Definition
An inertial navigation system in which the gyroscopes and accelerometers are fixed (strapped down) directly to the airframe rather than mounted on a gimballed, mechanically stabilized platform. The sensors measure motion in the aircraft's own reference frame, and a computer continuously calculates the aircraft's attitude, heading, position, and velocity from those raw measurements.
Plain English
A type of inertial navigation system where the motion sensors are bolted straight to the aircraft instead of sitting on a moving platform that keeps them level. A computer does the math to work out which way is up and where the aircraft is.
Context Anchor
Seen in discussions of instrument systems, inertial navigation, and electronic attitude or heading displays.
Derivation
From the plain English idea of being 'strapped down.' Earlier inertial systems used gimbals (a set of pivoting rings) to keep the sensors level no matter how the aircraft moved. In a strapdown system, the sensors are simply fastened to the airframe and move with it — hence the name.
Why Pilots Care
Strapdown systems are lighter, have fewer moving parts, and give reliable attitude and navigation data in modern glass-cockpit aircraft.
Analogy
It is like mounting a phone firmly to a car dashboard: the phone feels every turn and movement of the car, and software turns those movements into useful information.
Intuition Check
Do not read “strapdown” as a pilot action with actual straps. Here it means the sensors are fixed in place inside the aircraft system.
Example Sentence 1
The aircraft's attitude and heading reference system uses a strapdown system, with solid-state sensors mounted directly to the airframe.
Example Sentence 2
Before flight the crew verified that the strapdown inertial reference unit had completed its alignment sequence.