18th of December 1969
It was on this day in aviation in 1969, that a 2" (5 cm) piece of duct tape caused the crash of the 4th SR-71 Blackbird 61-7953.
The wreckage of 61-7953 on fire after the crash (source: www.thisdayinaviation.com)
The aircraft was on a mission, built up of three sections. The first [art was a functional test flight (after being modified with a new system). The test flight section of the mission was followed by an aerial refuelling session before the latter part of the mission, an high altitude flight at supersonic speeds.
After successfully refuelling the aircraft the crew radioed ATC for permission to climb to 60.000 feet (18288 meters), which was granted. The throttles for both engines were placed into the afterburner setting. Right at that time, both engines stalled. The lack of thrust slowed the SR-71 fast, she then violently pitch up and was uncontrollable.
The pilot was heard saying "let's go", and both crew members ejected from the aircraft. Both crewmembers safely landed by parachute near the town of Shoshone, Ca, USA.
During the investigation into the crash, the investigators found a 2" (5 cm) piece of duct tape that was rolled into a plug, inside one of the tubes of the pitot-static system, During the modification of the aircraft, the pitot-static system had been disassembled and a plug rolled from the piece of duct-tape, was inserted in one of the lines of the pitot-static system as a plug to prevent FOD from entering the line. During re-assembly and testing of the pitot-static system, the blockage was not observed. During a post-crash test, it was found that the blockage was not complete, but was reducing the airflow through the tube substantially.
it did cause an overreading altimeter and an overreading of the altimeter.
61-7953 in flight (Source: public domain)
With the overreading gauges, the aircraft was at a different altitude and speed than indicated. When the trhottels were advanced the aircraft was actually 2000 feet higher than indicated and 26 knots faster than indicated. When the throttles were advanced the thinner air caused a compressor stall on both engines. At this particular combination of weight, speed, & altitude, the aircraft needed full Afterburning Thrust to maintain a steady-state flight. But the engines were starved for airflow at the same time and the compressor stalled. At that point, a “pitch up” and loss of the aircraft was inevitable.