Can't speed up. That's the coffin corner. As an off-hour sailplane pilot, Cutting knew he had hit a mountain wave. He nursed the jet down just as if it were a sailplane. That episode marked the only moment in his long flying career when he genuinely feared a crash.
Atmospheric waves occur when air flows over a mountain or ridge.
The air tries to return to its original height, but its downward momentum causes it to fall too far. It then rebounds upward and overshoots again, climbing even higher. This oscillation repeats several times before the wave dissipates. Increase wind speed steadily with increasing altitude. Air in a mountain lee wave spends half its time ascending and the other half descending.