ET,
My understanding is that with a 'fully shrouded' engine such as the -A1, heat build up after shut down is particularly bad if there is no head or tailwind to move the hot air through the engine. In addition with such high pressure ratios (achieved with densely packed fan blades) the engine wasn't as good as others at clearing away that heat. With a long two-spool engine the heat build up was felt in the rotor shafts (obviously worse for the outer, high pressure, shaft) which caused the rotor (shaft) bowing with excess heat.
Cranking the engine for extended periods before putting fuel and ignition on allowed a stabilisation of the thermal load and allowed the shaft to straighten thereby minimising the blades rubbing on the case. This rubbing had caused blade failures in the engines early on which led to the discovery of the fault.
The fix on later engines was to pump up the oil pressure to the bearings in the middle of the shaft which 'squeezed' it straight while the heat was evening out across the engine and shaft.
Bump Thrust is possible because when setting thrust and beginning the TO roll there is a strong depression in the intake. This lower pressure (clouding on a damp day) temporarily increases the EGT due to lower gas flow. As the ac accelerates the pressure increases and the EGT reduces. If we were now to inject a little extra fuel we would 'restore' the engine to the limit EGT but in the process achieve a higher thrust rating.
If you watch the EPR closely during the roll you will see the EPR increase as the ac accelerates with bump selected. The A5 engine does the same thing automatically with TOGA selected.
This is why we should wait for 80 knots before checking to see that TO EPR has been achieved and more importantly check that the EGT is within the limits at this point.
Well that what I think anyway....standing by for other inputs!
