The initial concern here was to extend the APU turbine life.
The earlier B737’s (-100 through –500) were fitted with a Garrett… AlliedSignal… Honeywell GTCP85-129 of some sort. Some airlines such as United, Lufthansa etc. selected the APS2000 for their later B737 (classic) deliveries. A very few operators also selected the Honeywell GTCP36-280 (total flop) APU.
There are two distinct types of turbine wheels used in the GTCP85-129 series APU's; a two-piece forged wheel and a single piece cast wheel. The single piece cast turbine wheel has always been the Achilles heel of this unit. Although the cast wheel had better performance, it was susceptive to what we called “saddle” cracking and turbine wheel separation. This cracking occurred in the vicinity where the shaft was molded to the turbine wheel casting. Saddle cracking was a direct result of thermal cycling and also difficulties in getting a uniform casting material in such a relatively large piece. Several iterations were attempted and the Mar-M wheel with a TAFCU (Time to Accelerate Fuel Control Unit) comes to my mind …?
Anyway to the point! The APU turbine deterioration is primarily due to “time at temperature”, in other words running at elevated temperature for sustained periods. This does not differ from any turbo machinery using nickel based alloy turbines. The next worst scenario is thermal cycling and here I mean from deep cold soak to full electrical and ECS (Environmental Control System) or MES (Main engine Start) mode. Someone at Honeywell might argue this, but after years of observation, we have enough data to support this. I personally feel, and I have the support from some more unbiased Honeywell engineers, that the relatively small EGT thermal cycling going form electrical load and full ECS load, down to electrical load only, and back to electrical load and MES mode will have an absolutely MINIMAL impact on the life of the turbine.
They (today’s Honeywell) have always had a design issue with their 1950’s design GTCP 85 series APU and particularly the turbine wheel. The operators have been on their case about this for years and they were attempting to alleviate this by shifting the focus to the operation instead of addressing the real issue!
Fortunately the newer Honeywell GTCP 131-9B fitted to the B737NG’s is a absolutely “marvel of engineering”…
Dag