This came up during a recent IFR instructional flight. My student was cleared to "maintain at or above 3000 until established on the localizer, cleared localizer XX approach." He intercepted, and when I reminded him that the published altitude for the intermediate segment (and glideslope intercept) was at 2600, not 3000, so he should descend, he asked me why can't he intercept the glideslope at 3000? For the time being, "because I told you so" had to be sufficient, but clearly he wasn't satisfied with that answer for long after we landed!
There are a variety of reasons why it's best to intercept the glideslope at the published altitude, but the one I want to discuss here is the effect of high temperature on indicated altitude when on glideslope. The potential problem isn't from the FAF inbound, it's prior to the FAF.
Many ILS approaches have stepdown fixes in the intermediate segment prior to the FAF. A pilot is, of course, responsible to not descend below those minimum altitudes until crossing the respective fixes. What if the pilot intercepts the glideslope well before (and therefore well above) the FAF and published glideslope intercept altitude and follows it down? Will the aircraft meet all the stepdown fix crossing restrictions until getting to the FAF? Does it have to? Let's look at an example, KLAX ILS OR LOC RWY 25L:
|KLAX ILS OR LOC RWY 25L|
I've only included the profile view because it's the only part that's pertinent to this discussion.
Notice all the stepdown fixes before the FAF. If you do the math, the HUNDA and GAATE fixes are just about right on glideslope (the profile views are not to scale, so it doesn't look it), whereas FUELR at 7000 is below glideslope. If you pass FUELR at 7000 and stay there waiting to intercept the glideslope, then follow it down, normally you'd do fine and clear GAATE and HUNDA right on altitude.
But I said I was going to talk about non-standard HIGH temperatures, didn't I? This can take a bit of mental gymnastics...
So, remember that the glideslope is for all useful purposes a fixed line in space. It is not affected by pressure or temperature, it's an electronic beam. But our altimeters are affected by temperature and pressure. We adjust for pressure by changing the altimeter setting, but we don't adjust for non-standard temperature at all. And if we don't, then a higher-than-standard temperature will cause our altimeter to read lower than we actually are - or in other words we are higher than we think we are. Normally this is okay - it means more separation above terrain for one, and since everybody is using the same altimeter setting, it causes no problems with conflicting traffic either.
But back to that electronic beam that we know and love called the glideslope. It's fixed in space. If we cross GAATE on glideslope, we will be right at 5000, true altitude. What will our altimeter show? 4900? 4800? Depends on the difference from standard temperature. So when we cross GAATE on glideslope, if our indicated alitude is 4900, then guess what? We just busted our minimum altitude!
Okay, so, is this actually a problem? Well, at an airport in busy airspace like LAX, it proved to be. They have multiple traffic corridors both above and especially below some of these fixes going into other airports. If that crossing traffic is flying at the correct indicated altitude, it is (due to the temperature effects) also higher than indicated. But since you are maintaining that glideslope and therefore indicating a hundred feet lower than the stepdown fix altitude, you have just caused a loss of required separation!
This actually caused a problem a few years ago and they had many pilot violations as a result (see links at the end of this post). It was a real problem, because some pilots didn't realize they wouldn't meet the stepdown restrictions if they intercepted the glideslope at a higher altitude prior to the FAF and followed it on down.
(Notice that this doesn't apply to stepdowns in final, of course they're marked "LOC only" anyway, like LADLE on the example above. Only to stepdown fixes prior to the FAF.)
Here's what the NBAA has to say about it:
And here's the release from the FAA:
Fly safe and watch those intermediate segment stepdown fixes!