EFIS failures: a working screen isn’t always a working backup

You’re looking at a glass cockpit. Several screens, plenty of information, and a reassuring sense that there’s a backup for everything. Then the attitude display is flagged as invalid.

Your next move depends on what has failed. A display problem and a data-source problem can look quite different, and changing screens won’t necessarily give you fresh information.

That’s the point behind CPL Aircraft Technical Knowledge code 26.64.8: “Describe the management of EFIS system failures.”

It has appeared 33 times in reports uploaded to our KDR Refresher, tied for third among CPL Tech codes in the summary updated on 11 October 2026. Those are appearances in the cumulative uploads represented in that summary, rather than an exam-date window. They aren’t a national exam failure rate, and they don’t tell us how many questions on this topic a candidate answered incorrectly.

Start with the information you need to fly

EFIS means Electronic Flight Instrument System. The screen presents information; other parts of the system produce it. An attitude and heading reference system, or AHRS, supplies attitude and heading information. An air data computer supplies information such as airspeed and altitude.

So a live screen can still be displaying information from a failed source. Two displays can also share a source. Count the independent sources, not just the screens.

Don’t wait for a red X before questioning an indication. Garmin’s Cessna Nav III guide warns that incorrect operation can occur without a fault indication. Cross-check against independent standby instruments and other available information; two agreeing screens fed by one source aren’t an independent check. Use the applicable AFM/POH procedure to deal with any disagreement.

In flight, keep control of the aeroplane using information you have established is reliable. Move to the appropriate standby instruments or valid alternative source as trained, then work through the aircraft’s approved failure procedure. Don’t let fault-finding take over your flying.

A failed display and failed data are different problems

If a display fails, another display may be able to show the flight information you need. That’s often called reversionary or backup display mode. Whether it happens automatically, needs a button press, or is available at all depends on the installation.

If the attitude source fails, putting the same attitude information onto another screen doesn’t fix the source. You need the valid alternative provided by that aircraft: perhaps another selectable source, perhaps a standby attitude instrument. A magnetic compass can help with heading; it can’t tell you pitch or bank.

If air data is lost or unreliable, think separately about airspeed, altitude and vertical speed. A working attitude display doesn’t prove those other indications are sound. Equally, a failure affecting attitude doesn’t automatically mean every air-data indication has gone.

One example, with its limits

Garmin’s G1000 guide for the Cessna Nav III describes reversionary mode after a display failure and a manual DISPLAY BACKUP selection if the system hasn’t detected the display problem. It also describes red X flags over affected indications when data is invalid.

That’s an example of one installation. It isn’t a procedure for every glass cockpit, and it isn’t a description of the cockpit in our photograph. Use the POH or AFM, applicable supplements and checklist for the aircraft you’re flying.

The traps to avoid in revision

“The other screen is working, so I have an independent backup.” Check where its data comes from. A second presentation of the same source isn’t a second source.

“An AHRS failure always causes the same autopilot response.” Autopilot behaviour depends on the installation and the failure. Know the system you’re training on and follow its approved procedure.

“I’ll reset it and see what happens.” A reset is an aircraft-specific action, not a universal first response. Don’t substitute an improvised restart for the checklist.

“The standby instruments will cover everything.” Know what your standby instruments actually show, how they’re powered and which inputs they use. Standby equipment can be electronic or mechanical. Its independence needs to be understood rather than assumed.

A useful five-minute revision exercise

Draw three boxes: source → display → backup. Pick attitude first, then repeat for airspeed and altitude.

For each one, explain what supplies the information, how you would recognise missing or unreliable information, and what valid alternative remains. Then describe how you’d maintain control while following the approved procedure. Practise the explanation with your instructor on the ground or in an appropriate training device.

For the exam, the takeaway is simple: identify what has failed, establish what information remains reliable, and use the aircraft’s approved backup procedure. Learning the name of a backup button is only part of the answer.

Passed or failed, use the KDR to guide your next revision

A pass can still leave knowledge gaps worth closing. If you didn’t pass, the codes give you a place to start without rereading the whole subject at random.

Upload your result report to the KDR Refresher through Stoneybrook Training and use the flagged codes to focus your revision. Bring any system-specific questions back to your instructor and the aircraft manuals.