(CNN) -- Nine commuter aircraft were grounded for safety inspections Tuesday at Chicago's O'Hare airport after a federal security inspector climbed onto them by grabbing sensitive outside instruments, the Transportation Security Administration said Wednesday.
"It delayed a lot of folks getting to where they had to go yesterday," said American Eagle spokeswoman Andrea Huguely. "This was something we had never experienced before."
The inspector used the total air temperature probe and the jetway to haul himself through the open doors of seven of the nine planes he inspected, a TSA official said.
The inspector aimed "to look for and test, among other things, access vulnerabilities or areas were someone with ill intent could gain access to the aircraft," the TSA said, adding: "Aircraft operators are required to secure each aircraft when left unattended."
Sam Mayer, a spokesman for the Allied Pilots Union, said the maneuver could have damaged the probes. Pilots use readings from the probes to gauge the probability of icing.
"Apparently the TSA inspector hoisted himself up using the TAT probe as a stirrup. He repeated the same procedure with nine aircraft. Most TAT probes have a painted decal on the side that reads 'NO STEP,' " he said.
"We caught it this time, but who knows if this has happened other times ... and with other planes that are out there," Mayer added.
He said pilots examine their planes before take-off, but they might not spot something like a part damaged in an inspection.
"Pilots are the last line of defense. We do our pre-flight inspections, but we don't have a system in place to check for broken parts," he said.
Huguely said the airline's own inspectors checked out all of the aircraft, did not find any damage, and put them back in service by 10:45 a.m. Tuesday. She said the airline is in touch with the TSA "and we are working through this issue with them."
TSA spokeswoman Ellen Howe said the inspector did not intend any harm.
"It is not TSA's intent to cause delays or potential damage to aircraft as a result of our inspections," she said.
American Airlines, which owns and operates American Eagle, could face fines over the incident, she said.
"The inspector was able to gain access to the interior of seven of the nine aircraft inspected, which is an apparent violation of the airline's security program," Howe said. The inspector was not identified.
"TSA is reviewing the inspection results and, depending on the conclusion, could take action with the airline up to and including levying of civil penalties," Howe said.
American Eagle said in a statement that it was "confident that it followed all proper security procedures."
But, "if not observed by vigilant Eagle employees in Chicago, the actions of this inspector would have been unknown and could have jeopardized the safety of our customers and crew because of potential damage to the sensitive equipment mounted on the aircraft fuselage," the airline said. "American Eagle is also concerned that the TSA inspector's method of physically hoisting himself up to the jet bridge using a part of the aircraft could have placed him in physical danger."
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