FAR Part 97: Aircraft Approach Categories - IAS vs Ground Speed

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"Steven P. McNicoll" wrote:

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...

You are correct. There is no end to what some flight instructors will
dream
up or invent. Everything the FAA does in the world of charting is
predicated
on IAS.

Not quite everything. The approach timing table uses ground speed.

More correctly, the Jeppesen timing table states ground speed. NACO does
not.

Whether they state it or not, there's nothing but ground speed that they
*could* be using to calculate the time to traverse the stated distance.

--Gary

Gary Drescher wrote:

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"Steven P. McNicoll" wrote:

wrote in message
...

You are correct. There is no end to what some flight instructors will
dream
up or invent. Everything the FAA does in the world of charting is
predicated
on IAS.

Not quite everything. The approach timing table uses ground speed.

More correctly, the Jeppesen timing table states ground speed. NACO does
not.

Whether they state it or not, there's nothing but ground speed that they
*could* be using to calculate the time to traverse the stated distance.

--Gary

For the best accuracy, it obviously has to be ground speed. But, there is no
requirement to make the necessary calculations to arrive at ground speed. A
lot of folks over many years have simply treated the timing table values as
indicated airspace, on the premise there are a lot more important things to do
in the final approach segment than attempt to make conversions.

In recent years, RNAV has all-but-eliminated any need to use the timing table
in any case.

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A lot of folks over many years have simply treated the timing table values
as indicated airspace, on the premise there are a lot more important
things
to do in the final approach segment than attempt to make conversions.


What makes you think that?

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In recent years, RNAV has all-but-eliminated any need to use the
timing table in any case.


Assuming, of course, that your plane has an IFR-approved GPS that is
currently working.

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Gary Drescher wrote:
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...
"Steven P. McNicoll" wrote:

wrote in message
...
You are correct. There is no end to what some flight
instructors will dream up or invent. Everything the FAA
does in the world of charting is predicated on IAS.

Not quite everything. The approach timing table uses ground speed.

More correctly, the Jeppesen timing table states ground speed.
NACO does not.

Whether they state it or not, there's nothing but ground speed that they
*could* be using to calculate the time to traverse the stated distance.

For the best accuracy, it obviously has to be ground speed. But, there is
no
requirement to make the necessary calculations to arrive at ground speed.
A
lot of folks over many years have simply treated the timing table values
as
indicated airspace,

Whatever shortcuts pilots may take, the fact remains that the NACO timing
tables do use GS, just as the Jepp tables do. If wind is negligible and CAS
is close to IAS, then of course IAS closely approximates GS, but it's still
GS that's given in the tables.

on the premise there are a lot more important things to do
in the final approach segment than attempt to make conversions.

But why would you wait until the final approach segment to make the
conversion? I consider it part of the approach-briefing to calculate GS from
IAS and reported wind, and to write down the appropriate time to the MAP.

In recent years, RNAV has all-but-eliminated any need to use the timing
table
in any case.

Even if your RNAV is certified for IFR, I think it's still a good idea to
pre-compute your time to the MAP in case your RNAV fails during the
approach.

--Gary

"Gary Drescher" wrote in message
...
Whatever shortcuts pilots may take, the fact remains that the NACO timing
tables do use GS, just as the Jepp tables do. If wind is negligible and
CAS is close to IAS, then of course IAS closely approximates GS,

Only for sea level airports. You will be off by about 20% landing at a
6000msl airport.


but it's still GS that's given in the tables.

"Harlo Peterson" wrote in message
...
"Gary Drescher" wrote in message
...
Whatever shortcuts pilots may take, the fact remains that the NACO timing
tables do use GS, just as the Jepp tables do. If wind is negligible and
CAS is close to IAS, then of course IAS closely approximates GS,

Only for sea level airports. You will be off by about 20% landing at a
6000msl airport.

Yup, density altitude is a factor too; my flatlander assumptions were
showing. But even at 6000', the discrepancy is less than 10%, not 20%.
Still, you're right that the discrepancy is large enough to matter.

--Gary

"Gary Drescher" wrote in
:

Yup, density altitude is a factor too; my flatlander assumptions were
showing. But even at 6000', the discrepancy is less than 10%, not
20%. Still, you're right that the discrepancy is large enough to
matter.

Not to mention that your assumed GS for the approach will only be a WAG
anyway. The winds on the approach can be greatly different from the
winds at your cruise altitude, and from the reported surface winds.
TERPS gives you enough protection so that it really doesn't matter much
anyway. Using the ARP or the runway in the GPS for the MAP will be much
more precise than timing anyway.

--
Regards,

Stan

"They that can give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety." B. Franklin