Definition
An altitude-encoding device that senses pressure altitude and sends that data to the transponder for transmission to ATC, but does not display the altitude to the pilot. It works in the background to support Mode C altitude reporting without providing a visible readout in the cockpit.
Plain English
A small box that measures your altitude and feeds it to your transponder so ATC can see it on radar. It is called "blind" because the pilot cannot see what it is reporting -- it only talks to the transponder, not to you.
Context Anchor
Seen in Mode C altitude-reporting discussions, transponder checks, and instrument-panel equipment descriptions.
Derivation
"Blind" here means "without a display" -- the device does its job unseen, much like a "blind" valve or fitting in engineering means one with no opening or readout. The pilot never reads the encoder directly; only the transponder and ATC see its output.
Why Pilots Care
It enables required altitude reporting for ATC without adding cockpit panel space or requiring the pilot to monitor a separate display.
Intuition Check
Do not read “blind” as damaged or unable to sense altitude. In this term, “blind” means the unit has no pilot-facing altitude display.
Example Sentence 1
The avionics shop installed a new blind encoder so the transponder could report Mode C altitude to ATC.
Example Sentence 2
Before departure the pilot verified the blind encoder was operating by observing a valid transponder reply code.