Dealing with AHRS/ADC Failures

by HPA · August 26, 2011

What To Do When Murphy Shows Up, Part I

By Hank Gibson, CFI, CFII, MEI

Dealing With AHRS/ADC Failures

I was in hard IFR, 400 foot ceilings and a mile visibility. I had just passed the initial approach fix on the ILS. On this particular ILS approach, I had to follow an arc that led me to the final approach course from the initial approach fix. The approach was briefed and loaded into the G1000 and I thought all I had to do was just keep the needles centered. Then, BAM! Red X’s started showing up everywhere on my PFD. Altitude, airspeed, vertical speed, all gone. The attitude indicator and my heading information were both inoperative as well. Panic began to set in as the green ILS needle began to center, but pointing straight up. Where was I supposed to look? How did I know what my altitude was? I thought I was a good instrument pilot, but now it felt like I was tumbling into an unusual attitude! What was I supposed to do? With a smile, my instructor pushed the pause button on the Frasca Mentor and asked, “So what’d you think?”

The AHRS/ADC Failure

Up to this point in my G1000 instrument training, everything had been full panel. We were now up to that dreaded time in my training known as “partial panel” instrument flying. Those words strike fear into the most proficient instrument students, causing the weaker ones to tuck tail and run.

So, really, what is the big deal? Having done countless partial panel approaches since then, mostly as a flight instructor, they don’t seem that scary anymore. The standby airspeed and attitude indicators, along with the standby altimeter provide sufficient information to the pilot, while the compass and moving map on the MFD, together, give ample heading information. The navigation needle still works on the PFD, so that capability isn’t lost. Most pilots should see this as a non-event, not an emergency, life threatening situation.

Immediately following G1000 instrument training, most pilots are very proficient at flying partial panel. As time goes on, though, proficiency is lost so that when an actual failure occurs in “real life,” things don’t go quite as smooth as they did back in training.

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Some Helpful Tips in Handling Murphy

The best way to get that proficiency back is to look up an instructor and go fly several approaches with AHRS/ADC failures. Don’t just fly all ILS approaches, either. Those are the easiest with this particular failure, but easy isn’t always the best. Non-precision approaches cause pilots to think more and use more available resources in the airplane.

A proper scan in an AHRS/ADC failure should always start and end with the standby attitude indicator. A good practice is to stay on the attitude indicator, count to three, scan another instrument, then go straight back to the attitude indicator. Once proficiency builds, scanning two other instruments quickly before returning to the attitude indicator would be acceptable, but start simple. The attitude indicator is a pilot’s lifeline, so focusing on other instruments is asking for trouble.

In the Cessna Skyhawk I instruct in, the standby instruments are below the two GDUs, while the Diamond Twin Star has the standby indicators above the displays. Different manufacturers put the standby instruments in different places, so developing a proper scan is largely airplane dependent. With the standby instruments so far from the compass in the Skyhawk, the moving map on the MFD provides an excellent, quick reference for heading information.

One must remember the heading information on the MFD has a delay to it. This leaves the pilot with two options: timed turns or compass turns. Using standard rate, three degree per second turns will lead to rolling out on headings properly. Again, the compass should not be ignored, but the airplane’s resources should be utilized.

Unusual Attitudes

One last thing that must be addressed when discussing partial panel instrument flight is a brief talk about unusual attitudes. As with steam gauge unusual attitudes, when there is an AHRS/ADC failure, recovery should always be executed using the standby airspeed indicator. The standby attitude indicator is not trustworthy at this point since a steep enough bank or pitch attitude would cause the gyro to tumble. If needles are spinning wildly, the airspeed indicator will report the pitch attitude: accelerating means nose low, decelerating means nose high. If the standby attitude indicator is functioning, that can be used for bank information. If not, the MFD can give you an idea of which way the airplane is turning as well as the compass, though the compass would be harder to read.

Again, the best thing to do is go find an instructor to practice with. With the exception of engine problems in IMC, an AHRS/ADC failure is the most dangerous situation a pilot could face. Maintaining proficiency will cause this to be a non-event, just like going outside to pick up the paper every morning.

How it Started

One of my best friends in high school, (Doug Gray) was a private pilot. He offered to take me up for a flight in a 1967 Cessna 150, N6228S. We took off from Calhoun, Georgia, and he took me on a scenic tour of the area, I was hooked. I later found out that my English teacher, (Jan Haluska) was also a flight instructor and the school was offering a ground school course the next year, which he taught, along with flight training with the goal of becoming a private pilot. I managed to talk my dad into funding the training, at the time the total cost was right around $500. Cessna 150 rental rates where $12 an hour, and the 172 we used for cross country was $15 an hour including fuel.

Training Begins

It started August of 1977. The fall semester rolls around, and I am enrolled in ground school, and if memory serves me, we met twice a week. First came the paperwork for my student pilot certificate, which at the time was included with the 3rd class medical. I loved ground school, especially learning navigation, plotting courses on the sectional, again this was before iPads and GPS, so we did a lot of dead reckoning and VOR navigation for cross countries. At the end of the course, I made an 83% on the Private Pilot written exam, remember this was before we had the question-and-answer books that on future test I would consume before taking a written.

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Flying

The fun really began on August 22, 1977, it was my first flight. To my surprise we used the same Cessna 150, N6228S that my friend Doug Gray had taken me for a ride in. The Cessna 150 had a standard VFR instrument panel, with one NAV/COM, and one VOR CDI. No intercom, so no headset, at the time I didn’t know what I was missing. I didn’t get my first headset until I started my instrument training in 2003. I was a big guy, I was 6’ 4” and weighed 245 lbs. but I do not remember being uncomfortable in the 150 even with the instructor in the right seat.

First flight lasted .8 hours, and we accomplished orentation, shallow turns, stability, and effects of flight controls. Over the next few months we added steep 720’s, slow flight, stalls, turns around a point, S-turns, emergencies, landings, (short field, soft field and normal).

Learning

At this point I want to talk about a training experience that still stands out. We were close to solo and were practicing takeoffs and landings. Turning base to final Jan got very upset at the way I was cross controlling the aircraft. Cross control is when you are in, say a left turn, and use opposite rudder to line the nose up with the runway. So, he had me depart the pattern and head east to the practice area, and climbing up to 5500 feet MSL, he had me slow down to just above stalling speed, start a left shallow turn and add right rudder. As the airplane stalled I had the strangest sensation, no roller coaster has ever come close, instead of blue sky in the windscreen I was looking at brown ground, and as far as I could tell we were upside down, through the terror of the moment Jan talked me out of the spin, controls neutral, oposite rudder, pull slowly out of the dive. As we leveled off he asked me what altitude we were at, as I remember it was around 2,800 feet MSL. Then he asked me what would happen if I experenced this on base to final in the traffic pattern. The answer was obvious, I would be a pile of wreckage off the end of the runway with a very short-lived aviation career. Needless to say this cured my cross-control tendencies.

Another training event the sticks out in my mind was the first time we did takeoffs and landings at the High School runway. The runway was 1,500 feet long, not sure of the width, but it seems like we had about 5 feet on each side of the wheels when on the center line of the runway. Landing from the south you also had to go between a cutout in the trees to be able to stop in time on the runway. After getting confortable landing here every runway since seemed huge. I remember landing at Chattanooga (KCHA) on a night cross country and commenting to Jan, that I felt like I could of landed sideways on the runway, it felt that large.

Very soon after this I started wearing old shirts to all my flight lessons, the reason for this occurred on February 8, 1978. The lesson that day was stalls, takeoffs and Landing. As we were taxing back in Jan told me it was time for my first solo. I was very excited, and after some last minute advice from Jan, including watch for floating on landing the plane will be light with him not in the right seat, complete 3 takeoffs and landings to a full stop. It was a blast, and at the end of my shirt was shorter in the back because Jan cut the tail out of it signed and dated my solo [an aviation tradition]. Total flight time accumulated on the day of my solo was 18.1 hours. I was now officially a pilot with solo priveleges.

In my next article I start cross country training, when Jan decided that it would be best accomplished this in the 172. Checking out in N5970R was like moving into a 747, this started a love affair with one of my faviriote planes to date. Since this time I have flown over 900 hours in many different models of the 172 and feel like I am stepping into an old friend every time.

More about Randy here: https://www.flyhpa.com/team/randy-delong/

 

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