Locating the runway

[cowman]

So hypothetical... possibly common scenario:

You are inbound for landing at an uncontrolled airport you've never been to before. You know what runway you want based on winds, the gps is telling you it's right in front of you 5 or 6 miles ahead. You know about where it should be in relation to nearby landmarks but your eyeballs just won't locate the runway so you can set up for a proper pattern entry and landing.

What's the ideal response to this?

I'm thinking climb to 500-1000' above pattern altitude and just overfly the airport until you see it, then fly out a ways adjusting course so you come in where you want to be and descend to pattern altitude and make a normal midfield downwind entry(announcing position/intention on the radio).

What if it's a class D field with an active tower? Ask the tower if you can do the same, ask for directions/vectors? What if it's a class C?

 

[Jim Logajan]

See if you can find a video of a landing at the airport of interest - it may provide some visual clues when you do it in real life.

I was surprised to find videos on Youtube and Vimeo that pilots have made of their landings at some airports I would thought were too low traffic to have anyone videotape their landings. This was the case when I decided to fly into 5S1 (George Felt, OR), 6S6 (Powers, OR), and 9S3 (Lakeside State, OR). These are all grass runways. Finding videos may require use of some mix of keywords like "landing", "airport", with the airport ID or name.

I am new at this, so spotting grass fields has not been easy - I expect to see them sooner than I do. The result is that I typically end up overflying them and maneuvering into the pattern as appropriate. Since grass fields may have critters on them it is probably a good idea to overfly even if you spot the airfield from a distance. An additional complication is when the airfield is in a valley such that TPA puts you below the valley rim or close enough to cause a bit of ground shyness. The only viable entries are straight into upwind or downwind legs.

 

[Jim Logajan]

IF it works. If it uses GPS to determine attitude, it won't include WCA. And if it's anything like Google Sky Map, it will place stuff 10 deg off, probably bad enough to land at the wrong airport.

I'm willing to spend $10 on an experiment so I just bought it for my iPhone 4s. Have only had a chance to use it in my office so far. Really is kinda neat.

It uses the electronic compass in the iPhone/iPad to determine orientation, GPS to determine location, and an internal database of airport locations. Then it uses all that info and the camera to paint whatever the camera sees onto the screen and overlays markers on it showing where the airports should be. I really can't tell while sitting in my office how close it gets, but so far seems about right. My home airport appears to be behind the bookshelf....

Oh - something I didn't realize, but the compass in the iPhone 4s seems to work just fine oriented in all three axis and right side up or upside down.

 

[Jim Logajan]

Actually, that's a sign of BS from the phone. A real compass has two directions in 3D space where the measurement is poorly defined. It's very much like the zone of confusion for a VOR, except it's not zenith/nadir.

You are correct when the sensor senses on only 2 axis. However, according to net sources, the iPhone 4 uses the 3 axis AKM AK8975 hall effect chip:

http://www.akm.com/akm/en/product/datasheet1/?partno=AK8975

 

[Jim Logajan]

That doesn't change basic geometric constraints that magnetic field lines are inherently one dimensional, and attitude is not determined orthogonal to that. There must be additional constraints. If they assume you're looking at the horizon, that's only going to work for distant airports.

The app appears to also use either the 3 axis accelerometers in the iPhone or assumes the field lines are parallel with the earth surface (unless he's taking into account the local dip) - because when I tilt the phone up or down, the airport markers shift down or up so they try to maintain their locations on the background image being displayed. There is some hysteresis and lag that seems to cause as much as +/- 15 degree movement against the background if I pan rapidly - much less than that if I pan slowly. However, it doesn't work at all correctly when the phone is tilted so the horizon isn't parallel with bottom of the screen. If he did a better job of using the accelerometers the program probably could be made to handle a tilted screen.

And, as a toy, we don't know how accurately it can find the field direction, or if it has any knowlege of local distortion, which can be substantial.

Not sure the point of calling it a toy just because the accuracy isn't known - given the option of not having it at all, which is perfectly reasonable option. Since many pilots already fly for the enjoyment anyway, the whole bloody plane and all the stuff in the typical flight bag are "toys". I spend about a $90 for an hour of flying - a one time purchase of a $10 app isn't that big of a deal to me. I don't know that I'll ever find it helpful, but I do think it is neat and clever.

What it does mean is that it can sense the dip angle, and should therefore not show the usual mag compass errors.

It is true that Hall sensors will not exhibit any of the problems of a mechanical wet compass. They require power, but will have a great advantage during turns and turbulent conditions.

 

[Jim Logajan]

Heck, there's a local airport near here that I never did spot from the air until I saw it from the ground. Airport environment? No, just a cut in the trees.

Speaking of a cut in the trees, can you spot where the airport is in this photo attachment?