Originally Posted by
Zeffy
From the AvWeek article:
Sounds as if the "bypass" is done via software and could be re-enabled via software, no?
Some years ago, I discovered a way to reliably, and repeatedly, make a 767, with autopilot engaged in VNAV, fly through the MCP altitude. I reported this to our tech people, who passed it along to Boeing. Very quickly I heard that they’d been able to replicate it in a system sim, and a red bulletin was soon issued. It was fixed in an update a few months later.
Fast forward ten years, and I was now flying the 747. An update came out, and lo and behold, the MCP bug had reappeared. Apparently the software had simply been modified to bypass the offending code, and a later update, had removed the bypass.
The point is that the software fix itself was not permanent.