Few things are more important than being at the right altitude on an instrument approach. In the abstract that sounds easy, but in practice I often see pilots make mistakes in this most unforgiving aspect of instrument flight.
Most of my flying is in aircraft with moving maps, as part of either a GPS or a glass cockpit, so my observations are biased by what I see in these airplanes. Oddly, I recall fewer issues in airplanes equipped with just a VOR receiver and a DME receiver; watching a DME count down is unambiguous, resulting in fewer mistakes.
Modern electronics present more information, making it harder for pilots to focus on the data most relevant for each phase of flight. Most glass-cockpit aircraft in the United States don’t have DME. They do have GPS navigators that can substitute for DME, but pilots often don’t look at that information, or they fail to display it.
Most problems I see fall into three categories. The first is correctly determining the step-down altitude. I’m surprised at how cavalier pilots are about finding the altitude for an approach segment. Many will glance at a chart for two seconds, pick an altitude (often the wrong one) and then confidently proceed.
Whatever happened to the principle of double-checking the critical few things that can kill you? If your life is worth anything — and I’m sure you believe it is — it’s worth spending more than two seconds determining and confirming each altitude.
A frequent problem is misinterpreting the altitude to descend to when flying a course reversal at the initial approach fix (IAF). But, almost always, pilots choose an altitude that is too high, so in practice making this mistake is not a major safety hazard. Also, pilots generally choose vectors instead of their own navigation, and hence they don’t need to fly a course reversal.
But a surprising number of pilots misinterpret the step-down altitudes when reading the approach chart. The most frequent error is choosing the altitude for the next step-down fix, meaning they will be flying too low along the approach segment, which could be fatal. Generally, I see pilots make this mistake when looking at the profile view (or side view), which displays segment altitudes immediately adjacent to and just before the next step-down fix. I don’t know why pilots sometimes misinterpret that symbology.
Segment altitudes appear in two places, and I don’t recall a pilot ever misinterpreting the segment altitudes displayed in the plan view, which is essentially a bird’s-eye view of the approach. So, like a carpenter who knows to measure twice before cutting, pilots should look twice — once at the plan view and once at the profile view — to confirm that they know the correct altitude for each approach segment.