all three.
Holding the rotor at constant speed
Raise the collective and the rotor needs more power; without more fuel it would slow. The FADEC senses free-turbine speed and adds fuel to hold it.
It also reads collective position, so it can add fuel as you raise the lever rather than waiting for the rotor to droop. That anticipation is why rpm stays steady through large, quick power changes.
What the governor reads
- Nf — free-turbine speed, which is rotor speed through fixed gearing. The thing it is trying to hold.
- Collective position — the early warning of a power change.
- Ng and TOT — how hard the engine is already working, and how close it is to its limits.
From these it moves the fuel metering valve. Nothing here senses what the pilot meant — only what the levers and the engine are doing.
Limit protection, and its limits
A FADEC keeps the engine inside some limits as it governs — it will not, for example, keep adding fuel without regard to the engine's state. Turbomeca lists Ng and Nf speeds, turbine temperature and acceleration among what the Arriel 2 control protects, plus a torque limit set by the transmission. Exactly what, and what it leaves to you, depends on the engine and its software.
Do not treat it as a guarantee. Exceedances are still possible and the VEMD records them. Treat every limit the FLI shows as yours to respect.
The Arriel 2B: single channel, manual backup
The 2B's digital unit had one channel. Its backup was you: the twist grip is linked mechanically to the fuel metering, with ranges either side of a stop at the flight position.
On early engines the stop was disengaged by hand; from engine S/N 073084 an automatic stop was added. An AUTO/MANU selector exists only to simulate a governor failure for training — after a real failure, the grip controls fuel without moving the switch.
The 2B1: two channels, automatic backup
From engine S/N 073254 the B3's engine became the Arriel 2B1: a dual-channel governor and an automatic backup unit (the EBCAU), with a simplified twist grip.
Loss of one channel leaves the other governing. Loss of both hands fuel control to the backup without the pilot having to take over the fuel — but the aircraft is degraded. Eurocopter told AIN the backup loop is slower, so rapid collective movements are to be avoided; the RFM says how to fly and land it.
The 2D: two channels, independent backup
The B3e's Arriel 2D has a new-generation dual-channel FADEC and a backup control system that automatically controls the engine if both digital channels fail.
The principle is the same as the 2B1's: redundancy first, then an automatic backup, so a governor failure does not depend on the pilot's hand on the fuel.
What "manual" means
On aircraft with a manual backup, a governor failure hands fuel control to you. You match fuel to collective with the twist grip, watching rotor rpm, and every collective movement needs a matching grip movement.
It is a skill that fades without practice, and it is the reason training on those aircraft includes simulated governor failures. On the 2D the backup is automatic — but the pilot still has to recognise the failure and land as the RFM requires.
Twist grip and switches
On the B3 the twist grip has IDLE and FLIGHT detents. On a 2B it sits in FLIGHT for the start and a three-position ceiling selector selects flight; on a 2B1 it starts in IDLE, and the grip itself is turned to FLIGHT. On the H125 Airbus describes the grip as being for engine reduction after a tail-rotor failure and for autorotation training — your RFM has the detail.
The 2B1 has an approved autorotation-training supplement that sets the engine to idle with the grip. Airbus has stressed that after governor-failure training the selector and grip go back to normal.
Failure warnings, in principle
Engine-control failures come in grades:
- Loss of redundancy — one channel lost, the engine still governed normally. A caution: the next failure is the one that matters.
- Loss of normal governing — the backup (or, on the 2B, the pilot) is controlling fuel. A warning, with a degraded aircraft.
The warning-panel names and what each requires are in the RFM. In flight the first sign is often rotor rpm doing something you didn't ask for.
The grip left in the wrong place
AIN reported in 2006 that the first B3 engine-control version had suffered main-rotor overspeeds when autorotation training followed simulated governor failures. The simulation required moving the grip off its flight stop to set fuel by hand, and trainees or instructors sometimes forgot to return it.
The automatic stop added from engine S/N 073084 addresses exactly that. In 2016 Airbus Information Notice 2976-I-76 again reported overspeeds on 2B-engined B3s, naming non-compliance with flight-manual procedures as one possible cause.
Training configurations are a hazard of their own. Restoring the normal configuration is part of the exercise.
Tabusintac, 2004
An RCMP AS350 B3 with an Arriel 2B was in a turn when the alarm sounded and the red GOV light came on — a control-unit fault had frozen the fuel flow. Heading for a precautionary landing, rotor rpm rose past the maximum with severe vibration.
The pilot did not realise the twist grip still controlled the fuel, and returning it to the FLIGHT detent made things worse. He had not had in-flight governor-failure training. The VEMD logged the overspeed at its highest capturable value; all three aboard were unhurt.
After a governor failure the grip is the fuel control: small inputs, matched to the collective, watching rotor rpm. That is trained in the air, not learned on the day.
Source 6
Pampilhosa da Serra, 2019
A firefighting AS350 B3 in Portugal rolled over on take-off. The operator flew a mixed fleet of Arriel 2B and 2B1 aircraft, which differ in how flight mode is selected. This one had the 2B; the pilot had little experience on the 2B and more on the 2B1.
Investigators concluded he turned the twist grip as on a 2B1, which on the 2B engaged manual fuel control and drove the engine up. The aircraft yawed, lifted about a metre, struck its tail rotor and rolled onto its side. The automatic-stop modification was not fitted.
Mixed fleets need variant-specific training and checklists. "It's a B3" doesn't tell you which engine control you have.
Source 3
How does the FADEC anticipate a power demand?
From the collective. It adds fuel as the lever comes up, instead of waiting for the rpm to fall.
On the Arriel 2D, what happens if both digital channels fail?
An independent backup takes over automatically. Manual twist-grip control belongs to the earlier Arriel 2B installation.
Which B3 engine had a single-channel governor with a manual backup?
The 2B. The 2B1 and 2D have dual channels with an automatic backup.
On a 2B-engined B3, what is the AUTO/MANU selector for?
Training. The TSB noted that after a real failure the twist grip controls fuel without moving the switch.
What hazard follows a simulated governor-failure exercise?
Overspeeds have followed training sessions where the configuration wasn't restored.
Sources
- H125 Technical Description 2025 (baseline aircraft definition) · Airbus Helicopters · 125 B3 25.100.04 E / 25.101.01 E — General, Cockpit/Cabin, Power plant, Transmission
- Type-Certificate Data Sheet E.001, Arriel 2 series engines · EASA · Issue 14, 8 July 2026
- AS350B3 rolls — engine controls · Aerossurance
- Eurocopter addresses AS 350B3 engine-control failure recovery · AIN · 30 October 2006
- Arriel 2B/2B1 turboshaft engine — training manual · Turbomeca · Edition March 2003 (hosted by a flight school; facts cited only)
- Aviation Investigation Report A04A0050 (AS350 B3) · Transportation Safety Board of Canada
- Arriel 2D — the new standard from Turbomeca · Helicopter Maintenance Magazine
- Information Notice 2976-I-76 · Airbus Helicopters · Revision 0, 23 February 2016
- AS 350 B3 Arriel 2B1 Flight Manual Supplement 9-6 — Autorotation Training Procedure (investigation docket copy) · NTSB · Revision 3 (EASA approved 2 July 2010)
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 11, Helicopter Emergencies and Hazards
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.