Unaccompanied Minor by Hollis Gillespie

Fourteen-year-old April Mae Manning has spent her life on airplanes with her flight-attendant mother. After her father dies in a crash, April's mom marries a pilot who turns out to be an abusive jerk--and who connives for custody of April by getting April's mom confined to a psychiatric hospital. So April takes off, literally, living on airplanes, using her mother's work badge and flight benefits, relying on sympathetic airline employees who know she's been flying unaccompanied between coasts for years due to a divorce war. Then there's a hijacking, but why are April's stepdad and his girlfriend onboard? April flees to the cargo hold with another unaccompanied minor, and they fight to thwart the hijackers, faking a decompression, making weapons from things they find in the luggage, and improvising ways to use dead bodies to relay messages to the authorities. At last--locked in the cockpit with a wounded police officer, the boy, his service dog, and a killer--April tries to remember everything her mother told her to do in a crisis above the clouds. But she knows it won't be enough.