Transmission
The main gearbox is attached to the engine with a coupling shaft that has a hydraulic pump drive pulley, through which the shaft drives the MGB input coupling. The shaft and flexible coupling turn inside a flared coupling tube. The gearbox has three interchangeable modules – an epicyclic reduction gear module with five planet gears, a bevel reduction gear module in two casings, and a lubrication module attached to the lower casing. There is a pressurised lubrication system, with the bottom of the casing used as an oil tank.
The transmission cannot move vertically, so forces applied to it will be transmitted to the fuselage, or go back up to the head.
There is a two-piece drive shaft for the tail rotor – the second shaft is supported by five ball bearing assemblies.
The tail rotor gearbox is splash-lubricated.
The B1 and B2 have higher pedal force, hence the hydraulic accumulator on the tail rotors.

Hydraulics
The fluid must comply with AIR 3520 or Mil-H-83282. The total volume is 3 litres. The rated operating pressure is 40 bars, and is the same for all machines, regardless of gross weight. There is an accumulator system for each jack to provide acceptable loading if the hydraulics fail, rather than providing a second system which would add weight and cost. The AS 350 and AS 355 (and Gazelle) suffer from a condition known as “servo transparency”, when the jacks may stall under certain operating conditions and leave you with frozen cyclic and collective controls. The conditions are supposed to be extreme, but have also been relatively benign, as when gently flying round a fire at 4,000 feet ASL on a warm day. In another example, the machine was descending from a mountain top at 110 kts with the collective fully down and well below gross weight. Windshear took the speed up to 155 kts and it snapped 90° to the right. The cyclic froze solid and both arms could not bring it back. Although a nippy machine, the AS 350 should be flown more as a medium, that is, gently.
When the hydraulics fail, there is greater force as speed is increased, as the servos are directly connected from the main rotors to the top of the transmission casing. The accumulators are actually there to allow you to slow down to an acceptable speed before the feedback comes in. It is not a good practice to try and land on accumulator pressure alone – for one thing, they don’t discharge at the same rate, and, for another, they only work if you have fluid in the system, and you have no way of knowing what you have in there until you land – this is the reason for the run-on landing on any machine. You might easily find yourself in the hover with only fore and aft control, for example. With regard to different discharge rates, the daily accumulator check should show them even all round.
You could find Dunlop or Samm servos. Dunlops allow better control fore and aft if the hydraulics fail, and leak a lot less.