Hydraulics

Around the circuit

A gear-type pump, belt-driven from the main gearbox, draws fluid from a reservoir and delivers a constant flow. A regulator holds system pressure by opening and returning the excess to the reservoir; a filter keeps the fluid clean.

Alongside sit a low-pressure switch, which drives the cockpit warning, and solenoid valves, controlled from the cockpit, that dump pressure — how the system is tested and how hydraulics-off training is done.

Each main servo has its own accumulator. The tail servo has the yaw load compensator, with an accumulator of its own.

Sources 1, 4, 6

System figures

The yaw load compensator

The tail rotor has a single-cylinder servo. Behind it sits the yaw load compensator — a jack with its own accumulator that keeps assisting the pedals if hydraulic pressure to the tail is lost.

Without it, holding the tail rotor against its aerodynamic loads takes far more pedal force than a pilot expects; the Tail rotor chapter has the NTSB's test figures. With it depleted and the tail servo unpressurised, the pedals can feel jammed.

That combination — compensator empty, tail servo off — is exactly what a forgotten switch after a ground check produces.

Sources 3, 4, 9

The cockpit controls

Two controls matter.

• A hydraulic cut-off switch on the collective, which works the solenoid valve to depressurise the system — for the check, for training, and in some failures.
• On aircraft with a yaw load compensator, an accumulator test pushbutton (ACCU TST), which bleeds the compensator's accumulator so the check can prove it held a charge.

On dual-hydraulic aircraft the collective switch isolates the circuit that feeds the tail. Names, positions and guards vary by variant and modification: learn them on your aircraft.

Sources 3, 10, 11

What the checks prove

The hydraulic checks are a series of questions, each with a physical answer.

• Main accumulators: with pressure cut, do the servos stay boosted for a few control movements? Then each accumulator holds a charge and its non-return valve seals.
• Yaw load compensator: do the pedals stay light without system pressure, then go heavy when its accumulator is bled? Then the compensator works.
• Restoration: does everything come back when the switch is reset?

The checks create their own risk: a switch or button left in the test position. Whatever the checklist says, the last look before lift-off is at the configuration itself.

Sources 3, 4, 11

Dual hydraulics

The optional dual system has two circuits, each with its own pump. Both boost the main-rotor servos, so losing one leaves the main controls boosted. Only one circuit boosts the tail rotor — and the yaw load compensator backs that circuit up.

That asymmetry is the trap. On a single system, a cut-off switch left off makes every control heavy, which is hard to miss. On a dual system, the main controls feel normal and only the pedals are wrong — and you find out at lift-off, when torque demands right pedal.

Sources 3, 4, 11

After Frisco: the procedure

The FAA issued an emergency AD in October 2015, revised two days later as AD 2015-22-53, after two accidents and an incident on dual-hydraulic AS350 B3s. In each, the aircraft yawed anticlockwise — nose left, the way torque turns this aircraft — just after take-off. The pilots of the two non-fatal events said the pedals felt jammed.

The AD moved the yaw load compensator check to after landing, with the rotor stopped, and required the yaw servo switch to be confirmed on before take-off. A check you can't fly away from can't leave you unprotected.

Sources 9, 11

After Frisco: the hardware

Procedures rely on memory, so the hardware changed too.

• Mod 074622 gives each circuit its own amber caution and makes the HYD2 light flash whenever the yaw servo switch is off. It became standard on new B3e aircraft and can be retrofitted.
• EASA AD 2016-0220 required dual-hydraulic AS350 B3s to embody both mod 074622 and mod 074719, which also replaces the latching ACCU TST button with a spring-return one. It cited four occurrences in five years. A 2018 FAA AD required the flashing light and the spring-return button too.
• The NTSB recommended visual and aural alerts for loss of pedal boost on existing aircraft.

A flashing light in the panel is a configuration error, not a curiosity.

Sources 10, 11, 14, 15

Hobart: what the investigation found

The ATSB's report on the 2017 Hobart accident — an AS350 BA on a training flight practising a hydraulic failure — found that the helicopter was flown on a crosswind approach into a high hover without hydraulic assistance, and became uncontrollable.

It drew on Royal Australian Air Force findings that, hovering without hydraulics, the AS350 is subject to random perturbations and a loss of control authority. It also found that an intermittent fault in the hydraulic cut-off switch may have delayed restoring pressure, and that the lack of a pre-flight brief may have left the two pilots unclear about who had control.

Sources 2, 7, 16