Broken Glass

The modern PFDs and MFDs being installed in personal airplanes provide awesome functionality. But what will you do when they break?


By Thomas P. Turner

There will probably never be another new store-bought airplane certified for IFR flight without some version of a glass cockpit. Computer-driven Primary Flight Displays (PFDs) provide an instantaneous level of situational awareness superior to the mental picture even a seasoned Instrument-rated pilot can develop without them. The multi-instrument and navigation scan is replaced with focus on a single, all-inclusive display that literally draws a picture for the pilot. The magnitude of the change in instrument flight is similar to the move from open cockpits to enclosed cabins, or even to the change from piston engines to jets in the commercial world.

The revolution has already succeeded. Already there are pilots who have earned their Instrument rating solely in glass-cockpit airplanes, never having flown steam gauges. But, like any other technology, glass cockpits will eventually fail. Will you be ready to respond with pre-revolutionary skills?

Modern PFDs dont use traditional gyroscopes. Instead, they employ a series of rate sensors and magnetometers that together are called the Attitude/Heading Reference System, or AHRS. The AHRS sensors drive the attitude, rate of turn and slip/skid presentations, and a magnetometer (similar to a slaved HSI) provides heading information.

Some manufactures add Air Data to the acronym, calling it ADAHRS to acknowledge critical speed and altitude input to the primary display. Air Data Computers, common to all glass cockpit designs, turn conventional pitot and static-port inputs into electronic airspeed, altitude and vertical speed depictions, and also provide digital outside air temperature readings.

ADHARS data displays on the PFD with a fairly traditional (although much wider and VFR-looking) attitude indicator. The HSI display, in its usual place below the attitude, can be enhanced with all sorts of navigation, weather and traffic information overlays. Harder to get used to, vertical tapes left and right of the horizon/HSI show airspeed, altitude and vertical speed, often with trend arrows and other helpful information. Each manufacturer will display other flight data on the PFD.

Big Red X
Glass cockpit layouts contain not only the PFD immediately in front of the pilot, but also a Multi-Function Display, or MFD, usually near the center of the panel. The MFD is used primarily for navigation displays-huge moving maps with chart overlays-and/or depiction of engine management and other aircraft systems information. The PFD and MFD talk to each other and, usually, a total failure of the PFD screen will trigger a reversion mode that takes over the MFD, displaying PFD instrumentation as a backup to the failed display. All the information is still there in one grouping, and the autopilot is still able to use it.

Instead of a massive failure of a PFD or MFD, its far more likely that a single ADAHRS sensor will fail with time. If the system detects a failed sensor, it will display a very obvious big red X (or other obscuration) over that part of the PFD display. The autopilot may or may not continue to work depending on the make, failure mode and installation. The pilot then must use the required back-up instrumentation-a traditional airspeed indicator, battery-powered attitude indicator and altimeter located somewhere on the panel-to fill in the blanks.

The big red X is very helpful because we pilots tend to do a fairly good job of partial panel flying once we have the airplane stabilized. Our biggest problem comes from identifying the failed instrument in the first place-a task done immediately, and very conspicuously, by the ADAHRS.

Where You Gonna Look?
Flying a steam-gauge Beech Bonanza inbound outside the marker on a practice ILS approach, I inform my student that hes identified an attitude indicator failure and disengage his autopilot/flight director for him. He dutifully covers the dead instrument with a Post-It note to avoid being distracted by its errant display and presses on with the approach.

Like most high-performance airplanes, this owner has equipped his Bonanza with an electric-powered backup attitude indicator (AI) mounted where room permits-on the far right side of the panel. As an instructor, Ive seen literally dozens of times what happens next as the pilot passes the Final Approach Fix and continues down the glideslope.

One can usually predict the scans he makes between the remaining flight instruments and the backup AI. One turn of the head, then, two, then three … and then he decides its too much trouble to incorporate one, distant instrument in his scan while trying to maintain a precise approach.Maybe hes even a little dizzy from the rapid head movements; virtually all pilots Ive put through this exercise revert to partial-panel-only flight after about three scans to the far side of the panel, ignoring the backup instrument for the remainder of the approach. The conclusion? The backup AI, so placed, is worse than useless-it creates disorientation and distraction.To learn if placement of the backup AI so far across the panel can be fatal, one only needs to look as far as the October 16, 2000, crash of a Cessna 335 that killed Missouri Governor Mel Carnahan, his son and an aide.

My subjective feeling, also, is that pilots trained in steam gauge airplanes can more quickly interpret airspeed, attitude and altimeter gauges aligned horizontally and in that left-to-right order…the way were used to seeing the top row of sacred six instruments.

With all that in mind, the sidebar on the following page details where manufacturers of glass cockpit airplanes are placing their mechanical back-up instruments.

From my instructional experience it seems like Cirrus has the most pilot-friendly arrangement, although bobbing the head up and down to integrate the bolster-mounted back-ups with remaining PFD data might be disorienting.

Adam Aircraft, Cessna and Diamond are close to optimal. Piper and Lancairhave a good layout also, albeit vertical instead of the usual horizontal.Eclipse is in a class of its own, placing the backups embedded in the exclusive Avio MFD in the center of the panel.

Mooney and Beech get the better than nothing award for vertically stacking the backup instruments as far from the pilots primary scan as the instrument panel allows, seemingly placed more for styling or fit considerations than for emergency functionality.

Are The Backups Enough?
Are a back-up altimeter, attitude indicator and airspeed indicator enough to safely complete an IMC trip following PFD failure? Only the Chelton incorporates a traditional slip/skid indicator (as opposed to an electronic, on-screen representation) to help out in the case of an AHRS malfunction or a total electrical failure. None of the glass cockpit designs have a turn and bank or turn coordinator for recovery from a partial-panel unusual attitude that tumbles the attitude gyro.

All this capability, new out of the box, might create complacency with flight bag backups, but for true safety and redundancy youll still want to carry along a handheld radio (preferably the airplane will have an external antenna connection) and a handheld GPS to get you down in an emergency. Dont forget enough spare batteries-either AA-size alkalines or full recharged battery packs, for example-to power the portables.

Owners might also consider a turn-and-bank gyro with a slip-skid indicator as the first addition to their powerful instrument panel-this might be critical for maximizing single-engine performance in Diamonds DA42 twin, as another example.

To be ready to deal with a glass cockpit failure, those of us lucky enough to be flying behind PFDs will need to spend even more time than usual training and practicing partial panel flight as that failure mode is now even further removed from normal operation. Instructors providing instrument training and proficiency checks are well-advised to spend a significant amount of time drilling students on partial and total PFD failure modes, flight in reversion modes and by reference to the backup gauges, and procedures for flying emergency approaches using only the steam-gauge instruments, a portable GPS and a handheld communications radio. And that would be a good idea for pilots with moving maps, too.

Training For Failure
As we become more and more comfortable with computerized flight displays, using separate, old-style steam gauges will become a less-and-less-natural task. Its not like transitioning to partial panel flight after identifying a failed attitude indicator. Its more like moving from use of a slaved HSI to a traditional magnetic compass-its possible to fly with precision but its critical that you have a solid understanding of backup system operation, and recent practice in its use.

Put another way, moving from full to partial panel in a steam-gauge airplane is like a right-handed person trying to write with his/her left hand. Going from a fully functioning glass cockpit to using only steam-gauge backups is more like trying to write holding a pencil with your toes. Success can come only with training that places a strong emphasis on the procedure, and with recent practice.

Partial data failures on glass cockpit displays may be even harder to deal with as a very unnatural (for the glass-savvy pilot) scan would be needed to incorporate out-of-scan steam gauge information into an impaired PFD display.

I embrace the tremendous advantages in cockpit management and situational awareness promised by introduction of glass cockpits in personal aviation. We must all be ready at a moments notice, however, to move from this all-inclusive information resource to basic attitude flight using a trio of ever-less-familiar steam gauges.

Also With This Article
“Common PFD Failure Modes”
“Backup Instrument Placement In Glass Cockpits”
“The Gimli Glider”
“SR22: PFD Failure?”

-Tom Turner is a CFII-MEI who frequently writes and lectures on aviation safety.


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