Is it standard practice to select the fuel pump off at that point during the start cycle where T4 begins to climb and then, once T4 stabilises, to return the fuel pump to on?
Earl
Earl,
Depends on a whole host of factors, but generally the Aztazou IIIN or IIIN2 is a civilized beast that seldom gives you a hot start. Normal practice (or certainly used to be) was switch the boost pump on for 30 seconds and then initiate the start cycle. As MD600 mentioned, you would only cut the pump if it looked like going over 600C, which was a very rare occurrence.
In many ten of thousands of hours service with the MoD, the number of genuine engine failures involving Gazelles can be counted on a few fingers. Causes range from broken turbine bearing oil feed pipes (due to misalignement and vibes) the odd FCU failure or two, but none of the traditional thrown turbine blades or exploding combustion chambers you might expect on an engine of this vintage. Very nicely engineered.
One of the main recurring faults with engine starting is that in the Start Control Unit (SCU), which is a little box of electrical gizmo's in the rear manhole, there is a relay that carries quite a bit of current during starting. This causes pitting and arcing across the contacts and eventually it will fail and prevent the engine starting. You can usually get a couple more starts out of it by cleaning the contacts with sandpaper, but you need a new SCU before too long.