Definition
A unit used to specify surface roughness, expressed as the root-mean-square (rms) average height of microscopic peaks and valleys on a finished surface, measured in millionths of an inch. One microinch equals one-millionth of an inch (0.000001 inch). The rms value is calculated by squaring each measured deviation from the mean surface line, averaging those squares, and taking the square root, giving a single number that represents how rough or smooth a machined surface is.
Plain English
A way of measuring how smooth a metal surface is. The smaller the number, the smoother the surface. It works by measuring the tiny bumps and dips on the surface and giving you one average figure in millionths of an inch.
Context Anchor
Seen in aircraft engine maintenance specifications for machined surfaces such as shafts, bearing areas, sealing surfaces, and cylinder parts.
Derivation
Micro- comes from the Greek mikros, meaning small. An inch is the standard unit of length, so a microinch is one-millionth of an inch — an extremely small distance. Rms stands for root-mean-square, a mathematical method of averaging that prevents positive and negative deviations from canceling each other out. The combination tells you the average size of surface irregularities at a microscopic scale.
Why Pilots Care
Correct surface finish on critical engine components prevents excessive wear, scoring, and early failure while ensuring proper lubrication and clearances.
Analogy
Think of running your hand over a tabletop that looks flat. Microinches rms is like a very precise way of measuring the tiny roughness your hand can barely feel, but at a much smaller scale.
Intuition Check
Do not read microinches rms as the size of one scratch or one bump. It is a calculated roughness value for a surface, expressed in millionths of an inch.
Example Sentence 1
The overhaul manual specified that the crankshaft journals be polished to a finish of 15 microinches rms or better.
Example Sentence 2
During cylinder inspection, any surface roughness exceeding 20 microinches rms required honing before the engine could be reassembled.