Moist air most likely caused the US Air Force to lose one of its prized Northrop Grumman B-2 bombers earlier this year, a loss calculated at $1.4 billion, the US Air Force reported on 5 June.

During a takeoff on a Guam runway on 23 February, moisture contamination on the fly-by-wire B-2’s air data sensors threw off the tailless bomber’s sensitive flight control system.

The onboard computer believed the bomber was pointed slightly downward on takeoff roll and rolling 10-12 knots faster than its actual speed.

These two errors set off a chain of events that caused the bomber to crash.

First, the faulty airspeed indication caused the bomber to depart the runway about 1,450ft sooner than normal. During the early rotation, the incorrect angle of attack data caused an uncommanded, 30-degree pitch-up. The bomber’s slow speed at this 1.6g-inducing angle caused the aircraft to stall, yawing until its left wingtip scraped the ground.

At that moment, the B-2’s two crewmembers ejected from the aircraft. The crewmember identified as “mishap pilot-2” in the USAF report pulled the ejection handle, although the first pilot also told investigators he knew it was time to “get out”.

In the next instant after the pilots ejected, the B-2 spun into the ground, ripping off the nose gear and left main landing gear. The second crewmember suffered a compressed spinal fracture during the ejection, but is expected to fully recover. The other crewmember was lightly injured.

The first crewmember attempted to recover the B-2 after the uncommanded pitch-up, but faced a perhaps impossible task with the low-speed and low-altitude conditions.

With no abnormal control inputs by the crewmembers, the USAF investigation focused on the aircraft’s maintenance procedures.

On a previous deployment, maintenance crews found that moisture could foul up sensors unless a special technique was performed. The technique involved heating a pitot tube to burn off moisture before the engine was started. Despite the severity of the problem, USAF investigators found that the technique was not widely communicated among the maintenance crews.

On the day of the crash, a maintainer performed an air data calibration without heating the pitot tube. Because the sensors were coated with moisture, the calibration actually drove transmitted latent errors into the flight control system, the USAF’s report says.

The first crash of the B-2 reduces the only combat-coded fleet of stealth bombers to 16 aircraft, and the total fleet to 20 aircraft. It accident also marked the single-most expensive military aircraft disaster in aviation history.

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