Approach Timing

Howard Nelson wrote:




Legally your IFR approved clock for timed approaches and NDB for NDB
approaches are what you WILL use.

Legally I will use the IFR approved GPS and the little mileage number on
the plate. Screw the timing.

"Newps" wrote in message
...


Howard Nelson wrote:




Legally your IFR approved clock for timed approaches and NDB for NDB
approaches are what you WILL use.

Legally I will use the IFR approved GPS and the little mileage number on
the plate. Screw the timing.

Here I thought we were discussing portable GPS's. My bad.

Howard



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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?

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

Roy Smith wrote in message ...
(C Kingsbury) wrote:
The conservative choice would be to listen to the first box that cries
"miss." snip


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.

You're begging the question: "If I listen to the GPS and land safely,
isn't it safer to listen to the GPS and not head back up into the
clouds?" Of course it is, but you don't know in advance that following
the GPS will lead you to a safe landing. Check out this approach:

http://204.108.4.16/d-tpp/0409/00654VG23.PDF (VOR-23 @ LWM on Airnav)

Stright-in this will bring you down into a real minefield of stacks
and towers. Now let's say on the way down you plan to dial the VOR
into the GPS to get a DME reading to use as the MAP. But in the heat
of the moment you put LWM the airport instead of LWM the VOR in. This
means you'll wait until you're past the airport to miss. You're
probably OK so long as you don't go down any further, but you've
unquestionably increased your risk.

Or perhaps you're used to an approach at your home field where the DME
counts down, instead of up. You get distracted and see 3DME here, and
think, OK, I have 1.5 to go. So you putter on until you're 5 miles
away. Right about where that 606' obstacle is. Downdraft anyone?

My situation is purely hypothetical, but not at all unrealistic. I've
made every one of these mistakes in isolation. Even if I used the GPS
as a primary means of determining MAP, I would back it up with the
timer, which would quickly catch the gross errors described above.

Second, your response assumes that diverting to the alternate actually
increases risk. This is a variable situation. Where I fly in the
Northeast, you usually don't have to go more than 20-30 miles to find
an airport serviced by an ILS, which is usually what you'll put in as
an alternate if you're headed to a field with no precision approaches.
If your alternate is an asphalt patch with an NDB on the field, well,
then maybe you're better off trying to limbo your way in.

Best,
-cwk.

In article ,
(C Kingsbury) wrote:

You're begging the question: "If I listen to the GPS and land safely,
isn't it safer to listen to the GPS and not head back up into the
clouds?" Of course it is, but you don't know in advance that following
the GPS will lead you to a safe landing. Check out this approach:

http://204.108.4.16/d-tpp/0409/00654VG23.PDF (VOR-23 @ LWM on Airnav)

Stright-in this will bring you down into a real minefield of stacks
and towers.

All of which are below the MDA. Which is a good reason not to descend
below the MDA unless you have the runway in sight.

Now let's say on the way down you plan to dial the VOR
into the GPS to get a DME reading to use as the MAP. But in the heat
of the moment you put LWM the airport instead of LWM the VOR in. This
means you'll wait until you're past the airport to miss. You're
probably OK so long as you don't go down any further, but you've
unquestionably increased your risk.

You can always screw up. That's why you brief approach procedures and
double-check your setup. What if you calculated the FAF-MAP time as
3:10, but put 4:10 into the clock instead?

I'm still going to believe a handheld GPS is more accurate than a DR
track. It may not be legal, but it's common sense.

Keep in mind that starting the missed too early can be as bad as turning
too late, it the procedure involves a turn predicated on you already
being past an obstacle.

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

Roy Smith wrote:
In article ,
(C Kingsbury) wrote:


You're begging the question: "If I listen to the GPS and land safely,
isn't it safer to listen to the GPS and not head back up into the
clouds?" Of course it is, but you don't know in advance that following
the GPS will lead you to a safe landing. Check out this approach:

http://204.108.4.16/d-tpp/0409/00654VG23.PDF (VOR-23 @ LWM on Airnav)

Stright-in this will bring you down into a real minefield of stacks
and towers.


All of which are below the MDA. Which is a good reason not to descend
below the MDA unless you have the runway in sight.


Now let's say on the way down you plan to dial the VOR
into the GPS to get a DME reading to use as the MAP. But in the heat
of the moment you put LWM the airport instead of LWM the VOR in. This
means you'll wait until you're past the airport to miss. You're
probably OK so long as you don't go down any further, but you've
unquestionably increased your risk.


You can always screw up. That's why you brief approach procedures and
double-check your setup. What if you calculated the FAF-MAP time as
3:10, but put 4:10 into the clock instead?

I'm still going to believe a handheld GPS is more accurate than a DR
track. It may not be legal, but it's common sense.

Keep in mind that starting the missed too early can be as bad as turning
too late, it the procedure involves a turn predicated on you already
being past an obstacle.

Maybe someone can tell us how a hand-held GPS behaves with lots of
moisture in the air? I've been doing a lot of geocaching recently and
know for sure that trees cause outages. Maybe aircraft-certified GPS
units get around this somehow or at least flag the unreliable situation.
But how do you fly if you can't trust the GPS?

"William W. Plummer" wrote:
Maybe someone can tell us how a hand-held GPS behaves with lots of
moisture in the air? I've been doing a lot of geocaching recently and
know for sure that trees cause outages. Maybe aircraft-certified GPS
units get around this somehow or at least flag the unreliable situation.
But how do you fly if you can't trust the GPS?

Trees block the frequencies that GPS uses. Rain and water vapor doesn't.

Presumably, if you're using the GPS for an instrument approach, you're
above treetop level, so the leaves shouldn't be blocking the signal :-)

It is certainly true that IFR certified GPS's detect and alert on
unreliable signals (that's what RAIM is all about). I still think a
handheld GPS is more accurate and reliable than a stopwatch.

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.