The US FAA has finalised a rule that requires operators of 741 older generation US-registered Boeing 737s to install new warning systems within three years.
"We are issuing this [airworthiness directive] to prevent failure of the flight crew to recognize and react properly to a valid cabin altitude warning and takeoff configuration warning horn, which could result in incapacitation of the flight crew due to hypoxia and consequent loss of control the aircraft", says the FAA.
The AD, which requires installation of new warning lights in the centre cockpit console of some aircraft and activation of cabin altitude warning and takeoff configuration warning lights in other aircraft, is linked to the 2005 crash of a Helios Airways Boeing 737-300.
In that crash, the aircraft climbed to its cruising level without automatic cabin pressurisation selected and the pilots misinterpreted the cabin pressure alert when it operated.
The aircraft flew its programmed route from Cyprus to Greece on flight director/autopilot with all passengers and crew on board unconscious from hypoxia, crashing in an uninhabited area near the approach to Athens airport when the fuel was exhausted.