To Pull or Not to Pull

Posted by in IFR, Instructing

It’s hard to believe a full decade has elapsed since the launch of the GA glass panel revolution. But as I recall, the first relatively high-volume GA aircraft with a fully integrated glass cockpit was the 2003 edition of the Cirrus SR22. That was the same year that Diamond brought the Garmin G1000 suite to their DA-40. The race was on, and we haven’t looked back since.

While this technology is a blessing, it’s also more complex than traditional analog gauges. Each product line has it’s own failure modes and redundancies, it’s pluses and minuses. Those are the things which dictate how partial panel scenarios should be simulated. It ought to be based on the way failures are expected to occur in real life, right?

With the Entegra suite, for example, that means dimming the Primary Flight Display. It has no reversionary capabilities, so whether the offending component is the display itself or the AHRS, dimming the PFD forces the pilot to fly with (and without) the appropriate equipment, as I discovered during my own partial panel situation in 2011.

As the years have gone by, Garmin’s G1000 suite has claimed a larger and larger share of the market. You’ll now find the G1000 and it’s variants in everything from diminutive LSAs to Citations and Learjets. Because the Garmin suite has reversionary capability, dimming the PFD isn’t good enough. During a real-world failure of that screen, the MFD would automatically display the critical flight instruments. In other words, Garmin has turned display failure into a non-event. Therefore, the critical scenario in the G1000-equipped airplanes has been the loss of the AHRS sensors.

Since no aircraft I’m aware of contains a switch for shutting off the AHRS, instructors learned to disable the sensors by pulling the appropriate circuit breaker. This forces the trainee to fly off the standby instruments, just as they’d have to do during a “real-world” loss of the primary flight instrumentation. The system seemed to be working just fine.

But a few years ago, the FAA (or perhaps it was the FSDO in my area) decided that circuit breakers should never be pulled to simulate partial panel scenarios and passed that guidance along to Designated Pilot Examiners. The new recommendation is for instructors and examiners to dim the Primary Flight Display, even though that does not accurately model an AHRS failure in the Garmin installation.

In fact, Garmin has published similar guidance in their instructor/examiner guide. While they don’t expressly prohibit pulling a circuit breaker, it isn’t exactly encouraged.

Pulling circuit breakers—or using them as switches—has the potential to weaken the circuit breaker to a point at which it may not perform its intended function. Using circuit breakers as switches is also discouraged in Advisory Circulars 120-80, 23-17B, and 43.13-1B. Additionally, a circuit breaker may be powering other equipment (such as avionics cooling fans) that could affect the safe operation of other equipment.

The guidance from those Advisory Circulars is not new. But since the FAA changed the way examiners administer flight tests, it has forced instructors to alter the way they simulate partial panel scenarios with students. And my concern is that they are no longer being properly prepared to use the information in front of them during an actual AHRS failure. That’s a huge problem, because a partial panel situation in actual instrument conditions can be a serious emergency if not handled correctly.

I’m not the only one who feels this way. AVweb recently published an article positing that “realistic scenarios make more sense than challenging the pilot with failure modes that are unlikely to happen in the real world.” I couldn’t agree more.

This problem is specific to the Garmin suite and others which operate the same way. Avidyne’s Entegra utilizes separate GPS/NAV/COM radios, and those displays will remain functional during a partial panel emergency. That provides an additional place to display vital navigation data, as on the default nav page of the Garmin GNS4/500 series. But on the G1000 suite, everything is integrated into two main displays. There is no CDI available anywhere else, so if the AHRS fails, the Primary Flight Display remains critical because that’s where course guidance information will come from.

Also, consider the fact that the G1000′s HSI becomes a fixed-needle CDI when heading data is lost. Should we be training pilots to fly with that presentation? How are we to do that if the entire display is dimmed?

Dimming the PFD is also unrealistic because while the reversionary mode would bring up the HSI on the MFD, it would also display all the primary flight data that would be unavailable in a partial-panel situation. Dimming the PFD means giving up your only source of course guidance sufficient to be used for an instrument approach. Without the HSI, how is a partial panel aircraft supposed to navigate? I’ve queried Garmin about this and never received a satisfactory answer.

Some instructors have settled on using stickers, hanging cardboard cutouts over the display, or other such methods. I’ve tried those and found them to be problematic. Stickers leave residue on those $10,000 displays. They fall off. They interfere with the pilot while they’re being placed on the display — it’s not anywhere near as sudden or simple as the old 3″ instrument covers. Cardboard or plastic overlays are cumbersome and tend to shift or drop off in turbulence. And they’re a real pain to store when not in use. I bought one overlay, designed another, searched the Web for a better mousetrap, and still came away without a good solution.

As much as I hate to say it — because Garmin and the FAA do have a point — pulling that darn circuit breaker still seems like the best option. It’s often within easy reach of the right seater, and provides a higher fidelity simulation to the trainee.

While I grant that circuit breakers may not be designed for use as switches, how often are we really simulating this scenario? If the condition of the CBs is a concern, I can’t imagine a dedicated CB/switch combination with a heavy-duty guard to protect it from accidental deactivation couldn’t be installed or designed to solve this problem.

The difference in instrument scans between functional glass and partial panel can be significant. Whether you favor pulling circuit breakers or not, one thing we should all be able to agree on is that pilots should not be left to discover the intricacies of instrument failure until they experience it on a dark and stormy night.