Hi lomapaseo;
the problem is the reliable determination that the reverser is in transition.
Well, in my experience, reverser systems, including indications, were extremely reliable. Once in a while we saw a "sticky" reverse cascade on a DC8-53 (PW, not RR) aircraft but the system was not prone to anomalies or failure. FADECs, as has been pointed out, have contributed enormously to reverser reliability in more complex (sensor/software) systems. There was just never a time in the aircraft I flew when one could get more than idle reverse thrust until the reversers were solidly in position. So a system in which this can happen is a significant anomaly in the industry and isn't primarily a "pilot problem". No matter what the pilot does, it should never be possible to obtain more than idle thrust until all reversers are deployed, and that has been my experience in all types flown. The systems are extremely reliable in both the takeoff and landing cases described.
BOAC;
Sorry for the late reply.
Yes, I do recall something in the thread regarding the monitoring and calling of "reverse green" or whatever the verbal affirmation was/is, and I see Karel_x has provided information on the SOP - which seems pretty straightforward and in line with what other familiar procedures for use of reverse on landing are.
This isn't a DC8-era airplane, it is a relatively modern, computerized airplane. It is entirely within industry design experience that if there is no indication that reverse is fully engaged then in system design that should preclude the application of more than idle thrust, period.
Until now I've never even heard of a system that would permit otherwise. Even if just the cockpit indication fails then only idle reverse should be available and with FADEC that is what happens, at least on Airbus aircraft.