Definition
The dome-shaped cover and its associated parts that fit over the top of a valve body or fuel cell fitting in an aircraft system, providing a sealed housing for the valve stem, packing, and operating mechanism.
Plain English
A protective cap-and-parts unit that bolts onto the top of a valve, holding the moving stem in place and sealing the system so fluid or fuel cannot leak out.
Context Anchor
Seen in aircraft engine and carburetor maintenance descriptions, especially when discussing pressure carburetors and fuel metering.
Derivation
From the French 'bonnet,' meaning a soft cap or hood. Mechanics borrowed the word because the cover sits on top of a valve body the way a bonnet sits on a head.
Why Pilots Care
If a bonnet assembly leaks or is improperly torqued after maintenance, it can cause fuel or hydraulic fluid loss in flight. Pilots check for leaks around these fittings during preflight inspection.
Intuition Check
Do not read bonnet here as the hood of a car. In this aviation maintenance use, it means a cover or housing on a carburetor assembly.
Example Sentence 1
The mechanic replaced the bonnet assembly on the fuel selector valve after finding a slow leak around the valve stem.
Example Sentence 2
During the preflight the pilot checked that all latches on the bonnet assembly were secure.