The US Federal Aviation Administration (FAA) ordered emergency tests after a rudder-control valve jammed in a laboratory experiment under extreme conditions.
In the event of a jam, tests suggested the aircraft could roll to one side and possibly go out of control if the pilot reacted incorrectly, or if the plane was too close to the ground.
British Airways owns 66 of the aircraft and will carry out checks on them over the next few days.
Boeing said the problem, which it referred to as "very unlikely", had never been encountered in more than 69 million flights worldwide. But the 737's rudder has been a focus of investigation into the still unexplained crashes of the Boeing aircraft in Colorado Springs in 1991 and in Pittsburgh in 1994. Crash investigators did not find conclusive evidence that rudder malfunctions caused either crash. But they did compile extensive evidence of potential 737 rudder-control problems, and prepared a list of recommendations they believed would make 737s safer.
Boeing has now issued a service bulletin directing a test of the rudder- control system on all of the 2,700 737s in service within 10 days, and then again each time a plane has flown 250 hours. The FAA will review, and possibly enlarge, Boeing's suggested inspection plan.
Statistically the 737, known as the "Guppy" and "Fat Albert" in the airline business, has a better than average safety record, with 1.21 crashes per million flights for older models and 0.51 for newer models. The figure for all passenger jets is 1.83.
The aircraft had its inaugural flight in 1968 and is the most popular commercial jet ever built. Every hour more than 500 737s take off and land worldwide. Since 1968, 67 have crashed, mostly in Third World countries, but no single safety issue had, until now, emerged.
"This event is extremely rare," said Guy Gardner, the FAA's associate administrator for certification and regulation. "(Boeing) are going with what's in their service bulletin, and we are analysing it."
Despite the unusual order and the National Transportation Safety Board (NTSB) tests that prompted it, the FAA still believes the Boeing 737 is a plane "with a high level of safety", Mr Gardner said.
The FAA said the inspection order does not suggest that rudder failure caused the still-unexplained 1994 crash of US Air 737 at Pittsburgh. But aircraft safety experts have questioned whether rudder failure could have caused both the Pittsburgh and Colorado crashes.
In both cases, the planes were about to land when they suddenly plunged to the ground, without their pilots having reported any malfunctions. Of 12 other 737 crashes worldwide in between, errant rudder movements have been suspected in two. And in June this year a pilot in Virginia, USA, claimed he had fought off two "rogue" rudder swings, narrowly avoiding a crash as he prepared to land a 737. In tests afterwards, a Boeing test pilot suggested the pilot may have overreacted to a rogue signal.
Verifying a problem with the jet's rudder control has proved difficult because of the transitory way it works and the small size of its controlling mechanism.
Boeing has always maintained that the design is safe and requires no alteration. It offered alternative theories for the two crashes and persuaded the NTSB to conduct a series of flight tests to back this up. However, these proved that rogue rudder swings could not be easily corrected by a pilot, as Boeing had insisted.
In the latest tests, Boeing officials said a secondary valve - which is called into use only if the primary valve fails - in the rudder-control unit jammed in a laboratory test under extreme temperature differences.
"This is the nature of our business," said Charlie Higgins, a Boeing vice-president. "We identify very unlikely possibilities, and take steps to eliminate them, or at least to further reduce their likelihood of ever happening."
Mr Higgins, who heads airplane safety and performance for Boeing, said no jam was detected in either rudder-control unit from the 737s in the Pittsburgh and Colorado Springs crashes.
But the NTSB said that "while it supported Boeing's efforts", it was studying the data and its relevance to the crashes.
The new inspections are not expected to cause any scheduling problems for airlines.Reuse content