Structural Testing Northrop Grumman constructed two additional aircraft without engines or instruments for fatigue testing. On the second of those test aircraft (the B-2 on display at the museum), engineers attached computer-controlled, hydraulically driven plates along the airframe to simulate all flying conditions. They calculated that the structure would reach 150 percent of the design specifications, but the wing withstood stresses over 161 percent before it finally cracked. Early production