A retired airline captain, Barry Schiff has been writing for AOPA Pilot since 1963.
The pilot departed Runway 30 at Santa Maria, California, and pointed the nose of his 1995 Piper Seneca IV into an overcast sky dripping with rain. He turned left to a heading of 100 degrees in compliance with the departure procedure and waited to intercept the designated radial.
Suspicion and doubt infiltrated the cockpit as the pilot began to realize that course interception was overdue. This is when the aircraft also began to dive insistently and pegged the vertical speed indicator at 2,000 fpm. The pilot pulled mightily on the control wheel with his left hand and reached between the front seats to find the trim wheel with his right. This confirmed that the pilot was experiencing a runaway electric trim, an uncommanded and steady, nose-down rotation of the trim wheel.
The pilot continued to click the spring-loaded trim button on the back side of the control wheel with his left index finger, but this had no effect. He wanted to pull the circuit breaker controlling the pitch trim system, but he could not read the circuit breaker labels in the dim ambient light. Besides, the circuit breaker panel was on the lower right side of the instrument panel, and a passenger's knees made it almost impossible to locate and trip the appropriate breaker. (The pilot subsequently keeps a photocopy of the breaker panel with his checklist.)
As he continued to haul back on the control wheel, the pilot began to catch intermittent, peripheral glimpses of the ground and knew that he had descended close to the base of the overcast that obscured the surrounding hills.
With an urgent tone, the air traffic controller asked, "Why are you descending? Say heading."
"I've got a runaway trim," the pilot blurted. "I'm heading 100 degrees."
The controller advised that the aircraft was below the minimum safe IFR altitude and that the heading appeared to be in error by more than 50 degrees.
The pilot pulled so hard on the control wheel that his arms ached for days, but he finally managed to claw his way to the assigned altitude of 9,000 feet, which thankfully was on top of the undercast. He also noted that his slaved heading indicator and magnetic compass were in gross disagreement.
"Center, I've got serious problems. Can you vector me to the nearest VFR airport?"
After landing at Bakersfield, California, the pilot taxied to an avionics shop, where the technician found nothing wrong. Mysteriously, all systems worked as advertised. Nor did the problems recur during subsequent flights.
Although it may never be known with certainty, the problem apparently was caused by rainwater that shorted, faulted, or otherwise affected one of the two avionics buses and resulted in the misbehavior of the trim system, the slaved horizontal situation indictor (HSI), and other avionics items on that bus. The problems could not be duplicated after the water had evaporated.
The pilot in this factual account was an experienced multiengine pilot and had recently been checked out in his newly acquired Seneca by a respected instructor. Although his insurance company required that he obtain 15 hours of dual instruction and 10 hours of solo flight in type before carrying passengers, the pilot was not required to demonstrate any particular knowledge of aircraft systems.
All airline and most corporate pilots spend as much as two weeks in ground school studying the systems of aircraft in which they are about to become qualified. This is before they see the inside of a cockpit (simulated or otherwise). After completing initial qualification, they undergo recurrent training every year to review what might have been forgotten.
When general aviation pilots get checked out in a new airplane, however, system knowledge is given short shrift, especially avionics systems, which include the autopilot and its related electric trim. Instructors typically spend little time explaining systems and instead tell them to "read the book," which is dry and boring. As a result, many pilots learn only what they need to know for normal operations.
On the Seneca IV, the small button on the control wheel is used almost exclusively to disconnect the autopilot. But that switch has a second function. When depressed and held, it serves as a trim-interrupt switch. In other words, it removes power in the event of a runaway trim. The pilot in this case was accustomed to achieving the desired result (an autopilot disconnect) with a momentary activation of the switch and never learned that it had to be held in to deactivate the electric trim system. This is one of those systems nuances that professional pilots learn in ground school and that many general aviation pilots may not learn until it is too late.
The pilot also was never taught that a runaway trim can be arrested by simply turning off the avionics master switch or the primary master switch, which would be an excellent example of prioritizing aircraft control ("aviate") over the need to "navigate" and "communicate."
It is difficult to know how many accidents might have been caused by a lack of system knowledge, but it should not be difficult for any pilot to prevent the one that could involve him.
Visit the author's Web site ( www.barryschiff.com).
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