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As instrument pilots, we are taught how to intercept and follow a glide slope during our instrument training. Whether an ILS or LPV, there’s little debate about whether to intercept from above or below. We always want to intercept from below. But the question of where along the approach course to intercept is often debated. Depending on your instructor, you may have been taught different strategies.
There are two common approaches—pardon the pun—to intercepting the glide slope. The first option is to descend to the glide slope intercept altitude while on the intermediate approach segment and level off. You then fly at that altitude until you intercept the glide slope at the final approach fix. The second is to maintain your assigned altitude and intercept the glide slope before the final approach fix.
Dive and Drive Method
The first method is frequently called “dive and drive.” It refers to the fact that the pilot dives for the minimum altitude then drives along at that altitude until he can descend lower—by virtue of a lower minimum altitude on the next segment or breaking out of IMC. This technique is usually discussed when talking about descending to MDA on a non-precision approach, but it also applies to any intermediate step-down altitudes as well.
Proponents of this method point out that the sooner you get to a lower altitude, the sooner you may break out of the clouds. Once out of the clouds, you can descend visually. For the average private pilot, getting out of the clouds sooner seems like a pretty good idea.
Detractors point out that a stabilized approach is the safer way to go rather than dragging along at the minimum altitude fighting to keep the plane from descending. They also say dive and drive is more work at a time when the pilot’s workload is already high. Additionally, descending lower early in the approach may not be a good idea even if you’re in visual conditions. There may be obstacles on the final approach path that could be hard to see.
Stabilized Approach Method
The stabilized approach method means setting up the plane for the proper approach speed and proper descent rate to reach your target altitude when you reach your missed approach point. For approaches with vertical guidance, this means following the glide slope.
So why not intercept the glide slope as soon as possible, then use it to descend all the way to your decision altitude? Champions of the stabilized approach method often like to point out that if you can set your airplane up for descent early, then you have less to do at the final approach fix.
While that makes sense, critics point out—correctly—that the glide slope is technically only valid for the final approach segment. Most importantly, the glide slope doesn’t provide obstacle clearance prior to the final approach fix. Nor will it guarantee that you won’t descend below any intermediate step-down altitudes.
What’s the Right Answer?
Like many things in aviation, there’s not really an easy answer. Personally, my instrument instructor taught me the dive and drive method. So that’s the way I did it for a long time. Then an old airline pilot CFI talked me into trying the stabilized approach method. I prefer it, so that’s what I use now.
As I mentioned above, the biggest problem with intercepting the glide slope early is making sure you don’t descend below any step-down altitudes. Depending on where you fly, you may or may not encounter many of these. I fly in the relatively flat midwest where you don’t often see step-downs on an ILS approach. Those that I know of locally are all below the ILS glide slope anyway. So my argument is why would you descend, then stop your descent, only to have to start another descent—possibly multiple times—when you could just follow the needle?
Obviously, if you fly in an area where terrain and obstacles are a factor, then you are going to have a different opinion. Every pilot should be capable of descending and maintaining a target altitude for as long as necessary. If you can’t, you shouldn’t be flying in actual instrument conditions.
And that view isn’t wrong. Every instrument pilot should be capable of doing that. My view, though, is why should you make the approach harder than it needs to be? If the approach has step-downs, just be aware of them. After, regardless of which method you choose, you can’t descend below any minimum altitude. So what if I use the glide slope to get to that step-down?
The problem is that following the glide slope can give a false sense of security. When you’re following the needle, it’s easy to forget that you have to watch for more than just your decision altitude.
What Does the FAA Say?
While the FARs don’t specifically tell us, the AIM does have this to say about it:
The ILS glide slope is intended to be intercepted at the published glide slope intercept altitude… Interception and tracking of the glide slope prior to the published glide slope interception altitude does not necessarily ensure that minimum, maximum, and/or mandatory altitudes published for any preceding fixes will be complied with during the descent.
Additionally, this document from the Aeronautical Charting Forum states it very bluntly:
Based on industry observation, there appears to be a mistaken belief among pilots that the ILS glide slope descent angle is established such that the published minimum altitude for step-down fixes in the intermediate segment that precede the glide slope interception altitude will be achieved if the pilot elects to intercept and track the ILS glide slope during the intermediate segment descent.
That would seem to be the final word. Except (again from the AIM):
If the pilot chooses to track the glide slope prior to the glide slope interception altitude, they remain responsible for complying with published altitudes for any preceding stepdown fixes encountered during the subsequent descent.
And from the ACF:
Should the pilot elect to intercept and track the ILS glide slope at a point prior to the glide slope interception altitude, this action does not necessarily ensure that subsequent fixes will be crossed at or above the published minimum crossing altitude, an expectation which is stated in AIM 5-4-5 (b)(1).
So the FAA seems to be telling us that while it’s legal to intercept the glide slope early, they recommend not doing it that way.
Personally, in the absence of a compelling reason not to, I prefer to intercept the glide slope as early as possible and fly a stabilized approach to the decision altitude. This is especially true when I’m flying a faster plane—which means less time to make changes—However, I am well aware of the potential hazards. If there are step-downs before the final approach fix, I make a decision when I brief the approach on whether or not to intercept the glide slope early or to use the dive and drive.
Of important note is that there are approaches where the pilot must adhere to maximum altitudes as well as minimums. In some cases (generally for traffic separation) there are exact attitudes the pilot must maintain. Additionally, ATC may assign a specific altitude to maintain until you intercept the glide slope. In these cases, it’s simpler to dive and drive.
So Which Method Do You Choose?
In the end, you’ll have to decide for yourself which method works better for you and your aircraft. I know that sounds like a copout, but it’s the best answer. I don’t believe there’s a one size fits all solution. Go try both in VFR conditions. Even if you don’t put on the hood, you’ll get a good feel for how each method works for you.
There are going to be clear situations where the dive and drive approach is the best (and maybe only) option. There are other times when choosing to intercept the glide slope early might be better. But whatever you choose, where you should intercept the glide slope is something you should decide well before you’re established on the approach. As with any approach, that’s the wrong time to figure out what to do.