You would have thought that the high loads on all three controls would have been a clue.
Not sure how you could mistake hydraulic failure with TR failure I agree! As Crab says surely the control loads would be obvious.
Think about this one.
HYD Pump failure will remove assistance from the TR servo immediately yet the accumulators on the MR servo's would result in no immediate change.
Of course you would have a HYD caption and HORN blaring. If you didn't notice the caption and were a bit slow on the uptake as to what is going on in
the first instance you could suspect a TR control failure as the pedals would be pretty stiff and thats all. (AS 350 B & BA sans TR accumulator)
Continuing on the MR accumulators can deplete rapidly and asymmetrically which makes for more fun and if you are slow as was the case here very
interesting especially if you have further effect of yaw rate from stiff pedals.
On the 350 if you have a HYD failure you need to get the aircraft sorted, slowed down if necessary and then turn the HYD OFF on the collective. This will dump the
accumulators and silence the HORN.
Some people get confused with the use and purpose of the HYD TEST switch. Reason being it is the only one they actually use regularly during pre-takeoff checks
and they assume it is the HYD OFF which it isn't. Correct use of the HYD TEST is imperative.
To all intents and purposes to be sure the HYD TEST switch should never be used in flight operationally. It has a training function in flight but that needs careful consideration.
extract form EASA EXPERT DEPARTMENT / CERTIFICATION DIRECTORATE - OEB Report -
Simulated Hydraulic failure
(Except EC 130 B4 & T2 and AS 350 B3 equipped with the optional dual hydraulic system):
- In steady flight conditions, simulate the hydraulic failure by depressing HYD TEST push button on the System Control Unit (Honeywell or SCU console panel):
HYD + Gong sounds while the student adjusts speed to obtain between 40 and 60 Kt.
CAUTION: The Instructor must ensure that the trainee adjusts the speed and attitude prior to isolating the hydraulics.
- Once safety speed is set, and prior to activating the hydraulic isolation switch reset HYD TEST pushbutton to restore hydraulic pressure in tail rotor accumulator (except for AS 350 B3e, B and BA).
- If necessary during the training exercise, hydraulic assistance can be recovered immediately by setting the HYD TEST push button to the UP position or by resetting the hydraulic cut off switch to ON.
- If the HYD TEST pushbutton is not reset on the control panel, no hydraulic assistance can be restored. Before hydraulic isolation with the switch on the collective lever, do not forget to reset the HYD pushbutton on the console.
- Do not let the student attempt hover flight or low speed maneuver, as the intensity and direction of the control feedback force will change rapidly. This will result in a loss of control
On previous versions of AS350 equipped with a HONEYWELL console control, do not silence the HORN by using the HORN switch.
The HORN will be silenced when the pilot selects the hydraulic cut-off switch to off.
If the pilot uses the HORN switch to silence the HORN before using the hydraulic cut-off switch, this crucial step could be forgotten. This could then result in significant unbalanced lateral cyclic feedback forces, especially at low speed, if one of the lateral accumulators depletes before the other one.
In addition, de-activating the HORN using the HORN switch, makes it unavailable to warn the pilot of low or high rotor RPM.
Pay attention to the following:
- Hydraulic accumulators gives energy during approximately 20 seconds, in proportion of controls movements, so reduce to safety speed in this time frame,
- Anticipate to perform a shallow approach,
- Perform a running landing,
- Hover flight or any low speed maneuver must be avoided,
- Keep in mind that higher All Up Weight increase the risk of aircraft loss of control at low speed,
- The statistics show that failure to strictly comply with the procedure consequently increases the risk level.
Notes:
Left hand collective lever is not equipped with “HYD” switch,
- To be well prepared, brief your Trainee for setting the collective lever HYD switch to on, if necessary.
- If the Instructor decides to take over the controls, he must plan to continue the flight up to the landing
without the hydraulic assistance.
- CAUTION: when hydraulic pressure is restored in flight, the forces disappear which can lead to an abrupt left roll movement.
- Anticipate the power application to avoid induced increase in nose-up attitude.