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hy do some King Airs have the Rudder Boost (RB) system installed and others do not? No, the answer is not that Rudder Boost was an extra-cost option, similar to a particular piece of avionics. Instead, some models were certified with RB as standard equipment and others were not. From the first King Air 65-90 model in 1964 up until the model 200 that received its Federal Aviation Administration (FAA) approval in late 1973, RB did not exist. The 200 was the first model that had it installed.
Realize that the 200 was a rather huge step up from its large-cabin predecessor, the A100. Among other improvements, it gained nearly 10 feet of wingspan, 1,000 pounds of maximum gross weight, 30 knots of speed, 1.4 psid of maximum differential pressure and 170 SHP (shaft horsepower) per side! With the engines mounted 25 inches further outboard from the fuselage centerline and with 170 more horsepower, keeping Vmca down to a reasonable figure was a challenge. The T-Tail came as a result of the Vmca-lowering efforts. The engineers had a worry: Would the force required to deflect the rudder to its maximum limit during Vmca testing exceed the FAA-mandated limit of 150 pounds?
Because there was a distinct possibility that the 150-pound limit would be exceeded, the engineering team came up with the design of the RB system and had it installed on the two model 200 prototype airplanes, BB-1 and BB-2. As the flight test program progressed, it was found that the maximum cockpit rudder pedal force required was 147 pounds. Thus, RB was found not to be required. However, it was decided to make it standard equipment on all 200s. Why? I think there were two main reasons. First, since Beechcrafts are known for excellent handling characteristics in all regimes, why make the pilot apply a force so close to the limit? Second, what about when the 200 evolved with more powerful engines, as it surely would someday? Since the 150-pound force limit would be exceeded, why not have the system already in use with a proven track record?
The King Air F90 model came out in 1978 and it shared many design features and systems with the 200, including the RB system. Required? No. The maximum required rudder force was well below the FAA limit, due to having 100 horsepower less per engine than the 200 and with the engine back in its previous location, more inboard. In 1984 the C90A replaced the C90-1. It shared many systems with the F90 and 200, including RB. Necessary? Of course not. Not only do we have the closer-mounted engines but they are putting out 200 HP less than the F90!
The same year that the C90A appeared, the 300 also arrived. For the first time, RB became mandatory to allow certification of this high-performing model. Now we had 1,050 horsepower mounted further out, at the same location as on the 200. The rudder force exceeded 180 pounds in the worst-case scenario. No longer was there an MEL (Minimum Equipment List) allowance to operate without RB. It became a mandatory item, rendering the airplane no-go when it was inoperative.
In a previous article and in my first King Air Book, I presented details about the design and operation of the three different RB systems that Beechcraft used. In short, the C90A (which includes all subsequent C90 variants manufactured after 1984), the F90-series, and the 200-series (including the present-day 250 model) all use a very similar system. The difference in left and right unregulated compressor discharge pressures (P3) is the trigger that tells the RB to operate and the actual force applied to the rudder cable comes from a pneumatic servo filled with regulated P3 pressure. It is an all-or-nothing system, meaning that when the difference in P3 between the left and right engines gets great enough to trigger the P (Delta P) switch, the rudder force applied is constant, helping apply about 40 pounds of force to the “good” side’s rudder cable.
The first 1,000-plus 200s and B200s and all of the C90As were manufactured with three-blade propellers. These props had no minimum idle speed restrictions since they were not prone to the “reactionless vibration” concerns that came with the four-blade propellers. Thus, the Low Idle prop speed ran a little below 1,000 RPM, making the airplane quieter on the ramp and with less tendency to taxi too fast. Low Idle N1 or Ng speeds were just slightly above 50%. P3 pressure relates to N1 speed but not in a linear fashion. When N1 goes from 50% to 60%, P3 increases more than the 10% you might expect.
Do you understand why it is easier to trigger RB during a ground test when equipped with three-blade, and not four-blade props? It follows this logic: When an engine is actually shut down in flight, P3 becomes basically ambient pressure. To achieve the 60 psi differential pressure that the P switch requires to trigger RB – a little less than 60 psid in the 90-series – requires only moderate power on the operative engine, with its commensurate relatively low N1 speed and with torque well below its maximum limit. For the ground test, the low power engine is usually at Low Idle, putting out a relatively low P3 pressure but a value significantly above ambient pressure. Hence, to achieve the needed P, the “good” engine needs to be turning faster, putting out more P3 and creating more torque. Fortunately, the torque required to trigger RB is still easily attainable.
Using rough numbers for a 200, we might experience RB kicking the good side’s rudder pedal in flight with an engine actually shut down at about 1,400 ft-lbs. of torque. However, we will need perhaps 1,600 to 1,800 ft-lbs. to trigger the kick when doing the ground test with the other engine at Low Idle, near 50% N1. Now put on four-blade propellers and adjust the Low Idle speeds up to near 60%. Since the idling, lower-power, engine is now putting out more P3 pressure because of its elevated Idle speed, the other engine must put out even more power to create enough P3 to trigger the P switch. Now the torque can easily exceed 2,000 ft-lbs. before the rudder kick is felt! In fact, on hot days with the air conditioning operating – requiring an Idle speed of 62% or even more – it can become impossible to check RB without torque exceeding the 2,230 ft-lb. limit. (The chapter in my book discusses ways to still achieve a proper test.)
A slight diversion: The higher Low Idle speeds associated with the four-blade propellers led to another “problem.” The autofeather system originally used 200 ft-lbs. as the setting for the low-pressure switch, the point at which the automatic feathering actually takes place. It was impossible to reach this low torque value in some 200s with the four-blades during the autofeather test, done with the low-power engine at Idle, not actually shut 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.
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.