AS350 B3e course · Power and systems
The Arriel 2D
Two turbines, two shafts, one computer
The essentials
The B3e's Safran Arriel 2D is a free-turbine turboshaft. The gas generator — compressor, combustion chamber and its own turbine — keeps the engine running. A separate free turbine takes energy from the hot gas and drives the rotors.
Three numbers describe it: Ng (gas-generator speed), TOT (turbine temperature) and torque. Whichever is closest to its limit is the one that limits you, and the First Limit Indicator shows it.
A FADEC meters the fuel to hold rotor rpm. B2s and earlier B3s have different engines and engine controls — know which one you are in.
The Arriel 2D
A Safran free-turbine turboshaft. The gas generator — compressor, combustion chamber and the turbine that drives the compressor — keeps the engine running.
A separate free turbine, on its own shaft, takes energy from the hot gas and turns the gearbox.
Inside the engine
Air flowing through a free-turbine turboshaft. This diagram needs JavaScript.
Air in, fuel burnt, two turbines turning at their own speeds. Switch to take-off power and watch everything speed up.
Source 3
The numbers
| Take-off power, published | 952 shp / 710 kW (Airbus and Safran figure)4 |
|---|---|
| Max continuous, published | 856 shp / 638 kW (Safran figure)1 |
| Certified minimum take-off power | 598 kW (EASA TCDS E.001)5 |
| In the B3e | Installed limits are set by the type certificate and the RFM, not by these figures6 |
| Control | Dual-channel FADEC with an independent automatic backup7 |
FADEC
Full Authority Digital Engine Control: a computer meters the fuel to hold rotor rpm as you move the collective.
The Arriel 2D's has two channels, plus an independent automatic backup.
Source 2
Five modules
The Arriel 2 is built as five modules that can be changed separately:
- Module 1 — the accessory gearbox and the output drive, with the freewheel.
- Module 2 — the axial compressor.
- Module 3 — the gas generator: centrifugal compressor, combustion chamber and the turbine that drives both compressors.
- Module 4 — the free (power) turbine.
- Module 5 — the reduction gearbox.
The point is maintenance: a damaged or time-expired module is swapped rather than the whole engine going to the shop. On the 2D, Safran says modules 1, 2, 4 and 5 have no calendar limit, and module 3 — the hot section — is restored at periodic visits (Safran figure).
Following the air
Air enters the intake and is compressed in two steps: one axial stage, then one centrifugal stage. It burns with fuel in an annular combustion chamber.
The hot gas first drives the single-stage gas-generator turbine, which turns the compressors. What is left drives the single-stage free turbine, then leaves through the exhaust.
The free turbine drives the reduction gearbox at the back of the engine. An external transmission shaft, in a protective tube, carries the power forward to the front of the engine, through the freewheel, to the output that drives the main gearbox.
The thermodynamics, briefly
A turboshaft does three things to the air: squeeze it, heat it by burning fuel, then let it expand through turbines. Expansion of hot gas yields more work than compressing cold air costs; the difference is your shaft power.
The gas-generator turbine takes only what the compressors need. Everything left over goes to the free turbine — so more fuel means hotter, faster gas, more Ng and more power to the rotor.
Hotter or thinner air means less mass through the engine for the same Ng, so the engine has to run hotter for the same power. That is why TOT is the limit that matters hot and high.
Ng, TOT and torque
- Ng — gas-generator speed. How hard the core of the engine is working.
- TOT — turbine temperature. The hot section's life depends on it, and it rises with outside temperature and altitude.
- Torque — the twisting force delivered to the gearbox, as a percentage. Mostly a transmission limit.
The free turbine's speed, Nf, is held constant by the FADEC, so it is the rotor rpm you see rather than a power setting.
Arriel 2 speeds
Ratings
An engine rating is a power level with a time attached:
- Take-off — the highest power, time-limited. The Arriel 2 type certificate gives it five minutes; the 2D also has a 30-minute take-off rating. The installed limits in your aircraft are in the RFM.
- Maximum continuous — usable for as long as you need it.
- Transient limits allow brief excursions above take-off values for a few seconds.
Twin-engine helicopters add one-engine-inoperative ratings. The AS350 is single-engine, so there are none — the take-off rating is the most you have.
Why the limit changes with altitude
The Arriel 2D can make more power than the main gearbox is allowed to take. On a cool day at low altitude, torque is usually the first limit — a gearbox limit, not an engine one.
As altitude and temperature rise, the engine has to work harder for the same torque, and TOT or Ng reaches its limit first. That spare engine capacity at low level is why the Astar holds its performance so well up high.
Keeping the intake clean
Sand, dust and debris erode compressor blades, and an eroded compressor needs more TOT for the same power. The H125 comes with protective grids on the air intake; barrier and sand filters are added equipment.
Added inlet protection comes with an RFM supplement. A filter can cost power or change the performance figures — the VEMD's power check even accounts for whether a sand filter is fitted — and it may have a clogging or bypass indication you need to understand first.
Three engines, one airframe
- Arriel 1D1 (B2): the earlier Arriel 1 family — axial plus centrifugal compressor, a two-stage gas-generator turbine, a hydromechanical fuel control.
- Arriel 2B / 2B1 (B3): the Arriel 2 core with a single-stage gas-generator turbine; a digital governor — single-channel with a manual backup on the 2B, dual-channel with an automatic backup on the 2B1.
- Arriel 2D (B3e / H125): a new axial compressor, a revised compressor diffuser, new gas-generator turbine blade material and a new dual-channel FADEC.
Same airframe, three ways of starting, governing and failing.
The 2D remembers
The Arriel 2D carries an engine data recorder: it holds the engine's configuration data and records engine parameters, which maintenance can download.
It also has a creep counter that estimates, in real time, how much life the gas-generator turbine blades have used — those blades stretch slowly under heat and load.
None of this changes how you fly it. It does mean an exceedance you didn't notice is very likely recorded anyway.
Same family, different engine controls
AS350 B3s have been built with three generations of engine control: the Arriel 2B's single-channel governor with a manual twist-grip backup; the 2B1's dual channel with an automatic backup; and the 2D's dual-channel FADEC with its automatic backup.
The start procedures differ — twist-grip positions, mode selectors — and pilots moving between versions have been caught out, including in a rollover accident.
'I know the AS350' isn't enough. Know which engine control is in THIS one, and fly its RFM.
The VEMD and the FLI
The Vehicle and Engine Multifunction Display shows engine and aircraft parameters. Its First Limitation Indicator puts three engine limits — torque, turbine temperature and gas-generator speed — on one needle, showing whichever you'll reach first.
Source 17
In a free-turbine engine, what drives the compressor?
The gas-generator turbine shares a shaft with the compressor. The free turbine is on its own shaft and only drives the gearbox.
The First Limitation Indicator shows…
Torque, turbine temperature and gas-generator speed on one needle — the first limit you'd hit is the one it shows.
On a cold day at sea level, which limit are you most likely to reach first at high power?
Torque. Low and cold, the engine has power to spare and the gearbox limit comes first. Hot and high, TOT or Ng take over.
On the Arriel 2, how does the free turbine's power reach the main gearbox?
The reduction gearbox sits behind the free turbine; an external shaft takes the power forward, through the freewheel, to the output at the front.
Which Astar engine has a hydromechanical fuel control rather than a digital one?
The Arriel 1D1 in the B2. The Arriel 2 engines all have a digital governor.
Sources
- Arriel 2D — Airbus H125 engine · Safran Helicopter Engines
- AS350B3 rolls — engine controls · Aerossurance
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 4, Helicopter Components, Sections, and Systems
- H125 technical information · Airbus Helicopters
- Type-Certificate Data Sheet E.001, Arriel 2 series engines · EASA · Issue 14, 8 July 2026
- Type-Certificate Data Sheet R.008, AS 350 / EC 130 · EASA
- 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
- Arriel 2B/2B1 turboshaft engine — training manual · Turbomeca · Edition March 2003 (hosted by a flight school; facts cited only)
- Airbus and Safran roll out major competitiveness boost to H125 and H130 · Safran · 27 June 2018
- Type Acceptance Report TAR 12/21B/2, Turbomeca Arriel 2 · Civil Aviation Authority of New Zealand · Revision 1
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 7, Helicopter Performance
- Aviation Investigation Report A04A0050 (AS350 B3) · Transportation Safety Board of Canada
- AS350 B3 / EC130 B4 VEMD training · Eurocopter
- Type-Certificate Data Sheet E.073, Arriel 1 series engines · EASA · Issue 05, 20 February 2025
- Arriel 2D — the new standard from Turbomeca · Helicopter Maintenance Magazine
- Information Notice 2976-I-76 · Airbus Helicopters · Revision 0, 23 February 2016
- The greatest guide: Airbus AS350 helicopter · Winair
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.