It's hard to have an eye on every single mechanism on an airplane at all times — pilots are only human, after all. Luckily, there's a little device called a “stick shaker” that alerts pilots when the airplane is about to stall.
A recent Qantas flight to Hong Kong put the focus on this tiny life-saver, when 15 people were injured due to extreme turbulence right before the Boeing 747 stalled, according to Business Insider. The turbulence lasted about two minutes before the pilots were able to guide the plane to more stable airspace.
So let's talk about what happens when an airplane stalls. This generally happens either because there's an engine failure or because the nose of the airplane is at too high of a pitch, according to pilots on Aviation Stack Exchange. In the latter case, this causes what's called airflow separation, meaning the conditions aren't right for the airplane wing to create any significant lift (which is how airplanes stay in the sky).
If a pilot — or the autopilot — guides the nose to an improper angle or the engine begins to stall, a tiny device on the bottom of the steering wheel in the cockpit will begin to shake, causing the entire wheel to vibrate and alert the pilot that something is off.
Passengers experienced unexpected in-flight turbulence strong enough that it set the stick shaker alarm off. The Australian Transport Safety Bureau labeled the incident “serious,” which, according to The Sydney Morning Herald, means that it was an “accident causing loss of life or aircraft damage nearly occurred.” Passengers said the entire plane shook before dropping sharply, according to the Herald.
In case you're wondering what a stick shaker alert sounds like. Spoiler: It's loooooouuuud.
Generally, the fix is taking the airplane off of autopilot and tipping the airplane nose downward to increase speed and airflow to the wings, according to pilots on Aviation Stack Exchange. In the Qantas incident, the alert was derived from “airframe buffeting,” or airflow separation.
The Australian Transport Safety Bureau will be interviewing the flight crew and releasing a report on the incident.