Subscriber question:
"Is it OK to intercept a glideslope or glidepath above? Assume you're still outside of the final approach fix."
Martin:
“Many instrument approaches have a very simple vertical profile once aligned with the final approach course – maintain an altitude to the final approach fix (FAF), then descend to the decision altitude or minimum descent altitude.
At other times, there can be one or more step-down fixes on the way to the FAF. When that happens, do you have to fly every step-down altitude, or can you simply wait until glideslope intercept and then ride down on the ILS or LPV?
A lot of pilots like to do the latter. And there are advantages of doing it this way: it means less workload for the pilot than multiple separate short descents, and the passengers will probably appreciate the constant rate of descent all the way down. Those are good arguments, but: you have to make sure that doing this complies with the altitude crossing restrictions for all step down fixes prior to the FAF. In many cases those constraints are not a problem when on the glide slope, but occasionally you’ll find an approach where due to terrain or obstacles or air traffic management needs there can be altitude constraints which conflict with the ILS glide slope.
An example where the potential problems are quite obvious is the ILS Z approach to runway 6 at Teterboro (KTEB). DANDY, which is two miles ahead of the FAF, has a mandatory crossing restriction of exactly 1,500 feet, and you will violate that crossing restriction if you are on the glideslope.
There are less obvious examples, as well as marginal cases where the glideslope works sometimes but not always. How is that possible, you ask? Well, if a step-down fix has to be crossed at, say, 7,000 feet or above, remember that’s a barometric altitude, and the actual point in space which results in an altimeter indication of 7,000 feet is higher on a warm day than on a cold day. If you just follow the glideslope, you may cross at 7,000 feet or above on a cold day, but maybe not on a hot summer day.
Another potential issue could be that the glideslope reception is sometimes not reliable from too far out. And if you fly an RNAV LPV approach, remember that you won’t even see a glide path indicated on your instruments until the final approach fix becomes your active waypoint, so you may have to sequence that leg to the FAF manually and skip the step-down waypoints on the way there.
So what does it all mean? Here is my take on it: if you want to have only one way of dealing with these step downs, one way that always works, then fly each step altitude individually, and only follow the glideslope or glide path from the final approach fix. If on occasion you want to deviate from this, study the vertical profile carefully, and only do it after convincing yourself that all altitude constraints along the way will be met.”
