The Helios 737 Precursor Events

The earliest Symptom is Impairment of Judgment

INSIDIOUS comes from the Latin "insidiae" (ambush)



Aer Lingus737-500 EI-CDF on 16 Jan 2000 AAIU Report 2003-013 of 29 Sep 03

Multiple system faults. After flight trouble-shooting was unable to locate the exact cause for loss of pressurization at 37,000ft due to widely varying modification states of the components. See also 2003/012

Ryanair 737-800 EI-CSC on 07 Oct 2000

AAIU Report 2001/0018 of 30 Nov 01

Eng Bleed Air Switches left OFF and  pax masks deployed (incident left unreported). Flight-crew failed to go onto oxygen during descent and cabin crew failed to report any abnormalities.

Aer Lingus 737-548 EI-CDB on 07 Dec 2000

AAIU Report 2001/014 oof 21 Sep 01

Continued climb unpressurized with Captain electing not to go on oxygen after a Cabin Alt warning. Both packs had been left OFF

Aer Lingus 737-500 EI-CDD on 09 Dec 2000

AAIU REport 2003/012 of 29 Sep 03

Mayday call at FL370 after pressurization failure. Trouble-shooting found an AUTOFail Transfer fault in the cabin pressure controller and a further fault in the AC actuator for the outflow valve

Ryanair 737-204 EI-CJE on 28 Sep 2002

AAIU Report 2003/010 of 06 Aug 03

5th serious Irish 737 pressurization incident since 2000

Bleeds OFF t/off with F/O accidentally later turning Aircon Pack Switches OFF. Flt continued on up to FL270 and pilots both vaguely recollect hearing warning horns....which they mistook to be the configuration warning horn.

Crossair MD-82 HB-INW on 18 Dec 1997

AAIU Report 1998/011 of 26 Nov 98

Loss of cabin pressure control at FL370 followed by an emergency descent. Outflow valve sticking due to contaminant build-ups

Ryanair  737-204 EI-CJC on 08 Nov 2004

AAIU Report 2005/009 of 23 May 05

APU ON but Engine Bleeds OFF take-off and climbed to FL320. APU left ON & managed to maintain cabin altitude up to FL170 (but this eventually ran away to 15,000ft). Diverted Biarritz with hypoxic passengers. After t/off checklist is silent (not challenge/reply)

Alaska Flt 506 737-700 on 25 Mar 2000 Flight-crew sacked & their Licences revoked for continuing the flight with pax oxy masks down


ASTREUS 737-300 Flt AEU952 of

19 Aug 2005 ()@ 1930L) - NO LINK YET

Mahon to Leeds Bradford flight diverted Brest after depressurization in the cruise (pax masks dropped at Flight Level 360)
Hypoxia Information High altitude respiratory physiology
Hypoxia Study as linked from EI-CJE Report (above)
AAIU Report No. 1999/0019 Incapacitation of a Captain

A Scenario That Explains Much


 From the available accounts, a scenario suggests itself for the crash Aug. 14 of the Helios Airways B737-300 near Athens: the aircraft was never pressurized to begin with (see ASW, Aug. 22). Consider that the aircraft took off from Cyprus at 9:07 a.m. and at 9:15 a.m., only eight minutes later, it was at 14,500 feet with the cabin altitude warning (triggered passing through 10,000 ft. in the B737) annunciated. To reach 14,800 ft. in eight minutes is just under a 2,000 feet-per-minute (fpm) rate of climb, which is probably about right, and the only way for the cabin altitude to get there that fast is if there was no source of pressurization at all.

Additionally, without the air conditioning packs, there would have been no cooling air for the avionics, which would likely precipitate problems with flight displays, the flight management computer (FMC), and so forth. Those problems occurring at the same time as the pressurization warning might cause a crew to mis-attribute the source of the warning to the avionics problem, and not to cabin pressurization (and lack thereof). If the computer cooling reset instructions received from the ground engineer seemed to be having some effect, then it's possible that the flight crew decided that continuing to climb from 10,000 ft., where they may have leveled off briefly, was acceptable, still not recognizing that the real source of the problem lay with the lack of an air supply from the packs, and not a failure of the avionics cooling system or ground/air sensing.

As the climb continued without pressurization, the hypoxic effects would quickly become more severe and the warning would continue to be ignored, having already been attributed to the avionics cooling. With mental capacities decreasing and pilot fixation still centered on the avionics cooling problem, it's quite possible the crew lost consciousness without ever realizing what the source of their problem was (as exemplified by the apparent fact that they never donned oxygen masks).

It's likely that the cabin crew were informed about the reason for any interim level-off or problem, and that would have allayed initial flight attendant alarm about the rubber jungle (passenger oxygen masks) having come down. The flight attendants may have noted the aircraft continuing its climb and assumed all was well. Then too, the flight attendants (F/A's) would probably also be unaware that the aircraft was continuing its climb. Only when the passengers started dropping off into unconsciousness would they have definitely realized that all was not well. Unfortunately, it would only take about 4 to 5 minutes (in that continued climb) for the pilots to succumb to hypoxia (time of useful consciousness is measured in seconds above 30,000 ft.). By the time the airplane reached 34,000 ft., it's quite possible that the pilots were brain dead.

Despite airline procedures for F/A's to remain seated on oxygen, and/or a few to attend the passengers while using a portable bottle themselves, one or more flight attendants burst into the cockpit. They would have tried desperately to revive the pilots with the pilot's own oxygen, but probably would not have been knowledgeable enough to select 100% or ensure a good facial seal. Eventually, they'd have removed the captain to the rear to free up his seat for the attempt to fly a recovery. The inability to transmit via the radio probably relates to the microphones being diverted to the oxygen masks as much as still being on Nicosia frequencies. The weak MAYDAY calls reportedly heard on the cockpit voice recorder (CVR) were possibly made on the intercom and/or just picked up on the cockpit area microphone (CAM). The flight attendants may not even have been able to identify the transmitter selector switch or the VHF radio's press-to-talk (PTT) button.

It would seem that a +/-2,000 fpm cabin vertical speed should have been noticeable to the flight crew, but then again there was a very similar incident at Alaskan Airlines on March 25, 2000, in which a B737NG departed Portland, Ore., for San Jose, Calif., with engine bleed air switches left at OFF. The crew failed to recognize the condition until the alarm went off at 10,000 feet and the rubber jungle made its appearance at 14,000 feet. The pilots were later fired for pressing on to destination (see ASW, Sept. 11, 2000; see also; for a case where air-bleeds are left off, and the pilot doesn't don the oxygen mask but does descend, see www.aaiu/upload/general/3496-0.pdf; see extract from this investigation in box at this page, above).

In any event, the indicators are that the Helios jet possibly departed with the engine bleeds OFF (a common practice in runway/weight limited situations), or with air conditioning packs off.  However quite neatly, to complete this deadly package, when the engine bleed switches are left OFF, it is all the more likely that some avionics overheating will occur, because of the lack of air conditioning. That is, the lack of air conditioning serves to induce the confusing scenario - although the likelihood is that the only indication the crew had was the "take-off config" warning horn. The fact that engine bleed air switches are left off so easily is exemplified by the Alaskan Airlines incident. Then again, the Helios 737's pressurization problem could have been a repeat of its Dec. 4 door-seal glitch (as that same door was alleged to have been causing a hissing noise on the flight prior to the crash - and was written up as a defect). The fact that there was no condensation fogging the windows would seem to indicate that some aircon air was being pumped in and that the pressure vessel itself may have been compromised (leaking door-seal, cracked door frame, thermal-acoustic blanket jamming the outflow valve (per the Miami A300 accident)). Equally, it may have been a pack or bleed switch out of position, or an outflow valve left at OFF (and not at AUTO for setting of the correct cabin differential pressure). The warning horn that they did not cancel was a sufficiently mind-numbing distraction for the flight crew, as their minds were oriented towards avionics cooling - which sometimes happens on the ground, and as evidenced by what they said in their radio call to their maintenance.

Three points are worth considering:

1. The dual function of the warning horn. The altitude warning horn serves a dual purpose, also sounding on the ground (only) in the event of a take-off configuration problem (such as forgotten flaps, etc.). Two horns being one and the same for two completely different abnormal situations may help to complete the deadly loop quite nicely. This is particularly so as the problem on the ground is addressed by simply retarding the throttles - yet the cabin altitude warning horn can only be cancelled by pressing a not very obvious button (labeled "Alt Horn Cutout"). The fact that the horn was never cancelled on the accident flight of the Helios jet is another clear pointer to the crew's confusion about the true (and completely different) reason for that familiar horn sounding. It might be better to have a voice message clearly annunciating (twice) the problem as "Pressurization/Check Cabin Altitude, Beware hypoxia, Don oxygen mask immediately." That might have helped avoid this accident. visual annunciation of the cabin altitude warning horn's advisory would also be appropriate.

2. Descend for depressurization. An automatic 90 degree turn off the airways and descent, enabled by the FMS, is featured on the Cessna Citation VII (7-8 passengers) and the Citation X (8-10 passengers). This emergency descent and automatic level off at 15,000 feet can be activated any time the aircraft is cruising above 35,000 feet (see ASW, Nov. 8, 1999). For large jets like the 737, this feature could include a turn and descent after three minutes if the crew had not retarded thrust levers for an emergency descent. Locked cockpit doors make this facility even more desirable.

3. Training and hypoxia experience. The Helios accident makes the case for some aviation medicine training and hypoxia experience in an altitude chamber being made mandatory for airline pilots. The military has long recognized that individuals have highly personal onset symptoms for hypoxia, and that they need to be intimately familiar with them. (For a good reference to high altitude respiratory physiology, see A listing of hypoxia-related incidents and discussions may be found at This collection shows that the Helios accident was not a one off occurrence and that action needs to be taken by manufacturers and regulators.)

Courtesy: "Air Safety Week" of  29 Aug 2005

Addenda:  The anomaly supposedly found thus far by Greek investigators (via Greek Chief Air Accident Investigator, a Capt. Akrivos Tsolakis) was that the Flt ZU522 pressurisation was in manual not that the bleeds were off. Capt. Tsolakis advised that the crew "took off with" the pressurisation outflow valve 1/3 open and in 'manual' mode (instead of Automatic) and therefore that there was not enough air differential being generated to pressurise the aircraft and that hypoxia remains a strong possible explanation i.e.  if it was 1/3 open then the cabin would have climbed at a slower rate than the aircraft. He said that the cabin never made the max differential pressure, not that it was not pressurised. This would go a long way towards explaining why the windows were not fogged and why passengers were unconscious but alive at impact. A possible and plausible reason for crew selection of Manual is given below. It may also explain the recent pax complaints about "being cold" in that particular airframe.

These very recent Helios revelations point towards the possibility of a repeat of that earlier 20 Nov 2000 Miami A300 incident where the thermal-acoustic blanket jammed the outflow valve after landing, leading to an explosive decompression at door-opening (a male F/A was ejected onto the tarmac and killed). Seeing as all airlines have had to recently replace (due to an AD) the metallized mylar blankets with Tedlar (or similar), poor blanket re-installation during that swap-out must be considered a candidate.

If the captain did leave his seat and went aft (on a red herrings quest) to check or trip/reset avionics fan CB's, leaving the relatively inexperienced F/O in his seat to continue climb, then that would explain much. Captain soon passes out in cabin or rear of flight-deck circa a cabin altitude of 20,000ft (with a/c altitude somewhere above that).  The relatively inexperienced F/O would've been then quite nonplussed by developments - never ever figuring why the horn was blowing, how to cancel it and so soon/finally succumbing himself.

   Deadly Doors (and a Possible Connection to the Helios Flight ZU522's Crash)


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