Mechanical problem forces Obama plane to land

ST. LOUIS (Reuters) - A Midwest Airlines MD-80 carrying U.S presidential candidate Barack Obama made an unscheduled landing on Monday in St. Louis after pilots found they were having difficulty controlling the plane, U.S. safety investigators and airline officials said.

After landing, an inspection found the unusual cause of the problem -- an emergency evacuation slide deployed inside the plane underneath the tail. The tail cone, the very rear of the fuselage where passengers would exit in an emergency, did not pop off the plane when the chute inflated.

Climbing out of Chicago en route to Charlotte, North Carolina, at about 10:30 a.m. EDT, the pilots were flying around thunderstorms when they noticed heavier than normal forces on controls that move horizontal flaps atop the plane's tail.

"We detected a little bit of controllability issue in terms of our ability to control the aircraft in the pitch, which is the nose up and nose down mode," the pilot said before diverting to Lambert-St. Louis airport.

The jetliner was carrying 48 passengers, including Obama, aides and journalists.

Midwest Airlines said in a statement there "was never an issue" with the safety of flight and the decision to land in St. Louis was a "precautionary measure."

Obama spoke with reporters afterward.

"So everything is fine guys ... just thought we'd spice things up a little bit today," Obama joked with reporters later. "Any time the pilot says something's not working the way it's supposed to, then you make sure you tighten your seat belt."

The MD-80, also used to ferry sports teams, is a substitute for the usual campaign plane now being overhauled ahead of the November election push.

The National Transportation Safety Board has launched an investigation.

(Reporting by Caren Bohan, John Crawley and Andy Sullivan, editing by Jackie Frank)

(To read more about the U.S. political campaign, visit Reuters "Tales from the Trail: 2008" online at blogs.reuters.com/trail08/)