Up to now in my career, I have flown airplanes that I steered on the ground through pure mechanical linkages powered by my own muscles. I was using levers to physically shove the nosewheel into pointing the way I wanted to turn the airplane. The Screaming Whippet one is too big for me to kick around by myself, or they don't want to make levers that big, so it uses electrically controlled, hydraulically activated, variable authority nosewheel steering. I have to remember that description in case I'm asked about it on the ride. (Ride is pilotspeak for flight test). I also have to remember a whole lot more about how it works.
First pass, what I remember and understand from the class and the reading without looking anything more up:
Nosewheel steering is on the non-essential DC electrical bus. It must be armed and activated to be used. You arm it with a toggle switch, the location and position of which I can't picture now and it becomes activated when the squat switch indicates that you are on the ground and one of the following things happens. Either a) the right speed lever is moved into the low position or b) a button on the side of the left power lever is pressed and held. (Yes, I have both speed and power levers: resolving units, you'd think you'd get a power lever by forcing the speed lever, but that would be a geeky joke and not the reality of this airplane). Once the system is activated, you steer with the rudder pedals, just like old times.
Moving a rudder pedal sends a signal to a potentiometer. There's a pile of potentiometers, I think it was four. One of them represents the input you gave to the rudder pedal, one the position of the nosewheel, one the target position and one for good luck. So maybe it was three. The difference between the target and the position is sent to a selector valve which shuttles the appropriate way to send hydraulic fluid to a piston operating rack and pinion steering to pivot the nose gear in the appropriate direction. The channels in the shuttle valve go straight through for left turns and criss-cross to reverse the pressure for right turns. When the commanded steering meets the actual steering, the system holds that input until a new input is received or the system is disarmed. In the later case it smoothly recentres. The maximum steering in this mode is ten degrees either side of centre, but you can get up to sixty-three degrees deflection in park mode, by pressing and holding the park button. This is for manoeuvring into a tight parking spot. I really hope I don't often have to parallel park this airplane.
There's an annunciator for the system being armed, an annunciator for it activated and a flashing one if it fails. Because it's an electrical system, there is a chance of transient weirdness, and transient weirdness is not something you want to see in large vehicle steering If there's a three degree disagreement between two of the potentiometers, representing some form of where it's requested to be and where it thinks it is going, the system disconnects. You can still steer with braking and power.
Now let me see if by re-reading the chapter I can fix that up, and add things I forgot.
There is a command potentiometer (my "input you gave the rudder pedal") on the left side of the rudder pedal linkage, and a fault protection monitor potentiometer (my "target position") on the right side. There are two more potentiometers on top of the steering actuator, a control follow-up potentiometer (my "position of the nosewheel") and another one for the fault protection monitor circuit. So I was right, there are four, and if monitoring faults is good luck, I got them all right. Well, let's see.
There is a "Nose Steer Computer/Amplifier" which "contains Servo Valve Solenoid drive circuitry to initiate steering based on Rudder Pedal Potentiometer and Nose Gear Follow-up Potentiometer Position." (Why yes, this training manual is apparently so old that it predates the late eighteenth century English move to distance itself from its Germanic roots and not capitalize all nouns).
Okay, reading the quasi-incunabulum more closely, I surmise that the follow-up potentiometer simply turns with the nosewheel steering column, and has the same type of electrical connections as the rudder pedal potentiometer. When the "wiper resistance," a term I am not familiar with, but which I'll read as "the combined effect of the electrical connection and the turning," of the two is the same, the steering is deemed to have turned as far as commanded. The hydraulic supply ports close, so the wheel is locked in the position it was steered to.
I think the Rudder Pedal Monitor Potentiometer must be at the same wiper resistance as the Rudder Pedal Potentiometer--perhaps one potentiometer can only be compares with one other potentiometer, so the latter needs the former to shadow it. My manual names former, and the fourth potentiometer, the Hydraulic Actuator Monitor Potentiometer only once, in the sentence, "In either case, if a difference greater than approximately 3 degrees exists between the Rudder Pedal Monitor Potentiometer and the Hydraulic Actuator Monitor Potentiometer, the protective circuitry within the computer will shut down the system." I can't quite follow that all the way, but I know it means that if the monitor circuit thinks that the commands sent to the hydraulic system are incorrect, it will disconnect the steering to avoid having an electrical irregularity steer an airplane off a runway.
Absent positive steering, you still have directional control of an airplane through braking and differential power, so this is better than definitive by incorrect steering inputs. The "in either case" above refers to the case of a wheel being locked in place despite commanded steering versus uncommanded steering occurring.
The arming switch is located on the left hand console.
I also need to remember the fault test switch and that the system acts as a shimmy damper when not activated, but left in what is known as castoring mode, and that on retraction it automatically centres because the centre position is the low point in the race ... ah I guess I don't understand that well enough to explain it clearly. Also variable authority. I'll say that again to make sure I remember it.
Today is, I believe, International Women's Day. There is one male on our course and the rest are females, who happened to all be connected. The man had a eureka moment when the women all had connections to one another, either having previously met, knowing people in common, or just the familiarity of being females in aviation. We were all talking about then and catching up. He suddenly realized what it must feel like to not be part of the "old boys' network," and the girls saw it at about the same time. We promised not to exclude him from our network, and of course had no intention to do so, but through the week I noticed that our conversations would drift into discussions of female interest topics like clothing, or career planning in the context of postponing or forgoing pregnancy and he would be unintentionally excluded by dint of not having experience or opinions. It must have been for him exactly as it is for me when my usually all male colleagues start discussing football, or someone getting drunk while hunting and rolling his truck. It's hard to get into a conversation when you can't identify with the decisions being made or discussed. I can sort of feel for the men who feel threatened about women in traditionally male-dominated fields, but I don't think it will hurt them to learn to say "aww" when they hear about strangers' babies any more than it has hurt the women to feign interest in football for social purposes.