VarioVoice

A flight instrument for paragliding and hang gliding that you listen to instead of looking at.

It runs on an iPhone that stays in a pocket with the screen off, and it talks into whatever earphones the pilot likes to fly with. There is nothing to read in the air and nothing to set on the ground: the altitude calibrates itself on launch, so there is no QNH to enter.

It is being built and flown by one pilot. There is no release yet. The sections below describe the instrument it is becoming; most of the flight logic already runs on recorded flights, and the phone already records, sounds the vario and sends its position.

What it says

First of all it sounds a variometer, and in flight it has turned out more sensitive than the dedicated vario its author carries.

While you are circling, it tells you every so often how much height you have gained or lost over the last minute. That is the number a pilot works out in their head from any instrument, to decide whether the thermal is worth staying in.

When you leave a thermal, it says the course you have set off on and the wind relative to it, as a headwind and a crosswind from one side.

On a long glide it says the altitude each time it has changed by fifty metres, so a flat glide stays quiet and a long sink is always announced.

And it answers a question. Two knocks on the phone, through the trouser or the harness, ask for altitude, course, ground speed and wind there and then.

How it gets there

The variometer combines two sensors that complete each other. The barometer measures height to the centimetre and takes a few seconds to say so. The accelerometer feels the wing start to climb at the very moment it happens. A filter fuses the two, the accelerometer giving the signal and the barometer confirming it and keeping it anchored, and the beep comes a third of a second after the air, in time to tighten the turn onto the core.

The wind comes from four independent estimates: the drift of your circles in a thermal, the drift of the thermal itself, the difference between where the glider points and where it actually goes, and the ground speed on different headings. Each one does best in different conditions and reports how sure it is. The app weighs them by that confidence and sets aside the one that disagrees with the rest, so the wind is available for most of the flight, including the stretches where you are not turning.

The altitude calibrates itself on launch and corrects itself in flight for the temperature of the air, which it works out by comparing the climb the barometer measures with the climb the GPS measures. In summer the air is warmer than the standard atmosphere every barometer assumes, and by the time you land the difference reaches twenty or thirty metres. With the correction, the error at the landing field on the test flights came down to a couple of metres.

Every rule was tuned by replaying dozens of recorded flights and then checked in real flight. During each flight the app records every sensor, the motion sensors at a hundred readings a second, so every second can be reconstructed afterwards at a desk. More than four hundred automated tests make sure a change breaks nothing that already works. Every flight is also kept as an IGC track.

Live tracking

Whoever is coming to collect you, waiting for you on the ground or looking for you can see where you are on the maps of the Open Glider Network, the open network that follows gliders, paragliders and hang gliders across Europe. The measured vertical speed and turn rate travel with each position, so the map also shows when you are climbing in a thermal.

Tracking comes on when you decide, and you choose what the map shows about you. Positions travel over the internet and anyone watching those maps sees them.

Later

The connection that sends your position can also receive the traffic around you. The plan is a spoken warning when something faster is approaching, a glider, a tug, a helicopter or a powered aircraft, saying where to look.

It speaks Italian today. Other languages will follow, with units of measure chosen separately, since a pilot may well want the climb in metres per second and the wind in kilometres an hour.

Then the Apple Watch. The flight logic has been written from the start so that it does not depend on how the device is oriented, which is what lets the same code run on a wrist.