AIN Blog: Boneheaded Engineering Department
A recent Aviation Maintenance Alerts published by the FAA highlights a problem that should never, ever come up in aerospace: a design that allows mechanics to install something opposite the way intended. In this case, according to AC 43-16A No. 407, mechanics installed the elevators on a Piaggio Avanti upside down. After doing so, the mechanics were even able to rig the elevators according to the aircraft maintenance manual (AMM) instructions.
Although the elevators had been installed upside down, the twin turboprop was able to fly—and fly it did. According to the FAA’s maintenance alert, “During flight, this reversed elevator installation greatly influenced elevator trim authority—additionally causing the airplane yoke to be in a noticeably different longitudinal position.” The alert goes on to note that Piaggio has added a note to the AMM, warning mechanics about this potential problem. The FAA added, “A simple way to ensure the correct elevator is installed on the proper side is to verify the location of the static wicks—they must be on the upper surface of the elevator.”
That this upside-down elevator installation and what mechanics thought was proper rigging could happen is a dangerous example of poor engineering practices. It reminds me of an incident that happened to my friend, the acclaimed cinematographer Robert E. Fulton, when he flew his Cessna 180 to South America to do some aerial filming. While there, a local maintenance outfit offered to strip the paint off the 180; Robert liked the polished-aluminum look, and the price was right.
When he returned to the U.S., a puzzled Robert relayed to Fred, his expert mechanic, that he couldn’t figure out why the 180 seemed to fly funny. During landing, he just couldn’t get as much elevator authority and couldn’t raise the nose as much as before. Fred spent hours looking at the airplane, checking cable runs, control travel and so on. As he was measuring the travel of the elevators, Fred happened to look underneath the airplane and he noticed a big dent in one of the elevators. He mentioned this to Robert, and finally the rest of the story became clear. The South American mechanics, it turned out, had dropped something heavy on one of the elevators while stripping the paint. The clever fellows figured out that they could flip the elevators over to conceal the dent; they didn’t realize that although the elevators looked identical after being flipped, the reversal changed the way the airplane flew. And they never told Robert about their little accident. Luckily, Robert managed to fly from South America back to Massachusetts without crashing.
This is another example of engineers designing a product that can easily be messed up by inattentive mechanics. But wait, there’s more.
The AMM illustration showing how the trim-tab cable should have been installed on the trim drum in the Beech 1900 was backwards, according to the FAA Airworthiness Directive that resulted from an Aug. 26, 2003, fatal crash. “The figures in the applicable maintenance manuals depict the elevator trim cable drum at 180 degrees from the installed position and show the open, keyed side of the drum instead of the flat side of the drum. Following these figures when installing the control cables on the forward control cable drum could reverse the action of the elevator manual trim system,” the AD stated. The text of the AMM was correct, but the illustration was wrong. What was really wrong with this situation is that the system was designed such that a maintenance error could cause the elevator trim system to function in reverse.
One more example, which almost killed an A320-load of passengers and crew. This is the Lufthansa Papa Whiskey incident on March 21, 2001, where the pilots skillfully figured out that the captain’s aileron controls were backwards, while the copilot’s controls worked normally. The A320’s left wingtip dropped within a few feet of the ground during takeoff before the copilot took over and straightened the airplane. It turned out that a mechanic mistakenly reversed two wires on an electrical plug assembly that contains 420 connections. No one detected the error because cockpit instruments showed that the controls worked correctly; nobody moved the captain’s sidestick and watched for correct aileron travel after the job was done.
Certainly, there are holes in the maintenance procedures in all of these situations. Mechanics must follow the AMM carefully and verify proper operation after any job, but the fundamental issue is that engineers designed these flawed systems. And the flawed design of these systems allowed mechanics to cause a problem, instead of making it impossible for mechanics to create a problem where there was none before.
The solution to this is simple. Engineers need to get out from behind their computers and spend more time not only watching mechanics work, but maybe even working with mechanics. Engineers need to see how easy it is to take their wonderful designs and inadvertently mess them up. Their designs can’t rely on an alert mechanic or pilot noticing that the static wicks are on the top or bottom of the elevators. At the same time, it wouldn’t hurt for engineers to see how confoundingly difficult they make it to get at certain components and the deleterious effect that has on safety, but that’s another story.