61-7952

The story of the first Blackbird lost.

61-7925 over a mountain range.
#952 on a left bank angle.

Construction of the third Lockheed SR-71A (61-7952) began on 27 January 1964. Ten months later, the complete airframe was rolled out on 8th December. It made its first flight on 24 March 1965 from Air Force Plant 42 at Palmdale, California.

The USAF designated #952 the systems and sensors test aircraft and spent its entire service life engaged in developmental flight test activities while assigned to the SR-71/F-12 Test Force at Edwards Air Force Base. It carried cameras, radar and electronic intelligence (ELINT) systems for initial flight-testing and performance evaluation. Later, electronic countermeasures (threat warning and jamming systems) were installed.

61-7952 was also the first SR-71 to run inflight refueling tests, after meeting with a KC-135A on 29 April 1965.

61-7952 refueling from a KC-135 via boom.
SR-71A (S/N 61-7952) refuels from KC-135A (S/N 58-0094). (U.S. Air Force photo)

A Presage of Trouble

The engineers at Skunk Works designed the SR-71’s inlet configuration to automatically adjust during supersonic flight. The inlets needed to decelerate airflow, slowing it to subsonic speed before reaching the engine’s compressor face. To accomplish this, the inlet’s cone-shaped spike moved forward or aft, as required. The inlet bypass doors also opened and closed to control airflow around the engine.

Normally, there were mechanisms that performed these functions based on the aircraft’s Mach number, positioning the shock wave inside the inlet to ensure optimum engine performance.

Without proper scheduling, disturbances inside the inlet could result in the shock wave being expelled forward. This would result in an inlet unstart and lead to instantaneous loss of thrust, explosive banging noises and violent yawing motion.

In November 1965, Col. Robert L. “Silver Fox” Stephens and Maj. Kenneth D. Hurley departed Edwards in 61-7952 for a test flight along what was known as the Northern Route. While travelling at Mach 3.1 at 80 000 ft near Klamath Falls, Oregon, the aircraft’s left inlet suddenly unstarted during a 40 degree bank left, leading to an uncontrolled left roll.

Stephens struggled to regain control but eventually did so. He then headed south, to land at Edwards AFB. After he started the descent, the left engine began to malfunction. Stephens set it to idle and proceeded with an emergency landing at Edwards. The investigation later found that 3 of the 6 bleed air bypass tubes on the left engine had split open, allowing 650ºC air to blast inside of the titanium nacelle structure. Fortunately, no catastrophic failure resulted.

Lockheed repaired the airplane and returned it to the Test Force. It made a total of 41 flights and logged 79.47 flight hours.

The last flight of 61-7952

On 25 January 1966, #952 took off from Edwards AFB to test several systems and also investigate procedures designed to reduce trim drag and improve high Mach cruise performance as a function of center of gravity (CG) position. This involved flying with the CG further aft than normal, reducing the Blackbird’s longitudinal stability.

Lockheed test pilot William Weaver and flight test reconnaissance and navigation systems specialist James Zwayer departed at 11:20 with mission call sign “Dutch 51”.

After they completed the initial part of the mission and the refueling from a KC-135Q tanker, the pilot climbed to 78 000 ft to continue the tests.

During a 35 degree right bank turn at Mach 3.17, an unstart occured on the right engine. This led to the SR-71 rolling into a 60 degree right bank and pitching up. Weaver reacted, pushing the controls left and down, without response. He new instantly they would have to eject, but didn’t think it was survivable at that speed and altitude.

The cumulative effects of system malfunctions, reduced longitudinal stability, increased angle of attack and supersonic speed imposed forces on the airframe that exceeded the Stability Augmentation System’s ability to restore control and stressed the airframe to failure.

Several seconds after the unstart, the 61-7952 suddenly disintegrated at 78 800 ft, 60 miles north of Tucumcari, New Mexico. Both occupants flung into free fall by the snapping of their seat harness restraints.

When Weaver regained consciousness, he could hear the sounds of rushing air but due to frost on his helmet’s faceplate, he couldn’t see anything, much less estimate his altitude. Luckily the automatic parachute opening system, designed to trigger at 15 000 ft, worked and deployed his chute.

Relieved by the feeling of his chute opening, he raised his frozen faceplate. The breakup damaged the helmet mechanism, so Bill had to keep holding the faceplate up while trying to control his parachute. Eventually landing safely, a local ranch owner picked him up with his helicopter and flew him to the nearest hospital.

James Zwayer was found dead with a broken neck, making him the only fatality in any SR-71.

Weaver’s full account of the event is here.

The information on this page was gathered from The X-Hunters, Habu.org and Aviation Safety Network.