The June 2016 issue of Business & Commercial Aviation published an accident report to a PA44 Piper Seminole. It departed with an MEL on one of the two engine driven vacuum pumps which was inoperative.
During cruise in IMC the second vacuum pump failed. With the loss of the artificial horizon and HSI, the pilot lost control and the aircraft broke up. The aircraft was equipped with an electrically powered Turn coordinator as a back up source of attitude.
In October 1997, a Moonee M20 VH-KUE was lost in similar circumstances 113 kms NE of Balranald NSW. During dark night cruise flight the pilot reported vacuum pump failure but that he had back-up electrics. Shortly after the pilot's last radio transmission he lost control and crashed. The ATSB report stated:
“An examination of the wreckage indicated the aircraft had impacted the ground at high speed, in a steep nose-down attitude, consistent with loss of control. With the exception of the vacuum system, the aircraft was considered to have been capable of normal operation prior to impact.” See ATSB link:
https://www.atsb.gov.au/publications...aair199703221/
For some general aviation aircraft, flying on limited panel instruments such as a typical Turn Coordinator or Turn and Balance indicator with no AH or HSI/DI, requires not only considerable instrument flying skill but also to be current on limited panel flying. Few pilots have both. Today’s synthetic trainers used by most flying schools are ideal for keeping current on limited panel with minimum cost. It is good life insurance.
It is all too easy to become complacent on the aircraft you currently fly. For example:
A private pilot owner was practicing full panel instrument flying in a flying school synthetic trainer in preparation for his command instrument rating. All his practice was on full panel. His instructor suggested he needed practice at limited panel as well. The student disagreed saying his own aircraft had a glass cockpit with reversion modes and limited panel would never happen.
During the conduct of a holding pattern prior to an ILS, the instructor failed the artificial horizon without first warning the student. In less than 45 seconds after the AH failure the student went into an ever steepening spiral dive and lost control. Worse still, the student accused the instructor of deliberately setting out to cause him to crash. The student had the wrong attitude in more ways than one.
It is easy to practice things you are good at. It takes dedication and a good attitude to practice something you are not good at. Today’s synthetic trainers are excellent for instrument flying practice. You can’t cheat by looking outside. Practicing limited panel flying on the standby flight instruments is best done in a synthetic trainer until you are truly competent. Then keep in regular currency