Approach Timing

On 9/7/04 4:02 PM, Brien K. Meehan wrote the following:

Mitty wrote:

Doesn't seem like the MAP is that important.


On an instrument approach, it's the most important thing in the world.


So what if you are a bit
early or late when you decide you can't see where you're going?


You'd better study this a bit, because if you manage to get through
your IFR checkride without busting it a bit, this attitude may soon
make you dead a bit.


I don't want to turn this into a flame war, but I'll take the bait he

How (calculator, flight computer, mentally, etc.) and how precisely,
during an approach, do you calculate your time to the MAP based on your
ground speed crossing the FAF? Finally, to you, what is an acceptable
"bit?"

My checkride went fine, thanks.

Mitty wrote:
How (calculator, flight computer, mentally, etc.) and how precisely,
during an approach, do you calculate your time to the MAP based on your
ground speed crossing the FAF? Finally, to you, what is an acceptable
"bit?"

A perfectly reasonable question. Lacking any better instrumentation
(DME, or GPS), here's what I do:

1) Estimate the surface wind based on the best information you've got
(which usually means ATIS or AWOS).

2) Add some random fudge factor to account for the fact that the winds
at 500 - 2000 AGL (where you're going to be flying the approach) will
tend to be a bit stronger than on the surface.

3) If it's not a direct head or tail wind, take a WAG at the
head/tailwind component.

4) Add this to the indicated airspeed you plan on flying the approach at
(at the speeds and altitudes you usually use for approaches, IAS is
close enough to TAS that you shouldn't worry about the difference).

5) You now have a reasonable estimate of your groundspeed. If you're
trying to refine this estimate to anything closer than the nearest 10
kts, you're fooling yourself.

6) Now, look at the FAF-to-MAP timing table and do a rough interpolation
between the listed entries.

If you spend more than about 30 seconds on the whole process, you're
working too hard. Without a reliable way to measure GS, the best you
can do is a reasonable guess, so don't knock yourself out trying to do
anything fancier.

With a handheld GPS, you're be insane not to take advantage of the
information it gives you. If the MAP itself is not in the database, set
it navigating to the ARP, or the FAF, or the last stepdown fix, or
whatever makes the most sense for that approach. Start your watch to be
legal, but use your GPS to be safe.

A buddy of mine does this. Drives me nuts. He'll sit and screw with
the throttle to get 90 knots ground speed. I want to reach over there
and smack him upside the head. He makes himself so busy trying to fly a
certain groundspeed and he can't see that.

does your friend fly a groundspeed or fly the plane? I know the plane
flies through the air and doesn't know which way the ground goes
(up/down/left/right/forward/backward). If he is doing an
circle-to-land approach say in a 152 with 50 knot tail wind, does
he really have a IAS of 20 knots (or whatever a 152 does an approach
at). If so, his ground speed might be zero knots when he stalls.

Gerald

On 9/7/04 6:21 PM, Roy Smith wrote the following:

Mitty wrote:

How (calculator, flight computer, mentally, etc.) and how precisely,
during an approach, do you calculate your time to the MAP based on your
ground speed crossing the FAF? Finally, to you, what is an acceptable
"bit?"


A perfectly reasonable question. Lacking any better instrumentation
(DME, or GPS), here's what I do:

1) Estimate the surface wind based on the best information you've got
(which usually means ATIS or AWOS).

2) Add some random fudge factor to account for the fact that the winds
at 500 - 2000 AGL (where you're going to be flying the approach) will
tend to be a bit stronger than on the surface.

3) If it's not a direct head or tail wind, take a WAG at the
head/tailwind component.

4) Add this to the indicated airspeed you plan on flying the approach at
(at the speeds and altitudes you usually use for approaches, IAS is
close enough to TAS that you shouldn't worry about the difference).

5) You now have a reasonable estimate of your groundspeed. If you're
trying to refine this estimate to anything closer than the nearest 10
kts, you're fooling yourself.

6) Now, look at the FAF-to-MAP timing table and do a rough interpolation
between the listed entries.

If you spend more than about 30 seconds on the whole process, you're
working too hard. Without a reliable way to measure GS, the best you
can do is a reasonable guess, so don't knock yourself out trying to do
anything fancier.


OK, that's basically what I do too and the consequent errors are what I
meant by the "a bit" comment that led to Meehan's smart-ass shot. "A
bit" is maybe a 10-15% error on the time sans. I have no interest in
studying the TERPS design rules but I gotta believe that they leave us
plenty safe with that size error.

With a handheld GPS, you're be insane not to take advantage of the
information it gives you. If the MAP itself is not in the database, set
it navigating to the ARP, or the FAF, or the last stepdown fix, or
whatever makes the most sense for that approach. Start your watch to be
legal, but use your GPS to be safe.

No argument. But the original poster's idea sounded like head-down
during an approach managing airspeed as it gets updated on a GPS -- that
idea still scares me. I'm not smart enough to fly an approach while
trying to do that, too. & I think it's unnecessary.

"Howard Nelson" wrote in message . com...

I really, really, really would recommend flying an airspeed under all
circumstances.

Ditto. Planes fly airspeed, not groundspeed. I was taught to fly
standard configurations of pitch, power, and trim. These should
produce known airspeeds which set the plane up well for the specific
phase of flight.

Do you use the distance to the airport to determine/verify the MAP, even
though the time may not have expired?

Legally your IFR approved clock for timed approaches and NDB for NDB
approaches are what you WILL use. Rationally if my clock or NDB disagreed
with my GPS and I had to make a choice I would trust the GPS.

The conservative choice would be to listen to the first box that cries
"miss." Let's say the wind shifts and your groundspeed changes, the
handheld says you're there. Might be good to miss it. If OTOH the
clock says you're there and the GPS doesn't, you're not going to
increase your risks by missing on that basis.

I can understand the FAA wanting to avoid a free for all by regulating GPS
usage so carefully. But each pilot, if faced with conflicting data, needs to
decide which technology they trust their life to. The best way to do that is
lots of practice under VFR checking clock vs. GPS, NDB vs. GPS, VOR vs. GPS,
Localizer vs. GPS and make up your own mind.

I use handheld GPS and panel Loran to cross-check everything else in
my non GPS-equipped C-172.

Like you say, there's no way the FAA will ever allow handheld GPS as
an official navigational device for IFR, but they've also made a wise
decision to not prevent it from being used anyway. A Decent handheld
GPS and COM radio give a pilot a fighting chance to get down through
the clouds in a total electrical failure, which is one of the
emergencies I worry about most in a middle-aged plane like mine. (NB-
I have a Precise Flight standby vacuum already)

Also if you are not a renter you might consider spending 15-20K for an IFR
certified GPS linked to your autopilot.

You could probably put a good used box in for a lot less, though
perhaps not linked to the A/P. 6K range for a used King/Apollo unit +
install/certification is the estimate I've seen from multiple sources
for a 172.

Mitty wrote:
No argument. But the original poster's idea sounded like head-down
during an approach managing airspeed as it gets updated on a GPS -- that
idea still scares me.

And it should. Fly whatever airspeed you're used to flying
non-precision approaches at (90 KIAS is typical for common trainer
types). The instrument to watch for managing airspeed is the ASI. Your
groundspeed is whatever it works out to be.

(C Kingsbury) wrote:
The conservative choice would be to listen to the first box that cries
"miss." Let's say the wind shifts and your groundspeed changes, the
handheld says you're there. Might be good to miss it. If OTOH the
clock says you're there and the GPS doesn't, you're not going to
increase your risks by missing on that basis.

I don't know if I agree with that. Let's say your calculations are
wrong and the timer runs out when you're 1/2 mile from the MAP. Which
has less overall risk:

1) Continuing another 1/2 mile to the real geographic MAP based on your
GPS, finding the runway, and landing uneventfully.

2) Going missed, and being back in the clouds trying to decide if you
should try the approach again or divert.

#1 sure sounds safer to me.

Do you use the throttle to increase/decrease power to match the
ground speed to the approach speed table so the time is correct to the
MAP?

I find this concept astonishing. Surely very, very few CFII's are
teaching this? I have run into one or two.

You use the time appropriate to the ground speed you have, when flying
an appropriate indicated airspeed.

Newps wrote in message ...
john smith wrote:

For those of you using handheld GPS's when you fly IFR:

Do you use the throttle to increase/decrease power to match the ground
speed to the approach speed table so the time is correct to the MAP?

A buddy of mine does this. Drives me nuts. He'll sit and screw with
the throttle to get 90 knots ground speed.

*** Even with a brisk tailwind?

- Jerry Kaidor ( )

You could probably put a good used box in for a lot less, though
perhaps not linked to the A/P. 6K range for a used King/Apollo unit +
install/certification is the estimate I've seen from multiple sources
for a 172.

Yep. It's that install/linking/cert that runs the price up. Other than a
430/530 box the labor may well equal the price of the unit.

Howard


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