Southwest Airlines Flight 8684 takes off at 8:30am from Honolulu for Los Angeles with a hundred and seventy passengers aboard, three flight attendants, and two cabin crew. The plane itself is a Boeing 737, ten years old, handles well. They flew it in the previous night. The first three hours of the flight are uneventful. The first officer is flying, the autopilot's engaged, and they're cruising at 34,000 feet when there's the first sign of any trouble, in the form of an alert on the flight management computer.
"GPS," Davidson reads it out. He is in the middle of eating a chicken sandwich. He reluctantly sets it down. By the time he's done troubleshooting the GPS it'll probably be cold. He pulls out the manual. GPS, page 271.
Condition: One or both GPS receivers are failed.
Note: The FMC uses only IRS or radio inputs.
Look-ahead terrain alerting and display are unavailable due to position uncertainty.
Continue normal operation if ANP meets the requirements for the phase of flight.
He reads this out too. They're over the ocean, not in busy airspace, and actual navigation performance should be comfortably within requirements for this phase of flight. He double checks the heading, which is correct.
He confirms that the inertial reference system, which tracks the plane's position based only on the plane's own acceleration with no communication with anything on the ground, claims to be functioning normally and that the backup inertial reference system agrees with the primary inertial reference system. It's a good modern IRS; fly for an hour with the GPS out, you'll be less than a mile off course.
That completes the checklist.
He finishes his chicken sandwich, and gets up to get another one.