Definition
A condition in which one or more flight instruments display readings that fluctuate, jump, lag, freeze, or otherwise fail to reflect the aircraft's actual state, typically caused by a malfunction in a sensing system such as the pitot/static system, gyroscopic instruments, or electrical supply. The indications cannot be trusted as a reference for flight or navigation.
Plain English
The instruments are giving readings that don't make sense or keep changing in ways the aircraft isn't actually doing. You can't rely on what they're showing you.
Context Anchor
Seen in instrument flying and emergency-procedure discussions when a blocked or leaking pitot/static system may make some flight instruments misleading.
Derivation
"Erratic" comes from the Latin errare, meaning to wander or stray. An erratic indication wanders away from what's actually happening. "Unreliable" simply means cannot be depended on. Together they describe instruments that wander from truth and can't be trusted.
Why Pilots Care
Recognizing these indications allows the pilot to transition to backup instruments or visual references before losing aircraft control.
Analogy
It is like a car speedometer needle bouncing between 20 and 70 mph while the car feels steady. The number is still visible, but it is no longer something you can safely trust by itself.
Grounding Statement
If the pressure reaching an instrument is blocked or leaking, the display in the cockpit may no longer match the airplane’s real speed, height, or climb/descent.
Intuition Check
Do not assume “erratic” only means wildly jumping around. In this context, an indication can also be unreliable if it freezes, lags, or quietly shows the wrong value.
Example Sentence 1
After the pitot tube iced over, the airspeed indicator showed erratic and unreliable indications, so the pilot referred to pitch and power settings to maintain control.
Example Sentence 2
The crew cross-checked the standby instruments once erratic and unreliable instrument indications appeared during the approach.