Ask the Expert: Elevator Trim Tab Alignment

A recent 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 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.

Hence, one way to check the accuracy of the pitch indicator wheel in the cockpit is to set it at zero and then to manually lift the elevators into a neutral position and verify trailing edge alignment. But if you are quite short or if the tail is hanging out over some drainage ditch at the airport, this can become impossible to do. Only with a helper in the cockpit who could pull the control wheel back halfway, could the visual check be accomplished. Recognizing that we may be alone with no cockpit helper, the FAA devised a fool-proof method that covers all the conditions, even when the tail is unreachable. You know what it is: Those funny-looking stripes on the elevator tab pushrods and on the bottom of the horizontal stabilizer.

Knowing that the trailing edges will not align, due to the anti-servo action of the tab, when the elevators are in the at-rest, down position, the stripes are applied so that they do indeed align when in that down position.

And, wow, have I seen some weird stripes through the years! I think some stripe-painters didn’t get copied on the memo and did not realize what was to be accomplished here! The stripes should be red with triangles painted on the stabilizers and a single red band painted on the pushrod (see photo on oppostie page). When an apex of the triangle points to the leading edge of the band, the trim should be at zero.

Now for the T-tails. Since the tabs here have no anti-servo nor servo action, the alignment between the elevator and the tab does not change as the elevator travels up and down. So no painted stripes are needed nor desired. Instead, the pilot merely leaves the trim wheel in the cockpit at zero and visually confirms that the trailing edges of tab and elevator are in side-by-side alignment. (Caution for the F90-series pilots: Realize that the entire range of trim settings the F90 has are “Up” settings. Zero trim is as far “Down” as it goes. Strange, but true. So you must take care to reset your normal takeoff trim setting after the exterior check is completed.)

There is one exception about setting zero trim for the visual check of the 350 or B300. Due to its longer fuselage, it was determined that neutral trim actually needed to be slightly nose down. Zero trim, therefore, has the trim tabs just slightly up, with their trailing edges higher than the trailing edge of the elevators. (Trim tab up … elevator down … nose down. Make sense?) Therefore, the 350’s procedure is to set the trim wheel indicator in the cockpit to a two degrees nose up (+2°) position before doing the exterior walk-around. Now the trailing edges should align perfectly.

I hope this article gives you a better understanding of this important preflight procedure … why it’s there and how it is accomplished.

Fly safe!

Note: In the June issue of King Air, my friend and colleague Dean Benedict wrote a fine article about the Overspeed Governor’s Test Solenoid and its disconcerting tendency to stick in the test position. I’d like to add a comment about that.

Not many pilots realize that the speed at which the Overspeed Governor moves into the test setting is extremely different than the speed at which it resets back to normal. It comes down fast, but it resets back to normal very s-l-o-w-l-y. It’s never a good idea to have the propeller turning at a high speed when the test switch is activated. Man, it’s like someone did a forceful, sudden yank back on the prop lever! But the reverse is not true. When the test switch is released, the RPM just creeps back up. So I strongly suggest, and teach, that after you have done an OSG test, let go of the switch while power is still applied and watch for the creep up to begin before reducing power. Now you know with 100 percent certainty that the solenoid has not stuck. For what it’s worth, I also believe that the extra power and vibration of the higher RPM still existing when the switch is released makes it more difficult for the solenoid to remain activated/stuck.