FAA Aviation Mechanic Airframe (AMA)Hydraulic and Pneumatic SystemsHard
A hydraulic system relief valve begins to crack open (initial opening) at 3,000 psi and does not reach full flow (fully open) until system pressure reaches 3,250 psi. What is the percentage differential between the crack pressure and the full-flow pressure of this relief valve?
- A8.3%
- B25%
- C12.5%
- D3.3%
Show answer & explanationAnswer & explanation
Correct answer: A. 8.3%
The pressure differential is 3,250 psi − 3,000 psi = 250 psi. Expressed as a percentage of the crack pressure: 250 ÷ 3,000 = 0.0833, or 8.3%. This spread, sometimes called the valve's 'range' or 'differential,' is a normal characteristic of relief valve design and must fall within acceptable limits during a functional check.
Why the other options are wrong
- B. 25% would result from an incorrect calculation such as 250 ÷ 1,000.
- C. 12.5% would come from dividing 250 by 2,000, not the actual 3,000 psi crack pressure.
- D. 3.3% is too low and does not match the correct division of 250 by 3,000.
Relief Valve Crack/Full-Flow Differential
The percentage difference between a relief valve's initial cracking pressure and its full-flow (fully open) pressure, calculated as (full-flow − crack) ÷ crack × 100.
- Crack pressure is where the valve first begins to open
- Full-flow pressure is where the valve is fully open
- Differential/spread must be within the manufacturer's tolerance during checks
Memory trick: Divide the gap by the starting point to get the percent spread