Page 17 - Volume 15 Number 1

a gauge reading of 0.20 would indicate a current output from the generator of 50 amps (250 amps maximum x 0.20). In more recent years these gauges – load meters – are marked in units of percent, so the 0.20 reading would now show as 20%.
It is important to realize that there really is no actual limit on maximum generator output. If a direct ground short were to occur on the generator output line, current will immediately rise to 1,000 amps or more! This excessive workload would create too much heat that could, given enough time, cause the generator to undergo an ugly and expensive death. A lot of King Air pilots and even instructors suffer under the misconception that any ground short will be handled automatically without the need for pilot action. Not true. Although the melting of a large fuse – called a current limiter – will protect the opposite generator and the battery from harm, the only thing that saves the shorted generator from its ugly demise is the pilot turning off its switch ... in reaction to the load meter on that side being pegged out at more than a 100% reading.
So, when it is stated that 250 amps is the maximum continuous output, it is really saying that this is the maximum output that may be sustained, under certain conditions, while still being able to maintain the correct output voltage and without overheating taking place. The “certain conditions” referenced here has to do with how much airflow is pumped through the generator by the cooling fan built into the back of the unit, where the cooling duct connects. The two factors that most determine the airflow are N1 speed and altitude. The fan is turning at the same percentage of its maximum speed as the N1 gauge reads. At an idle speed of 60%, for example, the fan is also turning 60% of its highest speed so it cannot provide the same cooling as it would at cruise power. (Actually, that is not totally correct since the maximum Ng speed of PT6s is either 101.5% or 104%, but close enough.) Of course, since air
JANUARY 2021
density decreases with altitude, less airflow takes place the higher one flies, even for the same N1 speed. This explains why the POH typically shows lower sustained load meter limits on the ground with idle speeds and in flight at very high altitudes.
The previous paragraph included the phrase, “... while still being able to maintain the correct output voltage ...” The target voltage is 28.25 volts with an allowable tolerance of ± 0.25 volts. Thus, any voltage between 28.0 and 28.5 is proper. No instrument can ever be totally accurate and the typical King Air voltmeter can have up to a 0.5 volt error and yet still be considered acceptable. That explains why in some POHs the statement appears that, during the electrical system After-Starting checks, the voltage must be between 27.5 and 29.0. These numbers are derived by including the possible gauge error into the correct voltage range.
Early 90- Series
Prior to LJ-678 (C90), LW-157 (E90), and B-224 (A100), the voltage output of the generators was controlled by a carbon-pile type of device that was not nearly as accurate and predictable as what we have come to expect in the modern digital age. To keep voltage within proper limits and to make the generators share their current output equally – i.e., provide good paralleling of the load meters – were difficult aims to achieve and almost impossible to maintain over any significant period of time. However, beginning with the serial numbers noted above, a solid-state Generator Control Unit (GCU) was made standard equipment and what a nice difference it made! Now voltage rarely strays out of the expected range and generator paralleling is so perfect that an imbalance between the left and right load meters becomes the exception rather than rule.
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KING AIR MAGAZINE • 15