The National Transportation Safety Board (NTSB) has issued another safety video highlighting the need for general aviation pilots to use "extra vigilance" in performing preflight inspections prior to flying for the first time after maintenance work has been performed on a plane.
Over the last two years, the NTSB has investigated multiple incidents in which the incorrect rigging of flight control and trim systems has led to serious in-flight emergencies, in some cases resulting in injury and death. Two common threads were found in these incidents:
- Maintenance personnel in charge of checking the flight control and trim systems failed to recognize that the control or trim surfaces were moving in the wrong direction.
- Pilots flying the aircraft in these incidents failed to recognize the control anomalies during preflight inspections.
In one example shown in the safety video, a college-aged pilot flying a Cessna T182T airplane in December 2014 experienced extreme nose-down control forces shortly after takeoff. Both the pilot and the pilot-certificated passenger were forced to pull hard on the yoke to prevent the plane from pitching down, even though the elevator pitch trim indicator showed a nearly full nose-up trim position.
The Cessna had just undergone maintenance work on the elevator trim system, and the mechanic had briefed the pilot preflight on what had been done. As the two were considering whether to declare an emergency, the pilot figured out that there might be some kind of control-reversal problem. While positioning the plane for an emergency landing, the pilot applied nose-down trim control inputs and found that the extreme control forces lessened. He was then able to successfully land the plane without incident.
It turned out that the pilot's hunch was right - the elevator trim control cables were misrigged, which caused the elevator trim control to be reversed. The mechanic who performed the maintenance work had 26 years of experience, proof that this type of mistake can happen to anyone.
So, what can maintenance personnel do to ensure these types of incidents don't happen?
1) Maintenance personnel should familiarize themselves with the normal directional movement of the controls and surfaces before disassembling. This will make it easier to recognize the difference between normal and abnormal.
2) Carefully follow manufacturer instructions to make sure that the work is completed as specified using only up-to-date instructions and manuals.
3) Be aware that some maintenance information-especially for older aircraft-may be nonspecific. If this is the case, ask another qualified person questions if something is unfamiliar.