FLARM, ADS-B, and what actually sees you

Two questions that keep getting merged

Nearly every confused conversation about gliders and electronic conspicuity is two separate questions wearing one coat:

1. What am I legally required to carry?
2. What will actually stop me hitting someone?

The first has a precise answer that most pilots get roughly right and slightly wrong. The second has a completely different answer, and the gap between them is the entire subject.

What the exemption actually says

14 CFR 91.225(e), word for word:

"The requirements of paragraph (b) of this section do not apply to any aircraft that was not originally certificated with an engine-driven electrical system, or that has not subsequently been certified with such a system installed, including balloons and gliders."

Note the first six words. The exemption is from paragraph (b), which is the ADS-B Out requirement for the airspace listed in paragraph (d) — Class B and C, the Mode C veil, Class E above 10,000 feet, and so on.

Paragraph (a) is a different rule and the exemption does not reach it. Paragraph (a) governs Class A airspace, and it reads "unless otherwise authorized by ATC, no person may operate an aircraft in Class A airspace unless the aircraft has equipment installed that…". Your glider's lack of an electrical system does not excuse you there. What excuses you there, if anything does, is an ATC authorization — which is exactly the machinery behind a wave window, and a subject of its own. See wave windows and Class A.

The exemption also comes with its own boundaries written into it. Exempt aircraft may operate in the Mode C veil airspace only if those operations are conducted "outside any Class B or Class C airspace area" and below the ceiling of that Class B or C area, or 10,000 feet MSL, whichever is lower.

The transponder rule works the same way

91.215 has the same shape and produces a result that surprises people from the powered world:

• A glider may fly above 10,000 feet MSL without a transponder. The rule that catches everyone else specifically excludes aircraft without an engine-driven electrical system, balloons and gliders.
• A glider may operate inside the 30-nautical-mile Mode C veil around a major airport, provided it stays outside the Class B or C airspace itself and below that airspace's ceiling or 10,000 feet, whichever is lower.
• A glider may not enter Class A, B or C on the strength of the exemption. There the exemption simply stops.

So the honest summary of the legal position: in the airspace where gliders actually soar, you are free to carry nothing. At the edges of that airspace — the wave climb into the flight levels, the ridge run that clips a Class C shelf — the freedom ends abruptly, and it ends at a boundary you cannot see out of the cockpit.

Three systems, three populations

Here is the thing that is genuinely hard to hold in your head, and it is worth the effort because it determines what equipment is worth buying.

These are not three grades of the same product. They are three separate technologies, and each one makes you visible to, or shows you, a different set of aircraft.

• Transponder. Replies when interrogated — by ATC secondary radar and by airliners' TCAS. This is what makes you exist to air traffic control and to the traffic-alerting system in a jet. It shows you nothing at all.
• ADS-B Out. Broadcasts your position continuously. Received by ATC and by any aircraft with ADS-B In. In the US this is either 1090ES or UAT on 978 MHz, and the two do not directly hear each other — the ground infrastructure bridges them, which matters more than it sounds.
• FLARM. A low-power, glider-specific system on its own frequencies. Units exchange position and short-term predicted flight path, and the prediction is the point: it is built to reason about aircraft that are circling, which is what gliders spend their day doing and what every other system handles badly. FLARM sees FLARM. That is its limit and it is a real one.

The asymmetry that catches people

This is not a criticism of receive-only equipment, which is genuinely useful and is what most gliders sensibly carry. It is a warning about the sentence "I've got ADS-B", which gets said in clubhouses and means one of two opposite things.

There is a further subtlety worth knowing. The ground-based traffic service that fills in non-ADS-B aircraft is provided to aircraft that are broadcasting — it is triggered by an ADS-B Out signal. A receive-only glider therefore sees the direct broadcasts of ADS-B Out aircraft nearby, but does not get its own tailored picture of everything else the ground system knows about. The screen is not empty, and it is not the whole picture either, and the difference is invisible while you are looking at it.

Why FLARM exists, and why "it only sees FLARM" is less damning than it sounds

Consider what actually kills glider pilots in midair collisions. Not, mostly, airliners. Two gliders in the same thermal, converging on the same ridge, or on the same task on a contest day — aircraft with similar performance, in the same lift, at the same time, because they are all looking for the same thing. Soaring concentrates traffic by design.

That is the problem FLARM was built for, and it is why the criticism that it "only sees other FLARM" carries less weight than it first appears. In Europe, where adoption is effectively universal in gliding, "only FLARM" means very nearly every aircraft you are realistically going to conflict with. The system's value is a direct function of how many aircraft around you have it — which makes the decision to fit one partly a club decision rather than an individual one.

In the United States adoption is patchier, which weakens the argument in exactly the same proportion. It is strongest at the places where density is highest: contests, cross-country camps, and busy weekend sites. A FLARM-compatible collision avoidance device is required at US National Championships, and individual regional organisers may require one too.

PowerFLARM, the unit most US pilots mean when they say FLARM, combines FLARM with ADS-B In reception and transponder detection — so it addresses both the glider-glider case and, in the receive direction, the powered traffic case. It still does not make you visible to ATC or to an airliner's TCAS. Only a transponder or ADS-B Out does that.

The power budget is a real constraint, not an excuse

The reason gliders were exempted in the first place still applies. An unpowered glider runs everything from a battery it must carry, on a flight that may last five or six hours, already supporting a vario, a radio, and a flight computer. Adding a transmitting device to that budget is an engineering decision with consequences, and pilots who dismiss the objection have usually never landed out with a flat battery and no radio.

It is a genuine constraint. It is also a smaller one every year, and it is worth re-testing rather than inheriting. If your reason for carrying nothing is a power calculation somebody did in 2009, do the calculation again.

So what should you fit?

We are not going to tell you, because the answer depends on where you fly and it would be a safety recommendation from a software company. What we will do is give you the questions that produce the answer:

1. What do the other gliders at your site carry? FLARM's value is proportional to local adoption. If nobody has it, one unit protects nobody; if most people have it, being the exception is a choice you are making about everyone else's day as well as your own.
2. Where does your flying take you? A site under a Class B shelf, a ridge that runs toward a Class C, or wave that goes to the flight levels each generate a hard equipment requirement rather than a preference.
3. Do you fly contests or camps? Then the requirement is already decided and you are only choosing the model.
4. Is the aim to see, or to be seen? Answer this one explicitly. It is the question that most often gets answered by accident.

And one point for club committees: this is a fleet decision as much as a personal one. A club that equips its two-seaters and its tug changes the environment for every member and every visitor, and it is one of the few safety investments whose value genuinely rises with the number of people who make it.