Every once in awhile over the last few years, I have wondered what would be a good way to determine how long a piece of dirt is from the air, that you want to land on.

I would imagine that if you do this enough, you probably develop an "eye" for this, but I haven't reached that level of developement yet.

So my best plan is to figure how many feet/sec. I am traveling at a certain speed, then time myself over the piece of dirt that I am interested in.

So, here's what I came up with:

kts/mph '/sec.
60/70 ~100
70/80 ~120
80/90 ~135

I Figure to get my speeds from the GPS.

So, there ya go

Have Merry Christmas Gary

Thanks for this. I know I'd seen the 70mph=100'. Nice to see a few other numers.

Good reminder it's groundspeed, not airspeed. It explains why my home airport is 2800 one way and only 1200 the other direction.

And, since what we're really concerned with is length of the "runway", you need to conduct runs BOTH directions to come up with a relatively accurate result. Timing a runway has always been a good way to get a ball park idea of the length.

Don't forget to measure actual useable length, not just the part with no trees....

MTV

I don't see anything wrong with doing it in both directions, but why "must" you do it in both directions. I would probably do it 2-3 times, whether in 1 direction, or both, wouldnt think it should be any different???, to get a solid average#. Using the gps should correct for outside variables, wouldn't it?? And just give you the true speed over the ground?

Gary

You could measure it from Google Earth's ruler tool if you want to be extra safe about it.

My situation is that I am out flying and find something I want to land, that I may not have noticed before. I think that timeing the distance is the easiest, quickest, simpleist way, atleast for me, sofar.

That being said,if there was a better way, I'd be happy to hear about it.

Gary

Shortfielder asked "why "must" you do it in both directions"

The short answer is because of wind. If there's no wind, no worries, one pass will do. Assuming precision, of course

Remember, with a wind down the runway, your ground speed will be decreased on upwind, and increased on downwind, thus evening out the differences. And the GPS measures groundspeed, not airspeed.

And, while a headwind on final does shorten up the landing distance some, it's still best to KNOW how long the landing surface is.

Google Earth is also a great tool, depending on what the image coverage clarity is. I've used it some to get a basic idea of a few places that I knew I was going before I launched. It doesn't help much when you are pioneering a spot, though. Unless you've got WiFi in your bus...

MTV

Why make two runs? Line up on the runway a little ways out, get yourself slowed down, check what airspeed you need to see 70mph on the gps, then pin that airspeed. I find it way easier to pin the airspeed on the ASI than to try to pin the groundspeed on the GPS. A stopwatch is way easier than trying to eyeball the second hand on your watch-- I carry a little digital IFR timer for timing airstrips, it also doubles as a digital clock. I hit the timer button as the approach end of the strip comes into view in front of the wing strut, then hit it again when the departure end comes into view. Not perfect, but if the small degree of error in my method makes a go/no-go difference, I'm probably cutting it too close landing there anyway.
Checking airspeed versus GPS groundspeed is helpful in determining wind direction if there's no windsock or anything else to reference.

Eric

shortfielder wrote: kts/mph '/sec.
60/70 ~100
70/80 ~120
80/90 ~135

Thanks for sharing that, shortfielder. I can't argue with your results, but I hate the lookup table. (#@& near vision). For me it's easier to multiply or divide in my head by really small numbers. So, I use the (my?) "one and two/thirds" rule instead.

So, for 60 kts,

60 plus two thirds of 60 = 60 + 40 = 100 fps


As shortfielder obviously knows, you can find the true answer by first multiplying knots by 6076 (ft per nautical mile) to get feet per hour.
Then, divide the feet per hour by 3600 to get feet per second.

So, you are really multiplying by the fraction 6076/3600, which is the same as multiplying by 1.687...
And since 1 2/3 is equal to 1.666... ,
it's a very good approximation, about 1.3% error.

Guess this fellow had an error in his arithmetic.

MTV

The reason I chose to use the GPS is because what ever speed it says I'm going, is the speed I am going over the ground. Isn't that correct? So, wind, temp, alt, etc, are already built into the #.

My gps is mounted in front of me on the panel, so it is as easy to see as anything else.

I guess others could modify this idea to whatever is easy for them. If they even want to use it at all.

Gary

Gary,

Uh, part of determining whether to land or not has to do with the TAKEOFF. So, if you build in that headwind for the landing, and decide to land, the question then remains:

A) Can I take off from this strip, with the existing wind? Remember that takeoff numbers are often different than landing numbers.

and

B) Can I take off from this strip if the wind dies suddenly? I have had this happen several times over the years. I want to know if I can safely land AND take off, before I put a tire on it.

Finally, in answer to your question, no, the GPS doesn't compensate for everything in this equation, but the biggest issue is what comes next: The takeoff.

MTV

Here's another way to judge the length of an airstrip using your gps track. Fly perpendicular to the downwind end, make a 180 and fly perpendicular to the opposite end. Then come around and fly the length of the airstrip. Now zoom in until you get a scale of 500' or whatever is appropriate to the length you are measuring. Compare the scale to the track log and estimate the length. Here's an example:



Looking at the track, it looks like a little less than 3 scale lengths of 500' each would fit in the loop made by my track. This produces a length just under 1500'. The Flying Tom is listed as 1400'.

This technique should be pretty accurate and as long as you can get the plane over the ends accurately, the wind and your speed won't effect the result. One place this technique would not work very well is down in a canyon. Most of the strips in canyons I've been to run parallel with the canyon and trying to fly perpendicular would be difficult.

tom

That's a good idea, Tom. One of the older hand-held eTrex GPS units let you measure distance via feet...so you can start and end the countdown just like a stopwatch. So, say you are flying parallel to the spot you want to land on...start the countdown on the GPS unit and then end it once you pass abeam the end of the spot. Do the newer GPS units let you do this? I know real estate agents use some kind of "hiker" GPS units to measure acreage.

TexasNick wrote:That's a good idea, Tom. One of the older hand-held eTrex GPS units let you measure distance via feet...so you can start and end the countdown just like a stopwatch. So, say you are flying parallel to the spot you want to land on...start the countdown on the GPS unit and then end it once you pass abeam the end of the spot. Do the newer GPS units let you do this? I know real estate agents use some kind of "hiker" GPS units to measure acreage.

Mine doesn't, but you can mark a waypoint at two spots and measure the distance between. It's too much messing around for me.

I've got an older aviation clock on the instrument panel, and it has a nice stop watch function with a single button to operate it. It works fine for measuring as outlined above. I'm not so good at holding a set speed while maneuvering so the gps is easier for me.

tom

Gary, just get an S7 with fat tires and you can just land most anywhere

Savannah-Tom wrote: Compare the scale to the track log and estimate the length.

The Garmin X96 series software has a handy "measure distance" option from the map page menu. Once in Measure Distance mode, you position the cursor, hit Enter, and then move the cursor anywhere on the map and it will display the relative distance.

Em
I like the way you are thinkin , and agree with you, but doesn't seem to be in te cards for me at this time.

Zane

I had heard of that, but seemed like I might have to pay too much time looking at my gps, and not out the window. Sofar, for me, it seems simpler to just just fly over an area of interest, time it, and do the math.

Gary

1SeventyZ wrote:

Savannah-Tom wrote: Compare the scale to the track log and estimate the length.

The Garmin X96 series software has a handy "measure distance" option from the map page menu. Once in Measure Distance mode, you position the cursor, hit Enter, and then move the cursor anywhere on the map and it will display the relative distance.

I had forgotten all about that feature. Thanks for the reminder. I just used it to measure the example above. The result is .3 mi. I don't see a way to change the units of measure. It seems to use feet for measurements below 500' and miles above that. 1/10th of a mile is approximately 500', so I guess that is what drives the change from feet to miles. Anyway besides having to do a conversion in your head, getting the length in miles will only give you 500' resolution.

tom

I don't see any way to enforce feet as a unit for distances over 500 ft. I don't think the software guys at Garmin had this in mind.

I wonder if the Ohio Bush Planes guys do this.