Monday, June 30, 2014

When to slow to approach speed?

There is a discussion taking place on the Pilots Of America forum that is revolving around when to slow the aircraft to final approach speed on an instrument approach. Unfortunately, the Instrument Procedures Handbook, the Instrument Flying Handbook, and the Aeronautical Information Manual do not provide much guidance on this.

One way or the other, you need to be slowed down to your final approach speed and configuration by the FAF. You of course can do this far away from the airport, even before the IAF, and the earlier you get things established the more time you have to fine-tune your configuration.

But there is a place that is expressly designed for this purpose in case you're not configured earlier - the intermediate segment! This segment, which starts at the IF (Intermediate Fix) and ends at the FAF, for the most part exists primarily to help you get slowed down, configured and ready to descend at the FAF. Once past the FAF you should have all those changes already made (gear, flaps, etc. as appropriate) so that you can concentrate solely on flying the plane, staying on course, and watching your altitude.

This is encapsulated in the FAA Order for approach procedure designers, FAAO 8260.3B, which states:

Para 240: [The intermediate segment] is the segment in which aircraft configuration, speed, and positioning adjustments are made for entry into the final approach segment.

Further on:

Para 242d: Because the intermediate segment is used to prepare the aircraft speed and configuration for entry into the final approach segment, the gradient should be as flat as possible. The OPTIMUM descent gradient is 150 ft/mile. The MAXIMUM gradient is 318 ft/mile [or higher if there is a greater than 3 degree ILS]. Note: when the descent gradient exceeds 318 ft/mile, the procedure specialist should ensure a segment is provided prior to the intermediate segment to prepare the aircraft speed and configuration for entry into the final segment. The segment should be a minimum length of five miles and its descent gradient should not exceed 318 ft/mile.

So the intermediate segment should be flat to allow aircraft configuration, around 150 ft/mile, but no greater than 318 ft/mile (which is a 3 degree glidepath), with some exceptions if, say, there is a 3.2 degree glidepath.

Let's look at some examples.

To me, this is an example of the ideal intermediate segment, at Dayton, Ohio (KDAY) RNAV (GPS) RWY 18.

Notice that once you do all the initial maneuvering, you have a completely flat, level segment (GILPE to WALMA) in which to get your speed set. Why is this important? Most of us train for our instrument ratings in something like a Cessna 172, which isn't too hard to get to slow down even while you're descending - it has a lot of drag. But something faster and more slippery, maybe like a Mooney or even a jet, will have a harder time both going down AND slowing down. So this level segment is a good place to get that done. Then, when everything's configured and on speed, you just wait until the FAF, make your power reduction, and start on down. This is the easiest type of intermediate segment to get set up for and fly.

It doesn't always work that way, of course. Many things can cause the intermediate segment to require some kind of descent. It might be obstructions like antenna towers or buildings, it could be surrounding airspace, or many other things. So sometimes it's unavoidable. Here's the profile view of the Appleton, Wisconsin (KATW) VOR/DME RWY 3:

You cross the Oshkosh VOR (the IF) at 3000, then descend to 2700 by the FAF. This is only 300 feet of descent, and yet you have 10 miles to do it! So that's not bad at all - 30 feet per mile. Most pilots would probably just descend to 2700 in the first mile or so, then spend the rest of the intermediate segment getting configured.

But sometimes terrain drives this descent gradient up much higher. Here's the profile view of the Arcata-Eureka, California (KACV) ILS Y or LOC/DME RWY 32:

The FAF has a minimum altitude of 2100, but if you do the math you'll see that each previous fix (OMBEE, HURDU, KORBE) is almost exactly 318 feet per mile higher. This is okay and meets the criteria in the 8260.3B, but might make it hard to "go down and slow down". Fortunately, prior to KORBE (as can be seen on the plan view below), there are some segments that are a bit more level (like that long arc from HIDAK to JEBGA) where your final approach configuration can be established.

This is yet another example of why it pays to take a good look at the procedure well beforehand and plan your arrival. Don't want to get to KORBE and then realize you don't have a good place to slow down!

Happy flying!

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