The Real Glass Cockpit Question
By J. Mac McClellan
December 2007
 There has been a great deal of discussion about the difficulty, or lack of it, of transitioning from conventional flight instruments to flat-panel primary flight displays (PFD). Many also worry that new instrument pilots who learn on a PFD will find it very difficult to fly safely with a conventional round dial set of gyros and air data instruments.
To my knowledge there has been no conclusive study that shows competent instrument pilots have problems going between flat glass and conventional instruments. I know that I have not experienced a problem flying whatever is in front of me. In fact, my very first flight with a PFD was in the Gulfstream IV, the first civilian airplane to have such a display, and the weather was 400-feet overcast. So in a few seconds after rotation I was on solid instruments with a totally new type of display, and it felt perfectly natural.
If I had to choose between conven-tional instruments and a PFD, I would always take the glass. I think it may be a little easier to scan the information without my eyes moving across the hardware that separates conventional instruments and the data they present. And certainly the airspeed and altitude trend vectors that are a part of a PFD make precise flying easier.
But a PFD is the baby step in making IFR flying less complicated and more precise. It doesn't change the fundamental pilot task, but does make it a little easier. But the real breakthrough is having the glass display and the computer behind it do the thinking for you, not just show you the same data in a different way. And it can do that now.
If you reduce instrument flying to its basics you find that only two questions must be constantly answered—where am I now, and where am I going. When a human flies IFR using conventional instruments, or a PFD, his brain must function as a computer that takes the available information and answers those two fundamental questions.
For example, it's essential to know the attitude of the airplane because that determines where the airplane is going. But airspeed, heading and vertical speed also must be considered to know where the airplane will be within the next few seconds. Altitude and course information tell you where you are now. It is a very complex task to mentally process all of this data in real time and do it almost subconsciously so that you have working memory available for that new heading, altitude, frequency or clearance the controller just read to you.
Whether the dynamic raw information is presented to a pilot on a PFD or mechanical dials is a small difference when you consider the enormity of the workload of taking that information and constantly computing and visualizing your airplane's path through the air and over the ground. The task is so complex, in fact, that computers really couldn't do it well until a very important piece of information the human pilot doesn't have is added, and that is acceleration vectors.
Acceleration is nothing more than a change in the flight path and speed of the airplane. The human mentally computes a crude acceleration vector by noting changes in the basic instruments. But there is quite a lag in the time it takes for the human pilot to note a change in altitude, for example, comprehend that change, and then consider if the change is desired or must be corrected. Flight director and autopilot computers with their unwavering electronic concentration do a better job of computing accelerations and correcting for them than we humans can, but it is still a second order solution based on change in data such as attitude, heading, airspeed or altitude, and there are lags in measuring those data.
Precise flight guidance really comes from inertial devices that directly measure acceleration vectors and thus compute the airplane's path through the air and over the earth. Inertial sensors were once exclusive to the most expensive civilian jets or the military. The first inertial navigation systems were among the most highly classified of military equipment because only they could guide intercontinental missiles to their targets. When the inertial equipment was made available to civilians it was so big and heavy, and expensive, that it could fit only in airliners, or the largest business jets.
The ring laser gyro shrank the size and weight of the inertial sensors, but not the price, so the equipment remained unavailable to even midsize business jets. But now high-speed computer chips and miniature electromechanical sensors have made it possible for even piston singles to carry inertial sensors. That's what the nonmoving attitude heading reference systems (AHRS) in the Garmin G1000, Avidyne Entegra and other glass cockpit systems are—inertial sensors.
With an inertial sensor onboard it is possible to compute a precise flight path without the pilot having to integrate and process the raw data we have been using. The inertial sensor measures very tiny changes in the acceleration vector and projects them many seconds ahead to show where the airplane will be. With this information the instrument pilot's job becomes something like point and shoot. Just keep the flight path pointed at where you want to go, and you will get there without processing much additional information.
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