Approach Timing

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.

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.

Mitty wrote
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.

Don't bet on it. Study the rules.

I used to believe that just because you flew a VOR approach to well
within instrument PTS standards, using a VOR that had easily passed,
and would easily pass again, a VOR check, that I could be assured of
not slamming into an obstruction if I was still 200 ft above the MDA.
WRONG.

Michael

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.

Greg Esres wrote
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.

It will be more popular. Watch and see.

This method is used by the flight control software of the Airbus A-320
series of aircraft. I **** you not. Straight from the lips of an
A-320 captain. The autothrottles adjust to a given groundspeed on
approach. Groundspeed, not airspeed. No, I don't know why either -
but he insists that it's true.

Once more aiplanes adopt this approach, I am willing to bet it will
become the norm at the big flight schools - especially as IFR GPS
becomes standard. And once the big flight schools start teaching
it...

Michael

(Michael) wrote

This method is used by the flight control software of the Airbus A-320
series of aircraft. I **** you not. Straight from the lips of an
A-320 captain. The autothrottles adjust to a given groundspeed on
approach. Groundspeed, not airspeed. No, I don't know why either -
but he insists that it's true.

Well...I have a pretty good idea why without ever having been
in an Airbus. MicroBurst. On an ILS using a "normal" autopilot
and autothrottles, when approaching a microburst, the autothrottles
(or human pilot) will retard the throttles to maintain Glideslope
and IAS and then shortly after passing the microburst, one finds
himself with reduced power and rapidly reducing airspeed. Been
there--Done that!
By flying a groundspeed that is proportional to the strength of the
microburst on the front side of the microburst, one just might survive
the backside. Without the Groundspeed input, one just adds some
arbitrarily chosen number of knots to the IAS, wanting at the same
time to keep it as low a number as possible because you had to get
rid of any excess prior to touchdown.
In really bad conditions, indicated by a larger than normal difference
between IAS and GS, I always kept the GS up.

Bob Moore
ATP B-727 B-707
PanAm (retired)

Fly the same airspeed and final approach configuration all the time. Legally
you need to interpolate the table and estimate a time to MAP. Also
very handy should the GPS take a dump during final approach.

Realistically .. look at the GPS distance to the airport when you cross
the final approach fix. Subtract the distance to the MAP
and when the GPS says you're there .. go missed if you don't
have the runway.

Don't make it any harder than it needs to be. To me jockeying with
airspeed using a GPS to match a 90k ground speed is kind of silly
when the GPS will tell you right when you're at MAP.




"john smith" wrote in message
...
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?

Or,

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

john smith wrote in message .. .
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?

Or,

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

Fly the same airspeed as always, measure the groundspeed from the GPS,
and interpolate the timing table for your ground speed.

One thing to remember is that the MAP does not have to be over the
airport. It may be a few miles before the runway, or past the runway.
This is a concern even when using an approach certified GPS because
the MAP may not be in the database. An example would be a MAP that is
a DME fix off a localizer. Most GPS databases do not contain the
lat/lon for localizer stations.

Andrew Sarangan wrote:

Most GPS databases do not contain the
lat/lon for localizer stations.

Not sure about most. Many do.

"Andrew Sarangan" wrote in message
om...

Fly the same airspeed as always, measure the groundspeed from the GPS,
and interpolate the timing table for your ground speed.


If you trust the GPS to provide accurate groundspeed why not trust it to
determine the MAP?