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Failure is an option

Handling curveballs on an instrument flight

As a designated pilot examiner, my job is to throw applicants the occasional curveball to ensure they can maintain aircraft control within standards during an uncomfortable or emergent situation.
Illustration by Andrew Baker.
Zoomed image
Illustration by Andrew Baker.
This system overview for a glass-panel retrofit of a Beechcraft Bonanza was adapted from Garmin's DPE and CFI Avionics Guide. Arrows indicate the direction information travels among the boxes.

One of the three approaches on a practical exam for the instrument rating, for example, must be flown without reference to primary flight instruments; according to FAA Order 8900.2C, that means losing, among other information, the primary attitude indicator. So, examiners will affix a sticky note over the attitude indicator or dim the entire primary flight display (PFD), the giant attitude indicator in a glass-panel. Of course, instrument candidates are privy to the airman certification standards, so they know that failure is coming.

Still, I issued two notices of disapproval last month alone because the candidates ended the instrument approach with a full-scale deflection of the course deviation indicator (CDI) at the missed approach point (MAP). Each chose to focus on minimizing the difference between Track and Desired Track on the GPS to fly to the MAP despite the backup CDI showing on the same navigator.

Once they had deviated from the final approach course, TRK and DTK gave similar values despite the CDI—the best reference for lateral guidance—remaining pegged to one side. With any system failure, pilots should know what information remains and how to appropriately adapt their scan for its optimal use. If such lack of facility can occur when it’s expected, I shudder to think how the flight would end with an unanticipated failure.

These experiences serve as a continual reminder of my own need to stay knowledgeable and proficient should a curveball come my way. With older panels, it was easier to anticipate the way failures happen—gyroscopic instruments driven by vacuum/pressure systems or electronics going dark after an alternator failure were, if not relatively common, at least not unheard of. I’ve experienced each of these events more than once in my own airplanes.

The good news for those of us who have ditched vacuum systems and sprung for full glass panels is that such failures happen less frequently. But they do still happen on occasion, so pilots need to anticipate and train for such events. (See “Instrument Tip: Partial-Panel in the PFD Era,” page 84.) Let’s see how one might anticipate problems in my Bonanza Niky’s panel.

As a first step in the process, I consulted the retrofit version of Garmin’s “DPE and CFI Avionics Guide” for a general overview of the way the boxes (officially, line replaceable units) behind my panel communicate, what failures might present themselves and best practices for simulating those failures during flight training. After creating a system overview for Niky’s panel (see next page), I consulted the manufacturer’s guides and the approved airplane flight manual supplement for each unit to understand the specifics of the installation. The following notes on the boxes explain how their failures may affect other components.

PFD. Niky’s new panel features a large G3X display that assembles position, attitude, heading, engine information, navigation/communication settings, and autopilot selections in one place. Errors can happen in myriad ways, but it might be an entire display failure, or one created by one of the systems that provides information to the PFD.

Pitot-static system. The air data computer merely computes values provided by the pitot-static system so a blocked pitot tube and/or static port will result in the same erroneous information it would on a non-glass panel.

ADAHARS. The air data (computer) and heading and reference system combines pitot-static information along with information generated by solid-state units to provide data such as airspeed, altitude, vertical speed, heading, pitch angle and bank angle to the primary flight display. Failures in this unit will result in red Xs over the airplane symbol or associated data fields on the PFD.

Engine monitor. In its panel makeover, Niky gained a brand-new system with which to monitor the fuel supply and the health of the engine, and those gauges sit front and center along a vertical strip on the right side of the PFD. The downside is that, if the PFD screen were to fail, it’s not possible to know power or propeller settings, or even that the oil pressure is within acceptable limits. With Garmin’s FlightStream 510 and access to Garmin Pilot on the iPad, it is possible to present those data on the iPad and that may be a future upgrade I make.

Standby flight instruments. Niky’s original work proposal involved the G5 as a backup flight display but as she was disassembled at the avionics shop, Garmin announced the new GI 275 as a replacement. I chose this as it featured a brighter screen with better resolution. The unit has its own air data computer so a failure of the main ADAHARs unit should mean that the backup unit offers reliable information. The GI 275 also has a VFR GPS that can be used in a pinch or as a complement to the navigators.

Autopilot. Unlike the G5, early versions of the GI 275 do not connect with the autopilot upon PFD failure. So, were such a failure to happen in Niky, I’ll expect the autopilot to shut off and plan to console myself with the beauty and striking resolution on the GI 275 as I hand fly to a nearby airport.

Transponder. I chose a remote transponder so all the units would fit neatly in the panel. With a PFD failure, I won’t have a way to make changes to the code such as indicating an emergency by squawking 7700.

Audio panel. Upon failure of the audio panel, the unit reverts to “fail safe” mode that allows a single frequency for transmission and reception. It defaults to the frequency loaded in Com 1 and can be controlled there.

Nav/Com 1 and Nav/Com 2. I chose the GTN 650Xi (Nav/Com 1), which offers both VOR and WAAS GPS capability, along with the GNC 355 (Nav/Com 2) as an additional WAAS GPS unit. Were Nav/Com 1 to fail, I would need to switch the nav source on the PFD to Nav/Com 2. From there, navigation would be restricted to GPS but virtually all airports with approaches these days feature RNAV/GPS approaches with minimums that often rival the low minimums of an ILS. In the event of a PFD failure, the Nav/Com CDI indicator is the go-to source for maintaining lateral guidance.

Standby alternator. Overhauling Niky’s alternator and installing a standby alternator in the space created when the pressure system was removed were no brainers. That provides extra assurance of a safe end to a flight, but the system has limitations. The standby alternator requires that the total sustained load be kept below 20 amps so I should be prepared to shed any items that aren’t essential to flight.

iPad. Because I don’t like flying with items attached to my yoke and looking down toward my lap is too disorienting, the shop mounted an iPad in the center of my panel. It’s a perfect place in the Bonanza since the space above where the yoke meets the panel cannot accept instruments as deep as some of the navigators. The mount has a dedicated avionics fan and keeps the iPad on ship’s power. If I lost everything else in my panel, at least I could commence the remainder of the adventure with a fully charged and cool EFB. I recently practiced an approach using only the EFB and backup flight display (“Flying Smart: New Year’s Resolutions,” AOPA Pilot January 2024).

These notes for Niky’s avionics package represent just the first step of anticipating the failures that can occur and what information remains, and the process can be adapted to your own aircraft panel. The important second step is to then simulate system failures, in a safe and controlled training environment, to effectively use what’s left when those curve balls head my way.

Catherine Cavagnaro teaches aerobatics at UOS and is the Gaston Swindell Bruton Professor of Mathematics at Sewanee: The University of the South.

Catherine Cavagnaro
Catherine Cavagnaro is an aerobatics instructor ( and professor of mathematics at Sewanee: The University of the South.

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