A320 APU Gen loses power with altitude
I assume you are a non pilot. When you ask a generator to produce electricity it requires power to do so. 500 HP can produce a lot more power than 5 HP. A gas turbine loses about half it’s available power from sea level to 20,000 feet. At some point in a climb the available power usually becomes less than the rated output of the generator. You must reduces electrical output at that point or the gas turbine will either overtemp or be unable to maintain the required RPM resulting in the generator frequency output falling below limits.
I'm also well aware of the behaviour of gas turbines.
But maybe you missed my point.
If the APU runs on constant RPM it simply means the gas turbine has enough power for the generator.
Yep, at some point the turbine may give up, but the generator doesn't "lose power with altitude" it will trip off.
Look at the #2 graph of Smythe. 90kVA all the way....
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"However, the maximum power output of the generator decreases with altitude."
The "Generator" referred to in the OP's question is the combination of APU gas turbine + Alternator. (i.e. ability to generate electricity)
"90 KVA all the way ...." depends on ISA deviation (for the gas turbine output). If the Alternator isn't required to deliver 90 KVA (say 45 KVA) then the graph would show ability for the Generator to provide that power to higher FLs.
If the APU runs on constant RPM it simply means the gas turbine has enough power for the generator.
Yep, at some point the turbine may give up, but the generator doesn't "lose power with altitude" it will trip off.
Look at the #2 graph of Smythe. 90kVA all the way....
Yep, at some point the turbine may give up, but the generator doesn't "lose power with altitude" it will trip off.
Look at the #2 graph of Smythe. 90kVA all the way....
"90 KVA all the way ...." depends on ISA deviation (for the gas turbine output). If the Alternator isn't required to deliver 90 KVA (say 45 KVA) then the graph would show ability for the Generator to provide that power to higher FLs.
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The battery is required to start the APU regardless of whether we are in EMER ELEC or not. So why is it that we are limited to FL250 for APU starts in EMER ELEC?
This is why an APU start on batteries only takes ages, and since the start (or start time) isn’t guaranteed above FL250, Airbus would prefer you to wait until you get below before trying.
In some aircraft installations the rated generator output is reduced with altitude due arcing as the atmosphere acts as an insulator which reduces with altitude. This is why magnetos need to be pressurised in high altitude piston aircraft. From (distant) memory the output reduction is in the order of about 0.86% per 1000'
Last edited by The Banjo; 7th May 2019 at 16:50.
