Tuesday, August 21, 2007

Stone Age Redux

In light of the recent Garmin AHARS woes, I decided to resurrect a post I made to my now defunct freight dog blog about flying with a simulated AHARS failure.

When I do G1000 checkouts for instrument-rated pilots, I like to give a demonstration of just how primitive things could get if certain equipment failed. After they have shown proficiency with full panel and reversionary mode, I give that a scenario that involves an instrument approach with the simulated failure of the G1000's Air Data Computer/Attitude Heading And Reference System (ADC/AHARS). Without the ADC/AHARS, the Primary Flight Display's (PFD) airspeed tape, altitude tape, VSI, attitude and heading are blank and covered with red Xs. And you thought there was no challenge to flying partial panel in a G1000-equipped aircraft?

You may find yourself wondering how likely the failure of the ADC/AHARS might be. While it's probably a lot less likely than a vacuum pump failure in a "steam gauge" aircraft, recent events have shown that unexpected AHARS failures are more likely than once thought.

Before you go pulling the two ADC circuit breakers in a G1000 172, it's important to know that when you do so your transponder will no longer have any altitude input and the transponder will lose mode C altitude reporting capability. Therefore, it's very important to 1) not do this exercise in airspace where mode C reporting is required nor 2) under IFR in instrument meterological conditions. I like to find a non-towered airport in Class E airspace. If we request a practice approach or flight following, I make a point of telling ATC that we will be practicing a simulated equipment failure, that our mode C altitude reporting will be temporarily unavailable, and that we'll report any altitude changes.

Without the ADC/AHARS, the KAP-140 autopilot becomes unusable except for ROL (wings level) mode and the pilot's scan becomes pretty weird. The pilot can look at the PFD to see their current track and desired track, but they must look at the steam gauge attitude indicator, airspeed indicator, and altimeter to control the aircraft. These steam gauges are not conveniently located and there is no rate-of-turn indicator, which adds to the challenge. And surely it wouldn't have cost Cessna that much to have put a mechanical slip indicator below the vacuum-driven attitude indicator. In fact, there is a turn coordinator mounted out of sight, behind the G1000 panels, which provides roll input for the KAP140 autopilot. Unfortunately, it's out of sight and not of much use.

Pulling circuit breakers to simulate failures is, as I mentioned in a previous post, controversial in some circles. The reasoning is that a circuit breaker is not a switch and shouldn't be used as such. I personally don't subscribe to this position, believing instead that providing a complete training experience for pilots outweighs concerns about a circuit breaker's useful life. I've been pulling circuit breakers for years to simulate gear failure and engine instrument failures with no ill effects. Having said that, I don't pull circuit breakers willy-nilly on each and every flight.

The least conveniently located instrument is the magnetic compass, which in a Cessna is mounted above the windshield. The good news here is that the GPS-derived ground track is probably more useful if you remember that when you change your heading there is a slight time lag before the ground track is updated. The pilot can also look at the Multi-Function Display (MFD) to see the moving map display, which can be a great help.

An even more difficult version of this scenario is to include a simulated failure of the PFD, which means you must use the MFD and you lose the inset moving map as well as the MFD's giant moving map display. If you want even more of a challenge, fly a non-precision approach in this configuration.

Flying this scenario in VMC is a great time to ask ATC for a practice no-gyro approach. If the controller grants your request, s/he will say something like "This will be no-gyro vectors to the Stockton ILS runway 2, make standard rate turns." The controller then instructs the pilot to "turn" or "stop turn." If the controller says "turn left," the pilot initiates a half standard rate turn to the left and responds "turn left." The pilot continues turning until the controller says "stop turn," which the pilot acknowledges and returns to wings level. This continues until the controller has the aircraft within 30 degrees of being established on the intermediate approach course segment.

A no-gyro approach requires skill on the part of the controller as well as the pilot. While there is no specific requirement for instrument-rated pilots to perform a no-gyro approach to maintain their instrument currency, the pilot must be proficient at flying straight and level, level turns, climbs, and descents with limited instrumentation. Air traffic controllers must perform a minimum of three ASR (approach surveillance radar) approaches every calendar quarter and many controllers appreciate the opportunity to practice them. Of course, the best time for a pilot to request a practice no-gyro approach is when the controller isn't busy. Don't ask for a no-gyro practice approach on a beautiful weekend when everyone is out flying.

Here are examples of intercepting and tracking the Stockton ILS 29R approach with the G1000 that has a failed ADC/AHARS.

This points up something that every pilot transitioning to the G1000 has pointed out - Why is the glideslope indicator only shown next to the attitude indication and not next to the HSI representation?

I've been asked a few times by pilots about to start training for their instrument rating whether it's better to start with steam gauges (the old fashioned, round indicators) or with a glass cockpit like the G1000. Starting initially with a primitive, steam gauge aircraft seems like a good choice and I think you can see why.

So get a safety pilot or an instructor to fly with you and try going back to the stone age. Yabba dabba do!


Ron said...

It's not that flying a G1000 partial panel isn't challenging. It certainly can be. But it has one HUGE advantage over steam gauges, and that's the way in which the failure mode presents itself to the pilot.

Flying steam gauges? A bad AI will slooooowly roll over and die. A very insidious way to lose a primary flight instrument.

In a G1000, however, the failure is annunciated with the loss of the horizon and replacement with a gigantic red X. Nobody could miss that.

That's one of the reasons I feel glass is a better choice. Most people who lose control due to an instrument failure seem to do so when they have difficulty detecting the bad instrument quickly enough.

As for the lack of an analog TC, it's true. But I teach students to figure out what the bank angle should be for a standard rate turn at typical cruise and approach speeds. That way they can roll in an approximate standard rate turn using the backup AI.

No, I think the big problem with glass is the possibility of information overload. There's nothing like getting an approach clearance while the KAP140 yells "leaving altitude" and the G1000 master caution annunicator chimes away on top of it all.

So much data coming at the pilot so quickly -- he has to be like a junk mail filter, quickly eliminating non-pertinent information in order to preserve focus on the critical instrument(s) required to do the job at hand.

Paul said...

Welcome back.

I was wondering how you pull CBs that are the flush "Piper style?" I can never seem to get a grip on them.


John said...


Flush circuit breakers are specifically designed so that you cannot pop them manually. You can however reset them if they pop out. If you decide to reset a CB that has popped, it's best to wait a minute or so before resetting to let it cool off or it might just pop out again even if the overload that caused it to popped has passed.


A 182 I occasionally fly has an AHARS that can be knocked out of alignment by a firm landing: Not a hard landing, a firm landing. It's always distracting when this happens, even though it's clear that the attitude and heading indications are no longer usable. After a minute or so, it re-aligns and all is well until the next "event." Go figure.

I don't think these units have been out there long enough for me to draw any conclusions about possible, bizarre G1000 failures. The designed failure modes are much clearer than steam gauges, but as a software developer I feel there must still be a world of odd behavior for us to discover. One example is the odd stuff that happens when the PDF and MFD have mis-matched databases

Sure, you can calculate a standard rate turn - (airspeed in knots)/2 * 1.5 - but that is just the kind of extra workload you don't need. And how about an inclinometer attached to the backup AI the way Cirrus did it? How much would that have cost Cessna?

You're right on the mark about information overload, though you don't have to have a glass panel to get that. I was flying with a instrument student last night steam gauge 172 equipped with a Bendix/King package and at the end of a challenging ILS approach, right as he began his landing flare, the system very helpfully barked out "TIS not available." Really helpful.

Harry said...

I wondered about the placement of the glideslope indicator.
I receently started making a calculation of my expected vertical descent on an ILS.
Now you have 3 parameters to keep you on the glideslope, lined up in one horizontal sweep of the eye:
AI, glideslope, VSI. A change in expected VSI gives you the earliest warning of an impending glide slope deviation.


Aviatrix said...

That's cool. I've never learned a procedure for an ATC assisted no-gyro approach. I learned to do timed turns, and our commercial candidates must be able to roll out on assigned headings with both AI and HI covered. I will have to ask a Canadian controller if they do that.

John said...

My instrument instructors never had me do no-gyro vectors during my instrument training and I only learned about it afterward. I make sure my instrument candidates have plenty of experience with no-gyro vectors either from me or from ATC during a practice approach.

BTW, if you ever want to test a controller's abilities, ask for no-gyro vectors on a practice approach. Some controllers will hesitate or say in advance that they haven't done one in a while. Doing multiple practice approaches one evening, we had just completed a missed approach from no-gyro vectors to an LDA approach and we actually had a controller ask if we'd mind doing another no-gyro approach. I said yes and then we heard a different controller's voice. It seemed that another controller wanted to try his hand.

You can find a description of no-gyro procedures here, under 5-4-11(c)(3).

Walt said...

Glass to grass

There has been a fair amount written on transition to TAA/glass/automation/crew CRM, but little on transition/reversion (in an “abnormal” situation) to basic aircraft/steam gauges/hand flying/SRM. Going “back” (if you’ve ever been there in the first place) can be perturbing, to say the least.

Several years ago (at age 60) I went from left seat in a CATIII A300-600R on 7-hr international legs to VFR fire suppression in an Ag-Cat with a hand-held VHF, flying off grass strips. The #1 jug blew on my 3rd takeoff at 300’ (landed OK). It took me several weeks to find a flight manual; that was OK, because there wasn’t much in it anyway. It took me at least that long to relearn the “upside-down wedding cake” and all it’s permutations, and get recurrent on reading sectionals.

Not harder, but definately different.

Try to find an instructor/mentor perceptive and experienced enough to guide “children of the magenta line” back to basics. It ain’t easy. If you find one, treasure him/her, and let the world know.