Page 24 - Volume 10 Number 10

Ask the Expert
Elevator Trim Tab Alignment
by Tom Clements
Arecent thread on the BeechTalk internet forum prompted the idea for this article. All models of King Airs, for many years now, require that the pilot include a first-flight-of-the- day preflight check of the accuracy of the airplane’s pitch trim indicator.
In the 1970s, in England, a King Air 90 model nearly had a loss of control on initial climb after takeoff because the pitch trim indicator in the cockpit was incorrect by a wide margin. Not realizing that the trim actually had a large “Up” setting, the pilot was surprised as the nose kept wanting to rise and only a forceful control wheel push prevented a departure stall. An investigation found that a previous pilot had manually forced the elevator trim wheel past its travel limit causing the trim cable to find a position that bore little resemblance to the cockpit indication. First the British aviation authorities, then the FAA, issued Airworthiness Directives that required (1) trim indication accuracy to be checked and verified, (2) red limit marks to be painted onto the trim indicator wheel with instructions not to force the wheel past the limits, and (3) a procedure for verifying trim indicator accuracy during a preflight inspection.
Before I continue, I should remind my readers that the 100-series of King Airs do not have elevator trim tabs. Instead, trimming in the vertical axis is accomplished by moving the horizontal stabilizer ... just like on a J-3 Cub or a Cessna 180. But unlike the Piper and Cessna, the stabilizer in the King Air 100, A100, and B100 is moved by an electric motor, not by a manual cable, operating the jackscrew. In fact, there are two motors, “Main and Standby,” with appropriate cockpit activation switches.
The 100-series airplanes have no manual pitch trim wheel on the side of the power quadrant and the indicator, down on the pedestal, is an electrically driven pointer moving on a scale. The trim is so powerful on these models that a takeoff-out-of-trim warning horn comes as standard equipment and operates independently of the indicator, sensing actual stabilizer position. There is a small pointer attached to the center of the leading edge of the left side’s stabilizer and a rivet fastened to the fuselage. When the pointer points smack dab at the rivet, zero trim is
22 • KING AIR MAGAZINE
The stripes specified to show pilots if the elevator trim tab is aligned – red triangles on the stabilizers and a single red stripe on the elevator tab pushrods. When an apex of the triangle points to the leading edge of the band, the trim should be at zero.
obtained. The Normal Procedures checklist instructs the pilot to leave the trim at the zero position after the cockpit preflight checks are completed. Then, one of the steps on the exterior walk-around is to verify that the pointer and the rivet are side-by-side ... or at least very close!
So although the incidence of a misadjusted pitch trim cable cannot exist in a 100-series King Air, it is still important that the crew crosschecks indictor versus actual trim position.
There is a big difference between the elevator trim tabs on most of the 90-series and those on the 200- and 300-series. The tabs of the T-tail King Airs (including the F90 and F90-1) have no servo nor anti-servo action; the rest of the 90-series have tabs that operate with an anti-servo action. Remember, a servo tab assists control surface movement whereas an anti-servo tab resists movement. The anti-servo elevator trim tabs on 65-90s, A90s, B90s, C90s and E90s are geared such that when the elevator goes up, the tab goes further up, and when the elevator goes down, the tab goes further down. At an exact zero, neutral trim setting, the trailing edge of the elevator and the trailing edge of the trim tab are in perfect alignment only when the elevator is in its neutral position, streamlined with the horizontal stabilizer.
OCTOBER 2016