The latest alert has FBI and Homeland Security officials warning pilots and small airports of possible terror attacks.
"Two FBI agents actually came in and introduced themselves and told me about the possibility of, not a specific threat, but that al Qaeda was planning to use general aviation to harm us here in America," said Diamond Aviation flight instructor Bob Wood.
There are 228,000 general aviation planes at the 4,000 airports across the U.S. It is too many to monitor them all, but officials are working to alert pilots.
"The threats are real and the stakes are high," said John Pistolewe from the TSA. "We face a determined enemy who has not given up."
Experts say /*al Qaeda*/ is no longer determined to pursue only massive attacks, such as Sept. 11, but they're also focused on smaller attacks that are less complicated
"These planes are small," said ABC News Security Consultant Brad Garrett. "You might not be able to put some huge device aboard them. But the other side of that is if you were to put some biological or chemical agent in one of them, clearly that could cause some major damage depending on your target."
Threats to small airplanes are nothing new. Following the 2001 attacks, the government grounded thousands of crop dusters amid fears the planes could be used in terror attacks.