The spectacular crash of a B-2 stealth bomber in February could have been prevented by a simple, unofficial "bootleg" maintenance procedure that some ground crews have used for years.

Small errors, it now turns out, caused a large accident. A B-2 has four computers, called the flight control system (FCS), that translate the pilot's cockpit inputs into movement of the plane's control surfaces. The $1.4 billion warplane was brought down by a few drops of water in three of the 24 air-pressure sensors that feed data to the FCS. The moisture distorted the plane's air-pressure readings and confused the FCS badly enough to cause the crash, the first one of the B-2's career.

February's crash was caused by maintenance crews trying to do the right thing: They saw the wrong data and recalibrated the sensors. However, once the moisture evaporated, the sensors "fixed" by the crew were actually set incorrectly and were feeding the flight computer false data on airspeed and air pressure, which is used to measure altitude. "The pressure differences were miniscule, but they were enough to confuse the FCS, " Maj. Gen. Floyd Carpenter, who headed the Air Force's investigation, tells PM.

The FCS then took control; triggering a premature takeoff, automatically driving the airplane into a 30-degree, nose-up pitch and overruling the pilot's efforts to regain control. Fortunately, the pilot and commander were able to eject safely just before the B-2, the Spirit of Kansas, crashed in flames.

The accident might have been avoided­ if the crew that readied the B-2 for?takeoff knew of an unofficial fix that had been used by maintenance personnel for at least two years. During the occasional, temporary B-2 deployments to rainy, humid Guam, where the planes were often stored outside, some B-2 ground crewmen noticed that air data calibration was required much more often.? In 2006, a Air Force engineer based in the United States suggested turning on the heat before the calibration to boil off any water in the system before adjusting the sensors.

The procedure worked, and some ground crewmen adopted it,? but it was never formalized into a technical order change or captured in any after-action reports. Unfortunately, the Spirit of Kansas' pilots and ground crew were out of the loop.

Even those ground crewman who used the bootleg procedure had no inkling of the potential dangers of a slight miscalibration. Before the crash, air data calibrations were considered a benign way to double-check the altimeters.? Apparently no one made the connection that the air data sensors also fed info to the FCS computers about airspeed, angle of attack, and sideslip--and that the FCS flew the plane based largely on those numbers.

The B-2's computerized fly-by-wire wizardry is a supreme technical achievement, but the Guam crash--the only one in the stealth bomber's 19-year flying history--underlines the vulnerability of even sophisticated computer systems to mundane glitches. And it doesn't matter how sophisticated a military system is if the people taking care of it can't communicate.

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