At 42.21 minutes of this video about the crash of Helios' Flight 522 (a Boeing 737) caused by cabin depressurization, it was established that a major cause of the crash was a design flaw -- the main wire and the backup of a wire running from the cockpit to the depressurization valve runs through the same channel. So, if one wire burns up and fails, they both do. This is evidently contrary to safe design practices. Also, it seems that the cockpit instruments in the cockpit do not give sufficient indication during the preflight and takeoff checks that this problem exists.

From this video, it appears that this problem has not been addressed by Boeing. Is this still the position on current date?

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    $\begingroup$ Using the term "depressurization" with regard to the Helios crash is a little dubious... the cabin never pressurized in the first place. While various factors contributed to the crew's mistake, the bottom line is that they had a fully functional 737, albeit with one switch in an uncommon position (that they failed to catch & correct). Compared with every other 737 out there, there was nothing wrong with theirs that caused the crash. Discussions of wiring & backup wiring & etc is all hypothetical, and might explain another accident. This one, however, is entirely explained by human factors. $\endgroup$
    – Ralph J
    Commented Oct 12, 2015 at 18:47

2 Answers 2


The Wikipedia article on Helios Airways Flight 522 also mentions this wiring issue, and cites a separate report regarding a May 2003 incident in which a 737 had issues with pressurization. During the investigation it was discovered that wiring had shorted and burned in a particular bundle of wires, called a loom, which was important for control of pressurization. The AAIB issued a safety recommendation regarding this issue:

Safety Recommendation 2004-33

It is recommended that in order to prevent failure of the cabin pressure control system in the event of damage to wiring loom W298, the Boeing Commercial Airplanes should consider, on the Boeing 737-436 and similarly configured models, separating or protecting the wiring associated with the different modes of operation of this system, which connects the cabin pressure controller to the rear outflow valve, such that any single point failure of the loom would not result in effective failure of the pressurisation control system.

In their 2006 progress report, the AAIB followed up on this recommendation:

After a review of the issue and the service history of the pressurization system for the 737-100 thru - 500 model airplane, Boeing felt that modifications of the wiring with the intent of separating the outflow valve functions consistent with the subject safety recommendation would not significantly contribute towards a reduction in the overall rate of depressurization events. Their rationale was based on the basic design philosophy and concept of the original system which includes:

1) A single, mechanical outflow valve which is controlled by a single multi-mode analog pressure controller and driven by independent AC or DC powered electrical motors. Each motor can also be manually commanded open and closed via separate switch on the pressure control panel.

2) In the event of a complete power/control loss (e.g. - electrical interruption or damaged wiring) either motor will remain in last commanded position. The airplane is equipped with an independent aural warning system (cabin above 10,000 feet) and visual cues (cabin above 14,000 feet) should this condition lead to a cabin pressure loss.

3) The airplane is equipped with both fixed and portable oxygen supply systems for passengers and crew.

4) If pressure cannot be reliably controlled/restored within a short period of time, the flight crew will utilize procedures (such as rapid reduction in altitude, declaration of emergency and landing at nearest suitable airport) which mitigate the exposure time to low pressure.

Status - Rejected - closed

So the rationale is that if the system fails, the crew is alerted and may take appropriate action. Note the following regarding an AD published in 2011 with a deadline of March 2014:

The FAA released Airworthiness Directive AD-2011-03-14 requiring all Boeing 737-100, -200, -300, -400 and -500 types to install two additional warning lights, one for takeoff configuration and one for cabin altitude warning, and introduce updated documentation within 36 months following March 14th 2011.

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    $\begingroup$ Bottom line: The design is good, it allows to control each valve with two different motors and circuits, and in addition it can be manually operated. In case both circuits are damaged, an already low likelihood event, the crew is still alerted and can operate the valve manually and/or proceed to emergency descent. It's enough to make the alert redundant to respond to the accident. $\endgroup$
    – mins
    Commented Oct 12, 2015 at 18:16

I would like to add that the linked video deviates in major ways from the truth - or at least from the Greek accident board's assessment.

Firstly, the final report of the Air Accident Investigation & Aviation Safety Board (AAIASB) does not mention the wiring problems at all. Nor does it mention anyone noticing a "burning smell" as is said in the video. If they ever considered it a possibility it had obviously been ruled out before the final report was issued. In fact the report does reference several prior 737 pressurization incidents, each of which were attributed to failures in the operation of the controls and not any malfunction. Their recommendations all pertain to changes in the wording of procedures and changes in the cockpit warnings, i.e. a different warning horn and more noticeable indicators on the panel.

The firm conclusion of the investigation was that the pressurization switch had been left in the manual position and the pilots misinterpreted the warning horn. This was evidenced by

1) the position of the switch as it was found in the wreckage

The cabin pressure mode selector was confirmed to be in the MAN (manual) position by physical witness marks on the mode selector input shaft and by electrical continuity checks in the mode select circuitry. *

2) the Non-Volatile Memory (NVM) chip inside the controller

The NVM recording showed that on the accident flight, the cabin pressure control system was being operated in the manual mode. *

3) conversations with ground personnel

The Helios Dispatcher reported that the Captain referred to the Takeoff Configuration horn, and then the Equipment Cooling lights. *

The video also said that the the F-16 pilots saw two people struggling in the cockpit when they arrived. I have no idea where they got this (made it up maybe?) because the investigation states,

When the flight HCY522 was intercepted by the F-16s, the F-16 lead pilot reported that there was no visible damage to the Boeing 737 aircraft, that the Captain’s seat was vacant, the person in the First Officer’s seat was not wearing an oxygen mask and was slumped over the controls, and some seated passengers in the cabin were observed wearing oxygen masks. *

It appears to me that the linked video, which is titled "Aircrash Unsolved: The mystery of Helios 522" is taking a lot of liberties with reality to make a dramatic "mystery" where no real mystery exists. The fact that the YouTube account that posted it misrepresents it as an episode of "Air Crash Investigation" is bothersome. The latter show usually does a much better job of being accurate.

* Taken from the AAIASB final report.

I realize this is really not an answer to the actual question. Fooot's answer covers the question very well. This should really be placed as a comment but it is way too long for comments.


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