NTSB Safety Alert SA-042 - Mechanics Prevent Misrigging Mistakes

Mechanics: Prevent Misrigging Mistakes

Verify correct directional travel of controls and trim

The problem

 Incorrect rigging of flight control and trim systems has led to in -flight emergencies,

accidents, and even deaths.

 Four such mishaps within a 2 -year span share common safety issues:

o Maintenance personnel who serviced or checked the systems did not

recognize that the control or trim surfaces were moving in the wrong direction.

o Pilots who flew the airplanes did not noti ce the control anomalies during their

preflight checks.

 Anyone can make mistakes. In some cases, the mechanics who performed the work

incorrectly were highly experienced.

Related incident and accidents:

 In December 2014, the pilot of a Cessna T182T airplan e experienced extreme

nose -down control forces on the yoke shortly after takeoff. The airplane had just

undergone maintenance on the elevator trim system, and the mechanic briefed the

pilot about the work before the flight. Both the pilot and the pilot -certificated

passenger needed to pull hard aft on the yoke to prevent the airplane from pitching

down, even though the elevator pitch trim indicator showed a nearly full nose -up trim

position. The pilot remembered that work had been done on the elevator trim system,

and he thought that there might be some kind of control -reversal problem. While

maneuvering for the emergency landing, the pilot applied nose -down trim control

inputs and found that the extreme control forces lessened, and he successfully

landed th e airplane. The elevator trim control cables were misrigged such that the

elevator trim control was reversed. The mechanic who performed the work had 26

years of experience and described that he had carefully checked the trim tab

deflection angles. However, he did not recognize that the trim tab moved in the wrong

direction. The mechanic who checked the work had 24 years of experience and noted

that the cable rigging problem could not be clearly visually identified through the

airplane’s access panel . The maintenance manual provided the following information:

SA-042 March 2015 , revised December 2015 “Make sure that the trim tab moves in the correct direction when it is operated by the

trim wheel. NOTE: Nose down trim corresponds to the tab UP position.”

(CEN15IA079 )1

 In May 2013, the pilot of a Schweizer SGS 2 -33A glider received serious injuries after

the glider crashed due to imprope r rigging of the rudder control cables. The glider had

just received an annual inspection, and its rudder had been removed and reinstalled.

The mechanic who conducted a flight control check after the annual inspection with

assistance from another pilot sea ted in the cockpit stated that he confirmed

movement of the rudder while at the rear of the glider; however, he did not see which

pedal the pilot was pushing and did not verbally confirm the corresponding position of

the rudder pedals. The mechanic had 35 years of experience and had assembled

hundreds of gliders. ( ERA13LA229 )

 In October 2013, the private pilot and passenger in a Piper PA -22 received serious

injuries when the airplane crashed during takeoff on its first flight since an annual

inspection in which frayed elevator control cables were replaced. During the takeoff

roll, when the pilot ap plied forward stick controls to raise the tail, the tail did not rise,

and the airplane instead “jumped” off the runway. The pilot reduced engine power

and attempted to control the airplane’s pitch, but it responded abnormally and

crashed. The elevator con trol cables were installed incorrectly such that the elevator

moved in the direction opposite to that commanded. (ERA14LA015 )

 In July 2014 , the pilot of a Piper PA -12 airplane was fatally injured after his airplane

pitched up steeply during takeoff and crashed after the airplane had undergone

extensive maintenance. The elevator control cables were installed incorrectly such

that the elevator moved in the direction opposite to that commanded. (ANC14FA050 )

What can maintenance personnel d o?

 Become familiar with the normal directional movement of the controls and surfaces

before disassembling the systems. It is easier to recognize “abnormal” if you are very

familiar with what “normal” looks like.

 Carefully follow manufacturers’ instructions to ensure that the work is completed as

specified. Always refer to up -to-date instructions and manuals —including

airworthiness directives, maintenance alerts, special airworthiness information

bulletins, and unapproved parts not ifications —when performing a task.

 Be aware that some maintenance information, especially for older airplanes, may be

nonspecific . Ask questions of another qualified person if something is unfamiliar.

 Remember that well -meaning, motivated, experienced te chnicians can make

mistakes: fatigue, distraction, stress, complacency, and pressure to get the job done

are some common factors that can lead to human errors. Learn about and adhere to

sound risk management practices to help prevent common errors.

1 These pilots and the mechanic are featured in the NTSB’s safety video, “ Airpla ne Misrigging: Lessons

Learned F rom a Close Call ,” which is available on the NTSB’s Safety V ideos web page at

http://www.ntsb.gov/safety/safety -alerts/Pages/Safety -Videos.aspx .

SA-042 March 2015 , revised December 2015  Ensure that the aircraft owner or pilot is thoroughly briefed about the work that has

been performed. This may prompt them to thoroughly check the system during

preflight or help them successfully troubleshoot if an in -flight problem occurs.

Interested in more i nformation ?

The reports for the accidents referenced in this safety alert are accessible by NTSB accident

number from the Aviation Accident Database & Synopses web page at

http://www.ntsb.gov/_layouts/ntsb.aviation/index.aspx .2 Each accident’s public docket is

accessible from the NTSB’s Docket Management System web page at

http://dms.ntsb.gov/pubdms/ .

Education and training are cri tical to improving GA safety. The Federal Aviation

Administration (FAA) Safety Team ( FAASTeam ) provides access to online training courses,

seminars, and webinars to provide mechanics with recurrent training that focuses on

accident and incident causal factors, special emphasis items, and regulatory issues. The

courses listed below (and many others ), as well as seminar and webinar information, can be

accessed from the FAASTeam website at www.faasafety.gov . (Course access requires login

through an existing free FAASTeam account or creatin g a new one .)

 Dirty Dozen: Human Error in Aircraft Maintenance

 Human Factors Prim er for Aviation Mechanics

 Failure to Follow Procedures: Installation

 Failure to Follow Procedures: Inspections

 Aircraft Maintenance Documentation for AMTs

 Fatigue Countermeasure Training

Aviation Maintenance Technician Handbook — General (FAA -H-8083 -30) includes an

“Addendum/Human Factors ,” which discusses the human factors -related conditions behind

most maintenance errors. The addendum (which is also called chapt er 14) can be accessed

from the FAA’s website at www.faa.gov .

“Acceptable Methods, Techniques, and Practices —Aircraft Inspection and Repair ” (FAA

Advisory Circular 43. 13-1B) contains a Personal Minimums Checklist in chapter 13 that lists

actions to reduce maintenance errors.

Aircraft Safety Alerts , including airworthiness directives, maintenance alerts, special

airworthiness information bulletins, and unapproved parts notifications, can be accessed

from www.faa.gov/aircraft/safety/alerts .

This NTSB safety alert and others can be accessed from the NTSB’s Safety Alerts web

page at http://www.ntsb.gov/safety/safety -alerts/Pages/default.aspx or searched from the

NTSB home page at http://www.ntsb.gov/Pages/default.aspx .

2 Less-recent fatal accidents that may also be of interest are CHI05FA038 , CHI08MA270 , and

DCA03MA022 .