Definition
Behaving in an irregular, unpredictable, or inconsistent manner with no steady pattern. In the context of flight instruments, an erratic indication is one that fluctuates, jumps, or changes in ways that do not match actual flight conditions.
Plain English
Acting unpredictably and without a steady pattern. An erratic instrument is one whose needle or reading wanders, jumps, or behaves in ways that don't make sense for what the aircraft is actually doing.
Context Anchor
Seen when describing unusual airspeed, altitude, or vertical speed indications during a pitot/static system problem.
Derivation
From the Latin erraticus, meaning 'wandering' (same root as 'err' and 'error'). Originally used to describe stars that appeared to wander across the sky compared to fixed stars. The 'wandering' image fits well: an erratic instrument needle wanders rather than settling where it should.
Why Pilots Care
Erratic readings can mislead a pilot into wrong control inputs, increasing the risk of spatial disorientation or loss of control if the failure is not recognized and alternate instruments are not used.
Intuition Check
Do not read erratic as simply “wrong.” Here it means irregular or unpredictable; an erratic instrument may sometimes look close to correct and then suddenly wander or jump.
Example Sentence 1
When the pitot tube began to ice over, the airspeed indicator gave erratic readings before failing completely.
Example Sentence 2
The pilot noticed the airspeed indicator acting erratic after entering icing conditions and immediately cross-checked with the standby instruments.