Mach tuck is a change (usually nosedown) in an aircraft’s pitching tendency during transonic flight relative to its pitching tendency in below-transonic flight, caused by the appearance of areas of supersonic airflow (with attendant formation of shockwaves, sometimes accompanied by regions of flow separation) around the aircraft when it exceeds its lower critical mach number. It first showed up immediately prior to and during World War II, as a completely unexpected, and quite deadly, type of loss of control at high speeds (especially on aircraft using the very thick airfoils then in fashion); however, by the mid-1950s, advances in wing and airframe design had reduced mach tuck to little more than a nuisance, allowing even first-generation jetliners to safely fly at transonic speeds without risking a loss of control.1
In contrast, 20-series Learjets, although dating from the 1960s (by which point we already knew very well how to make aircraft highly mach-indifferent), tuck like an aircraft from the 1940s, pitching down suddenly and violently at approximately mach 0.81. This extreme susceptibility to mach-induced loss of control at high speeds caused a rash of crashes in the 1970s and 1980s, and is one of the many factors that make first-generation Learjets some of the most unforgiving aircraft ever sold to the public (at least, if the NTSB’s AAR archive is anything to go by).2
It is quite hard to see what about these aircraft would cause them to have such evil overmach characteristics; admittedly, they do have straight, rather than swept, wings, but this should impact more where the lower boundary of the transonic flight regime is - not so much how the aircraft behaves once it’s already in transonic flight - and, besides, the thinness of the wings should compensate for their lack of sweep.3
What is the cause of the extreme susceptibility of first-generation Learjets to mach tuck?
1: First-generation jetliners are, nevertheless, generally kept below their lower critical mach numbers in normal line operations, due to the greatly-increased drag associated with transonic flight; however, further design advances, first seen on the Convair 990, greatly mitigated even the transonic drag penalty, with the result that second-generation and later jetliners routinely cruise at transonic speeds.
2: Other factors included a balky autopilot, spoilers that make the aircraft go faster when extended (by causing a large pitch-down tendency of their own), and the fact that their original certificated service ceilings were well into the coffin corner.
3: The reason swept wings help with transonic flight is that they make the wing appear, to the air flowing over it, to be thinner than it actually is, thereby helping to delay and mitigate shockwave formation and flow separation. These same benefits can also be gained by actually making the wing thinner.