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Old 4th May 2019, 10:59
  #18 (permalink)  
Join Date: Feb 2002
Location: UK
Age: 54
Posts: 2,678
If the gen is 'on load' then it needs a finite amount of power to drive it at a constant rpm, if you increase altitude you will get to the point where the required demand load on the gen is too much for the reducing available power from the gas turbine. The APU will begin to slow down and once the frequency drops off below its limit, EG 398Hz, then the APU gen breaker will trip leaving you on battery power only until the RAT comes on line.
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