FAF to MAP timing…AIM

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John Dreylick

Filing Flight Plan
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RoarkJr
Hello, new to the forum here. Where exactly in the AIM does it say that the speed/time chart for the FAF to MAP chart on approach plates is GS and not IAS? Thanks.
 
Country Flier said:
I know this doesn't answer your question, but, does it need to say it? IAS makes no sense.
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EdFred said:
You are flying at 90kts IAS at < 2000 MSL and have a 90kt headwind, do you really think it's going to take you 3 minutes to go 4.5 miles?
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I believe at that point you are in a hurricane and should consider reviewing your flight planning process!

Point taken, though. I guess I just wanted the warm and fuzzy of having a reference to cite. Common sense will do I suppose.
 
MauleSkinner said:
Yet another reason not to use government charts. ;)
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Lol. Yeah. But one could look at like this. Jepp users need to be told the obviousness of that it would be Groundspeed. The Gov gives them a little credit for knowing that. Not serious here, just funnin around a little.
 
I was gonna suggest the Aeronautical Chart User's guide but I see someone beat me, though I would have been too lazy to look up which page it was on anyway.

Not saying it's right, but a lot of people just use their IAS (which is probably why the OP started this thread), as over the distance of a few miles the difference isn't that much unless the wind is particularly strong. The fact that you are only expected to maintain airspeed within +/- 10 knots and that the wind is going to be changing during your descent means that attempting to be exact about your timing is like measuring with a micrometer and cutting with an axe.

MauleSkinner said:
Yet another reason not to use government charts. ;)
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Jepp also gives you a more logical breakdown of groundspeeds which are closer together. Pretty much the only thing I like better about the FAA charts is the mini airport diagram, and last I heard they were strongly considering removing it, if they haven't already.
 
EdFred said:
Primacy and all that. But I don't like the layout or look of Jepp charts. I also like the airport diagram on the gov chart.
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I do admit that I’ve done two approaches with government charts in my career. They were scary approaches.
 
dmspilot said:
attempting to be exact about your timing is like measuring with a micrometer and cutting with an axe.
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It really is. There is just no way (sans GPS) to be that precise about ground speed.

I mean, you have IAS, which is going to be a little off of CAS.
Then you have TAS, which is going to generally be a few knots faster than CAS.
Then you have the wind, presumably a headwind. I've seen it vary by 20 knots or more over the course of flying final (not talking about wind shear).
And inaccuracies of how well you fly - holding speed, starting the timer, and even whether you're weaving back and forth across final or not.

Pretending that timing is anywhere near remotely accurate is a fantasy.

Fortunately, some of these errors tend to cancel themselves out in a lot of situations. For example, if 100 kias = 100 kcas, and the airplane is at 3000 feet on an 85 deg F day, then TAS is about 107. If there's a light headwind, then there you are back to 100 kts GS which is the same as indicated.
 
dmspilot said:
Not saying it's right, but a lot of people just use their IAS (which is probably why the OP started this thread), as over the distance of a few miles the difference isn't that much unless the wind is particularly strong. The fact that you are only expected to maintain airspeed within +/- 10 knots and that the wind is going to be changing during your descent means that attempting to be exact about your timing is like measuring with a micrometer and cutting with an axe.
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There used to be a VOR-A approach into Toluca, Mexico, that used a table like this to time your outbound leg on the missed approach, keeping you within 10 miles. I had a couple of guys run about 3 miles beyond the 10 because they used IAS instead of GS. At 12,000 feet there’s almost a 25% increase in TAS over IAS.
 
EdFred said:
Primacy and all that. But I don't like the layout or look of Jepp charts. I also like the airport diagram on the gov chart.
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I lived close to Jepp for 20 years. They used to give us free Trip Tiks for Angel Flights so I became very familiar with both FAA and Jepp. 20+ years ago, I think In Jepp was better - the paper was definitely better :D But, on balance, I never really saw much of a difference in approach plate presentation to call one "better" than the other or easier to understand than the other. Still don't.
 
Martin Pauly said:
And yet, there’s this thread… :)

- Martin
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Heck, how many even realize the table is just a basic time/speed/distance calculation. Just the distance from the FAF to the MAP divided by ground speed? A charting convenience.
 
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