First and foremost you should read the accident report which you can find in full here. I will pull some excerpts from that below.
Helios Flight 522 crashed when the crew unknowingly suffered hypoxia.
Although the masks in the passenger cabin deployed, the crew were
unaware of this and didn't realize the extent of the problem.
This is the one of the debated/interesting aspects of the accident. The crew would have been alerted by alarms to the depressurization event
...Non-identification of the warnings and the reasons for the activation
of the warnings (Cabin Altitude Warning Horn, Passenger Oxygen Masks
Deployment indication, Master Caution).
Many might say they did in fact know there was a problem but for some unknown reason they did nothing about it. Generally speaking "Don Oxygen Masks" is the first item on the emergency checklist following the mishap of the South Dakota LearJet crash in 1999. One can conclude from the report that its possible the pilots assumed the cabin pressure system was in the proper place (auto) when in fact it was set to manual (MAN) and subsequently were confused by the alarm or thought it was faulty.
...that the following factors could have contributed to the accident:
omission of returning the cabin pressurization mode selector to the
AUTO position after non-scheduled maintenance on the aircraft;
This is a procedural failure on the implementation end. The report would cite bad procedures for this
...the report also identifies a number of additional safety deficiencies
pertaining to: maintenance procedures; pilot training, normal and
emergency procedures; organizational issues of the Operator;
organizational issues related to safety oversight of maintenance and
flight operations by Cyprus DCA, EASA/JAA and ICAO; issues related to
the aircraft manufacturer’s documentation for maintenance and flight
If I was a passenger on a plane at cruise altitude and the oxygen
masks deployed, I know that there is only about 10 minutes oxygen
available. I would expect the plane to immediately descend to a lower
There are some assumptions here of course first and foremost that the plane is still maneuverable. There are any number of things that could cause a depressurization and some may also cause flight control issues as we saw Aloha Airlines Flight 243 a situation where the crew did not immediately know what was going on. However generally speaking, once the issue has been identified the plane should start a decent if possible.
If I don't notice any change in attitude and there is no passenger
announcement after a few minutes, would it help to alert a flight
attendant to check that the pilots are aware of the issue? Or would it
be best to assume that 'they know what they are doing' and just keep
This of course brings up another debate. In almost all cases you should alert the crew to anything you feel is strange our out of place. Of course in this instance it may be hard to move about the cabin, and potentially hard to shout or get your message to the crew. In some cases the crew may be working the issue and getting in the way of that can be problematic. Again generally alerting them is a good thing but its a situation by situation basis for sure.
In regards to the comments on other answers you will not die, at least one flight attendant (who was also a pilot) survived the ordeal and made his way into the cockpit as was heard on the CVR
At 08:54:18 h, the following distress was recorded by the CVR “MAYDAY,
MAYDAY, MAYDAY, Helios Airways Flight 522 Athens … (unintelligible
word)”. A few seconds later, another “MAYDAY, MAYDAY” with a very weak
voice was recorded.When the Boeing 737 was at about 7 000 ft, the
person in the Captain’s seat for the first time appeared to
acknowledge the presence of the F-16s and he made a hand motion.
Could a knowledgeable passenger have saved Helios Flight 522?
Unlikely as these days its hard to get in the cockpit, or even get a message to the pilots. But you should, in any event at least try. This saved some lives on Air Florida 90 when a passenger realized icing on the wings and assumed the crash position and informed the other around him.