Systems: Hydraulics, Fuel, and Oil

Key Components of the Hydraulic System in the Cessna 172:

1. Brake Pedals:
   • The pilot applies pressure to the top portion of the rudder pedals to activate the hydraulic brakes.
2. Master Cylinders:
   • Located behind the brake pedals. When you press the brake pedals, the master cylinders convert mechanical force into hydraulic pressure.
3. Hydraulic Lines:
   • Transport hydraulic fluid from the master cylinders to the brake assemblies.
4. Brake Assemblies:
   • Located at each main wheel. Hydraulic pressure moves pistons within the brake calipers, causing the brake pads to squeeze the brake discs (rotors) and slow the aircraft.
5. Hydraulic Fluid:
   • Typically uses MIL-H-5606 (red aviation hydraulic fluid). This fluid is fire-resistant and operates effectively across a wide temperature range.
6. Parking Brake:
   • Uses the same hydraulic system. Engaging the parking brake traps hydraulic pressure in the brake lines to keep the brakes applied.

Normal Operation:

• Pressing the brake pedals applies hydraulic pressure to the brake calipers.
• The system is closed-loop, meaning hydraulic fluid is cycled within the system without any external pump.

Common Issues and Troubleshooting:

1. Soft or Spongy Brakes:
   • Indicates air in the hydraulic lines. This requires bleeding the brake system to remove air bubbles.
2. Hydraulic Fluid Leaks:
   • Signs include reduced braking effectiveness or visible fluid around the wheel or brake lines.
3. Brake Fade:
   • Can occur during prolonged braking, such as after a heavy landing or when taxiing downhill. Avoid overusing brakes to prevent overheating.
4. Parking Brake Failure:
   • If the parking brake doesn’t hold, it may indicate a leak or insufficient hydraulic pressure in the lines.

Preflight Checks:

1. Brake Pedal Feel:
   • Ensure the pedals are firm when pressed. Soft or spongy pedals may indicate air or a fluid leak.
2. Hydraulic Fluid Levels:
   • Check the fluid level during inspections (usually done during maintenance, as fluid reservoirs are not pilot-accessible).
3. Visual Inspection:
   • Look for fluid leaks around the brake calipers and hydraulic lines.

Key Questions for Oral Exams:

1. What is the primary use of hydraulics in a Cessna 172?
   • The braking system.
2. What type of hydraulic fluid is used?
   • MIL-H-5606 (red hydraulic fluid).
3. What are signs of hydraulic system failure in the Cessna 172?
   • Spongy brake pedals, reduced braking performance, or visible fluid leaks.
4. What would you do if one brake failed during taxi or landing?
   • Use differential braking with the remaining brake and the rudder for directional control. Reduce speed before taxiing off the runway.
5. How is the parking brake engaged?
   • Apply pressure to the brake pedals and pull the parking brake handle to trap hydraulic pressure in the lines.

Common Questions for Oral Exams in regards to engine oil.

1. What type of oil does this engine use?
   • Typically 20W-50 ashless dispersant aviation oil. (check POH)
2. Why is oil important in the engine?
   • Lubricates, cools, cleans, seals, and prevents corrosion.
3. What happens if the oil level is too low?
   • Inadequate lubrication can lead to overheating, wear, and potential engine failure.
4. How do you check the oil level?
   • Use the dipstick during preflight, ensuring the aircraft is on level ground.
5. What are signs of oil system problems in flight?
   • Low oil pressure, high oil temperature, or engine roughness.