FAA Aviation Mechanic Airframe (AMA)Hydraulic and Pneumatic SystemsEasy
A technician is bleeding air from a hydraulic system after component replacement. Air trapped in the system can cause which of the following operational problems?
- ANo effect, since air and hydraulic fluid behave identically under pressure
- BSpongy or erratic actuator movement due to compressibility of trapped air
- CIncreased system operating pressure beyond relief valve settings
- DImproved actuator response due to compressibility of trapped air
Show answer & explanationAnswer & explanation
Correct answer: B. Spongy or erratic actuator movement due to compressibility of trapped air
Air is compressible while hydraulic fluid is essentially incompressible; trapped air in the system absorbs pressure changes, causing spongy, delayed, or erratic actuator movement. This is why systems must be properly bled after maintenance.
Why the other options are wrong
- A. Air and hydraulic fluid have very different compressibility characteristics.
- C. Trapped air does not cause pressure to exceed relief valve settings.
- D. Compressibility of air degrades, rather than improves, response.
Air in Hydraulic Systems
Trapped air is compressible and causes spongy, delayed, or erratic operation of hydraulic actuators, unlike the essentially incompressible hydraulic fluid.
- Systems must be bled after component replacement
- Air causes spongy brakes/actuator response
- Bleeding removes trapped air via bleed valves
Memory trick: Air is a sponge, fluid is a rock — bleed the sponge out.