“It is the closest all of us passengers ever want to come to a plane crash,”
a Qantas flight QF1889’s passenger said after the plane suddenly descended about 20,000 feet on Monday September 22, and diverted back to Darwin.

The Embraer 190’s crew received a pressurisation warning, followed the procedures, and landed normally – but in the cabin, that rapid drop felt anything but normal.

The truth is, in-flight technical problems such as this one are part of flying. Pilots train extensively for them. Checklists contain detailed instructions on how to deal with each issue. Aircraft are built with layers of redundancy, and warning systems alert pilots to problems. It is because of these safety systems that the vast majority of flights that experience technical issues end with a safe arrival rather than tragic headlines.

Here are four scary-sounding failures you might hear about (or even experience) and how they are actually dealt with in the air.

1. Air-conditioning and pressurisation hiccups

What it is

At cruising altitudes (normally around 36,000 feet), aeroplane cabins are kept at a comfortable “cabin altitude” of 8,000 feet using air from the engines that is cooled through the air conditioner.

This artificial air pressure allows us to survive while the atmosphere outside the plane is highly hostile to human life, with temperatures around -55°C and no breathable air. However, if the system misbehaves or the cabin altitude starts to rise for whatever reason, crews treat it as a potential pressurisation problem and initiate the preventive procedures immediately.

What you might feel/see

A quick, controlled descent (it can feel dramatic), ears popping, and sometimes oxygen masks – these typically drop automatically only if the cabin altitude exceeds roughly 14,000 feet. Similar to QF1889, a rapid descent without masks being deployed is the most common outcome.

What pilots do

As soon as they notice a problem with the cabin pressurisation, the pilots put on their own oxygen masks, declare an emergency, and follow the emergency descent checklist, bringing the aircraft as quickly as possible to about 10,000 feet. This is usually followed by a diversion or return to the departure airport.

2. Most feared: engine failures

What it is

Twin-engine airliners are certified to fly safely on one engine. Yet, one-engine failures are treated seriously and thoroughly rehearsed in flight simulators at least annually.

Dual failures, however, are exceptionally rare. The 2009 “Miracle on the Hudson”, for example, was a once-in-a-generation bird strike event that led to both engines stopping. The plane safely landed on the Hudson River in New York with no casualties.

What you might feel/see

A loud bang, vibration, sparks coming out of the engine, smell of burning or a sudden quietening. This may result in a turn-back and an emergency services welcome. Recent headlines on engine failures – from a 737 in Sydney to a multiple bird-strike-related return in the United States ended with safe landings.

What pilots do

After being alerted by the warning system, pilots identify the affected engine and follow the checklist. The checklist typically requires them to shut down the problematic engine, descent to an appropriate altitude and divert if in cruise, or return to the departure airport if after takeoff.

Even when an engine failure damages other systems, crews are trained to manage cascades of warnings – as Qantas A380 flight QF32’s crew did in 2010, returning safely to Singapore.

3. Hydraulic trouble and flight controls

What it is

The many aeroplane flight controls move because of multiple hydraulic or electric systems. If one system misbehaves – for example the left wing aileron, which is used to turn the aircraft, won’t move – redundancy keeps the aeroplane flyable because the right wing aileron will still work.

Crews use specific checklists and adjust speeds, distances and landing configurations to ensure a safe return to the ground.

What you might feel/see

A longer hold while the crew troubleshoots, a return to the departure airport or a faster-than-normal landing. In July, a regional Qantas flight to Melbourne made an emergency landing at Mildura after a hydraulics issue.

What pilots do

After the warning system’s detection, pilots run through a checklist, decide on the landing configuration, request the longest suitable runway and emergency services just in case.

All these resources are available because lessons learned from extreme events – such as United 232’s 1989 loss of all hydraulic systems – were brought into the design of modern aeroplanes and training programs.

4. Landing gear and brake system drama

What it is

Airliners have retractable landing gears that remain inside a compartment for most of the flight. Those are the wheels that come out of the aeroplane belly before landing. Assembled in the wheels are the brakes. They aim to reduce the aircraft speed after touchdown, like in a car.

With so many moving parts, sometimes the landing gear doesn’t extend or retract properly, or the braking system loses some effectiveness, such as the loss of a hydraulic system.

What you might feel/see

A precautionary return, cabin preparation for potential forced landing, or “brace for impact” instruction from the cabin crew right before landing can happen.

While scary, these are preventive measures if something doesn’t go as planned. Earlier this year, a Qantas flight returned to Brisbane after experiencing a problem with its landing gear; passengers were told to keep “heads down” while the aircraft landed safely.

What pilots do

They’ll use long checklists and eventually contact maintenance engineers to troubleshoot the problem. There are also redundancies available to lower the landing gear and to deploy the brakes.

In extreme cases, they may be required to land at the longest runway available (in case of brake problems) or land on the belly (if the landing gear can’t be lowered).

The big picture

Most in-flight failures trigger a chain of defences aimed at keeping the flight safe. Checklists, extensive training and decades of expertise are backed by multiple redundancies and robust design. And these flights typically end like QF1889 did: safely on the ground, with passengers a little shaken.

A dramatic descent or an urgent landing doesn’t mean disaster. It usually means the safety system (aircraft + crew + checklist + training + redundancy) is doing exactly what it’s supposed to do.