Engineering: There are many different APUs available for the 737. The Garrett GTCP (Gas Turbine Compressor [air] Power unit [electrics]) 85-129 was standard for the series 1/200 but when the -300 was introduced it was found that two to three times the energy was needed to start the larger CFM56 engines. Garrett produced the 85-129[E] which had a stretched compressor, ie the impellers were lengthened and the tip diameters increased. When the 737-400 was introduced, even more output was required and Garrett produced the 85-129[H]. This has an Electronic Temperature Control which limits hot section temperatures depending upon demand and ambient temperatures. By 1989 the 85-129[H] was the most common APU, although there are actually 14 different models of the 85-129 in service with 737s (see table below).
Other APUs available for the Classic were the Garrett GTCP 36-280(B) and the Sundstrand APS 2000; NGs have the Allied Signal GTCP 131-9B. The main difference between them is that the Garrett is hydro-mechanical whereas Sundstrand and Allied Signal are FADEC controlled. I am told by engineers that whilst the Garrett is more robust, the Sundstrand and Allied Signal APUs are easier to work on. On the 3/4/500s, we pilots prefer the Sundstrand because it has no EGT limits and faster restart wait times. The easiest way to tell which is fitted is to look at the EGT gauge limits; the GTCP 85-129 has an 850C limit and also runs at 415Hz, the GTCP 36-280 has an 1100C limit if no EGT limits are marked you have a Sundstrand. Later aircraft have MAINT instead of LOW OIL QUANTITY and FAULT instead of HIGH OIL TEMP warning lights.
The AlliedSignal APU has a 41,000ft start capability and incorporates a starter/generator, thus eliminating a DC starter and clutch. In practice this means that it can be started either by battery or AC transfer bus 1 (the classics are battery start only). It has an educter oil cooling system (see Bottom of page advert) and therefore has no need for a cooling fan. It is rated at 90KVA up to 31,000ft and 66KVA up to 41,000ft. The Garrett and Sundstrand APUs are only rated to 55KVA.
The fuel source is normally from the No 1 main tank and it is recommended that at least one pump in the supplying tank be on during the start sequence (and whenever operating) to provide positive fuel pressure and preserve the service life of the APU fuel control unit. Boeing responded to this need by installing an extra DC operated APU fuel boost pump in the No 1 tank on newer series 500 aircraft which automatically operates during APU start and shuts off when it reaches governed speed
There is no CSD in the APU because it is a constant speed engine.
If the APU appears to have started but no APU GEN OFF BUS light is observed then you may have a hung start.
The current limit is 125A -air and 150A -ground, due to better airflow cooling on the ground. The galley power will automatically be load shed if the APU load reaches 165A. Because of these limits, the APU may only power one bus in the air. However, if you should accidentally take-off with the APU on the busses then it will continue to power both busses. If the APU EGT reaches 620-650°C, the bleed air valve will modulate toward closed. (This can lead to an aborted engine start if the electrics do not load shed first.)
LOW OIL QTY/MAINT – When illuminated, you may continue to operate the APU for up to 30 hrs. Note: this light is only armed when APU switch is ON.
FAULT – Although the malfunction will cause the APU to auto-shutdown, additional restarts may be attempted.
Max recommended start altitude – 25,000ft Classics; No limit NG’s.
Garrett 85-129 limitation:
Max alt Bleed & Elec: 10,000ft
Max alt Bleeds: 17,000ft
Max alt Elec : 37,000ft
Bleed Pack Operation 1 pack