TCAS Unserviceability Periods
TCAS equipment serviceability has proved to be pretty good, but when a fault does appear, it tends to be something that cannot be fixed by a simple box change. For example, a problem with an aerial may not easily be rectified at a line station.
It is therefore reasonable to allow the aircraft time to return to its major base where rectification can be carried out. Remember, some aeroplanes may be on the other side of the world when the fault materialises. Therefore, the 10 day period has been thought reasonable either to allow that aeroplane to return to base or, if it is a small/medium sized aeroplane without comprehensive spares support facilities at home, time to get a repair kit shipped out.
One other point worth bearing in mind. It is extremely unlikely that two aeroplanes, both with unserviceable TCAS, will find themeselves in a situation that needs TCAS to resolve. Where only one of a pair has serviceable TCAS, the manoeuvre perfomed by its pilots should produce a sufficient difference of altitude to ensure that no collision occurs. (This would be also true if the other aircraft didn't have TCAS anyway.)