Qantas A380 uncontained #2 engine failure

[Old Engineer]

EGT-- Exhaust Gas Temperature:

If this EGT were actually that, meaning measured at the exhaust, it would have a significance in measuring the maximum theoretical thermal efficiency of the engine. That is, it sets a limit of efficiency in burning the fuel which the actual efficiency of the engine cannot possibly exceed.

If the combustion of the fuel takes place at 1600-C as reported here, and the rejection takes place at 850-C in cruise (it doesn't as this is the LPT inlet limit, so the rejection is at some lower temperature), then the efficiency is found by first adding 273-C to each number to convert to degrees K (Kelvin) above absolute zero. Thus 1873-K and 923-K are used in the following formula:

eff = (combustion temp, minus exhaust temp) divided by combustion temp.

For the HPT and IPT stages combined, this is 50.7 percent in this example.

To this, there is the added percent efficiency gained in the LP stage. The formula for this is:

+eff = (delta temp at LP stage) divided by combustion temp.

Theoretical efficiency is a measure against which to evaluate attained actual efficiency. No point in striving to exceed it, although theoretical efficiency can be increased by raising the combustion temp or by lowering the exhaust temp.

Of course, for running the engine, the LP inlet temperature is more useful.

Edit: When the LPT inlet temp rises, that is because gas temp in the IPT is rising. We are not told whether the alarm limit is because 900-C has been reached in service for an excess of 5 minutes, or whether 920-C has been exceeded. I would think the latter. We know that 950-C was reached, which I would think indicates a rise of 30-C in somewhere between 49 and, I think, 35 seconds. The post (p92) of engine timeline events does not include all the alarms to FD. The interview with Sr Check Capt indicates that the engine shutdown was commenced from the FD following the FO's personal timing of 30 seconds from the initiation of the first overtemp warning. The explosion (SCC's choice of words) of #2 occurred some very little time after the first 30 seconds as measured by the FO.

We are not told whether the 920-C not-to-be-exceeded limit is more significant to the IPT or the LPT. But given the appearance in picture of the LPT, and the absence of the IPT, I think we can infer that this temp is more significant in regard to the IPT, or to events in the IPT. Obviously the temp on the leading edge of the IPT blade disk (1 stage) will be higher than that exiting that disk. But the IPT entry temp appears impractical to measure directly.

Note that the interview puts to rest (IMO) any question of whether on the FD the crew was waiting through the reset of the timer to begin again the 30 seconds from a delayed start. The crew did not delay their actions in response to the initial alarm beyond the procedural 30 seconds, as I read that interview. And they were very alert in doing so. That also suggests that the indicated procedure needs to be revisited. Among other things, obviously.

OE