Behind the scenes: What goes on in the flight deck during a rapid descent

Sep 28, 2019

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Last week, Delta Air Lines Flight 2353 was on its way from Atlanta to Fort Lauderdale when it experienced problems with the pressurisation system. The pilots descended the aircraft from its cruising altitude of 39,000 ft. down to 10,000 ft. in accordance with their Standard Operating Procedures (SOPs) before diverting to Tampa.

My colleague, Katherine Fan, explained beautifully why the use of words such as ‘plunge’ and ‘plummet’ by the mainstream media are neither accurate nor helpful in these situations. The aircraft performed what is known as a ‘rapid descent’ and you may be surprised to know that you probably have already experienced something similar before.

The Outside Atmosphere

The the outside atmosphere at 39,000 ft. is a pretty inhospitable place. Temperatures are a chilly -60C and the air is so thin that breathing unassisted is impossible. It’s for these reasons that airliners have a pressurisation system to ensure that you’re kept comfortable in the cabin. The system is so advanced on aircraft such as the 787 Dreamliner that you can experience the same air composition as being on the ground in Denver, Colorado.

The cabin altitude on the 787 is much lower than on other types

During the flight, the pressurisation system is automatic. As the aircraft climbs away from the ground, computers regulate the amount of air entering and leaving the cabin to ensure that the optimum equilibrium is maintained.

Onboard Oxygen

“Should the cabin pressurisation system fail, oxygen will be provided. Pull a mask towards you and breathe normally.” How many times have you heard this during the safety briefing but never really understood what it meant?

Depending on the aircraft type, supplemental oxygen can be provided in different ways. On the 787 Dreamliner, there are two independent oxygen systems: one for the flight deck and one for the passenger cabin. At 43,000 ft., the cabin altitude is around 7,100 ft. Should the cabin altitude reach 15,000 ft, the masks in the cabin, galleys and toilets will drop automatically. Small oxygen cylinders in the unit above your heads contain enough oxygen for around one hour use. When you start breathing through the mask, they provide pulses of oxygen — the higher the aircraft altitude, the longer the pulse of oxygen.

Don’t believe everything you see on TV

A loss of cabin pressure is commonly known as a decompression and these can be divided into two kinds: explosive decompressions and slow decompressions.

The dramatic sounding ‘explosive decompressions’ are the type most movies tend to depict. Something exciting has happened in the cabin and all of a sudden there’s a hole in the fuselage and all hell has broken loose. The aircraft starts ‘plunging’ and everyone is screaming surrounded by ‘the rubber jungle’ — the oxygen masks hanging from the ceiling.

Unfortunately for Hollywood, this kind of decompression rarely, if ever, happens. Even if there was to be a sudden loss of cabin pressure, the aircraft would not start to fall out of the sky. Sorry to ruin the illusion.

Slow Decompression

What is more likely is a ‘slow decompression’. And when I say ‘likely’ it’s all relative: In 10,000 hours of flying, I’ve never had a pressurisation problem.

In normal situations, in order to maintain the desired cabin altitude, air is pumped into the cabin from the outside and is then vented back out into the atmosphere. A number of outflow valves control just how much air is escaping, thus controlling the cabin altitude. Should more air be escaping than required, the cabin altitude will start to rise. This is what’s known as a ‘slow decompression’ as it takes place over several minutes.

The outflow valve on this Lufthansa 747 can be seen just below the German flag
The outflow valve on this Lufthansa 747 can be seen just below the German flag

In most cases, as I’m sure was the case with the Delta flight, the pilots will have known that the cabin altitude was slowly deviating from the norm. The EICAS (Engine Indicating and Crew Alert System) alerts the pilots that there is a problem with the pressurisation system with the ‘CABIN ALTITUDE’ alert. This then sets us into a flow of well practiced procedures.

In the Flight Deck

As soon as the pilots are aware of a problem with the pressurisation system, before doing anything else, they immediately don their oxygens masks.

In the flight deck, the oxygen masks are a little different (and more complicated!) to those in the cabin. In order to protect us in the event of smoke or fumes, the masks also contain fully enclosed goggles to protect our eyes. Once the masks have been donned, establishing communication with the other pilot is key.

The pilot’s mask is more complex than those in the cabin and requires regular practice in its use.

Due to the goggles, our peripheral vision is drastically reduced and all audio conversation via the intercom is conducted over a background of heavy breathing. If you think of a standard Darth Vader sound effect, you won’t be far off understanding what it sounds like. This is part of the reason why we will delay making an announcement to the passengers too soon.

With the masks on, the adrenaline is certainly flowing. Now is a great time to take a deep breathe and ‘cage the chimp.’

Once we’ve established communication and have taken a few seconds to take the situation in, it is time to first make sure that the aircraft is doing what we want: Fly. Navigate. Communicate.

Fly: Who is flying the aircraft? Is it maintaining speed and altitude as required? Is the autopilot still on? Navigate: Are we going where we want to? Are we over any high terrain (more on this later)? Communicate: Do we need to talk to anyone? Can we hear each other alright?

Only after all this is it time to look at the problem by opening the CABIN ALTITUDE electronic checklist.

Slowly slowly, catchy monkey

The first two items of the checklist have already been completed so now is the first time we actually have a look to see what’s going on. By checking the current cabin altitude and the rate at which it is climbing, we can make an assessment of how urgent the situation is.

Remember, in the cabin you may still be unaware that anything untoward is going on. The oxygen masks will only drop automatically once the cabin altitude reaches 15,000 ft. Our aim, if possible, is to stop this from happening.

Depending on what the problem is, we may not be able to stop the cabin altitude from climbing. If this is the case, we can deploy the masks in the cabin before we start the next stage of the procedure. The PASS OXYGEN button is a guarded switch, with a plastic cover over it. This is to ensure that both pilots confirm the correct switch before activating the system.

The passenger oxygen button in the flight deck.

Rapid Descent

The Boeing training manuals use the technical term ‘rapid descent’. It’s not an ’emergency plummet’ or an ‘abnormal plunge’. It is a controlled descent at a rate a little faster than a standard descent. The training manuals even go as far as to say that the manoeuvre is designed to “bring the aircraft down smoothly” to a safe altitude in the minimum time.

To start the descent going, the same FNC principles are applied.

Fly: What altitude do we want to descend to? Ideally, we’d like to descend to 10,000 ft. Here, with a total decompression of the aircraft, the air is safe to breathe for both passengers and pilots so we can all take our oxygen masks off. We use the autopilot control panel to instruct the aircraft to start the descent down to 10,000 ft.

Navigate: Are we okay on our current course or do we need to make a turn? Over the Atlantic, there are set procedures to turn off our route in the case of a deviation from our cleared altitude.

Communicate: Who do we need to tell about the descent at this stage? ATC are the first people who need to know so we will tell them exactly what we are doing so that they can move other aircraft out of the way.

You’ve probably experienced this before…

The wing on the 787 is so efficient that it just wants to fly. This is great in most situations, but not great if we need to get down fairly quickly. In order to increase the descent rate, we use the speed brakes. These are big panels on the top of the wing which increase the drag and enable us to descend at a faster rate.

With the use of full speed brake, we can achieve a descent rate of around 4,000 ft./min., enabling us to get down to 10,000 ft. in around eight minutes.

This may sound dramatic, but I’d be willing to bet that you’ve actually experienced a 4,000 ft./min. descent before without actually realising it. Every so often, ATC leave us high on our desired descent profile to land. In order to lose the excess altitude and get back onto the profile, we use the speed brakes to increase our rate of descent — a rapid descent.

Descent over Mountainous Terrain

Descending down to 10,000 ft. is fine if you’re over the ocean, but what if you’re flying over mountains when you experience a decompression?

At all times of flight, pilots are aware of their MEA (Minimum Enroute Altitude). This is the altitude down to which we can safely descend and be clear of obstacles on the ground. However, if the MEA is above 10,000 ft., we must take a different course of action.

MEAs (in red and purple) over the Himalayas can be substantial

Flying over the European Alps, Mont Blanc gives an MEA of 17,800 ft. Over mainland USA, the Rockies can give a MEA of 17,000 ft. Parts of the Himalayas have MEAs of 31,100 ft. These are some pretty serious mountains.

If the MEA is above 10,000 ft., we must first descend to the MEA before making any further descent. Over the Alps, the area of high MEA is fairly short lived. However, in other areas, the MEA could be high enough for long enough to exhaust the one hour of oxygen provided in the cabin. In these situations, we have ‘escape routes.’

Escape Routes

If a segment of a route would cause problems in the case of a rapid descent, airlines publish ‘escape routes’ for their pilots to follow. Good examples of these are flying in proximity to the Himalayas and across the Andes in South America.

Approaching these areas, the crew will review the details of the escape route and load this into the secondary flight plan of the Flight Management Computer, ready to activate should the need arise.

The escape route will necessitate a turn off the planned track and onto a new prescribed route before the pilots can start the descent from the higher MEA to 10,000 ft. This will naturally take longer to achieve than a normal rapid descent so the passengers and will crew will have to keep their masks on until told otherwise.

Descent Complete

Once we have brought the aircraft safely down to 10,000 ft., we can take our own masks off to breathe and communicate normally. Once again, now is a good time for another FNC and finally it’s time to communicate with the cabin.

During the descent, flying the aircraft is the most important task. We don’t want anything to distract us from this. In addition, when we talk to the cabin, we want to be heard clearly and reassure our passengers. Transmitting a muffled announcement with a Darth Vader voice achieves neither of these.

At this stage, we can clearly explain what has happened and what the plan for the safe continuation of the flight is.

Bottom Line

Should the highly unlikely happen for real, we are well practised in what we need to do to bring the aircraft safely down to a lower altitude.

There are plenty of masks for all passengers on board all aircraft and enough oxygen to last longer than would ever be required. That said, if you ever see a mask appear in front of you, please put your own mask on before helping anyone else. At 43,000 ft., you have around 15 seconds before you’ll be unable to put your own mask on. Put yours on first then help others if they require assistance.

Whilst a rapid descent may seem alarming with all the masks down, the descent itself is no different to one you’ve probably already experienced before. The reason why your pilots won’t speak to you for a while is because they are ensuring that they are focusing 100% on the task of flying the aircraft safely.

Once the descent is complete and the aircraft is maintaining an altitude where it’s safe to take the masks off, that is the time when your pilots will speak to you.

So, next time you fly, please take a few moments to read the safety card and watch the safety demonstration. The crew are doing it for your benefit.

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