Definition
In a multi-spool turbine engine, the low pressure compressor is the forward set of compressor stages that takes in outside air and gives it a first, modest squeeze before passing it to the high pressure compressor for further compression. It is driven by the low pressure turbine through its own shaft (the low pressure spool), which rotates independently of the high pressure spool.
Plain English
It is the front section of the engine's compressor that does the first stage of squeezing the incoming air, before another compressor squeezes it further.
Context Anchor
Seen in turbine engine instrument discussions, especially when interpreting N1 or engine compressor section indications.
Derivation
Low pressure here refers to the stage of compression, not the absolute pressure inside the engine. It is called low pressure simply because the air has only been partially compressed at this point, compared with the much higher pressure produced by the next compressor stage.
Why Pilots Care
Pilots encounter references to this stage when reviewing engine performance, thrust output, and related instrument indications.
Analogy
Think of it like the first pump in a two-pump air system. The first pump raises the air pressure partway, then the second pump raises it more.
Intuition Check
“Low pressure” does not mean the air pressure is low in an everyday sense or that something is wrong. It means this compressor section operates at a lower pressure than the high pressure compressor and does the first stage of compression.
Example Sentence 1
On this engine, the N1 gauge shows the rotational speed of the low pressure compressor.
Example Sentence 2
Engine specifications list the low pressure compressor ratio as a key performance parameter for the turbofan.