I recall that on both 747 Classic and 744 that the Fuel Temperature Indicating System (sensed in No.1 Main Tank) could be u/s for dispatch (but not ex-Main Base), provided that TAT was substituted as indication of fuel temperature. However I must say that I do not remember any instance in over 20 years of 747 flying when a Fuel Temperature Indicating System was found unserviceable.
There can indeed be a significant lag between a fall in TAT and it being reflected by a corresponding drop in fuel temperature, it being dependant both on the difference between the two temperatures and also on the quantity of fuel involved. Therefore, using TAT in place of fuel temp will still be an effective (but more restrictive) method of ensuring that fuel temperature limitations are not exceeded.
I imagine that Boeing design philosophy in this regard in the 777 is likely to be similar to the 747.
However, I do agree with
M.Mouse on this - I think talk about need for dual sensors is not justified - indeed it is almost certainly highly irrelevant to this 777 event.
JD
