FAA Aviation Mechanic Powerplant (AMP)Fuel, Induction and LubricationEasy
A mechanic checks the oil level on a wet-sump reciprocating engine immediately after engine shutdown and reads 4 quarts, well below the 6 quarts specified in the AFM. After waiting 10 minutes and rechecking, the dipstick reads 6 quarts. What most likely explains this difference?
- AResidual oil in the lines, cooler, and cylinder heads had not yet drained back into the sump
- BThe engine consumed 2 quarts of oil during the short ground run
- CThe dipstick was defective during the first reading
- DOil was leaking from the sump during the first check
Show answer & explanationAnswer & explanation
Correct answer: A. Residual oil in the lines, cooler, and cylinder heads had not yet drained back into the sump
After shutdown, oil remains suspended in galleries, the cooler, and upper engine components before gravity drains it back to the sump. Checking too soon after shutdown gives a falsely low reading; AFMs specify a wait time before checking for this reason.
Why the other options are wrong
- B. Oil consumption occurs during running, not while sitting idle after shutdown; it would decrease level, not increase it.
- C. A defective dipstick would not read correctly the second time either without repair.
- D. A leak would not correct itself in 10 minutes without repair.
Oil Drainback Time
The waiting period after engine shutdown required for oil in lines and components to return to the sump before an accurate dipstick reading can be taken.
- Checking oil too soon after shutdown gives falsely low readings
- AFM/POH specifies required wait time (commonly several minutes)
- Applies to wet-sump systems where sump quantity is the total system indicator
Memory trick: Give the oil time to come home before you count it.