Definition
The decrease in engine revolutions per minute observed when switching from operating on both magnetos to operating on only one magneto during the pre-takeoff ignition system check. A small drop is normal and confirms each magneto is firing its set of spark plugs; an excessive drop, no drop, or an uneven difference between the two magnetos indicates an ignition problem.
Plain English
When the pilot tests each magneto on the ground, the engine slows down a little because only half the spark plugs are firing. That small slowdown is the rpm drop. How much it drops, and whether both magnetos drop by similar amounts, tells the pilot whether the ignition system is healthy.
Context Anchor
Seen during the before-takeoff engine check, when the pilot checks the ignition system at a set engine speed.
Derivation
Rpm stands for “revolutions per minute,” meaning how many times the engine’s crankshaft turns in one minute. “Drop” means a decrease, so “rpm drop” means a decrease in engine turning speed.
Why Pilots Care
A small, controlled RPM drop confirms each magneto is producing power; an excessive drop or zero drop signals an ignition problem that must be fixed before flight.
Grounding Statement
A small rpm drop is normal because the engine is running on one ignition source instead of two.
Intuition Check
Do not assume an rpm drop means the engine is failing. In this check, a small drop is expected; an excessive, missing, or uneven drop is the warning sign.
Example Sentence 1
During the run-up, the pilot noted a 75 rpm drop on the left magneto and a 70 rpm drop on the right, both within the 125 rpm limit specified in the POH.
Example Sentence 2
Any RPM drop greater than the manufacturer's limit requires the magneto issue to be investigated before departure.