The general aviation industry has long known about the vulnerability of the engine-driven vacuum pump. This device supplies the energy to spin vacuum-driven primary flight instruments, typically the artificial horizon and directional gyro of a so-called “steam gauge” airplane. It’s an Achilles heel of an otherwise IFR-capable platform and, although conscientious pilots train for so-called “no-gyro” flight, it’s never as easy as it is in training.
All other things being equal, one of the benefits of a primary flight display (PFD, which presents flight instrumentation on an electronic panel) is its use of a solid-state attitude and heading reference system, sometimes known as an AHARS. By using an AHARS to determine which side is up and in which direction the airplane is pointed, the vacuum-driven system is avoided and usually only an electrical system failure or failure of the display itself can eliminate the flight instruments. (Certification rules require backup flight instruments when a PFD is present but not when steam gauges are energized by a vacuum pump.)
