Where do you get your information? Do you go to experts, or rely on what people tell you in a Facebook post?
I try not to spend too much time interacting on social media because to me it’s largely a waste of time. It can also be a huge source of wildly speculative, uninformed answers to questions that people pose. Often the correct answer to a question is in the comments, but how does one sort it from the other 95 comments that are incorrect?
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Coincidentally, I was reading the book Pains and Planes: A Doctor’s COVID-19 Memoir by Dr. Victor Vogel, and I noticed he has the same frustration. Vogel, who’s also a CFI, is the founder of Susquehanna STEM to the Skies, which uses aviation to inspire students in rural Pennsylvania. Here’s what he wrote:
“I am troubled by those patients who have lost their faith in medicine and in physicians who think they can research the proper treatment for their breast cancer. But who do they read? Who are the authorities they seek? Why do answers on the internet have more credibility than the recommendations of a physician?”
For most issues, relying on bad information found on the internet isn’t going to get you killed. But for some high-stakes research, such as into breast cancer treatment, or on how to properly fly an instrument approach, a wrong answer could. A recent Facebook post on instrument flying is a good example:
“Was test flying a 2019 Cirrus SR22T yesterday for a buddy, thinking of buying, and went to shoot an approach in VFR conditions back to his airport. It had an RNAV lpv approach. I loaded the approach and went direct to the IAF at the altitude on the plate. Once reaching the fix, I hit the approach button, and it started navigating the approach course.
“I could see the glideslope above me and slowly coming down to where it would normally grab the glideslope. On the scoreboard the GS was in white and would normally grab green and start descending on the approach with WAAS. Instead, the glideslope continued down below my altitude, and I had to hand-fly the approach in. I’ve shot this approach hundreds of times in my aircraft without this issue. After landing we found that the Garmin database is expired on the plane and needs updating. This is a G6 Perspective+. Has anyone experienced this same thing with expired database?”
- READ MORE: What Happens When Autopilots Fail?
As soon as I read the post, I knew what the issue was, as I’ve experienced this behavior multiple times. But as I read through the comments, no one had correctly identified the issue. Instead, there was a random collection of guesses, none of which were relevant.
I posted a comment which said in part:
“It sounds like you’re saying that you stayed level at the IAF altitude and expected to catch the glidepath at a much higher altitude than the glidepath intercept altitude published for the final approach fix. That’s not the proper way to fly an approach, though it will work for an ILS. However, that often won’t work for an RNAV approach.”
That elicited a comment from someone that was 100 percent wrong when they wrote:
“It is 100 percent proper to intercept the GP/GS [farther] out, with an ILS, as long as you intercept from below there’s no risk of false glideslope. No issues ever doing it with magenta guidance.”
It’s not proper to intercept an ILS glideslope at an altitude that’s above the intercept altitude, and there are issues with doing it on an RNAV approach, which is what I think he meant by “magenta guidance.” But his misunderstanding of how to properly fly an approach was at the crux of the issue the original poster was having.
AIM 5-4-5b says in a note: “The ILS glideslope is intended to be intercepted at the published glideslope intercept altitude.” So, while you can often get away with intercepting an ILS glideslope at a higher altitude, flying an approach that way won’t work on an RNAV approach. So why not fly both approach types the correct way?
In modern glass cockpit aircraft, an RNAV (GPS) glidepath is initially shown as a hollow, white diamond on the vertical deviation indicator. Later, as the airplane gets closer to the FAF, the white diamond becomes a solid magenta diamond, and at that point an autopilot can couple to the glidepath. Prior to that point, you can hand-fly the aircraft to follow the white diamond, but an autopilot won’t couple to it.
While I was aware of this behavior, I could never find documentation that stated at what point along the approach an RNAV glidepath indicator turns to a solid magenta diamond. Knowing that would be helpful.
So, after reading the Facebook post, I hopped into one of my club’s AATDs for an hour to find the answer. I flew four different RNAV approaches, including the one flown by the Facebook poster, and received consistent results for all four approaches. For each approach, I started at the IAF and descended at every stepdown. I displayed the flight plan on the PFD, and at the moment the glidepath diamond turned magenta, I took a photo. When I got home, I put together a spreadsheet that calculated the distance from the airplane to each fix on an approach, when the diamond changed colors.
Fortunately, I flew more than one approach, as I picked up a subtlety that could easily have been missed. I had thought that the transition would occur at some distance past the IF or before the FAF, but that turned out not to be the case. Instead, the diamond always changed to magenta while crossing the fix preceding the FAF. On most approaches, that is the IF. But a number of approaches have one, and possibly more, fixes between the IF and FAF. On those approaches, the switch does not occur at the IF but at the fix before the FAF. For my efforts, I was rewarded with three “likes,” which was fewer than some of the wrong answers had.
So where do you get critical information? And do you choose the answer with the most likes?
This column first appeared in the July Issue 960 of the FLYING print edition.
