When most of us learned to fly instruments the term partial panel had a very specific meaning. Flying partial panel meant the attitude gyro, and usually the directional gyro, were not working and you were left with only a turn coordinator or turn & bank gyro to know up from down if you were flying in the clouds.
Partial panel flying worked. Sort of. We had to demonstrate our partial panel skills to earn the IFR rating, and again on instrument competency checks, or when we advanced to higher ratings. But I always thought partial panel was really an emergency with no margin for error and high risk for loss of control in turbulent conditions. More than a few pilots dove out of the clouds after a vacuum pump failure left them with nothing but the partial panel.
But the new flat glass cockpits with the electronic gyros have redefined partial panel. Now, if that glass goes blank, or the non-moving electronic gyros shoot craps, we’re left with an attitude gyro plus altimeter and airspeed. Those are the minimum instruments that must be either mechanical—usually the case for airspeed and altimeter—or be powered by an independent source. So, the failure analysis shows you are very unlikely to be left in the clouds without an attitude indicator to show up and down, and wings level.
An independent backup attitude gyro has been required in jets for decades, and in many other larger airplanes. In most of those airplanes the backup was a two-inch gyro powered by its own dedicated continuously charged battery that would last at least 30 minutes if all other aircraft power was lost. We all called it the “peanut gyro” because of its size. And J.E.T.—now a part of L-3—made virtually all of the peanut gyros.
In jet simulator training during at least one session a cascade of very unlikely, but remotely possible failures, would leave you flying on only the peanut for attitude. But in jets we knew this was an emergency and it was treated as such. None of that bravado that partial panel flying is no big deal if you’re a really good IFR pilot.
Flying the approach—almost always an ILS—with the peanut for guidance was hard. The peanut was usually mounted high in the center of the instrument panel where both pilots could see it. But the altimeter and airspeed indicator were in their normal positions on either side of the panel. And usually there was an RMI or some other heading indicator that probably worked but was in the far corner of the instrument panel. The instrument scan from the peanut to the other gauges was very weird.
In most piston airplanes with flat glass the backup attitude gyro is a full size three-inch instrument. It can be powered by a battery, or vacuum, or in some other way be independent of the main glass display. It should be easy to fly with attitude information compared to the old by guess and by golly flying with a turn coordinator. And it is. Sort of.
The issue is that the instruments backing up flat glass can be located just about anywhere, in any order, just as long as they are pretty much in front of the pilot in the left seat. In my airplane the attitude indicator is up in the outboard corner where the ADF indicator once was, and the altimeter and airspeed indicator are along the bottom edge of the panel where the number two VOR/ILS indicator and a Stormscope used to be. An odd scan to say the least.
In my airplane if the flat glass PFD fails in some way I should still have the GPS navigators functioning over in the radio stack because they are independent of the PFD. So the drill is to scan the attitude gyro, keep track of altitude and airspeed, and look over at the GPS to maintain the desired track. It’s about two feet from the attitude gyro to the GPS. More than you can scan without moving your head at least a little instead of just moving your eyes.
Because I had flown so many emergency scenarios in the simulator using only a peanut gyro I was confident I could do ok with my backup instruments if the PFD quit. But until a few months ago when I was flying the American Bonanza Society pilot proficiency course I had not tried it under the hood. It was odd, at first, but my scan did adapt and I did okay.
The only issue that caused me to stop and think for a moment was when I made a course change on the approach. It was a very windy day and I just reflexively turned the nose into the wind on the new course thinking I would need more correction for drift. After a few seconds I realized that I was flying ground track on the GPS and that box had no idea where the nose was pointed, just how I was tracking over the ground. No need for that drift correction that I would have used when flying heading as normal.
Overall, the new partial panel is about 1,000 percent better than the needle ball and airspeed most of us grew up on. When you have a good attitude indication, the other stuff is easy to manage. Score one more for general aviation progress.