That the US Federal Aviation Administration has seen fit to write yet another airworthiness directive about the Boeing 737's cabin pressurisation control system is a symptom of the fact that not only did insufficient thought go into designing the system in the first place, but also that nothing has been done to correct the design shortcomings since. This is by no means the only example of that syndrome at the FAA.

Readers could say of this comment that hindsight is a wonderful thing. Yes, it is, but the FAA has frequently failed to act according to the wisdom available from hindsight. It has issued two amendments to the 737 flightcrew operations manual so far to try to deal with repeated occurrences of crews failing to recognise that the system is wrongly set, and/or failing to respond correctly to the warning that the cabin is failing to pressurise. Both the directives papered over the cracks rather than eliminating them.

Boeing 737 
 © Boeing

The 737 series is such a good basic aircraft design that it has survived in production longer than any other jet airliner in history. That is now becoming the nature of modern aircraft designs, so permitting indefinite "grandfather rights" for onboard systems that are, frankly, outdated is a more compelling issue than it ever was before. It is no longer good enough to change the manual when repeated evidence suggests the system needs improvement or change, especially when more modern systems have been proven in other types.

There are numerous options for change in this case. The first would be to change the audible warning from the same sound as the take-off configuration alert instead of a repeating horn, a voice repeating "cabin altitude"? Instead of requiring the crew to select the pressurisation control from manual to automatic when cabin altitude exceeds 10,000ft (3,050m), make the change automatic, with audio advice of its operation. Engineers could certainly come up with alternatives such as those.

There are other cases in which the FAA has still not acted on required changes years after the fatal events that flagged up the need for them - such as not requiring dry-running fuel pumps to switch off automatically in 737 centre-wing fuel tanks, or not requiring spoilers to retract automatically if the crews firewall the power levers. Or when the FAA allowed the 737 to have a rudder power control unit (PCU) that did not comply with the fundamental requirement that it should fail safe, and all on board a United 737-200 and a USAir 737-300 died because of it. In that case the body count was so high that the FAA and Boeing worked to redesign the rudder PCU for all 737s. But the world has moved beyond using body count to determine safety policy. Hasn't it?

Source: Flight International