For decades pilots of light airplanes have practiced flying instruments using only what we called a “partial panel,” meaning that several crucial instruments were not operating. Being able to control the airplane in instrument meteorological conditions (IMC) without all of the primary instruments functioning is absolutely crucial because a loss of control in the clouds invariably leads to a fatal crash.
Partial-panel flying emphasized failure of the attitude and heading gyros. It’s not that the spinning metal gyros common to airplanes until flat-glass displays took over a few years ago are particularly unreliable, but that the vacuum pump that powered them in most piston singles was very prone to failure.
