Going missed

I was in instrument conditions with moderate rain and a 700-foot ceiling, and the autopilot was not capturing the glidepath as I expected. I knew it was probably something I was doing wrong, and that now was not the best time to be figuring it out.

I had recently regained instrument proficiency, knocking the rust off after several years of VFR-only flying. My strategy to feel comfortable flying single-pilot IFR again included cross-country flights from Frederick, Maryland, to Florida and Ohio with an instructor in actual conditions terminating in instrument approaches. This was merely preparation for an intensive six-hour Beechcraft Pilot Proficiency Program (BPPP) ground school and training flight administered by the American Bonanza Society that doubled as an instrument proficiency check. It felt like I had received a type rating for the Bonanza A36.

Understanding the fundamentals of instrument flying has become second nature—I earned my instrument rating in 1992 and hold an instrument ground instructor certificate—but correct avionics buttonology is a perishable skill I was trying to hone.

Now I was flying back to Frederick from Nashua, New Hampshire, where, during arrival, I had broken out of an overcast layer at 2,000 feet but continued to use the Garmin GFC 500 autopilot to fly the RNAV LPV approach to minimums; the forecast for Frederick included lower ceilings and moderate rain and I wanted to be as proficient as possible.

Seventy-five miles north of Frederick the cloud bases lowered to engulf me, as forecast, but the air was smooth, and I used this quiet time to get the Frederick weather and set up the avionics for an anticipated RNAV Runway 5 LNAV approach. I was vectored to the JOVUR initial approach fix, told to maintain 3,000 until JOVUR, and cleared for the approach. I pressed approach mode on the autopilot and heard “advisory vertical guidance” in my headset, confirming I was getting LNAV+V (lateral navigation plus advisory vertical guidance) from the GTN 750. At top of descent, the autopilot started a descent to 2,600 feet and the airplane turned inbound on the final approach course. But as the glideslope slid down the vertical deviation indicator, the autopilot did not initiate a descent as I expected it would. Clearly, I had missed a step, but what was it?

My (incorrect) inclination was to troubleshoot my mistake by selecting several possible vertical modes on the autopilot—none of which resulted in capturing the advisory glideslope. In a final attempt to descend to the final approach fix minimum altitude, I programmed a vertical speed descent, but was already too high and would need to descend too rapidly; I conceded that my approach was no longer stabilized. The instinct to salvage a poor approach is powerful and can be deadly.

I had a flashback to a training flight where I forgot to press the approach button on the autopilot after being cleared for the approach and flew past the approach course, still in heading mode. The instructor said he was happy to see the error, because it enabled him to see how I would react. My corrective action that day was to turn off the autopilot and hand-fly the approach. This time, a missed approach was the best course of action.

I called missed and the tower handed me off to approach control. Approach asked my intentions and I said I wanted to fly the same approach. This time I had a different strategy. Although I knew I did something wrong with the autopilot setup on the previous approach, I made a firm decision that I was not going to attempt any more troubleshooting in the air. I would set up the autopilot exactly as I had done before, press approach mode when cleared for the approach (which gave me lateral navigation) and not even try to capture the advisory glideslope. I employed the proven dive and drive method of descending to each approach fix step down minimum altitude and waiting until the next fix to descend farther. I broke out 700 agl and made a normal landing in the driving rain.

I emailed my BPPP instructor and asked what I missed. He said this situation can happen when the VNAV profile is overlapping with the glidepath, and the easiest way to handle it is to turn VNAV off so that the autopilot will capture the advisory glideslope. That’s not a solution I would have come up with in the air.

I recounted the story to another CFII. He explained I could also have set the autopilot altitude to the MDA of 700 feet and VNAV would have continued the vertical descent. “The good news is that you’ll never make that mistake again,” he said. I would agree.

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