When the engine quits your panel draws a single ring and calls it your reach. The honest picture has three parts: airfields you will reach whatever the assumptions, airfields you will reach under none, and — the largest group — the ones the evidence does not settle. Today that third group is drawn inside the line, where it looks exactly like the first.
Everything here is decided on one pinned ADS-B trace: N412JS, a PC12 — a single-engine turboprop, which is why a glide ring is a live instrument and not a curiosity — cruising at 22,000 ft on 2026-08-26. Nothing happened on this flight. The question is what the panel would have been telling the crew if something had.
Scrub along the flight. The shapes lean forward because reaching a field behind you costs the height you lose turning onto it — taken from the aircraft’s actual ADS-B ground track, and charged to the dashed line too, so the comparison stays about method and nothing else.
Then change the aircraft. The two turboprop rows are the same aeroplane on the same flight, differing only in whether the propeller feathered.
| ident | airfield | distance | proved band | the line says | verdict | turn | needs L/D ≥ |
|---|
Four powered scenarios, same flight, same instant, drawn at one scale. Three of them are ordinary: the panel’s configured glide ratio sits inside the honest envelope, so its dashed line falls between the two boundaries and the picture is the familiar one — a solid core, a wide undecided annulus, nothing refuted. Watch the fourth.
The failure is not a bad number. It is an event the instrument cannot observe.
An unfeathered propeller is a large draggy disc, and it roughly halves what the aeroplane can reach. The ring on the glass does not move, because nothing in the system knows. Of the 1491 claims that line makes across the cruise, 425 — 29% — are now provably unreachable for every value in the envelope. The worst single row in that scenario: KN14, Flying W Airport, at 24,000 ft. The field is 114 km away, the line promises 115 km of reach, and the band ends at 92 km — not close, and not a judgement call.
Look again at the four ordinary scenarios. Not one of them contains a single refuted airfield, and that is not luck. While the panel’s assumed glide ratio, airspeed and wind sit anywhere inside the honest envelope, its line is mathematically guaranteed to fall between the two boundaries — this build asserts it across every such scenario and refuses to publish if it ever fails. So nothing inside that line can be proved unreachable.
The line is drawn from the very assumptions it would have to doubt. It cannot be caught being wrong — and it is never shown to be right either.
That is the whole case, and it is worth being precise about what it does and does not say. It does not say the ring is inaccurate. It says the ring is unfalsifiable: 54% of what it claims is undecided by the evidence it was drawn from, and it has no way to tell you which part. An instrument that cannot fail a test is not passing one.
What changed in the unfeathered case was not accuracy. The line became falsifiable, and was falsified. That is the difference between the two instruments, and it is why the undecided annulus is the product rather than a defect: a wider band is the same answer with its width shown. Narrow the inputs — real flight-test data, a propeller-state signal, a better wind — and the annulus shrinks. Its width is a measurement of what nobody knows yet.
| the hypothesis | which way it can be wrong | |
|---|---|---|
| H1 | Terrain is not modelled. The band is glide distance over ground at the field’s own elevation. | Asymmetric, and it runs the safe way. Rising ground can only REMOVE reach, so REFUTED is unaffected and stays proved; PROVED REACHABLE means “reachable if the path is unobstructed”. A terrain layer moves green fields to undecided, never the reverse. |
| H2 | Steady wind through the descent and a steady-state glide. | No pushover transient, no shear or thermal structure, and no credit for trading cruise speed back into height. |
| H3 | Great circles on a sphere whose radius is enclosed by [6356.752, 6378.137] km. | Deliberately crude and deliberately conservative — six orders of magnitude above the double-precision error of the haversine, so the geodesy needs no separate error argument. |
| H4 | The envelopes are illustrative classes in stated configurations — not manufacturer data. | No published performance figure for any aircraft is asserted anywhere here, and nothing claims what any product computes internally; what is compared is the point-estimate METHOD. Move the envelope and every percentage moves. The structural finding of §4 does not — it holds for any envelope wider than a point. |
| H5 | The turn: height lost turning onto each field at standard rate from the actual ADS-B ground track. | Progress during the turn counted as zero — conservative — and charged to both instruments equally. |
“Certified” on this page always means a mathematically certified enclosure: an interval proved to contain the true value. It carries no airworthiness meaning, no design assurance and no approval of any kind. The artifact says NOT FOR NAVIGATION on its face and means it — this is a demonstration of a decision procedure, on a flight that had no emergency and a crew who did nothing of the sort.
Terrain. H1 is the big one. A certified band with a terrain floor is the same arithmetic over a pinned elevation model — the next build, not a research problem.
Real performance data. The envelopes are stated, not measured. Flight-test data would replace H4 with something narrower and the annulus would shrink, which is exactly the point.
Design assurance. Nothing here is DO-178C evidence, and the distance between an exact enclosure and certifiable software is the real cost in avionics.
“We ran ten thousand sorties and saw no failure” has the same shape. It cannot be refuted by the evidence it rests on.
Which is the reason this is worth more than a display feature. The defect on this page is not really about glide rings — it is what happens whenever a point estimate is handed over as a decision boundary, and it is the same defect that makes a test campaign an unfalsifiable safety argument. The glide ring is simply the most legible instance of it: one line, one aeroplane, and a field you either reach or you do not.