Left Seat: The New Partial Panel

What to do when primary instruments fail.

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Garmin

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.

The engine driven so-called "dry" vacuum pump is a relatively low-cost and lightweight source of power for gyro instruments. The carbon gradually wears away, thus lubricating itself without need for the constant oil bath of the earlier "wet" vacuum pumps that used hard metal for vanes. But the carbon vanes are brittle and can break without warning, and the pump fails instantly.

It seems crazy now, but thousands of pilots were willing to fly in solid IMC with only a single dry vacuum pump to provide the air pressure to spin the two primary gyros — attitude and heading — in the instrument panel. If that pump failed while flying in the clouds, life itself was at great risk. The only escape was a pilot's ability to fly with only a partial panel to get the airplane out of the clouds and back to a runway.

Because the most important instrument — the attitude gyro — ceased to function when the vacuum pump failed, the turn coordinator or turn-and-bank indicator became the only available lifesaver. Either instrument contains a spinning gyro but can't determine airplane attitude — only that the airplane is turning. If a pilot can keep the airplane from turning, or turn it only on command, he is still in control.

The turn coordinator or turn-and-bank gyros are powered by an electric motor, so they have independence from the vacuum pump. Of course, the electric motor, or electrical system itself, can fail, but it is unlikely that both vacuum pump and electrical system would fail on the same flight. And the electric motors and electrical system have demonstrated much higher reliability than dry vacuum pumps have.

For these reasons the turn-indicating gyro has occupied the exalted status of lifesaver for light-airplane instrument pilots. In fact, the rules require that a turn-indicating gyro be installed and functioning in any airplane approved for IFR flight unless there are three independent attitude gyros. Jets have had three attitude gyros for many years, and a few turboprops have too, but every other airplane has to have a turn coordinator or turn-and-bank instruments.

With the advent of flat-glass primary flight displays (PFD), the emphasis on the turn-indicating gyro is now gone, and the attitude gyro has replaced it as the lifeboat for instrument pilots. The required instruments for backing up the PFD are airspeed, altimeter and an attitude gyro. So long to the turn and bank and decades of partial-panel instrument flying practice.

It makes sense to have an independent attitude gyro to back up a glass PFD because attitude has always been the primary information needed to maintain control of the airplane. In old-style partial-panel flying, the pilot had to cross-check airspeed, altitude, vertical speed and rate of turn to mentally calculate the attitude of the airplane. It is a big challenge, particularly if you throw in some turbulence. And no matter how good a pilot is at partial-panel flying, precision is lacking and the threat of loss of control hangs over the entire process of flying out of IMC.

Even a pilot as experienced and proficient in light-airplane flying as Flying editor emeritus Richard Collins gave up on partial panel as a reasonable backup to the vacuum pump in his Cessna P210 many years ago. After several vacuum pump failures while flying in the clouds, Richard decided it was just unacceptable to have nothing more than a turn coordinator to keep the airplane upright, and he installed an electrically powered attitude gyro to back up the primary vacuum-powered unit.

I have been lucky enough never to have lost both the attitude and heading gyros while flying in the clouds, but about 30 years ago I decided the traditional partial panel was not an acceptable backup. In the last three airplanes that I have owned, I put a second attitude gyro in the lower left spot traditionally occupied by the turn coordinator. To keep the airplane IFR legal, I installed a two-inch turn-and-bank instrument in the corner of the panel. The second attitude gyro had independent power, so I was unlikely to lose both it and the primary gyro on the same flight. Truth be told, I didn't really care if the turn and bank actually worked because I looked at its slip-skid ball only to trim the rudder. When you have attitude, rate of turn just doesn't matter.

So, with a flat-glass PFD the FAA is now requiring an independent attitude gyro, along with mechanical airspeed indicator and altimeter as backup. Rate-of-turn indication is displayed on the PFD, so the requirement for it to be there is met, but if the PFD or some of its sensors fail, rate of turn is gone. However, attitude indication remains. Light airplanes are now on the way to matching jets for instrument redundancy.

The new partial-panel challenge for pilots with glass PFDs is to understand the possible failures of their particular system and how to deal with them. The obvious failure is the display itself, which, like your flat-screen TV or computer display, can simply go blank. That's a problem impossible to miss, and some systems allow you to transfer primary instrument information to the MFD in a composite mode. In that case you have lost some functions on the MFD, and the PFD data is shown smaller than normal, but everything you need to fly the airplane normally is available.

Another failure mode is loss of the AHRS or air data computer. The AHRS is an electronic nonmoving gyro that uses accelerometers to calculate the attitude and heading of the airplane. The AHRS function is independent, and it can fail even though the glass display is working perfectly. In that case you will see red X's in place of the normal attitude and heading displays.

The air data computer also uses nonmoving electronic sensors to measure pitot and static pressure, and air temperature to calculate altitude, airspeed and vertical speed. The air data computer could conk out and its data would be lost, even though the AHRS continued to function.

As you can see, it is possible for a glass PFD to present partial-panel information even though the display itself is functioning normally. Since most piston airplanes with glass cockpits have only a single AHRS and air data computer, the reversionary mode that transfers data from the PFD to the MFD will not help after one of those sensors fails, and you will be forced to fly the new style of partial panel.

I have not had the opportunity to practice much partial-panel flying in a glass-cockpit piston airplane, but I have been doing it in jet simulators every year for a long time. I think the best techniques will be the same in either type.

It takes multiple failures and a very unlikely sequence of failures to get to partial panel in a jet, but the normal way of leaving you in that situation in the simulator is to somehow fail both generators. In that case you switch to the emergency bus, which powers only the minimum equipment for IFR flight, and only for 30 minutes because the battery is the only source of power.

Recently manufactured light jets usually have a miniature PFD that is about three inches square that is powered by the emergency bus. These instruments have their own AHRS and air data computers, so you see all necessary information on the display. But in some jets only a few years old, the standby attitude is still the classic two-inch "peanut" gyro that was the standard for decades. The peanut, mounted in the center of the panel where both pilots can see it, is supported by heading information on one of the mechanical displays, and by airspeed and altitude from basic instruments.

In these airplanes the instrument scan can be really weird, with attitude in the center of the panel, heading on the RMI on the lower left, airspeed and altitude in their normal places, and course guidance and glideslope somewhere else. Every year in recurrent training at FlightSafety there would be the "black tube" day, when we had to intercept and fly an ILS using only this scattered array of instruments, and the weather was usually set at one-mile visibility with a ceiling at 700 feet, and it was usually in darkness.

What this practice taught me is that attitude control is king. I would look at the peanut attitude gyro nearly all of the time with only the briefest of glances at the other instruments. What I found is that, if you were off course on the localizer, for example, it was best to bank back toward centerline for a few seconds and then level the wings on the tiny attitude indicator before glancing back at the course guidance. If I tried to look at progress back toward centerline while in a bank, I usually screwed up the bank angle. It is better to turn, level, then look, and then to initiate a bank and then divert your attention from the attitude gyro.

When in the right seat, I found it useful to give the pilot flying radar approach-style information. I looked at the course guidance, said "left bank," noted the progress and then said "level the wings," and he didn't need to scan.

In the glass-cockpit piston airplanes, I know we will find the same thing — that attitude is all-important. The scan will be unusual, to say the least, because GPS track display will replace heading in many airplanes, and course guidance may be on the other side of the panel. In my Baron, with its new Garmin G600 glass PFD (more on that to come), I would need to look at the radio stack to see track and course on the GNS 530, while attitude, airspeed and altitude would be grouped in front of me.

If you work to keep the wings level on the backup attitude indicator, make only gradual bank and pitch changes, and scan only briefly when the wings are level, I believe you will find the new partial panel one heck of a lot easier, and safer, than the bad old days when rate of turn was the only information that could allow you to stay right side up while flying in the clouds.