The second flight, on 30th December, did not fare as well. Early in the flight a chase plane observed smoke or fluid trailing the aircraft. As the chase plane came in for a closer look, Miller reported that the primary hydraulic system had failed. The aircraft turned to head home, and four miles from the Calverton runway the emergency nitrogen bottle was used to blow down the landing gear. At the same time the secondary hydraulic system failed, and the aircraft automatically switched to the emergency system. This is a minimal hydraulic system driven by an electric pump and designed to power the rudders and stabilators only. A mile or two later this system also failed, and the aircraft pitched into a dive, crashing a mile from the end of the runway. Both Smythe and Miller ejected successfully, and sustained only minor injuries, although the aircraft was totally destroyed.
The accident investigation showed that pipes in both hydraulic systems had failed due to harmonic fatigue. The cause of the failure was rooted in some exotic technology that Grumman had developed earlier for NASA’s lunar lander. In order to save weight, titanium hydraulic lines had been used in the prototype, and these were connected using innovative bimetallic sleeves which were chilled in liquid helium before installation. As the sleeves returned to room temperature, they shrunk, sealing the lines with a leak-proof connection. What was not fully appreciated was that the piping and connectors were extremely sensitive to how they were mounted on the airframe, both in terms of how they were attached to the fuselage structure, and in terms of the distance between attachments. At certain harmonic frequencies the pipes simply fractured. As it turned out, one of those frequencies was matched when an engine was idled in flight, and the second flight had tested single engine performance with one engine idling. A switch was made to more conventional aluminum tubing and threaded connectors in the second prototype and subsequent aircraft, along with a re-routing of some hydraulic lines to eliminate a ‘mirror image’ syndrome uncovered during the accident investigation.
The loss of the prototype did not greatly affect the flight test program, though the second aircraft (BuNo 157981) did not become available for flight test work until its first flight on 24th May 1971. The prototypes and initial production units were powered by Pratt & Whitney TF30-P-401 turbofans instead of the planned TF30-P-412s, although the newer engines were retrofitted into most aircraft.