On 18 December 1969, SR-71A (61-7953) was scheduled for a functional check flight (FCF), piloted by SR-71/F-12 Test Force director Col. Joe Rogers and his Reconnaissance Systems Operator (RSO), Maj. Gary Heidelbaugh. The mission callsign was DUTCH 68. The aircraft configuration included, for the first time ever, the Optical Bar Camera (OBC) nose assembly.
That morning the airplane was parked on the hardstand behind Hangar 1414 and readied for the mission. Left engine start was uneventful, but the right engine flamed out and had to be restarted. Rogers taxied the SR-71 to the departure end of Runway 04. Before he began engine trim runs, the astro-inertial navigation system failed. The RSO attempted to correct the malfunction without success. On the advice of DUTCH BRAVO (SR-71 Operations) a hot ground-start was attempted and the system began operating normally.
The pilot ignited the afterburners and the SR-71 rolled down the 15,000-foot concrete strip and leapt into the sky above Edwards. DUTCH 68 passed over the 44-square-mile expanse of Rogers Dry Lake, climbing to an altitude of about 15,000 feet.
Rogers and Heidelbaugh completed a subsonic FCF in the Edwards area with Lt. Col. William Campbell flying chase in an F-104. Weather conditions included clear skies but haze obscured the horizon, so Rogers had to rely on his instruments in order to maintain level flight. Campbell had noticed DUTCH 68 was wandering a bit in his altitude profile, but assumed Rogers was simply preoccupied with the FCF.
Rogers then requested clearance to climb to 25,000 feet for rendezvous with SAUCE 62 (a KC-135Q tanker) in the refueling track near Beatty, Nevada. Once in sight of the tanker, Rogers maintained altitude control by visual contact with the KC-135 and refueling was completed without incident.
After separating from the tanker Rogers initiated a pre-planned acceleration and climb, starting with a gentle turn to the left to correct his course. After receiving clearance from Los Angeles Center to climb to flight level 600 (60,000 feet) and above, the pilot notified the RSO that he was about to select afterburner and transition to supersonic flight.
"Okay, I'm going to light them off, Gary."
A few seconds after lighting the afterburner, Rogers and Heidelbaugh heard a loud bang and high-frequency vibration, accompanied by a loss of power and severe control difficulties.
"What caused all that?" asked the RSO.
"I don't know." Rogers responded.
The pilot, suspecting possible engine compressor stalls, pulled back on the throttles until the vibrations ceased and then re-engaged afterburners. The vibrations started again and the airplane's nose began to rise. As the aircraft pitched up, Rogers realized it was uncontrollable.
"Let's go." he said.
As the crew ejected and parachuted to safety the aircraft continued in a deep stall, making its grave near Shoshone at the southern end of Death Valley, California. As it fell, the airplane broke into three sections. The nose separated ahead of the cockpit and the forward fuselage broke just forward of the wings. The aft fuselage, engine nacelles, tail fins and wings stayed together until impact. At the time of its demise, the airplane had logged a total of 290.2 flight hours.
Volunteer firefighters from Shoshone were the first to arrive at the scene. The aft section had sheared through several Continental-Edison transmission lines that supplied power to the communities of Shoshone, Tecopa and Zabriskie. The airplane's fuel ignited an intense fire, but it was soon extinguished. When Air Force crews arrived from Edwards they secured the site and began cleanup operations. Fortunately the Mission Recorder System (MRS) survived mostly intact and provided important information.
The MRS tape showed altitude variations of plus-or-minus 2000 feet during the FCF and afterward until Rogers reported sighting the tanker. At that point, his altitude became steady because he was no longer relying on his altimeter.
The MRS indicated that, after refueling, DUTCH 68 continued to have problems maintaining altitude. Airspeed also varied with these altitude excursions. Additionally, the MRS data was considerably different than Rogers' recollections of speed and altitude during his testimony before the Accident Investigation Board.
VIEW LOOKING SOUTHWEST TOWARD THE MAIN CRASH SITE. NOTE THE TRIANGULAR PATTERN ON THE TRAILING EDGE SECTION IN THE FOREGROUND
The Board struggled to determine the nature of the discrepancy because finding the answer would be key to identifying the primary cause of the accident. Analysis of the engines, hydraulics, flight controls and other systems showed no signs of mechanical failure.
PHOTO OF THE SR-71'S DETACHED NOSE SECTION. ARROW POINTS TO THE AIRCRAFTS NEEDLE-LIKE AIR DATA PROBE
About seven weeks after the accident, the Board found a significant clue. When the airplanes nose was modified to carry the new Optical Bar Camera, new pitot-static lines were installed. These lines were stainless steel and about 3/8-inch in diameter. Inside one of the two static lines, investigators found a piece of Duct Tape rolled up into a tight cylinder. The Board assumed it was placed there as a makeshift dust-plug when the line was fabricated. It was apparently not removed when the new OBC nose was installed.
The affected static line fed air data to the primary flight instruments and was not the source for the Triple Display Indicator (the TDI was normally used only during high-speed flight) or the MRS. Normal ground testing of the pitot-static system before 953 flew was completed satisfactorily but since some air could pass though the obstructed tube, this steady state test (basically a leak test) did not reveal any problems.
Technicians at the NASA Flight Research Center at Edwards performed a line-lag test on the tube to determine how much the static pressure available to the flight instruments would lag behind the actual aircraft altitude during climbs and descents because of the obstruction caused by the Duct Tape. This analysis provided an exact correlation between what Rogers thought the airplane was doing and the actual speed and altitude as recorded by the MRS.
When DUTCH 68 finished refueling and descended about 3,000 feet, Rogers thought his altitude was about 2,000 feet higher. When he tried to level off at 25,000 feet, the altimeter and rate of climb continued to show a slight descent so he started to climb again. A little later, when he thought he had returned to 25,000 feet, he was actually at about 27,000 feet and 20 to 30 knots slower than indicated. At this weight/speed/altitude combination, the aircraft needed full afterburning thrust to maintain level flight, but the engines were starved for airflow and the compressor stalled. At that point a pitch-up was inevitable.