>It's to do with a dodgy valve, apparently. Subject item was installed on a DC-10 in 1999, but had been incorrectly assembled. Caused an uncommanded deployment of the reversers (possibly only partial, as the aircraft did not suffer an extreme loss of control, as the Lauda Air 767 tragically did a few years ago)
Now, why it did not fail on installation is a mystery, but that's the situation as it is at the moment. Presumably once any suspect valves have been identified and isolated, the restriction can be lifted.
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Not exactly correct. The valve in question is a redundancy against deployment and as such its failure "allowed" the in-flight deployment to occur in combination with another fault. The authorities expect at least one redundancies to work all the time. In this case the faulty valve, was installed with an unknown latent failure against its single most important task. Such a latent failure in combination with at least one other form of revreser lockout is sufficinet to provide against an unanted deployment, However, should the primary means of guarding against in-flight deployment fail, then this valve has to work and it apparently didn't due to its unknown latent failure.
Either there must be a routine way of checking for hidden failures (lots of luck) or the reliability of each device must be of a high magnitude to make it unlikely that unwanted deployment will occur. So far the industry has not met this requirement with most current reverser so the authorities are requiring all new designs to add yet another line of redundant defense (which will probably also have some latent failures)
On another issue, since the problem was discovered in a Boeing product, it's typical that the corrective action was already employed in the DC10 fleet.
It probably took the JAA a tad more time to figure out that even with the diference in the nacelle reverser design, that with the same dodgy valve, an unwanted deployment could still ocur on an Airbus even though there had never been a failed valve found in that fleet.