Improper aircraft modifications have proven to be fatal, in the past, taking the lives, innocent or not, of the men and women on board. Aircraft modifications are the responsibility of maintenance personnel, who ensure that the modification is done right, and checked before releasing the aircraft.
On December 3rd, 2013, the U.S. Department of Transportation’s Federal Aviation Administration (FAA) proposed a $325,000 penalty against Dallas-based Southwest Airlines for allegedly operating an aircraft that had been improperly modified, violating Federal Aviation Regulations. The aircraft is a Boeing 717, operating for US based low cost carrier AirTran Airways, which is in the process of merging with Southwest.
On Aug. 29, 2011, maintenance personnel improperly installed a switch that enables flight crews to test the windshield heating system on a Boeing 717. The Boeing 717, unlike most other Western airplanes in operation, has 7 cockpit glass panels (excluding the eyebrows). Of these, three (left, center and right) are the front facing windshields, and the rest windows.
Proper installation of the switch would have allowed personnel to isolate the windshield anti-ice system that was causing a warning that the windshield heater was failing. Instead, the center and left windshield warning systems were reversed. The right windshield warning system continued to operate properly. The aircraft was operated on 1,140 passenger flights before the problem was corrected.
Southwest spokeswoman Brandy King said that the installation error didn’t compromise safety as it was an extra system. The primary system for alerting crews to a potential window-heater malfunction was still working.
On the 20th of March, 2001, a Lufthansa Airbus A320 almost crashed when the captain’s sidestick was cross-wired in error by maintenance personnel. Although the captain commanded a right roll input, the airplane rolled left. Following the brief confusion, when the left wing-tip was around 2ft from the ground, the first officer, whose controls operated normally, took over by overriding the captain’s side stick input.
The Flying Engineer 
Another lesser known, “maintenance caused” fatal accident occurred at Colgan Air in a Beech 1900 back in August 2003. An elevator trim control cable was twisted when it was installed, resulting in reverse elevator trim from that which was commanded by the pilots. The plane took off from KHYA and crashed into Nantucket Sound shortly after.
Trust but verify!