Definition
A fire extinguishing system used in aircraft engine and APU compartments that releases its entire charge of extinguishing agent in a very short time, typically one to two seconds, through a network of distribution tubing and spray nozzles. The agent is stored in a sealed steel sphere or cylinder under pressure (usually with nitrogen) and is released by firing an electrically actuated cartridge that ruptures a frangible disc, flooding the protected zone with a high concentration of agent almost instantly.
Plain English
A built-in fire extinguisher in the engine area that dumps all of its fire-killing agent at once, in about a second, to smother a fire fast.
Context Anchor
Seen in aircraft maintenance material for engine, APU, and other built-in fire-extinguishing systems.
Derivation
The name describes how it works: the agent is discharged at a high rate, meaning all at once rather than metered out. This contrasts with low-rate-of-discharge (LRD) systems, which release agent more slowly over a longer period.
Why Pilots Care
It stops engine fires before they spread and damage critical components.
Analogy
It is like opening a large valve all at once instead of slowly pouring from a bottle; the goal is to get enough extinguishing material into the space quickly.
Intuition Check
Do not picture a handheld cabin fire extinguisher. An HRD fire extinguisher is usually a fixed aircraft bottle connected to lines that carry the agent to a protected compartment.
Example Sentence 1
When the crew pulled the engine fire handle and pressed the discharge button, the HRD fire extinguisher emptied its bottle into the nacelle and the fire warning light went out within seconds.
Example Sentence 2
Technicians inspected the HRD fire extinguisher bottles during the preflight check.