Engines Auxiliary Power Unit (APU) Practice Exam 2026 - Free APU Practice Questions and Study Guide

These indicators use a ground-versus-flight gating approach: they are meant to show status or faults only when the airplane is on the ground or moving slowly, not during flight. When the engines are running and you’re taxiing, the engine control lights illuminate to alert you to any abnormal condition that should be checked before takeoff. But once you accelerate beyond a certain speed, you’re in the takeoff/flight regime, so these ground-mode lights are inhibited to prevent nuisance alerts during higher-speed operations.

The threshold is 80 knots because it’s high enough to cover typical taxi and slow-ground maneuvering, yet well below normal takeoff speeds. This ensures you get the warning on the ground when it matters, and you won’t be distracted by these lights once you’re flying. If the limit were much lower, you’d get lights during ordinary taxi; if much higher, you could miss a ground issue while still on the ground during the roll to departure.