Creating RWY Go


Today I want to discuss some things that happen behind the scenes when creating a new app, and in this case, RWY Go.

When Apple announced the Watch, we did some brainstorming about what we would love to see on it, and came up with some screenshots:



Yes, very lame I know (particularly the old school watch!). We had no idea what the resolution or features the SDK would support, so this was all just guess work. It turned out the SDK was fairly restrictive so it actually led the design in a different direction, which I think turned out to be a good thing.

Goal and design philosophy

We wanted something useful to use on iPhone and Watch to give useful, time critical information normally scattered across ERSA and approach plates, with zero or minimal user interaction in a divert or emergency. Here’s what we came up with:

  1. Where to go – What is the most intuitive way to represent the distance & bearing in a glance.
  2. Arrival Plan – Runway orientation, elevation, windsock and runway numbers. The entire diagram should rotate for orientation. Layered based on priority.
  3. Frequencies – Important ones starting with CTAF/TWR, then ATIS and navaids. These are ordered based on usefulness.
  4. Met & Notams – Low priority, so accessible via a swipe.
  5. Searchable – One tap shows a list of airfields ordered by distance. A runway diagram makes selection easier.
  6. Contrast – The colour scheme needed to work in both broad daylight and in a darkened night cockpit.

Class Design

As soon as Apple released the watch and SDK to developers, we had a look through to see what it could do. We very quickly discovered that the entire binary was run on the iPhone, communicating wirelessly to the watch to update the interface. The interface needed to have a static design layout (created at compile time) and the iPhone sends some basic low-bandwidth numbers to update it. It could do some basic images and animations. We also needed to create a ‘container’ iPhone app, which we hadn’t planned on doing. As it turns out, I think the iPhone app for RWY Go ended up being the best bit.

So the basic concept is that the iPhone app launches and gets a location, then chooses the best airport nearby. It procedurally draws some images of the airport and saves it into a shared container both the watch binary (also running on the iPhone) can use. It also fetches the Met & Notams and saves those for display.

Here’s the first draft I used as the overall structure for the app (how all the bits and pieces connect together):


The implementation was a lot more complex as you work through issues & test the limits of what the SDK can do for you. This was my first project using Apple auto-layout and Swift which was awesome for this project. Here’s the auto-layout for the iPhone app in XCode:


Essentially, all of the elements on screen are built up with relationships to each other. It requires tons of testing and tweaking on all of the different devices in portrait & landscape, but once done, works perfectly in native resolution in any interface orientation.


The Watch interfaces were created in a similar fashion by dragging & dropping elements onto a screen. With some nested grouping of items, it’s possible to achieve a nice interface on both watch sizes. We didn’t actually have any devices to test on, so it was all done in the Simulator. Apple only allowed 1 day in Sydney playing with the watch prior to public release to check it worked.


The Watch doesn’t do animations the same way iPhone/iPad does (e.g. “Rotate this image by π/2”). You need to specify 360 individual images and have them flash up quickly. Yes – you need to create 360 images in Photoshop. Then another 360 every time you change the design!

compass_220-0@2x compass_220-1@2x compass_220-2@2x compass_220-3@2x compass_220-4@2x compass_220-5@2x compass_220-6@2x compass_220-7@2x compass_220-8@2x compass_220-9@2xcompass_220-10@2x compass_220-11@2x


The first lot of artwork we used was yellow, then a light blue theme, however we settled on a darker blue theme which worked better for iPhone at night in a dark cockpit. The centreline markings were also removed and as it looked too busy and cluttered. We wanted an interface that could, in a snapshot, be used to give you the orientation and direction of the runways without your eyes getting lost in the picture.

First version – Runways drawing!


Second version – circle concept
Runways don’t fit well yet


Here’s some other designs – some of these look really awesome and it was a difficult choice. At first the ones with high contrast circles look the best, however you’ll quickly realise that the large circle is redundant information and drowns out the actual data. The dark blue circle works better at night and allows the white arrow + text to stand out better.


And some different ideas for runway drawings and frequency design. It turned out these don’t really fit well on the watch and some other technical limitations made them impossible to do well. We went with green/brown lines for grass/dirt runways as it’s more intuitive.

This looks modern but which is the grass runway?

IMG_3487 IMG_3490


RWY Go Watch
Final Design.



Database creation

We had an extensive database of runway thresholds from Airservices Australia and other sources, however it didn’t cover a lot of the smaller airstrips or international airports. So we created our own data set.

Using Google Earth, we plotted the position of every single airfield we knew about, then went about mapping the thresholds and runway type (grass, dirt, bitumen) for each airport. The current count is 28,164 runways across 23,562 airfields around the world. Painstaking work! We managed to squeeze all of this data, plus navaid / frequency information into a 6.1 mb database which is updated every 28 days.

Drawing airfields

Probably the most nerd satisfying part of creating this app was procedurally drawing the airfields from a bunch of lat/lon pairs for runways. The easy bit is creating a class that converts lat/lon pairs to an x,y pixel coordinate based on the maximum and minimum latitude and longitude pairs found for that airport. Here’s a snippet with some corrections to squeeze the runways inside a circle:

        //Center any axis that would otherwise be at the top or on the left (so the image is centered)
        if new_xScale < xScale {
            let scaleDifference = 1.0 - (new_xScale / xScale) //(e.g. compressed 50% would be 0.5)
            xOffset += (scaleDifference / 2.0) * Double(clippedRect.size.width)
        } else if new_yScale < yScale {
            let scaleDifference = 1.0 - (new_yScale / yScale)
            yOffset += (scaleDifference / 2.0) * Double(clippedRect.size.height)
        xScale = new_xScale
        yScale = new_yScale
        //Correct so any diagonal runways outside of the circle are put back inside for rotations.
        // iPhone & iPad only (not Watch):
        if fitInsideCircle {
            let center = CGPoint(x: rect.width * 0.5, y: rect.height * 0.5)
            var longestThresholdFromCenter = 0.0
            for runway in apt.runways {
                let p1 = latLonToPoint(runway.threshold1.position)
                let p2 = latLonToPoint(runway.threshold2.position)
                let p1Dist = (p1.x - center.x)*(p1.x - center.x) + (p1.y - center.y)*(p1.y - center.y)
                let p2Dist = (p2.x - center.x)*(p2.x - center.x) + (p2.y - center.y)*(p2.y - center.y)
                longestThresholdFromCenter = max(longestThresholdFromCenter, sqrt(Double(p1Dist)))
                longestThresholdFromCenter = max(longestThresholdFromCenter, sqrt(Double(p2Dist)))
            let desiredRadius = sqrt(Double(center.x * center.x)) - Double(rect.width*0.16) //assume square image & reduce by a further 16%
            if longestThresholdFromCenter > desiredRadius {
                let overhang = longestThresholdFromCenter - desiredRadius
                let reduction = sqrt(overhang*overhang*0.5)*2.0
                xOffset += reduction * 0.5
                yOffset += reduction * 0.5
                xScale *= (1.0 - reduction/Double(rect.size.width))
                yScale *= (1.0 - reduction/Double(rect.size.height))

The runways are then drawn on as a simple drawing call (“draw a box from (a,b) to (c,d) width 4.0”). The entire thing is saved as a small image and cached. It also draws a diagram for the search results (with no labels, thinner lines) and the Watch, which can be slightly larger as the image doesn’t need to rotate.

The tricky part is positioning the labels as subviews to the runway diagram so they rotate the opposite direction to the runway diagram and don’t overlap. For example, in the diagram below, RWY 04 at EHAM (Amsterdam Int’l) is shifted to not overlap with the other numbers. It took a couple of hours of tweaking drop shadows, font size and colours of the labels to have them readable with a high contrast over the top of the runways. Bonus points for the reader if you can figure out how to dynamically place a windsock on a runway diagram (choose an x,y position), in a good looking spot, that doesn’t overlap with any of the runways or labels – the only data is N pairs of runway thresholds (a,b), (c,d) where N >= 1.

Runway1 Runway2List

The windsock appears after it has downloaded the TTF, TAF and METAR for the airport. It uses a regular expressions to search through and find text like 23025KT or 18030G42KT prioritised by METAR, SPECI, TTF then TAF. The result should be a windsock that is very close to the wind right now.

Hypoxia Alerts

Soon after releasing RWY Go, we looked at the altimeter on the iPhone6 and wondered how we could use it. Because it will be measuring cockpit static pressure which has loads of errors, it’s fairly useless as an altimeter. The best use you can use for cabin altitude is to use it for cabin altitude! For those that don’t fly above 10,000 ft on oxygen, hypoxia is a big deal. The time of useful conscious decreases rapidly from about 30 minutes at 18,000 ft to 3-5 minutes at 25,000 ft. And the clock starts ticking above 10,000 ft so if you have had a gradual ascent (perhaps loss of cabin pressure), your time at 25,000 may only be a minute or two. What RWY Go does is start a timer above 10,000ft that ticks down at a certain rate to simulate your O₂ saturation. The higher you go, the faster it ticks down. When it gets to a very conservative threshold (about 50% of the published times), it will show a message to check oxygen. It goes without saying this isn’t a certified system and shouldn’t be relied upon in place of proper equipment and procedures. It’s an extra help that may come in useful one day, or to provide education with how long it can take to become hypoxic. Hypoxia alerts can be disabled in iPhone Settings.



Well I hope that gives some insight into what goes on behind the scenes. This has probably been the most enjoyable small project I’ve ever worked on. Swift is a really awesome language to program in, plus working with our artist and brainstorming design paradigms is great fun. Since writing the app, Apple have released Watch OS2 which allows native binaries to run on the watch. I’ve updated the iPhone/iPad code already, but now need to do a massive rewrite of the Watch code to support this change. It should result in the watch app loading and displaying content much quicker than wireless transfers do at the moment.

RWY Go 3_small


Author: Rowan Willson

Founder of OzRunways; nerd; pilot etc.