Systems: Hydraulics, Fuel, and Oil

Quick refresher

Pushing the brake pedal creates fluid pressure that applies the brake at the wheel. The specified red hydraulic fluid can burn. If a brake fails, maintain control, stop safely and get help rather than continue taxiing.

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 specified red hydraulic fluid can burn, so leaks need attention.
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.
• Your foot supplies the force. The fluid transmits pressure through the lines to the wheel brakes; releasing pedal pressure releases the brakes.

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?
   • Maintain directional control, stop safely and get assistance rather than continue taxiing. Follow the airplane’s checklist.
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 Oral-Exam Questions About 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.

Fuel: the basic idea

• The fuel-tank vent lets air replace fuel as fuel is used. A blocked vent can restrict fuel flow.
• Know your airplane’s fuel-selector and shutoff positions, and use its fuel pump according to the POH.