Page 21 - Volume 14 Number 8

down. This caused Beech to change the low-pressure switch to one set for 260 ft-lbs.
Back to Rudder Boost: Realize that it is not required on any King Air besides the 300-series. This is unlikely to happen during your entire King Air flying career, but when and if you fly a single-engine ILS or LPV approach with strong, gusty wind conditions you may discover that you are making lots of dramatic rudder trim adjustments during final approach. Why? Because the RB is alternating between being “On” and “Off” as power changes are being made in the changing wind conditions. Make it easy on yourself by reaching down to the RB switch on the pedestal and turning it off! With the low power associated with the approach, rudder force won’t ever be high enough to require any help on the rudder pedals. In the rare situation of a missed approach, you can always turn RB back on when you want the rudder force help.
The 300 (“straight” 300, not the B300/350) has a very different RB system than the one found on the C90A/ F90/200-series. Although the trigger is still P3 pressure, no longer does a P switch exist. Instead, raw P3 pressure from both engines is fed into a computer and the same electric servo that the Yaw Damp system uses receives a command to activate. The more difference between left and right P3 pressure, the more force the servo applies. So, it is no longer an “all-or-nothing” situation but instead the rudder force applied increases the more it is needed. Cool!
A downside? Yes, there is a minor one: It is harder to verify the ground test is working correctly because there is no rudder pedal kick. Instead, the rudder pedal on the higher power side gradually moves slowly forward as enough difference in power is experienced. Ah, but there is a workaround technique that proves the system is indeed working properly.
Since the force comes from the AP’s rudder servo, depressing the red AP/YD disconnect switch on either of the cockpit control wheels causes the RB action to immediately cease. My suggestion is to achieve about 60% torque on the higher-power engine during the test and then depress the red button. Now you will definitely note that the rudder pedal on that side quickly moves backwards. Release the button and the pedal will suddenly return forward if the system is working properly. (The same effect can be experienced by turning the RB switch on the pedestal off and then back on or by moving either bleed air valve switch to the bottom position momentarily.)
The B300/350 system is the best of all. Instead of measuring power difference using P3 pressure as a substitute for actual power, the 350 system uses torque. Voila! Since power equals torque times propeller speed, this parameter – torque – is as close to actual power as we can get. Like the straight 300, the rudder force applied comes from the AP/YD servo and is variable, based on the difference in torque between the two engines. Again, if you ever find that you don’t like what it’s doing during an approach, turn it off.
AUGUST 2020
In no King Air is RB meant to alleviate the pilot’s use of the rudder pedals! It is a help only, not a replacement for the pilot’s feet! The force that RB supplies will usually be just right, by itself, with no rudder trim needed, when one engine is at maximum power, the other is shut down and with its propeller feathered, at Vyse (single- engine best rate-of-climb speed, blue line) and with a five-degree bank into the good engine. However, think of that as just icing on the cake. When an engine quits on takeoff, at a speed below blue line, with the propeller not yet feathered, it is going to take a lot of rudder pedal force to keep going straight! Fly the airplane as if RB did not exist. “Step on the heading!” “Step on the ball!” Just realize, however, that RB is helping make the job easier ... but it surely is not a replacement for your feet. I worry that some pilots may have erroneously thought that they could just sit back, hold 10-degree pitch, let RB and autofeather operate, and all would be well. Friends, it doesn’t work that way! Doing that can get people killed. 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.
KING AIR MAGAZINE • 19