Changing pressure: How the crash of Helios 552 made flying safer

Last week marked the sad anniversary of the 2005 accident of Helios flight 552. Operating a flight between Larnaca, Cyprus, and Athens, Greece, the Boeing 737 performed an uneventful departure on that August morning.

As the aircraft neared its cruising altitude, however, Air Traffic Control (ATC) lost contact with the pilots. Fighter jets were scrambled to intercept the aircraft but they were unable to make contact with the crew. It transpired that the 737 had lost cabin pressurization and the pilots had become unconscious.

A short time later, both engines flamed out and the aircraft crashed in the hills near Marathon, tragically killing everyone on board.

How could such an accident occur, and what has the industry done since then to stop this from happening again? Here's how Helios flight 552 made flying safer for everyone.

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Why is the cabin pressurized?

The outside atmosphere at 39,000 feet is a pretty inhospitable place. Temperatures are a frigid negative 76 degrees Fahrenheit and the air is so thin that breathing unassisted is impossible. It’s for these reasons that airliners have a pressurization system to ensure 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.

As you climb higher into the atmosphere, the air pressure also decreases. This decrease in air pressure makes it increasingly difficult to breathe, reducing the oxygen saturation in your blood. If your oxygen saturation becomes too low, you become at risk of suffering from hypoxia.

In this state you become disorientated, confused and start to lose control of your coordination. Untreated it can lead to unconsciousness, cardiac arrest and death. That's why climbers who ascend to the peak of Mount Everest (more than 29,000 feet) usually require supplementary oxygen.

Contrast all this against the relative comfort of the inside of the aircraft and you start to realize just how well-designed these machines are. With a cabin temperature of 68 degrees Fahrenheit, there could be an 176-degree Fahrenheit differential across that small piece of glass. So, how is this possible?

During the flight, the pressurization system is normally 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.

How is the cabin pressurized?

The engines on a modern jet airliner provide more than just thrust to drive the aircraft forward. One of the other functions is to provide air to pressurize the cabin.

As air progresses through the stages of the engine, the pressure and thus the temperature also increases. When these two are at their highest level, a certain amount of this air is “bled” off to provide a supply for a number of aircraft systems. This is called “bleed air" and, depending on the aircraft type, this air is then fed to the various systems — one of them being the pressurization system.

On an aircraft such as the Boeing 777, high-pressure bleed air is directed to the air conditioning packs, which sit in the belly of the aircraft. Using a combination of heat exchange methods, utilizing cooler (think: negative 76 degrees Fahrenheit) air from outside, the hot bleed air is cooled.

Once it has been cooled to an acceptable temperature, it is then directed toward a unit that removes moisture. After this, it heads to another unit where it's mixed with some of the original hot air, where the temperature required in the cabin is created.

Finally, the conditioned air flows into the cabin, providing the air to pressurize the cabin at a temperature that keeps you comfortable.

Next-generation air conditioning

As I mentioned, most aircraft use bleed air to pressurize the cabin. This means that, despite being filtered before entering the air conditioning systems, the air you breathe has still come through the engine. The Boeing 787 Dreamliner, however, is different.

Instead of using air from the engines, Boeing designed the aircraft to use air taken directly from the outside. This means that the ambient air on a 787 is fresh from the sky. As the engines aren’t then losing energy to power the air conditioning system, it also makes them more efficient. More efficient engines equal lower carbon emissions.

Air is taken into the aircraft by two dedicated inlets just below where the front of the wing meets the fuselage. To protect these inlets, two deflector doors deploy in front of them during normal ground operations and the landing phase of flight. This stops large contaminants such as stones and birds from being taken into the air conditioning system.

This air is then directed into four electrically operated Cabin Air Compressors (CACs). The air is pressurized and sent to two identical air conditioning packs. Each pack has two dedicated CACs, however, a single CAC is enough to power a single pack.

This airflow is controlled by regulating the cabin air compressors. CAC output is automatically increased during periods of high demand, for example, to compensate for a failed pack. It is also limited during times of low aircraft electrical output to ensure that there is enough power available to run other critical systems.

From here, the supply of air to the cabin is much the same as other aircraft, except when it comes to another important factor: moister air.

The air on the 787 is much moister than on other types, particularly compared to the 777. On the Dreamliner, the crew can input the exact number of passengers on board, and the air conditioning system uses this number to optimize the humidity of the air being directed into the cabin creating an environment much more like that on the ground.

What happens if the cabin pressurization system fails?

If you imagine the failure of the cabin pressurization, I'm sure you'll immediately think of dramatic movies where a window blows out and all of a sudden the cabin is in total chaos with everything being sucked out of the window. The reality is quite different (in a good way, for those of you nervous about flying), but in a bad way for those of you who like your action movies.

Instances of a rapid decompression, where the cabin altitude suddenly goes to that of the outside ambient air, are extremely rare. What is more likely is a slow decompression of the aircraft where the cabin altitude rises slowly.

Should the cabin pressurization system fail, oxygen is provided by an emergency system and, depending on the aircraft type, this 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 feet, the cabin altitude is around 7,100 feet. Should the cabin altitude reach 15,000 feet, the masks in the cabin, galleys and toilets will drop automatically.

Small oxygen cylinders in the unit above passengers' heads contain enough oxygen for around one hour of use. When you start breathing through the mask, they provide pulses of oxygen — the higher the aircraft altitude, the longer the pulse of oxygen.

However, in the flight deck, things are a little bit different.

In the flight deck

When the cabin altitude approaches 10,000 feet, an alert goes off in the flight deck. However, depending on the aircraft type this alert can be very different. On a modern aircraft like the 787, the aural alert will be accompanied by an obvious message on one of the screens notifying us of a problem with the pressurization system.

However, on older aircraft types without electronic checklists, the alert may be somewhat more basic, as was the case on the Helios 737.

Once we are aware of the pressurization problem, if we are unable to immediately control the cabin altitude, we don our own oxygen masks. At 43,000 feet, we all have around 12 seconds of useful consciousness before we start to become hypoxic. As a result, it's absolutely vital that we get our masks on before doing anything else. This is also why, in the cabin, passengers must fit their own masks before helping anyone else.

With our masks fitted, our next priority is to get the aircraft down to an altitude where it is safe for everyone to breathe the ambient air, normally 10,000 feet.

To do this, we start a rapid descent to lose height as quickly as possible. But please don't think this means we are falling out of the sky. All we do is close the thrust levers and extend the speed brakes, something which you will most likely have experienced on a normal flight without even realizing it.

The key point to note here is that there's a good chance that you may never even be aware that this is occurring. With the cabin altitude alert sounding as the internal pressure reaches 10,000 feet, the masks will not drop until it reaches 15,000 feet. As a result, depending on how high we are at the time, we may well have enough time to descend the aircraft down to 10,000 feet before the masks drop.

Helios Flight 552

The flight to Athens was scheduled to take around one hour and 30 minutes on the 7-year-old Boeing 737. Critically, the night before the flight, engineers carried out routine maintenance on the pressurization system.

As part of this process, they were required to change the system from automatic operation to manual operation. However, for unknown reasons, at the end of their checks, they did not return this to the automatic position.

It's always said that an accident never happens for one reason. When a number of factors line up, like the holes in Swiss cheese, then an accident occurs. Tragically, this was one of those events.

Boarding the aircraft the next morning, the pilots did not notice that the switch was still in the manual position. As a result, as the aircraft climbed away from the ground, the cabin did not pressurize. This meant that the altitude inside the aircraft continued to match the altitude outside the aircraft as it climbed.

As they passed through 10,000 feet, an aural alert did go off. However, the crew may have mistaken this for an erroneous take-off configuration warning, starting the confusion in the flight deck. As the aircraft passed 14,000 feet, the masks automatically deployed in the cabin and a caution light illuminated in the flight deck.

At the same time, due to a lack of cooling air normally provided by the pressurization, the crew received a warning about the temperature in the avionics bay. However, as they tried to troubleshoot this problem, the effects of hypoxia started to set in. Communication between the two pilots became difficult and they were unable to work out what really was the problem.

To glean some further information, they decided to contact the airline's maintenance base via radio. They told them that they needed to pull a circuit breaker to stop what they understood to be an erroneous take-off configuration warning. But as more time passed, the more hypoxic and confused they became and the less likely they were to realize the real problem: a lack of cabin pressure.

Not long after, both pilots became unconscious and the aircraft was left flying on autopilot until it ran out of fuel.

What has changed?

The events on that tragic day shocked the aviation world. How could an airliner simply fly along until it just ran out of fuel? Why did no one do something about it?

Whilst this situation was unfolding, there was still time to save the day.

As the aircraft passed through 14,000 feet, the masks in the cabin deployed automatically. We can only assume that the passengers and crew did as they are briefed and put the mask over their nose and mouth and breathed normally. And then they sat and waited. And waited. And waited.

Eventually, it appears that one crew member realized something was wrong and, using extra oxygen bottles, went into the flight deck.

From a few hundred meters away, the pilot of the fighter jet sent to investigate what was going on was able to see a figure moving in the flight deck. However, they were unable to make contact and by then it was too late. The aircraft soon ran out of fuel.

If only a crew member had made their way to the flight deck much earlier, they may have been able to alert the pilots as to what was going on.

As a direct result of this accident, airlines around the world reviewed their procedures in the event of a loss of cabin pressurization. Cabin crew are now taught to not only recognize the tell-tale signs of decompression in the cabin but are also taught what they expect to see, feel and hear the aircraft do: start an emergency descent.

If they do not sense the aircraft starting to descend within a certain time frame, they must make contact with the pilots to ensure they are OK and aware of what is going on in the cabin. With these new procedures in place, we should hopefully never see the sad events of August 2005 ever again.

Bottom Line

Air travel is a wonder of the modern age, but it's only possible due to the complex pressurization systems in today's aircraft. Should they fail, oxygen is provided for a long enough time for the pilots to descend the aircraft down to an altitude where it is safe to breathe the ambient air.

The crash of Helios flight 552 was an awful event that shocked the industry. There is, however, one thing the aviation industry does well — and that's learning from mistakes to ensure they don't happen again. With the change in procedures, both in the flight deck and in the cabin, commercial flying is safer today than it has ever been.

Featured Image by Charlie Page/The Points Guy.