Page 22 - February 2015 Volume 9, Number 2

Ask the Expert
The Ground Fine
Power Lever Stop ...
Why Some
King Airs Have It
and Others Do Not
by Tom Clements
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
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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
FEBRUARY 2015