The Federal Aviation Administration (FAA) allowed some flights to resume after an outage of the system that alerts pilots to any obstructions before take-off had earlier forced the civil aviation regulator to ground all aircraft in the United States. Over 4,000 flights were delayed and more than 600 were canceled because of the outage as of early Wednesday morning. US flights were slowly beginning to resume departures as a ground stop was lifted. Here is a brief summary of what the pilot warning system does, what we know about what went wrong, and background about the safety notices provided to pilots, known as NOTAM. What happened? The FAA system that is meant to distribute ...Keep on reading: Explainer: Why US flights were grounded by a FAA system outage