From available accounts, a scenario suggests itself for the Aug. 14 crash of the Helios Airways B737-300 near Athens: the aircraft was never fully pressurized to begin with. Why it wasn't fully pressurized gets to the duplicitous nature of the cabin altitude warning system.
Only eight minutes after the aircraft took off from Cyprus, it was at 14,500 feet with the cabin altitude warning annunciated (triggered passing through 10,000 feet in the B737). Reaching such an altitude in eight minutes requires just under a 2,000-feet-per-minute rate of climb. The only way for the cabin altitude to get there that fast is if there was incomplete pressurization or, worse, no pressurization.
Let's consider the lesser of the two scenarios--incomplete pressurization. A hissing around a door was written up after the previous flight, a problem that had plagued the airplane since December 2004. The Helios mechanics had to pressurize the fuselage on the ground to make sure their fix had worked. After doing so, unfortunately, the mechanics left the outflow valve in MANUAL. That means that (unlike AUTO), the outflow valve stays in the present position (found by the Greek investigators to be one-third OPEN).
After takeoff, the engine bleeds output would not have been able to achieve full differential; therefore the cabin altitude would have climbed rapidly. The pressurization warning would have sounded at 10,000 feet cabin altitude.
Without full use of the air conditioning packs, there would have been inadequate cooling air for the avionics, which possibly could have precipitated the reported problems with flight displays, the flight management computer (FMC) and so forth. Whether or not those problems actually occurred together with the pressurization warning, the crew may still have attributed the familiar warning horn to notification of an avionics "configuration" problem, and not to cabin pressurization (and lack thereof). If the avionics cooling reset instructions received from the ground engineer seemed logical, then a continued climb per their clearance would likewise have been acceptably routine.
One assumption is that at 34,000 feet, a 27,000-foot cabin altitude would have resulted. That's enough to put the crew to sleep but not to kill them. But over a few hours, they'd end up brain-dead. It also would explain why the F-16 pilots sent to intercept the "renegade" airplane commented that the windows were not fogged up (per the pressurization problem that killed golfer Payne Stewart and five others in a 1999 Learjet 35 crash). That was because the warm pressurizing air was being pumped into the B737 fuselage but full cabin differential could not be achieved because there was a largish hole for that air to escape: the one-third open outflow valve.
It's likely that the flight attendants were told something about there being a problem, and that would have allayed initial alarm about the passenger oxygen masks deploying. They may have noted that the aircraft was continuing its climb and assumed all was well. Only when the passengers started dropping off into unconsciousness would the flight attendants have realized that all was not well. However, it would take only about 4 to 5 minutes in that continued climb for the pilots to succumb to hypoxia. The time of useful consciousness is less than a minute above 30,000 feet, but exposure during the climb would have had them comatose well before the autopilot leveled the aircraft for cruise.
Despite airline procedures for flight attendants to remain seated on oxygen, one or more flight attendants may have burst into the cockpit. They would have tried desperately to revive the pilots with their own oxygen, but probably would not have been knowledgeable enough to select 100 percent or ensure a good facial seal.
The weak MAYDAY calls reportedly heard on the cockpit voice recorder possibly were made on the intercom and/or just picked up on the cockpit area microphone. The flight attendants may not even have been able to identify the transmitter selector switch or the VHF radio's press-to-talk button.
Alternatively, the captain may have left his seat to check the status of, or cycle, circuit breakers, and then collapsed. In any event, the entire flight crew and passengers were eventually overcome by hypoxia as the airplane flew on autopilot along its programmed track where, despite a belated attempt by a trainee pilot/flight attendant to assume control, it ran out of fuel and crashed north of Athens.
A shift of the outflow valve to AUTO is all it would have taken to save the day.
The crew had received two indications that something was amiss with pressurization, yet both indicators were missed.
One would think that a +/-2,000 feet-per-minute cabin altitude change 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 oxygen masks made their appearance at 14,000 feet.
There is also the case of the Ryanair incident on Nov. 8, 2004, in which the captain of the B737-200 made a BLEEDS OFF takeoff from Reus, Spain, in order to utilize maximum engine thrust for takeoff. He didn't don the oxygen mask, but did descend, discovering that the rise in cabin altitude was due to the lack of bleed air to the air conditioning packs. The airplane made a diversionary landing.
On the Helios jet, the warning horn that the crew heard but did not cancel was a sufficiently mind-numbing distraction for the flight crew, as their minds were oriented towards avionics cooling. Nor did they pick up on or divine the significance of the visual indication that the system was in the manual mode.
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 in the event of a takeoff configuration problem. Using one horn 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 canceled by pressing a not very obvious button, labeled "Alt Horn Cutout." The fact that the horn was never canceled on the Helios jet is another clear pointer to the crew's confusion about the true (and completely different) reason for that familiar horn sounding. Would a cancelable voice message clearly annunciating (twice) the problem as "Pressurization/Check Cabin Altitude, Beware hypoxia, Don oxygen mask immediately" have helped avoid this accident?
2. The green light. If maintenance technicians did not switch the pressurization system back to AUTO, the crew would have had to miss the MANUAL light on every scan of the instrument panel. But if they missed the deadly significance of this innocuous green light, perhaps an indication that something requiring attention and control shouldn't be a steady green illumination. It should be amber, maybe even flashing, but surely not merely "advisory" green. It should be noted that on other aircraft the light is amber or red, depending on the cabin altitude.
3. Descend for depressurization. An automatic 90-degree turn off the airways and descent, enabled by the flight management system, is featured on the Cessna Citation VII (7-to-8 passengers) and the Citation X (8-to-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. For large jets like the B737, 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.
As the Helios tragedy indicates, adequate warning to the crew of a pressurization problem needs to be unambiguous, explicit and unique. Failing crew corrective action due to incapacitation, prompt automatic descent to an altitude where they will revive is critical.