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power can be achieved. As you know, N1 is going up connected. One never wants to be in a hurry to turn the
every time we push the power levers forward in the climb. So, as odd as it first seems, one of the lower N1s we will observe when high power is requested is the one at takeoff, not the one in cruise flight at altitude. If the switches were set for, say 95 percent, there are lots of low altitude, winter takeoffs in which autofeather would not arm.
In reference to the starter, when it is engaged and
the N accelerates during the start sequence, is the 1
starter physically turning so long as the start switch is engaged, or at some point does it become freewheeling, or maybe just “along for the ride?” I’m not aware if there is a clutch feature inside the starter. It seems that the higher speed of the N1 would at some point drive the starter. The starter is located on the accessory gearbox. Does it direct-drive the compressor or is there a series of gears involved?
Realize that the starter is also the generator. There is no clutch and it never disengages from the accessory gearbox (AGB) unless the mechanic removes the nuts on the studs! Typically, the engine becomes self-sustaining – where the energy from the exhaust gases causing rotation of the compressor turbine is enough to run the engine, without starter motor assistance – in the low 40 percent N1 range, so after that it can be said that the starter is indeed “going along for the ride” even though it is still
starter switch off since doing so before self-sustaining speed is attained will result in N1 rollback and a hot start, if fuel is not immediately cutoff. The starter time limits do not apply once exhaust flow starts. The only problem with leaving the start switch on is, of course, that the generator circuit is prevented from operating! The POH says to turn the starter off at “50 percent or above,” but I encourage people to wait until full low or high idle stabilization has occurred. As you said, there comes a time that the starter is not doing the driving but is being driven by the AGB ... when that happens, it is setting the stage for it becoming the generator.
I thank Xandi and Galen for these interesting questions and encourage other readers to send questions my way whenever I may provide some clarification or guidance. KA
King Air expert Tom Clements has been flying and instructing in King Airs for over 44 years, and is the author of “The King Air Book.” He is a Gold Seal CFI and has over 23,000 total hours with more than 15,000 in King Airs. For information on ordering his book, contact Tom direct at twcaz@msn.com. Tom is actively mentoring the instructors at King Air Academy in Phoenix.
If you have a question you’d like Tom to answer, please send it to Editor Kim Blonigen at editor@blonigen.net.
JANUARY 2018
KING AIR MAGAZINE • 19