Useful GPS trick

Guest Discussion
1

Today my instructor and I flew from San Jose to Pine Mountain Lake, in the Sierra foothils. Pine Mountain is about 3000 feet, nestled in among mountains.

Runways are 9/28, with 28 being the usual. Inbound we heard no traffic on the CTAF. Overflying the airport we saw only a limp wind sock.

So we tried runway 28. To my dismay, I came in too high and did a go-around. So next time I set up on a longer final. All looked good . . . but once again, too high, and a go-around.

On our third pass we looked at the GPS during the downwind leg — and it told the real story. The wind at pattern altitude was 10 knots . . . IN THE WRONG DIRECTION! That is, our groundspeed on the downwind leg was 10 knots slower than the KIAS. So downwind was not downwind at all. We were trying to land with a 10-knot tailwind, on a downhill runway no less.

I was flying a new 172SP (not a bad plane, by the way). The problem would have been worse in the aerodynamically dreamy SR20/22.

Bottom line: GPS is really useful for determining wind speed in the pattern! It can save your butt in the mountains, with typically tricky winds that can lead one to fly a downwind final.

2

Er, make that 9/27, not 9/28.

Today my instructor and I flew from San Jose to Pine Mountain Lake, in the Sierra foothils. Pine Mountain is about 3000 feet, nestled in among mountains.

Runways are 9/28, with 28 being the usual. Inbound we heard no traffic on the CTAF. Overflying the airport we saw only a limp wind sock.

So we tried runway 28. To my dismay, I came in too high and did a go-around. So next time I set up on a longer final. All looked good . . . but once again, too high, and a go-around.

On our third pass we looked at the GPS during the downwind leg — and it told the real story. The wind at pattern altitude was 10 knots . . . IN THE WRONG DIRECTION! That is, our groundspeed on the downwind leg was 10 knots slower than the KIAS. So downwind was not downwind at all. We were trying to land with a 10-knot tailwind, on a downhill runway no less.

I was flying a new 172SP (not a bad plane, by the way). The problem would have been worse in the aerodynamically dreamy SR20/22.

Bottom line: GPS is really useful for determining wind speed in the pattern! It can save your butt in the mountains, with typically tricky winds that can lead one to fly a downwind final.

3

I like that trick, thanks.

Here is another one which will be perfectly obvious to some fraction of existing Cirrus / Garmin 430 owners. But to anyone for whom it’s not already obvious, it’s worth pointing out. (Dave Katz has alluded to it before).

Suppose you’re coming into an unfamiliar airport, or one with a confusing runway alignment. Concord, CCR, is in the latter category. Its runways are in an X-shape, with 14-32 crossing 1-19, and from listening to the tower it sounds as if it’s common for unfamiliar pilots to line up with the wrong one .

The Garmin usually plots the direct course to your next waypoint. BUT it can also plot a line representing any given radial heading to or from a waypoint. To do this:

* You make sure the waypoint you're interested in is the "active" one on the flight plan, by hitting the Direct button.

* You push the OBS button on the Garmin, so that it's plotting radial headings to/from the waypoint, rather than the direct course to it.

 * You dial in the heading you're looking for, using the HSI or DG display connected to the #1 Garmin. For instance, I use the yellow NAV arrow on my HSI.

If you do this with runway heading – for instance, 320 if you’re heading to runway 32 at Concord – you end up with a great big line across the center of the Arnav and Garmin displays, showing the extended centerline of the runway in question. Makes it a piece of cake to know which is the right runway, when you’ve crossed the center line, whether your base leg is at right angles, and so on. Tremendous plus for “situational awareness” at unfamiliar sites.

(You do the same procedure to have a graphic aid for flying certain radials to or from a VOR.)

Maybe all Cirrus owners already know this. But I didn’t know it for the first few weeks I had the plane, and I really wish I had.

4

Bottom line: GPS is really useful for determining wind speed in the pattern! It can save your butt in the mountains, with typically tricky winds that can lead one to fly a downwind final.

Good tip, Rich. Another similar & interesting application: anticipating wind shear. Case in point: last week on a long straight-in approach to rwy 14 at Santa Rosa I noticed that GS was 15-20 kt FASTER than IAS. This differential was maintained, to my increasing concern, down to 100-150 ft agl. From even 1 mile final and ~500 ft agl I could see the windsock showing a slight headwind at ground level.

Because of this I was ready for something “interesting” to happen on very short final. Sure enough just below 100 ft agl I encountered light-to-moderate turbulence and my 60 kt IAS (260se, not SR20!) and 77 kt GS suddenly became 80 kt IAS with no descent rate at all! I finally landed nearly halfway down the mile long runway. Several trailing aircraft including a King Air also reported similar funny business and had to land long.

I also have come to use the 430 as my primary course reference and use the heading indicator mostly for an indication of whether there is a significant x-wind component or not. At least for me, it’s much easier to fly an instrument approach with that digital track reference in my field of view than to use the less precise analog heading indicator and CDI needle. Of course, the latter are properly tuned and on display if I need them or if I have to be PC for a CFII…

5

Today my instructor and I flew from San Jose to Pine Mountain Lake, in the Sierra foothils. Pine Mountain is about 3000 feet, nestled in among mountains.

Runways are 9/28, with 28 being the usual. Inbound we heard no traffic on the CTAF. Overflying the airport we saw only a limp wind sock.

So we tried runway 28. To my dismay, I came in too high and did a go-around. So next time I set up on a longer final. All looked good . . . but once again, too high, and a go-around.

On our third pass we looked at the GPS during the downwind leg — and it told the real story. The wind at pattern altitude was 10 knots . . . IN THE WRONG DIRECTION! That is, our groundspeed on the downwind leg was 10 knots slower than the KIAS. So downwind was not downwind at all. We were trying to land with a 10-knot tailwind, on a downhill runway no less.

I was flying a new 172SP (not a bad plane, by the way). The problem would have been worse in the aerodynamically dreamy SR20/22.

Bottom line: GPS is really useful for determining wind speed in the pattern! It can save your butt in the mountains, with typically tricky winds that can lead one to fly a downwind final.

I have found on any approach a headheld or panel GPS is a great tool to give your groundspeed during final. Even more useful than downwind because the lower altitude on final frequently results in different wind conditions. On the other hand, the old style method is looking for the windsock. It should tell you which runway has the headwind. But the GPS is a nice feature when no windsock exists.

That looks like a VERY STRANGE AIRPORT! Are the runways REALLY 9/28? Is there a 10 degree bend in the runway somehwhere? In many years of flying I have never come across a runway whose ends are not 180 degrees apart. What gives?

Brian

6

If Cirrus does start offering the Shadin ADC-200 air data computer as rumored you’ll have a direct readout on the Garmin of actual real-time wind direction and speed.

Bottom line: GPS is really useful for determining wind speed in the pattern! It can save your butt in the mountains, with typically tricky winds that can lead one to fly a downwind final.

7

I did not know this. Like they say, you learn something new everyday.

Thanks Jim!

Larry, N171CD

I like that trick, thanks.

Here is another one which will be perfectly obvious to some fraction of existing Cirrus / Garmin 430 owners. But to anyone for whom it’s not already obvious, it’s worth pointing out. (Dave Katz has alluded to it before).

Suppose you’re coming into an unfamiliar airport, or one with a confusing runway alignment. Concord, CCR, is in the latter category. Its runways are in an X-shape, with 14-32 crossing 1-19, and from listening to the tower it sounds as if it’s common for unfamiliar pilots to line up with the wrong one .

The Garmin usually plots the direct course to your next waypoint. BUT it can also plot a line representing any given radial heading to or from a waypoint. To do this:

  • You make sure the waypoint you’re interested in is the “active” one on the flight plan, by hitting the Direct button.
  • You push the OBS button on the Garmin, so that it’s plotting radial headings to/from the waypoint, rather than the direct course to it.
  • You dial in the heading you’re looking for, using the HSI or DG display connected to the #1 Garmin. For instance, I use the yellow NAV arrow on my HSI.

If you do this with runway heading – for instance, 320 if you’re heading to runway 32 at Concord – you end up with a great big line across the center of the Arnav and Garmin displays, showing the extended centerline of the runway in question. Makes it a piece of cake to know which is the right runway, when you’ve crossed the center line, whether your base leg is at right angles, and so on. Tremendous plus for “situational awareness” at unfamiliar sites.

(You do the same procedure to have a graphic aid for flying certain radials to or from a VOR.)

Maybe all Cirrus owners already know this. But I didn’t know it for the first few weeks I had the plane, and I really wish I had.

8

BUT it can also plot a line representing any given radial heading to or from a waypoint.

Just to underscore this point – I find this great when I get “Turn right heading xxx to intercept radial yyy off SOMEWHERE VOR”; or the similar “… intercept V99”. In cases where the radial or airway was not already in the flight plan, the method you mention provides a quick and easy way to get from here to there. I got the hang of this some time back from a post by Clyde (thanks, Clyde); it’s proven very valuable.

I always glance at the GPS for my actual groundspeed while on an ILS. I would use it on a Localizer approach, too, when I really need it; just haven’t had any since I got my SR20.

The idea of determining wind in the pattern hadn’t occurred to me – thanks, Rich. I guess we all understand that the wind in the pattern may be different than the wind lower down, but I can’t imagine that it would be 180 degrees off very often.

  • Mike.
9

Today my instructor and I flew from San Jose to Pine Mountain Lake, in the Sierra foothils. Pine Mountain is about 3000 feet, nestled in among mountains.

Runways are 9/28, with 28 being the usual. Inbound we heard no traffic on the CTAF. Overflying the airport we saw only a limp wind sock.

So we tried runway 28. To my dismay, I came in too high and did a go-around. So next time I set up on a longer final. All looked good . . . but once again, too high, and a go-around.

On our third pass we looked at the GPS during the downwind leg — and it told the real story. The wind at pattern altitude was 10 knots . . . IN THE WRONG DIRECTION! That is, our groundspeed on the downwind leg was 10 knots slower than the KIAS. So downwind was not downwind at all. We were trying to land with a 10-knot tailwind, on a downhill runway no less.

I was flying a new 172SP (not a bad plane, by the way). The problem would have been worse in the aerodynamically dreamy SR20/22.

Bottom line: GPS is really useful for determining wind speed in the pattern! It can save your butt in the mountains, with typically tricky winds that can lead one to fly a downwind final.

I have found on any approach a headheld or panel GPS is a great tool to give your groundspeed during final. Even more useful than downwind because the lower altitude on final frequently results in different wind conditions. On the other hand, the old style method is looking for the windsock. It should tell you which runway has the headwind. But the GPS is a nice feature when no windsock exists.

That looks like a VERY STRANGE AIRPORT! Are the runways REALLY 9/28? Is there a 10 degree bend in the runway somehwhere? In many years of flying I have never come across a runway whose ends are not 180 degrees apart. What gives?

Brian

It is an unusual airport but not for that reason–see Rich’s post below correcting to 9/27. The surrounding terrain can make for some interesting wind conditions as Rich experienced. Moreover it’s virtually a black hole at night–only the rwy lights in the immediate vicinity–I would be very wary of going in there at night especially if there’s no moon and/or the sky is obscured by high clouds.

Nonetheless it’s a good fly-in lunch destination, the Corsair Coffee Shop offers up some fine fare.

10

That looks like a VERY STRANGE AIRPORT! Are the runways REALLY 9/28? Is there a 10 degree bend in the runway somehwhere? In many years of flying I have never come across a runway whose ends are not 180 degrees apart. What gives?

Brian

Well, actually, Brian, it seems you have never flown in Australia, where it would appear “Rich K” does his flying.

You see, they make the runways purposely that way to help the aborigines hunt on slow flying daze. The 10 degree variation serves perfectly as a boomerang that the aborigines are allowed to use between flights to hunt and kill kangaroo and other delectable goodies in the outback.

Of course, since a boomerang always returns to its starting point, they always have the runway available on a moment’s notice should an aircraft arrive for takeoff or landing. Quite extraordinary, actually.

Those Aussies are truly a marvelous breed; sorta like Americans were during their once imaginative and resourceful years before political power centralized in Washington.

Cheers!

Pete