AS350 B3e course · Emergencies
Hydraulic failure
Heavy controls, and the reason for the run-on landing
The essentials
A hydraulic failure shows as the red hydraulic warning, the gong, and — once the accumulators run down — heavy controls whose forces can change quickly.
The accumulators give a short time with boost: use it to reach the RFM's speed and set up. The procedure then trades the hover away for a shallow, into-wind run-on landing.
Never hover without hydraulics.
What the controls feel like
Without boost, the rotor's loads come straight back through the controls. The cyclic and collective are heavy, and the forces change size and even direction with speed and power.
In cruise it's hard work but manageable. At low speed, where precise small inputs matter most, it becomes very difficult — which is the whole reason for the procedure.
How it unfolds
First the warning and gong — while the controls still feel normal, because the accumulators are still boosting the main servos. That quiet interval is the trap and the gift: nothing feels wrong yet, and it is the best time you will have to get set up.
When the accumulators are exhausted, the forces arrive — possibly in the middle of an input. A pilot expecting them handles them; one surprised by them overcontrols.
Why not the hover
In the hover every correction is small and constant, and the unboosted forces vary with each one. The result is overcontrol: a pilot pushing against a force that changes as soon as the control moves.
Near the ground that becomes drift, oscillation and, at worst, a skid caught and a rollover. The run-on keeps the aircraft in steady forward flight, where the forces are steadier and smaller inputs suffice, all the way to the ground.
The run-on, in principle
The reason for the run-on landing is to avoid every phase the failure makes hard: no hover, no steep approach, no big power change at the bottom. A shallow, into-wind approach to a touchdown with forward speed does that.
It needs a long, smooth, firm surface. Choosing one — an airfield, a road, a hard field — is the real decision a hydraulic failure asks of you. The speeds and steps are in the RFM.
Hobart, 2017
During a flight with the hydraulic assistance switched off, an AS350 was brought to a hover in a crosswind — and it ended in an accident.
The Australian Transport Safety Bureau's message afterwards was blunt: don't hover without hydraulics.
The run-on landing isn't a nicety. The hover is exactly where unboosted, changing control forces are hardest to handle.
Dual hydraulics
On aircraft with the optional dual system, both circuits boost the main-rotor servos — so a single circuit failure leaves the main controls boosted. Only one circuit boosts the tail rotor.
So the consequences depend on which circuit fails, and the warnings and procedure are different from a single-system aircraft's. A pilot moving between the two has to know which one is under them.
Servo transparency is not a failure
Servo transparency — jack stall — also makes the controls feel heavy, but the hydraulic system is healthy: in a hard manoeuvre at speed and weight, the rotor's loads briefly exceed what the servos can deliver.
The cues differ: no hydraulic warning, a cause you can name (a hard turn or pull-up), and forces that ease as soon as the load comes off. The servo transparency chapter covers it.
Other hydraulic problems
Loss of pressure is the failure the warning light is built for. A control that binds, stiffens or jams with pressure normal is a different problem, and switching the hydraulics off is not a universal cure for it.
Read what your RFM says about control problems without a hydraulic warning — before you need to.
Training without hydraulics
Hydraulics-off training is done by cutting pressure with the cockpit switch. It's valuable, and it has its own risks: the training set-up must be briefed, the switch must be within reach, and the approach must be the run-on the procedure calls for.
Investigations have found training flights that ended in accidents because the exercise drifted from the procedure — into a hover.
With hydraulics off, where are the controls hardest to handle?
Low speed, where precise inputs matter most and the forces change quickly.
What's the accumulators' time for?
A short time with boost, to get established — not to land on.
Heavy controls in a hard turn, no hydraulic warning, easing as you roll out. What is it most likely to be?
Servo transparency: the system is healthy, the load is momentarily too high. Unload and it goes away.
On a dual-hydraulic aircraft, one circuit fails. What is most likely still boosted?
Both circuits boost the main rotor, so one survives. The tail is on one circuit only.
Sources
- Hovering without hydraulics · ATSB · 2020
- Investigation AO-2017-109 · ATSB
- AAIB report: AS350 B3e Ecureuil, G-MATH · UK AAIB
- Aérospatiale AS 350B Écureuil — in depth · Heli Archive
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 11, Helicopter Emergencies and Hazards
- Aircraft Accident Report AAR-17/01, Airbus AS350 B3e N390LG, Frisco, Colorado · NTSB · 3 July 2015
- Airworthiness Directive 2015-22-53 · FAA · 80 FR 74982, 1 December 2015 (superseded in 2018, 83 FR 2039)
- Servo transparency · SKYbrary
- Jack be nimble · Flight Safety Australia · August 2024
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 5, Rotorcraft Flight Manual
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.