I sometimes get questions along the lines of “Why doesn’t my King Air have the rudder boost system?” Many pilots flying an E90 or C90 receive a demo flight in a newer C90GTx and observe that the newer model has rudder boost whereas their airplanes do not. “It must be due to the higher power of the PT6A-135A engine as compared to the -20, -21 or -28 that we have,” is the common reasoning. And yet, all engine variants installed in C90s and E90s are limited to the same 550 SHP value. This article will attempt to shed light on this question and give some insight into the design and history of the rudder boost system. I wrote an article “Rudder Boost Ramblings” featured in the August 2020 issue of this magazine but there still seems to be a lot of confusion.
As I am sure you know, King Airs have the deserved reputation of being easy airplanes to fly with excellent handling characteristics. Even when operating OEI – One Engine Inoperative – they handle very nicely with plenty of trim authority to make “Hands Off” – and even “Feet Off” – flying quite easily accomplished. If this were not so, we’d probably see much fewer King Airs being used in the multiengine primary trainer role. Also, this good reputation applies even to the most basic of King Airs, those without such great “options” as yaw dampers, autopilots, rudder boost or autofeather.
A rule found under the requirements for FAA-certified airplanes addresses the maximum force a pilot is expected to apply to the flight controls. When it comes to the rudder, the highest force allowed – the push on either pedal – is limited to a maximum of 150 pounds. Do you weigh more than 150 pounds? Stand on one leg and do a slight knee bend and return to the fully upright position. That wasn’t that hard, was it? You just experienced at least the magnitude of force being discussed.
All King Air models with the original wing center section dimension – that is, all of the 90- and 100-series – have a worst-case rudder force requirement that remains comfortably below the Federal Aviation Administration (FAA) limit. What is the worst case? Almost always, it is found while doing VMCA flight testing.
When the model 200 King Air – the Super King Air, as it was originally named – was being designed, a lot of improvements were envisioned from its predecessors: More power to yield faster airspeeds and better rates of climb, a wider center section to provide room for the larger-diameter propellers and to move the propeller arc further away from the fuselage for less noise, more pressurization differential to allow the cabin altitude to be lower at higher cruise altitudes … and on and on.
When the design team realized that an additional 170 SHP (850 SHP for the 200 versus 680 SHP for the A100) was going to be placed 25 inches further from the airplane’s centerline, concerns arose about rudder force and effectiveness. As I have related in the “The King Air Book – Volume II,” in the chapter about the T-Tail design, the primary reason for going with that major change of the empennage was to maximize rudder effectiveness when it was most needed … at slow speeds, including VMCA .
A very pleasant event that is forever seared into my brain’s memory bank occurred in 1973 at the Beechcraft factory. Bud Francis, chief experimental test pilot for the 200 program, gave me (as I prepared to be the first factory 200 instructor) a one-on-one lesson about the new rudder boost system. He used chalk on a blackboard to help illustrate what the test program had shown and the changes that this new system brought about. Although the maximum rudder force during the test program remained below the FAA limit, it only missed it by 3 pounds … 147 pounds required of the 150 pounds limit! Believing that King Airs should retain their “easy to fly” reputation (and, I speculate, anticipating the more-powerful model 300 to come), the decision was made to make the newly-designed rudder boost system standard equipment on the model 200-series. However, the KOEL (Kinds of Operation Equipment List) in the POH – as well as the MMEL (Master Minimum Equipment List) – show that rudder boost is never a required flight control … in the 200-series.
How much rudder force should the boost system apply? Bud explained that the choice was made that it would apply enough force to allow “feet on the floor” flying with no rudder trim input while flying at VYSE (Blue Line) with the left engine feathered, the right engine at its full 850 SHP, and with the proper five degree bank into the good engine. The force value selected was close to 50 pounds. This brought the worst-case situation from 147 down to an easily-handled value of about 100 pounds.
Since the required rudder force decreases as the differential power decreases, there will come a point – as airspeed increases, making the rudder more effective, and as engine power reduces when a climb is no longer required – in which every ounce of required rudder force is being supplied by the boost system, even with no rudder trim input. In this situation, to maintain proper airplane coordination with feet on the floor, the “dead foot, dead engine” rule would cease to apply. Instead, we’d have to start pushing on the “wrong” pedal or crank in opposite rudder trim. Bud explained that this was the point that the design team selected at which rudder boost would stop working, and stop supplying any rudder pedal force.
An educational and fun demonstration I have conducted during many flight training sessions is to be in the situation of an engine shut down with propeller feathered, full power on the other engine, and at Blue Line airspeed in a clean configuration climb. As mentioned above, in most cases the rudder trim wheel is very close to being centered. I tell the student to put both feet on the floor, to hold heading using ailerons only, and to start reducing the engine’s power a little at a time.
Let’s assume the left engine is the one shutdown. As the right engine power is reduced, the airplane wants to turn right – due to the right rudder force being supplied by the RB system – so the student dutifully turns the control wheel to the left, counter clockwise, to maintain heading. Another minor power reduction leads to the same outcome. Soon it becomes obvious that airplane coordination is lacking. We are banked noticeably toward the dead engine with the skid ball displaced to the left. But then, as power is slightly reduced again, suddenly the heading swings left and the student roles the control wheel clockwise to get back on the assigned heading. Well, looky there! We have “raised the dead” and are again flying in relatively correct coordination … because the rudder boost just stopped working. Cool!
No, I am not advocating flying with feet on the floor and/or not using rudder trim! However, realize that rudder boost is helping when needed but is turning itself off when no longer desirable.
More than anything else, here is what I want you to retain about the rudder boost system: Pretend it’s not there!
I have come to worry that some King Air pilots think this great system – and especially when combined with the wonderful autofeather system – makes it easy for the pilot to handle an engine out emergency without forceful control input. Wrong! Fly the airplane! Step on a rudder pedal forcefully to maintain heading. Pitch for proper airspeed. Conduct the engine failure memory drill. Yes, you can rest assured that the rudder boost and autofeather systems are reducing your force inputs, but you still need to input whatever is necessary! It’s a rudder boost system, not a rudder replacement system!
More about rudder boost history: Once Beech had designed this fine system and the parts that made it up were readily available, the decision was made to make it standard equipment across the spectrum of newly-designed King Air models. Hence, all models that were developed and certified after the model 200 have RB as standard equipment: F90-series, C90A and after series, and the 300-series. Only the 300-series require it. Personally, I wish the later-90 series did not have it. Why? Because much too often I have observed pilots undergoing training use too much rudder input, not realizing that rudder boost was helping with a lot of the force. Again, this is shown by flying with the dead engine down, not raised up. In my first book, “The King Air Book” I made the crazy comment that a C90A needs rudder boost like a fish needs a bicycle … but it’s true. Sure, if it’s there use it, enjoy it, but please keep a close eye on the changing rudder force/rudder trim needed to fly properly coordinated.
The system in the 200-series and all of the later C90A and F90-series that have it, is an “all-or-nothing” system in which the pedal force applied is constant when working and zero when idle. The force that rudder boost applies comes from pneumatic servos in the tail of the airplane and the trigger that tells the system to operate measures differential engine power indirectly by looking at the difference (P) between unregulated bleed air (P3) pressure. It is easy to change the point at which RB activates in the various models by varying the value at which the P switch activates. Also, by adjusting the pressure going to the pneumatic servos the rudder force can be easily changed to the value needed for a particular model … less for a C90A than for a 200.
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.