The B777 Tyre Pressure Monitoring Unit is a pretty good piece of kit. It will generate a Status Message for the flight crew and also trigger a warning on Boeing's AHM (Airplane Health Monitoring) for the maintenance watch fellas on the ground with one of the following conditions:
* Tyre pressure below 100 psi
* Main gear tyre pressure different from average tyre pressure
by 18 percent (the TPMU calculates average tyre pressure by
discarding the three highest and the three lowest tyre
pressures and averaging the middle six tyre pressures)
* More than 25 percent difference in pressure between main
gear tires on the same axle
* More than 12 percent difference between the nose gear tyre
pressures.
There has been an issue with the wheel hubs cracking through the tie bolt holes, due to the tie bolts being "biffed" through the hubs if they are slightly misaligned during assembly, causing a stress raiser on the hub. Typically, the tyre deflates through the crack at around 10 PSI an hour, triggering the status message sometime before landing. Yes, the fire trucks have been on standby as a precaution, but on each occasion, the aircraft has landed safely.