Flying VFR, avoiding a collision falls to the pilot: “nothing in this Regulation shall relieve the pilot-in-command of an aircraft from the responsibility of taking such action … as will best avert collision”. No French airspace separates one VFR flight from another. Control sees the aircraft carrying a transponder, which is mandatory in controlled airspace, and passes traffic information; but informing is not separating. And under a flight information service — a SIV in France, where most light aviation flies — carriage is optional.
That leaves the pilot’s eye, whose parameters are published: field of view, the sector to be scanned, how long a scan cycle takes, the time available to react. This page gathers them, then looks at what an ADS-L device adds.
Four different angles are used interchangeably when lookout is discussed. They measure different things.
| What is measured | Angle |
|---|---|
| Anatomy, head still | ≈ 190–200° wide, 120–135° high |
| What the cockpit leaves visible | a forward arc — the wing, the door and window posts, the nose, the panel and the visor take their share; aft, the airframe |
| The scan pilots are taught | 60° left + 60° right, 10° up and down — in blocks of 10 to 15°, about a second each, nine to twelve blocks |
| Where the eye identifies an object | ≈ 10–15° at a time — five degrees off the point of gaze, acuity is a quarter of central |
The division of labour is fixed: peripheral vision detects, the fovea identifies. A target is first caught in the periphery, then brought onto the fovea to be identified. Hence the taught technique: a series of short movements with a pause on each block, rather than one continuous movement — the eye only sees during the pauses, and stays effectively blind between them.
Nine to twelve blocks at a second each, plus the instrument panel, plus the one to two seconds the eye needs to refocus from the panel back to infinity: a complete cycle takes ten to fifteen seconds. Over that cycle the recommended sector covers about a third of the horizon around the aircraft, and none of what is behind it. Moving the head widens the sector, at the cost of the one just cleared.
Lookout is therefore characterised by a period, not by a coverage figure. Every number that follows is a consequence of that.
A light aircraft has a wingspan of around ten metres. Here is the space it takes up, held at arm’s length.
| Distance | Apparent size at arm’s length | Equivalent |
|---|---|---|
| 0.5 NM | 6.5 mm | a pea |
| 1 NM | 3.2 mm | a grain of rice |
| 2 NM | 1.6 mm | a pinhead |
| 3 NM | 1.1 mm | the dot of an “i” |
| 5 NM | 0.65 mm | a full stop |
At two or three miles the target is a grey mark of one to two millimetres, against a bright or hazy sky, in one block out of a dozen, for the one second the scan spends there. Detection is also not yet recognition: the BEA measures half a second to carry the image to the brain and two and a half seconds for the brain to identify it as an aircraft — three seconds in total.
For the UK CAA, “it takes a minimum of 10 seconds for a pilot to spot traffic, identify it, realise it is a collision risk, react, and have the aircraft respond.” The FAA breaks the same sequence into steps.
| Step | Seconds |
|---|---|
| See the object | 0.1 |
| Recognise an aircraft | 1.0 |
| Realise it is a collision course | 5.0 |
| Decide to turn left or right | 4.0 |
| Muscular reaction | 0.4 |
| Aircraft response | 2.0 |
| Total | 12.5 |
Two light aircraft head-on at a hundred knots each are still one nautical mile apart eighteen seconds before they meet, and two miles apart thirty-six seconds before. Reaction time and scan cycle are therefore of the same order as the encounter itself.
And scanning to the letter costs more than that: at ten degrees per movement and a second per block, a sector of 180° by 30° takes fifty-four seconds. The study EASA commissioned on improving see-and-avoid does the arithmetic and draws the consequence — “therefore usually only the middle part is scanned properly.”
Peripheral vision detects movement. On a constant-bearing conflict there is none: the other aircraft holds a fixed point in the windscreen and grows. Peripheral detection therefore has nothing to report, the target stays in one block visited once per cycle, and its angular size grows as the inverse of range — imperceptibly for most of the encounter, then quickly at the end.
This is documented in every safety publication on the subject. It is also the case that scanning more does not resolve, because the constraint is not effort but direction: which block, and when.
A visual search in which the approximate direction is already known is about eight times more effective than the same search conducted without that indication. That is the figure measured in flight trials, and it is the whole function of electronic conspicuity: not to replace the scan, but to say which of the twelve blocks to scan, and when.
An aircraft fitted with ADS-L broadcasts its position at least once a second. That link ignores the wing, the door post, haze, sun and where the pilot happens to be looking; it is continuous over 360°, including aft, where the eye has no coverage at all. What arrives in the cockpit is a bearing, a relative altitude and a closure rate — typically some twenty seconds before a converging track becomes a problem. In practice: the glider circling in a thermal on track, the faster aircraft catching up from behind and below, and who else is calling in before the circuit gets busy.
Gliding equipped itself first. In 2010 the Swiss authority recorded that around 95 % of Swiss gliders were fitted with a collision-warning device and concluded that no rule was required, the take-up having come about “not through regulatory measures, but rather on a voluntary basis”. French gliding clubs followed.
The devices still have to understand each other. In 2016 a glider and a light aircraft collided in England; both were fitted with an electronic conspicuity device, and the investigators found that “the differing technologies meant that the pilots of both aircraft were reliant on visually acquiring each other’s aircraft”.
Europe now has an open standard, ADS-L, published by EASA and already recognised as a means of making oneself electronically visible in U-space airspace. A skyBlip transmits and receives ADS-L, and also receives traffic fitted with FLARM-type equipment; under skyPost ground coverage it adds the aircraft transmitting ADS-B.