Page 27 - Volume 15 Number 5

As for the model 300, the maximum rudder force required at VMCA can reach 180 pounds so rudder force assistance is definitely a required item. No longer are pneumatic servos used nor is there a P switch. Instead, the same electric servos that supply yaw damping by moving the rudder also supply the needed rudder boost force. P3 pressure from both engines is fed to the autopilot computer and the boost force increases as the difference in P3 pressure increases. Hence, the force applied varies as needed, no longer an all-or-nothing force.
Measuring the difference in unregulated P3 pressure and using it as a substitute for an actual differential power measurement is satisfactory in almost 100% of the cases. But how about a reduction gearbox (RGB) failure? If the N2 shaft coming from the power turbines no longer connects to the Np shaft connected to the propeller, then we have a catastrophic reduction in engine power and yet – at least until the engine destroys itself – we have normal P3 output. That may explain why the model 350 (B300) replaces the P3 sensors with actual torque sensors ... the best way to go, by far. I am surprised that Beech did not do this right from square one, but it was not to be.
Regardless of the details of your particular rudder boost system, remember what I am asking you to do ... pretend it’s not there when you encounter a suspected loss of engine power! Rudder boost allows your feet to not supply all of the rudder force, but it alone is not enough. KA
King Air expert Tom Clements has been flying and instructing in King Airs for over 46 years and is the author of “The King Air Book” and “The King Air Book II.” 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 books, 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.
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JUNE 2021
KING AIR MAGAZINE • 25