Definition
A fabric filter element inside a fuel filter assembly that removes water from aircraft fuel by causing tiny suspended water droplets to merge into larger drops, which then fall out of the fuel and collect at the bottom of the filter housing where they can be drained off.
Plain English
A cloth-like bag inside a fuel filter that pulls small water drops together into bigger drops so the water separates from the fuel and sinks to the bottom, where it can be drained away.
Context Anchor
Seen in aircraft fuel system maintenance, fuel filter servicing, and water-separator discussions.
Derivation
From the Latin coalescere, meaning 'to grow together.' The bag's job is to make small water droplets grow together into drops large enough to fall out of the fuel.
Why Pilots Care
Water in fuel can cause engine stoppage or corrosion; the coalescent bag is a key part of keeping fuel clean and safe.
Analogy
It is like tiny beads of water on a window joining together until they become drops large enough to run down the glass.
Grounding Statement
Fuel flows through the bag, and the water that was spread through the fuel gathers into larger drops that can be separated out.
Intuition Check
Do not think of this as a storage bag. In this context, the “bag” is a filter part that treats fuel as it flows through it.
Example Sentence 1
During the inspection the mechanic replaced the coalescent bag in the main fuel filter and drained a small amount of water from the sump.
Example Sentence 2
After replacing the coalescent bag, the fuel system showed no further signs of moisture in the separator bowl.