AS350 B3e course · Power and systems
Transmission and freewheel
From turbine to rotor, and the freewheel between them
The essentials
The engine's reduction gearbox drives two ways: forward through a coupling shaft to the main gearbox, which slows the drive to rotor speed and turns it up the mast, and aft along the boom to the tail gearbox and tail rotor.
A freewheel connects the engine when it is driving and lets go when it isn't — the reason autorotation works. The tail rotor is on the rotor side of it.
The main gearbox has its own oil system, with warnings for pressure, temperature and metal particles. All of them are serious.
From turbine to rotor
The engine's power turbine drives into the main gearbox, which slows it to rotor speed through bevel and epicyclic reduction stages and turns the drive up the mast.
A long shaft down the tail boom takes power to the tail gearbox and the tail rotor.
Follow the power
Engine, freewheel, gearbox and rotors, with an engine-failure switch. This diagram needs JavaScript.
Watch the shafts turn — then fail the engine. The freewheel lets go, and the rotor keeps turning on the air coming up through it.
No clutch — a freewheel
Because the Arriel is a free-turbine engine, the rotor isn't locked to the part of the engine that keeps it running, so no clutch is needed to start.
A freewheel unit connects engine to rotor while the engine is driving, and lets go the moment it isn't. That's what makes autorotation possible.
The drive in brief
Engine to gearbox
The Arriel's front-mounted reduction gearbox has a hollow two-way output shaft: one end drives forward to the main gearbox, the other aft to the tail rotor.
The forward drive reaches the gearbox through a coupling shaft with flexible couplings at its ends — sometimes called flectors — that pass torque while absorbing small angular and axial misalignment.
They're needed because the engine and the gearbox don't sit rigidly together: the engine is mounted on the airframe, the gearbox on its own flexible mounting, and both move a little under load and heat.
The freewheel, up close
A freewheel is a one-way clutch. Rollers or sprags wedge between an inner and an outer race when the engine side tries to turn faster than the rotor side, and slip free when it doesn't. No pilot action, no switch.
What matters is what sits on each side. Everything on the rotor side keeps turning when the engine stops: the main rotor, the tail rotor, and the gearbox-driven accessories — including the hydraulic pump. That's why you keep yaw control and boosted controls in an autorotation.
Exactly where the unit is housed on your variant is in the maintenance documentation.
Inside the main gearbox
The gearbox reduces speed in two stages.
- A bevel stage takes the near-horizontal input and turns it through about 90°, reducing speed as it does.
- An epicyclic (planetary) stage finishes the job: the bevel drives a central sun gear, planet gears roll around a fixed outer ring, and the carrier that holds them turns the mast.
An epicyclic stage gives a large reduction in a small, light package, and shares the load across several planets. The mast and its bearings carry the rotor's lift and moments down into the gearbox housing.
How the gearbox is mounted
The main gearbox is held to the airframe by suspension bars, which carry the rotor's lift, and a flexible mounting at its base, which takes the torque and filters rotor vibration before it reaches the cabin.
The mounting and bars are on the walkaround. Their condition affects both safety and how smooth the aircraft feels. A change in the aircraft's vibration signature is worth reporting even when everything looks intact.
Gearbox oil and its warnings
The main gearbox carries its own oil, separate from the engine's. An internal pump draws it from the sump and sends it through a filter and a cooler to the gears and bearings; the cooler needs airflow from its fan to do its job.
The cockpit warns of low oil pressure and high oil temperature, and electrical chip detectors warn of metal particles. Oil level is checked on the sight glass during the walkaround.
A gearbox without oil does not last long. These warnings sit near the top of the list for urgency — the Emergencies part explains why.
Pressure versus temperature
The two gearbox warnings point at different problems.
- Low pressure means oil may not be reaching the gears and bearings: a pump or drive failure, a leak, or a sensor. Real loss of lubrication is time-critical.
- High temperature means heat isn't being removed: low oil, a blocked or damaged cooler, a failed fan, or friction from distress inside.
Either one is confirmed — or not — by the other indications: the gauge, a chip light, noise, vibration, a smell of hot oil. A warning with supporting signs is a gearbox in trouble. The urgency for each is set by the RFM.
Chip detectors and magnetic plugs
Both are magnets in the oil, and both catch steel particles shed by a wearing gear or bearing. The difference is who finds out.
- An electrical chip detector has an insulated gap at the magnet. Enough metal bridges it, completes a circuit and lights a caution in flight.
- A plain magnetic plug collects particles silently. Nobody knows until a mechanic pulls it.
Fine fuzz is normal wear; flakes and chips are not. Which gearboxes on your aircraft have which, and whether a light can be cleared, is a maintenance-documentation question — ask before you need the answer.
The tail drive
The tail drive is a series of shafts joined by flexible couplings, which let the shafts follow the boom as it bends and flexes. The long section along the boom runs in bearings mounted on the structure.
At the end, the tail gearbox turns the drive through 90° and reduces it for the tail rotor. It has its own oil, and its level is checked on the walkaround.
A failing bearing or coupling tends to announce itself as a new noise, a high-frequency vibration felt in the pedals or floor, or heat and discoloration found on the ground.
The hydraulic pump's drive
The hydraulic pump is driven by a belt from the main gearbox, not by the engine. It turns whenever the rotor turns — so boost remains in an autorotation.
The flip side: the belt is a single link. A broken or slipping belt is a hydraulic failure, however healthy the pump. Belt condition and tension are walkaround items.
The rotor brake
Where fitted, a rotor brake stops the rotor after shutdown so it doesn't windmill in the wind or keep turning while people approach. On legacy AS350s its lever is on the floor near the base of the collective, next to the fuel shut-off lever; later B3s and the EC130 have it overhead.
It has limits on the rotor speed at which it may be applied, set in the RFM. Applying it too early heats and wears it and loads the drive. Know which lever is which by feel and position, not by grabbing.
G-XCEL: when a freewheel slips
In December 2003 an AS355F1 — the twin-engined Écureuil — flew a test flight after a newly overhauled main and combining gearbox had been fitted. Witnesses heard unusual noises, then the tail boom folded forward around the cabin. All three on board were killed.
The UK AAIB found that the freewheels had slipped under load and then re-engaged violently, repeatedly, until the structure failed. The rollers came from a batch coated by an incorrect process. The cause couldn't be proven, but slippage stopped once that batch was withdrawn.
A freewheel is a simple part carrying the whole engine's torque. Bangs, jolts or rpm splits after drive-train work are reasons to land and investigate, not to keep flying.
Source 8
What does the freewheel unit do when the engine stops?
It disconnects the engine from the rotor, so a stopped engine doesn't stop the rotor — the basis of autorotation.
Why doesn't the AS350 need a clutch to start?
The free turbine isn't mechanically tied to the gas generator, so the engine can start and run with the rotor held back only by its own inertia.
What do the main gearbox's suspension bars carry?
The lift. The flexible mounting at the base takes the torque and filters vibration.
Why do you still have hydraulic boost in an autorotation?
The rotor keeps turning the gearbox, and the gearbox keeps turning the pump.
What does an electrical chip detector do that a plain magnetic plug doesn't?
Metal bridging its gap completes a circuit and lights a caution. A plain plug only collects particles for the next inspection.
Sources
- The Aerospatiale Ecureuil / AStar · Air Vectors (Greg Goebel)
- Aérospatiale AS 350B Écureuil — in depth · Heli Archive
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 4, Helicopter Components, Sections, and Systems
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 11, Helicopter Emergencies and Hazards
- Aircraft power plant having a connection device for connecting together a main gearbox and an engine (US 2015/0232187) · USPTO (Airbus Helicopters application) · Background (flexible couplings, suspension bars)
- AS-350 Hydraulic System (investigation docket document) · NTSB
- Aircraft Accident Report AAR-19/04, Airbus AS350 B2 N350LH, East River, New York · NTSB · 11 March 2018
- AAIB report: AS355F1, G-XCEL, 2 December 2003 · UK AAIB · AAIB Bulletin 7/2006
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.