Mirroring a February rule for early generation Boeing 737s, the FAA on 24 March will propose new rules to prevent pilots of next generation 737 models from succumbing to hypoxia due to cabin pressurisation problems.

The classic 737 airworthiness directive (AD), finalised in February, gives operators of the older-variant Boeing 737s three years to incorporate new warning lights in the centre cockpit console of some aircraft or to activate cabin altitude warning and takeoff configuration warning lights in other aircraft.

The mandate is linked to the 2005 crash of a Helios Airways Boeing 737-300 near Athens after most of its passengers and crew experienced hypoxia. An investigation revealed that the pilots did not recognise the aircraft failed to pressurize as it climbed, an error the AD attempts to correct with specific warning lights and alerts.

As produced, the 737 makes use of a common intermittent horn warning system for takeoff configuration errors and cabin-altitude errors when above 10,000ft (3,048m), a combination the US FAA says can confuse pilots. The agency says it discovered 25 instances in which crews misinterpreted a altitude warning as a configuration warning despite the fact that the configuration warning is inhibited when airborne.

Under the proposed next generation 737 AD, operators of 650 US-registered 737-600/700/700C/800/900 and 900ER twinjets will be required to install specific altitude-warning lights in the cockpit and make changes to the aircraft flight manual within three years of the finalisation of the mandate.

Source: Air Transport Intelligence news