Hydraulic Systems Flashcards
Draw the hydraulic system
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How many and what are the different hydraulic systems?
4 First stage Second stage Wheel brakes Rotor brakes
What is unique about the first stage system?
It contains a low level switch and a tail rotor shut off valve. Combined they will maintain fluid for the main rotor servos in the case of a leak in both systems in the tail section.
What is unique about the second stage system?
It contains a priority valve which connects the utility system, landing gear and pedal trim/yaw damper, prior to the shut off valve.
How many pressure switches are in each stage?
5 in each
One after the shut off valve and one on each of the four servos to sense a jam.
What is contained in each of the identical hydraulic modules?
Pressurized reservoir Pressure switch Solenoid operated shutoff valve Pressure transmitter Pressure and return filters w/ LRBs Reservoir relief valve Fill and pump case check valves
What does the priority valve do and which hydraulic system is it in?
The priority valve restricts flow to the utility system depending on system demand giving priority to the flight control servos.
It is in the second stage.
What does the low level switch do?
Senses fluid loss in the first stage and isolates the tail rotor servo and lines aft of the tail rotor shut off valve. This is done to preserve fluid for the main rotor servos in the event of dual hydraulic system leaks in the tail section.
Describe the hydraulic pumps.
Two constant pressure, variable delivery, main gearbox driven pumps.
3000psi constant pressure
Full pressure available about 10% Nr
Pump case lines return fluid to reservoir for lubrication and cooling.
How many main rotor servos are there?
3
Forward, aft and lateral
How many pressure switches are on each servo assembly?
2, one for each stage
What is a pilot valve and what do they do?
The pilot valve converts mechanical control inputs into hydraulic pressure to act on the servos.
They automatically bypass if jammed and release hydraulic pressure so the remaining stage can move the servo.
What do the hydraulic pressure switches do?
Switches on servos illuminate the associated Servo light when pressure drops below 1600 psi, the light extinguishes when pressure returns to 2100 psi.
They also disable the shutoff valve of the opposite system when pressure is below 1600 psi.
When is the priority valve fully open and closed?
Fully closed at 2250 psi and below.
Fully open when pressure returns to 2700 psi.
What is the target airspeed in the servo problem emergency checklist?
125 or less
What is the target airspeed for the fixed pitch emergency procedure?
80 kts, which also complies with the unsafe gear EP.
What bus are the hydraulic shutoff switches on? What happens if power to that bus is lost?
DC Essential Bus
Servo systems are fail safe on, so without the DC essential bus the pilot cannot turn them off.
What pressures correspond to the hydraulic pressure display?
Normal (green) is 2800-3200 psi
Max pressure (red) is 3300 psi
Minimum pressure (red) is less than 2700
Which hydraulic filter can bypass?
The return, at 100 +/- 10 psid and the LRB will pop.
The pressure side does not bypass to prevent servo contamination. The LRB will pop at 70 +/- 10 psid
How is a servo jam indication cleared?
Since they are magnetically latched, the circuit breaker must be reset.
They are also connected to the landing gear so upon landing the light will extinguish if the jam has cleared.
What type of landing gear does the S-76 have?
Retractable
Tricycle type
Non-steerable nose wheel
Describe the landing gear retract system.
Electronically controlled and hydraulically actuated.
Emergency pneumatic extension
What bus is the landing gear system on?
DC Essential bus
When is the landing gear up warning system activated?
Below approximately 60 kts or when the radar altitude is lower than the highest radio altimeter decision height and the gear is up.
Which side of the helicopter is the weight on wheels sensor located?
Left, PNF side
What does the landing gear damper do?
Improves the touchdown stability and reduces the possibility of ground resonance by centering the landing gear laterally.
What systems are on the weight on wheels sensor?
Landing gear retract function
Servo jam warning system
Engine out audible warning system
Where is the landing gear control valve located?
Right main gear well
How much pressure should be in the emergency gear blow down bottle?
3000 psi, recharge if below 2700 psi.
What allows the air to lower the gear in place of the hydraulic fluid?
Shuttle valves located on each of the gear actuators.
When will the nose wheel not self center?
If swiveled more than 180 degrees, if greater than 90 degrees a slight forward roll should be used to straighten it before lift off.
