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The engine start procedure for the Beechcraft King Air B200 says (emphasis mine):

  1. Right Ignition and Engine Start Switch – ON [R FUEL PRESS] annunciator – OFF
  2. Right Condition Lever – LOW IDLE (after N1 rpm stabilizes; 12% minimum)
  3. ITT and N1 – MONITOR (1000 oC maximum)
  4. Right Oil Pressure – CHECK
  5. Right Condition Lever – HIGH IDLE
  6. Right Ignition and Engine Start Switch – OFF (at 50% N1 or above)
  7. Right Generator – RESET, then ON. CHARGE BATTERY until loadmeter reads approximately .50, then OFF
  8. Left Ignition and Engine Start Switch – ON [L FUEL PRESS] annunciator – OFF
  9. As Left N1 RPM accelerates through 12%:
    • Left Condition Lever – LOW IDLE
    • Right Generator – RESET, then ON
  10. ITT and N1 – MONITOR (1000 oC maximum)
  11. Left Oil Pressure – CHECK
  12. Left Ignition and Engine Start Switch – OFF (at 50% N1 or above)
  13. Left Generator – RESET, then ON
  14. Right N1 – REDUCE TO LOW IDLE

Why is the right engine's generator turned back off before starting the second engine (step 7) and then only turned on again after injecting fuel (step 9)? Doesn't this put unnecessary load on the battery?

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2 Answers 2

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Yes it loads up the battery, but the battery is sized to do this. The generator is not necessarily sized to do this, especially on the ground, so this procedure gets you to take the generator out of the system for each start.

On an Alternator, like on your car, there is a phenomenon known as 'forward-stripping', which is where there is such a demand for current out of the generator that it overloads the rectifiers (that convert the created AC electricity into DC for output). So here, during/after the engine start, there may be a large charge request by the battery which might forward-strip the gen's rectifiers.

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    $\begingroup$ @Bianfable You're not obligated to accept an answer if it isn't helpful to you. If you don't fully understand it, you can address the author (as you have) and leave it unaccepted. If somebody who could explain it better were to come by, seeing an accepted answer may dissuade them from answering. $\endgroup$
    – T.J.L.
    Commented Oct 31, 2019 at 12:55
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Prior to serial number BB1444 the Generator Control Unit (GCU) design and the associated current limiting system was not able to handle the load of a cross generator start. So we have the generator 'assist' procedure instead (to prevent the current limiters blowing during the second engine start).

From BB1444 onward they changed the design so that the current limiters (CLs) would not potentially blow and therefore later 200's went to a cross generator start procedure (allowing the pilot to keep the first generator on).

Tom Clements wrote about this in his book 'The King Air Manual' and you can find a discussion about it on KingAirMagazine.com:

[...] the GCU limits the output of the operating generator whenever the opposite start switch is activated. This current limiting began with the 300-series, but was added to the 200-series at BB-1444. [...]

Leaving the generator on increases the chance of experiencing a current limiter failure during the cross-start on the earlier airplanes, whereas doing the Off-On procedure does not harm the later aircraft in any way.

If you haven't read his book I highly recommend it.

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