Among the questions that investigators are asking about the lethal shoot-down of an Army Chinook helicopter (search) in Iraq is whether the chopper used the standard defenses against surface-to-air weapons, such as flares designed to deceive a heat-seeking missile.

Details of circumstances in which the dual-rotor CH-47D Chinook was shot out of the sky over central Iraq on Sunday, killing 16 soldiers and injuring 20, were incomplete Monday.

An Army spokesman at the Pentagon, Maj. Gary Tallman, said all CH-47D helicopters are equipped with missile defense systems (search), but it was not clear what defensive measures were taken in this case.

The question is important because hundreds of U.S. airplanes and helicopters move troops and supplies around Iraq daily, all inherently vulnerable to the kind of missile that destroyed the Chinook. In practical terms, there is no way to stop using air transport to move troops and equipment around Iraq, even in the most hostile territory north and west of Baghdad (search), officials said.

A Chinook normally flies with a crew of three and up to 33 passengers, making it an efficient means of transporting troops as well as equipment and supplies around the battlefield. Smaller Black Hawk helicopters as well as Air Force C-130 cargo planes also are used daily.

C-130 and C-17 transports have flare and chaff dispensers as well as radar warning and other defensive systems, and they can fly at altitudes well beyond the range of some anti-aircraft missiles. The threat to planes is enough, however, to have delayed the reopening of Baghdad International Airport to commercial flights, even though the airport has been ready for commercial operations since mid-summer.

Lawrence Di Rita, spokesman for Defense Secretary Donald H. Rumsfeld, said there are uncounted numbers of shoulder-fired anti-aircraft missiles in Iraq beyond the control of the U.S. occupation forces.

"We know they've been fired before" at U.S. aircraft in Iraq, he said. "Countermeasures are not perfect."

Rumsfeld said on Sunday that Americans should not be surprised that U.S. aircraft are sometimes hit.

"We all know that these so-called man-portable surface-to-air missiles are widely available in the world and do have the ability to shoot down aircraft and helicopters and that from time to time it happens in various locations," he said.

By apparent coincidence, the shoot-down came on the same weekend that Central Command, which is running U.S. military operations in Iraq, expanded the number of soldiers eligible for flights out of the country for two-week vacations. A number of those killed and injured Sunday where en route to Baghdad to begin their breaks.

Maj. Michael Escudie, a Central Command spokesman, said Monday the rest-and-recuperation program would continue unchanged.

Di Rita said it was unclear whether investigators had recovered any tubes from which the surface-to-air missiles were fired, and he could not confirm reports that they were SA-7s. That is a Russian-designed missile that finds its target by seeking the heat from an aircraft's engine exhaust.

Chinooks are equipped with a series of countermeasures devices designed to defeat hostile radar and missile threats. These include metallic chaff for confusing the signals returned to a hostile radar, as well as flares that confuse a missile seeking a heat source.

Tallman said all Chinooks also have an ALQ-156 missile warning system in the cockpit.

The Special Forces variant of the Chinook, known as an MH-47E, is fitted with a missile approach warning system as well as an electronic signals jammer and other sophisticated defensive systems.

The Chinooks also can be equipped with one M60 machine gun mounted in the left side door frame and another in the right side escape hatch, as well as one mounted on the rear cargo ramp. It was not known Monday whether the Chinook hit on Sunday had machine gunners at each station.

Dan Goure, a defense analyst at the Lexington Institute research group, said the Army is aware of the inherent vulnerability of Chinook helicopters and can vary their flight patterns to lessen the threat.

"The helicopters are big, relatively slow and noisy, but they provide the mobility you can't get any other way" except with ground transportation vulnerable to other threats, such as ambushes, land mines and homemade bombs that the U.S. military calls improvised explosive devices, he said.