GPS Nearest Airport feature

They all do. However, it is based on distance and runway length, not
"steepest glide". I'm not sure why you would want the steepest glide.
Don't you want to use your best glide speed and go to the nearest
airport? I guess the steepest glide would be straight down.

-Robert, Flight Instructor

I think he means if he's flying over a plateau, with an airport on top
of the plateau say 5 miles away, but over the edge of the plateau,
there's one 7 miles away, and 4000 feet lower in elevation since it's
down in the valley or something. If you can make the cliff edge, you've
got 4000 more to drop.

Robert M. Gary wrote:

They all do. However, it is based on distance and runway length, not
"steepest glide". I'm not sure why you would want the steepest glide.
Don't you want to use your best glide speed and go to the nearest
airport? I guess the steepest glide would be straight down.


I guess I wasn't very clear in my first post. I know they all have the
nearest airport feature and give runway length. Lets say that I am
flying along at 14,000ft and the engine starts doing bad things. Since
I was stupid and wasn't paying attention, I ask my GPS where the nearest
airport is. It tells me that there is an airport 5 knots to my left and
10 knots to my right. What it didn't tell me is the runway to the left
was at an elevation of 10,000ft and the runway to the right was at an
elevation of 5,000ft. Lets make the math simple and pretend there are
6000ft in a nautical mile. To make the airport on the left you would
have to maintain a glide ratio of 7.5 to 1. To make the airport on the
right you would need to maintain a glide ratio of about 6.7 to.
Obviously the runway to the right gives you more room to maneuver. This
is of course assuming there are no obstructions in your path. As
another poster pointed out, if you are flying over the rockies you
better always have a plan on where you are going to go if your loose
power, you are just asking for something bad to happen.

Even from a single simulated flight over Colorado, it's pretty clear to
me, careful planing should go into any such flight in real life.

--
Chris W

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Stefan

In article pu6Vd.84$Ci7.63@lakeread07, Chris W wrote:
Robert M. Gary wrote:

They all do. However, it is based on distance and runway length, not
"steepest glide". I'm not sure why you would want the steepest glide.
Don't you want to use your best glide speed and go to the nearest
airport? I guess the steepest glide would be straight down.


I guess I wasn't very clear in my first post. I know they all have the
nearest airport feature and give runway length. Lets say that I am
flying along at 14,000ft and the engine starts doing bad things. Since
I was stupid and wasn't paying attention, I ask my GPS where the nearest
airport is. It tells me that there is an airport 5 knots to my left and
10 knots to my right. What it didn't tell me is the runway to the left
was at an elevation of 10,000ft and the runway to the right was at an
elevation of 5,000ft. Lets make the math simple and pretend there are
6000ft in a nautical mile. To make the airport on the left you would
have to maintain a glide ratio of 7.5 to 1. To make the airport on the
right you would need to maintain a glide ratio of about 6.7 to.

One thing the GPS can't tell you is the wind speed. This could make a huge
difference whethe you would make an airport or not.
tom

Hopefully it wouldn't pick an airport above you!

"Robert M. Gary" wrote:

Hopefully it wouldn't pick an airport above you!

The "nearest airport" feature of GPS and LORAN units will happily list airports
above you. They have no reason to exclude them. They don't know the rubber band
just broke.

What you want is a "gimme all the airports I can glide to" feature; not "nearest
airport".

George Patterson
I prefer Heaven for climate but Hell for company.

Chris W wrote:

It tells me that there is an airport 5 knots to my left and
10 knots to my right. What it didn't tell me is the runway to the left
was at an elevation of 10,000ft and the runway to the right was at an
elevation of 5,000ft.

You sure? My old LORAN used to provide field elevations, radio frequencies, and
other data for the airports listed in the "nearest" feature. I'd be surprised to
learn aviation GPS units don't do as well.

My LORAN didn't provide the glide ratio data you want, though.

George Patterson
I prefer Heaven for climate but Hell for company.

"Chris W" wrote in message
newsu6Vd.84$Ci7.63@lakeread07...
I guess I wasn't very clear in my first post.

Nope.

I know they all have the nearest airport feature and give runway length.
Lets say that I am flying along at 14,000ft and the engine starts doing
bad things. Since I was stupid and wasn't paying attention, I ask my GPS
where the nearest airport is. It tells me that there is an airport 5
knots to my left and 10 knots to my right.

You mean "nautical miles"? "Knots" is a measure of speed, not distance.

What it didn't tell me is the runway to the left was at an elevation of
10,000ft and the runway to the right was at an elevation of 5,000ft.

There aren't many airports around at 10,000'. And in mountainous terrain,
airports are usually (though not always, granted) found in the valley
floors. Terrain between you and a lower-altitude airport is probably more
likely to be a problem than a significant difference in altitude between two
choices.

Lets make the math simple and pretend there are 6000ft in a nautical mile.
To make the airport on the left you would have to maintain a glide ratio
of 7.5 to 1. To make the airport on the right you would need to maintain
a glide ratio of about 6.7 to.

So you actually want the least steep descent, not the steepest descent.

Obviously the runway to the right gives you more room to maneuver. This
is of course assuming there are no obstructions in your path. As another
poster pointed out, if you are flying over the rockies you better always
have a plan on where you are going to go if your loose power, you are just
asking for something bad to happen.

Now, you've written "loose" twice. It's "lose". I only "loose power"
during takeoffs and climbs.

Even from a single simulated flight over Colorado, it's pretty clear to
me, careful planing should go into any such flight in real life.

Indeed. That said, as I mentioned, there's at least one product out there
that displays 3D volumes (cones) to aid you in the exact decision making
you're asking about. Since your position relative to the cone depends not
only on your lateral distance, but your altitude above the airport as well,
it gives you the information you're thinking of. (Actually, while the data
is 3D, I believe it simply shows you a 2D cross-section of the cone on your
top-down map display)

That particular feature is not common though (only on one GPS navigation
unit that I'm aware of). I'm honestly not convinced it's really all that
necessary or useful except in very rare instances.

Pete

Steepest ... An airport right under you would be about the steepest. If
I could choose between an airport 1000 feet below me and 5 miles away,
and another 7000 below me and also 5 miles away, the lower airport has
the steeper descent.