All King Airs currently being produced share a common power quadrant, the part of the cockpit that includes the power, propeller, and condition levers, which also includes the elevator trim wheel, the flap handle, and the friction control knobs. Inside the power quadrant, a pin protrudes from the side of the power levers and rides in a slot in a metal plate. The shape of this slot requires a lift to move the lever aft of Idle into the Beta range. Another lift is required to leave the Beta range and to enter the Reverse range. This second stop is at the position labeled “Ground Fine,” where the propeller’s low pitch stop (LPS) should be “flat,” yielding neither positive nor negative thrust when stopped on the tarmac.
Up until the appearance of the King Air model 300 in 1984, no King Air power quadrant contained the Ground Fine stop. Instead, the separation of the Beta and Reverse ranges was indicated merely by the inclusion of red and white stripes painted on the latter half of the range behind Idle. From 1984 until 1990, the model 300 was the only King Air to have the new style of quadrant with the Ground Fine stop. In 1990 the 350 replaced the 300, in 1992 the C90B replaced the C90A, and in 1993, the B200 underwent a major upgrade. All of these later models included the new quadrant with the Ground Fine stop.
Let’s do a quick review of what Beta and Reverse are all about. In Beta, movement of the power lever should cause the LPS to flatten from its largest bite of air to where it is acting as a flat disc with no bite being taken. Compressor speed, Ng or N1, should remain constant throughout Beta, maintaining whatever speed existed when the power levers were at Idle.
The Reverse range, conversely, sees the LPS going from flat to its maximum negative bite of air while N1 simultaneously increases in proportion to aft power lever movement, reaching a maximum speed of about 85 percent in Maximum Reverse, when the power lever is back and down as far as it can travel.
A PT6 powerplant mechanic who can correctly make all of this happen as it should, while matching left and right power levers together, is worth his or her weight in gold and can be somewhat hard to find! This is what engine “rigging” is all about and it can be a frustrating and time-consuming experience when done by a less-experienced and less-knowledgeable person.
The need for accurate and correct rigging skills ratcheted up a great deal with the introduction of the Ground Fine stop. Before that stop existed, no one cared if the transition from Beta into Reverse occurred exactly as the aft edge of the power lever lined up with the start of the red and white stripes. A half-inch or even more off either way, forward or aft? It is not a big deal so long as both power levers operate the same and so long as Maximum Reverse power can still be obtained.
Add that Ground Fine hard stop, however, and the rigging task becomes much more difficult. Being even a half-inch off is now a problem. Either the airplane won’t slow down enough taxiing at Ground Fine – the more common problem – or else N1 has already increased before reaching the Ground Fine position, leading to more propeller blade erosion.
Since the presence of the Ground Fine stop makes the rigging task more difficult and since we got along without that hard stop for twenty-some years, why was it introduced? There is a solid answer to this question that I can provide. Please read on.
As you undoubtedly know, the model 300 was the first King Air certificated under a different FAR than that which had applied previously, since its weight exceeded the “light twin” limit of 12,500 pounds. Although earlier King Air models, especially the 200, underwent rigorous flight testing and the POH contained the charts and/or graphs for things like Accelerate-Stop, Accelerate-Go, and Second-Segment single-engine climb, this information was not required to be provided to “light twin” operators and Beech had a rather free hand in their testing since the FAA did not have to “bless” the results.
One specific example is how the Accelerate-Stop procedure was done. In the previous 90- and 100-series, the stopping procedure involved heavy brake usage only with the power levers remaining at Idle. For the model 200, on the other hand, Maximum Reverse on only one engine, along with heavy braking, was utilized in the testing and is so reflected in the chart’s “Associated Conditions.”
As a side note, we at the Beechcraft Training Center – back in 1974 when the first 200s began being delivered – decided that we had better teach and practice single-engine Maximum Reverse usage since it was now in the POH procedures. To our great pleasure, we found that directional control was not too difficult. That was the good news. The very bad news, however, is that we almost always returned from this portion of the training with the outboard tire on the “dead” engine’s side totally blown, with a hole the size of a silver dollar! The student, unintentionally, would apply a little brake at too high of a speed, lock up the wheel, and scuff the tire right down to its air!
A meeting of the Beech factory instructors came up with a modified procedure that brought the incidents of blown tires to a stop. The procedure involved three changes. First, we would emphasize to the student to keep his or her heels on the floor and to stay off the brakes. Second, without even telling the student we were doing so, we would immediately move the flap handle to the Up positon right at touchdown. Third, we would ask the student to “steer” with the control wheel. That means, for example, that if the reverse thrust on the right engine is making the airplane tend to veer right, then in addition to pushing the left rudder pedal forward, we would also turn the control wheel fully counter-clockwise, left-wing-down, just as if you wanted a car to go left.
Why were the second and third steps of the procedure effective? Retracting the flaps right away killed a lot of lift and anchored the plane more solidly to the runway. When one side’s powerplant went to Maximum Reverse, it blocked airflow back over a large portion of the wing, making that side have less lift, putting more weight on the tires on that side, and aggravating the pulling tendency to that side. By steering with the control wheel, the aileron deflection helped lift up on the heavier side and push down on the lighter side, better equalizing tire weight and helping the plane travel straighter.
As I said, using these three procedures for training eliminated the blown tire problem. (Even now, on a normal landing, I have the copilot raise flaps at touchdown. That puts more weight on the tires right away, makes it less likely to scuff a tire if heavy braking is ever used, and helps keep rock dings on the flaps to a minimum.)
But what does this have to do with why the Ground Fine stop appeared on the 300?
No longer a light twin, the model 300 was required to have certain tests done and the resultant charts included in its POH. One of these was Accelerate-Stop Distance. Although directional control during this maneuver was satisfactory in the model 200, such was not the case with the 300. Not only did the engines have more power – 1,050 SHP versus 850 SHP for the 200 – but also the propeller had a larger diameter and an extra blade. (B200s still came with three-blade propellers back then.) The bigger propeller and more power made control more difficult, for sure, but there was another factor no one had foreseen … Rudder Boost working against the pilot. Let me explain.
In the 200, Rudder Boost uses a dedicated pneumatic servo that applies about 40 pounds of help on the “good foot” side. It activates in an all-or-nothing fashion when the differential between left and right side’s unregulated P3 Air gets large enough. Since N1 speed only reaches about 85 percent in Maximum Reverse – less than 50 percent power – the P3 differential was never great enough for rudder boost to kick in.
In the 300, Rudder Boost uses the autopilot’s electric rudder servo to provide the helping rudder push and instead of being an all-or-nothing device, it increases rudder force as the power (still sensed by P3 Air pressure) differential increases. (The model 350 operates the same except it uses torque sensors in place of P3 sensors.)
Low and behold, when the Beech test pilot experimented with single-engine reverse in the prototype 300, he found that at Maximum Reverse rudder boost was starting to operate. Now stay with me here, because it’s a little hard to grasp. Do you see why rudder boost was now working “backwards?” In flight, with the left engine dead and the right engine producing power, the airplane wants to yaw left, so we, of course, want help on the right rudder pedal. When that same engine is putting out reverse thrust after a single-engine landing or aborted takeoff, now the yaw is toward the good engine and we need to push the left rudder pedal to stay straight. Having rudder boost push the right one is definitely not a good idea!
“Okay,” Beech Test said, “we won’t use Reverse then; we’ll just stick with Beta, getting rid of the propeller thrust but not spooling up N1. We’ll just have the pilot stop pulling back when he gets to the red-and-white stripes.”
“What?!” says Mr. FAA guy. “You cannot expect a pilot to do this by feel alone or to direct his visual attention away from the runway and down to the power quadrant to see when he’s about to leave Beta and enter Reverse. Get real!”
And that, dear readers, is why the 300-series has the Ground Fine stop … so that the pilots may easily and quickly select the bottom of Beta during aborted takeoffs or following single-engine landings, as necessary. As for why the C90B and after and the 1993 and after B200-series also have it? For commonality of parts. Personally,
I would prefer that they still used the old style pedestal
with the stripes, not the second lift, but it is what it is.
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