Why you’ll never hear a pilot use the word ‘plunge’

Pilots intensely dislike the word plunge. It seems to be the word of choice when an aircraft experiences a depressurisation, eliciting a sense of being out of control. Yet the reality is almost always the opposite.

Modern jet aircraft fly at between 30,000 and 40,000 feet above the ground – or between nine and 12 kilometres in metric terms. It’s all about minimising fuel consumption, whether for the ultra-long haul flying or for short domestic routes. And less fuel means a lower operating cost.

The aircraft thrive in this thin air, but unfortunately those sitting within do not. At 20,000 feet, the time of useful consciousness for an average adult without supplemental oxygen is approximately 10 minutes. At 30,000 feet that’s reduced to just 60 seconds and at 40,000, it’s just a few breaths, at 15 seconds.

Pressurising the aircraft to artificially reduce the cabin altitude ensures passengers survive the journey. It’s achieved by air flowing through the engines that is then compressed at the right pressure, then diverted towards the air-conditioning packs where it is cleaned and cooled before flowing into the passenger cabin.

This conditioned air creates an equivalent altitude of about 5000 to 8000 feet, which is still higher than sea level but completely safe in terms of a human’s oxygen needs.

This conditioned air is being constantly produced, so to avoid the fuselage becoming too pressurised (you can’t keep filling up a balloon with air indefinitely), aircraft are fitted with outflow valves, often located at the rear. Each valve is about the size of a laptop with mechanical doors that move automatically in flight to regulate the cabin pressure.

The manoeuvre may feel to the passengers like a plunge, yet to the pilots, it is a controlled descent they have trained for over and over again.

This system is reliable and effective, but that doesn’t mean a depressurisation can’t occur. Depressurisations are rare, however the most common cause is the pressurisation system failing. An outflow valve may fail in the open position, and the cabin altitude slowly starts to climb. Air-conditioning systems may fail – this is even more rare – but if they do it means not enough pressurised air is pumped into the cabin.

And then there are the one-of-a-kind structural disasters. One of the most infamous occurred in February 1989 when a United Airlines Boeing 747 departed Honolulu bound for Auckland and eventually Sydney. Passing 22,000 feet on climb, UA811 experienced a forward cargo door failure. The door was ripped from the fuselage, taking part of the right side of the aircraft with it. Several rows of seats were sucked out of the hole, killing nine passengers, including two Australians.

While depressurisations of any kind are extremely rare, as with all potential problems, pilots train for how to handle them. On the flight deck, if the cabin altitude climbs above 10,000 feet, a warning horn and red light alert flight crew to the danger. The immediate first step is for the pilots to put on oxygen masks – an absolute must to ensure those controlling the plane don’t lose consciousness. Fitting the mask within seconds is practiced regularly in simulator training.

A sudden depressurisation is no doubt scary for the passengers. There will be increased noise, a drop in temperature and misting of the air. The rubber jungle of oxygen masks will drop from the ceiling. A more subtle decompression may not be as dramatic, but if the cabin altitude has risen above the equivalent of 10,000 feet, the computer system will deploy the oxygen masks. As everyone should know from the safety briefing, passengers should immediately fit their masks, then help those around them to do the same, make sure their seat belts are fastened and try to stay calm.

Once the pilots have fitted their oxygen masks, the initial steps are to troubleshoot the problem to see if they can regain control of the cabin altitude. This takes only 30-60 seconds but is vital to see whether an emergency descent is necessary.

If the pilots determine the cabin altitude cannot be controlled, an emergency descent happens. This involves reducing the thrust on the engines, extending the speed brakes to increase drag and accelerating to the highest speed allowed. The result is a rapid descent of around 5000 to 8000 feet per minute, normally to an altitude of 10,000 feet.

The manoeuvre may feel to the passengers like a plunge, yet to the pilots, it is a controlled descent they have trained for over and over again. They are still flying the plane. Once in the emergency descent, one pilot continues to monitor the flight path, while the other will make an announcement to passengers, liaise with air traffic control, assess any high terrain, turn on all the external lights and squawk “7700” on the transponder to indicate an emergency. Lastly, they’ll work together to review the emergency checklists to ensure every action has been completed.

Around five minutes is required to reach 10,000 feet and once safely levelled off, the supplemental oxygen is no longer needed to breathe. Pilots will then assess the nature of the depressurisation event to determine the safest manner to divert to a nearby available airport.

Depressurisation events are rare, but when they do occur, they can be dramatic and scary. And whilst the descent may be sudden and steep, passengers can take comfort in the fact it is a controlled and regularly practiced manoeuvre designed to get everyone on board to a safe altitude.