On modern aircraft, pressurization tests are conducted on the ground.
First you have to safety systems that are affected by pressure, noticeably the oxygen regulators. Then the doors are shut and the APU is started, the packs are run while following the instructions in the maintenance manual about how to control the system. Some aircraft will not pressurize on APU air (the Boeing 737-200 won’t) the easy way then is to start an engine and run it until you get over the “hump”. This is when all the doors’ seals. Then you can shut the engine down and the APU is sufficient.
The pax doors hang in, and with APU supply the air rushes out round the doors until there is too much flow and the doors are pushed into the frames and seal.
“This is an example of what happens when we do not pay attention to detail, and do not follow instructions and checklists!”, says an excerpt from military “lessons learned” presentation which appears on discity.com.
According to the excerpt “a KC-135 Aircraft was being pressurized at ground level. The outflow valves which are used to regulate the pressure of the aircraft were capped off during a 5 year overhaul and never opened back up. The post-investigation revealed: that a civilian depot technician who, “had always done it that way,” was using a homemade gauge, and no procedure.
The technician’s gauge didn’t even have a max “peg” for the needle and so it was no surprise he missed it when the needle went around the gauge the first time. As the technician continued to pressurize the aircraft, and as the needle was on its second trip around the gauge the aircraft went “boom” – the rear hatch was blown over 70 yards away, behind a blast fence!
An incident like this is never funny and is further regrettable when we consider that this mistake is one that we (the taxpayers) will end up paying for. Fortunately, no one was reported as being injured.
This was a good “Lesson Learned” for making sure we have trained people, who have the right tools, and who are following detailed procedures. And it should serve as a reminder that just because you’ve always done it that way, it does not make it the “right” way!”
Photo credit: U.S. Air Force
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