The Equator Flight Data Challenge, Answered

1 · The measurement he asks for is made at the gate, before every flight

The instrument this experiment would be validated against is the aircraft’s inertial reference system, a ring-laser-gyro package, and it answers the question before the aeroplane moves — on the ramp, during the ten-minute alignment that precedes every departure. The accelerometers find the gravity vector: local down. The gyros find the Earth’s rotation vector, fixed in inertial space along the spin axis. The unit then measures the angle between those two, with nothing about the shape of the Earth entering the measurement. Only the label put on the answer uses a model; the method notes say how little that matters.

Fly London to Cape Town and that angle changes by 85.4 degrees: two stationary measurements, ten minutes each, at the two ends, the route between them irrelevant. The axis is above the horizon at one end and below it at the other — a swing of 482.8° in daily carry between the ends of one scheduled route. Every figure in the table is a total, not a rate, established while parked. The day is the sidereal one — 23h 56m 04s for a full 360°, rotation measured against the stars rather than the Sun — because the instrument measures against inertial space.

The pattern even fails where the geometry says it must. North-finding depends on the horizontal component, which goes as cos φ and thins toward the poles, so the Boeing 737 flight manual carries a hard limit: alignment must not be attempted above 78° 15′, where what is left has fallen from 15.04°/hr to 3.06. Above it you align further south and carry the alignment north against GNSS — satellite positioning — because at the pole gravity and the rotation axis line up and there is no north left to find. On a flat surface there is nothing for a cosine of latitude to be a function of. Nor is the signal marginal: Cape Town’s vertical channel stands about 840 times clear of a navigation-grade gyro’s own drift, and the alignment is pass/fail — enter a latitude that disagrees with the one the unit has just derived and it is rejected. Provenance of the 78° 15′ figure, the competing 78° 14.75′, and the separate ~82° maximum operating latitude set by magnetic variation rather than by the cosine: in Sources.

The book’s reply, sixty pages later, does not reach the measurement

At pp. 183–187, in the run headed “Evidence of Earth spin?”, he takes the ring laser gyroscope seriously. He does not dispute that it reads something; he reattributes what it reads:

“So if the gyroscope measures anything while standing still to the observer, what you have detected is that the medium of light travel is drifting.”

That is an aether, and against this particular instrument it is the right objection: a ring laser sends light both ways around a loop, so a drifting medium of light is a live rival explanation for what it reads. It fails three times over, each sufficient alone.

• 1. Instruments with no light in them read the same rotation. A spinning-mass marine gyrocompass finds true north from angular momentum; a gyrotheodolite does it with a suspended rotor, to arcseconds, underground; a hemispherical resonator gyro senses Coriolis forces on a vibrating quartz shell; a Foucault pendulum precesses at 15.04°/hr × sin φ. Not an optical path among them. A medium that drags rotors, quartz and a pendulum bob by exactly the amount a rotating Earth would is rotation under another name, and still owes an explanation for the sine of the latitude.
• 2. The substitution has been run on the aeroplane itself. Before ring lasers, airliners aligned on gimballed platforms spun by mechanical gyro wheels — the Delco Carousel IV of the early 747, whose manual describes the same ramp procedure: “During alignment, the gyros on the platform sense the earth’s rotation… the computer uses the resulting outputs to determine the platform’s relationship to true north,” parked only, degrading above 80° of latitude. The same cosine, found by spinning metal, before GPS existed to seed the answer from. Those airframes were later refitted with ring-laser units in the same bay and navigated identically.
• 3. Grant the aether anyway and both observables survive it. His vortex turns about the vertical — the text names the Z-axis twice and the diagram stacks its speed rings vertically — and on a flat plane every local vertical is parallel to the global one, so it predicts a sensed axis pointing straight down at every airport on Earth. The instrument finds it 38.5° from straight down in London, 88.6° in Singapore, and below the horizon altogether in Cape Town. A rotation about the vertical also contributes nothing to the horizontal channel, which is the whole of north-finding and the thing that runs out at 78°.

One thing his account gets right: the Sagnac effect — the shift between light sent both ways round a rotating loop — is what these gyros exploit, and asking whether the sensed rotation is the Earth’s or the medium’s is the right question to ask of a ring laser. It is answered by walking away from ring lasers. His supporting data and altitude test are taken up in §5.

The box also carries the size of the Earth, with no windows

The correction his island pilots (p. 122) say does not exist — earth rate, 15.041°/hr × sin φ — is the first row of the inertial unit’s mechanisation, the equations that turn sensed rotation and acceleration into position, applied several hundred times a second in a box the size of a shoebox and watched against GPS to about a nautical mile per hour. Pilots never see it because of where it happens. The planet’s radius sits in the same loop. An inertial platform is Schuler tuned: its error loop is set so the instrument behaves like a pendulum whose length is the Earth’s radius, with a characteristic period,

T = 2π √(R ÷ g) = 84.4 minutes

which is the period at which every inertial navigator’s error is observed to oscillate. Solve backwards for R and out comes 6,370 kilometres — not read off a dial, but the value that must be in the loop for the platform to hold vertical at all, in a sealed instrument with no view of the sky.

Mechanise it with the flat map’s 6,225 miles (§2) instead and the platform neither goes unstable nor tilts without limit, because a feedback loop bounces a steady error rather than accumulating it. The rate mismatch is V × (1/R_true − 1/R_assumed), which at 560 mph is 2.94° an hour of commanded-rate error — 2.9° of tilt after an hour and 5.9° after two if it accumulated. Instead the tilt turns over at an amplitude of 0.66°, breathing on the 84.4-minute period — a signature, not a smear. The navigator converts measured travel into angular position using the radius it was handed, so it reports 5.15° of movement per hour where the truth is 8.10° — a ratio of 0.6366, which is 2 ÷ π, the constant §2 finds inside the chapter’s own table — and its position solution falls 203 miles behind the aeroplane in the first hour, against a GPS cross-check and a certified drift envelope of about one nautical mile per hour. Riding on that ramp is a position excursion of about 46 miles, oscillating with a period of 84.4 minutes — 2π √(R ÷ g) for the true R. The error a wrong radius produces arrives carrying the correct one.

Which answers the obvious objection — that the box was programmed with the globe, so of course it reports one. The shape is a term in a closed loop, not a conclusion written into the output. Put the wrong term in and the loop stops closing, in a specific direction, at a specific rate, with a specific period.

2 · The number in the table

The setup as printed (p. 121): Quito to Pontianak, 8,250 miles at 560 mph, 14.73 hours, 119° of turn at 8.08°/hr, one steering input every 7.4 minutes, $715,000. Where does 119° come from? Not from the airports. Mariscal Sucre is at 78.36°W and Supadio at 109.40°E — 187.8° apart eastbound, 172.2° westbound, about 13,000 or 11,900 miles. The route and the 8,250 miles are not describing the same flight. It comes from the distance: 8,250 ÷ 24,901 × 360 = 119.3°, and 24,901 miles is the circumference of the equator on a globe. The figure presented as the flat model’s turning requirement was obtained by measuring the route on the globe.

So how long is the equator on a flat Earth? p. 120 declines to say — the Gleason map is “just a distorted version of the globe,” its distances possibly “elongated to conform to a predefined Earth radius.” That is a careful position, and it is fatal to the table: the flat side of the comparison then has no scale, no distance and no angle, and pp. 118–120 distrust the distance scale that p. 121 trusts to four significant figures.

That is the Gauss–Bonnet theorem — for any closed circuit, turning supplied by the traveller plus curvature enclosed sums to 2π. A flat plane supplies none so the pilot supplies all; a globe supplies all so the pilot supplies none. Map scales, road lengths, who controls the tracking data — all drop out. One exit closed: on bipolar or rectangular flat maps the equator is not a loop and eastbound circumnavigation is impossible — refuted by the Whitbread voyage the chapter itself cites on p. 121.

If a number is wanted for the partial leg, the only layout consistent with his own requirements — parallels as circles about the pole, spaced by true north–south distance, which is the azimuthal-equidistant layout of the usual flat-Earth map — gives an equator of radius 6,225 miles and circumference 39,115 miles, longer than the globe’s by exactly π ÷ 2. On it, 8,250 miles is 75.9°, at 5.15°/hr: 76 one-degree inputs rather than 119. Anything that can tell 119° from 0° can tell 76° from 0°.

3 · Why the equator, and what “turning” means

One definition first, because the dispute lives in it: turn here means yaw about the local vertical, which is what a rate gyro senses. The nose-down rotation of following the curve is §4’s business.

“But you can just drive straight west without turning”

This is the right objection. Hold a heading of 270°: the compass does not move, nothing says turn. And you can indeed drive perfectly straight. You just stop going west when you do. Stand ten metres from the North Pole and walk due west: you walk a circle twenty metres across, a lap every forty-five seconds, turning at eight degrees per second — compass reading 270° throughout. Walk south and the turn gentles — 30°/hr at 75°N, 8.1°/hr at 45°N, 2.2°/hr at 15°N — until at the equator it reaches exactly zero, the one latitude where hold the parallel and do not steer are the same instruction.

The instinct is right about everything a pilot would feel, though. At 45°N and 560 mph the bank needed to hold the parallel is 3.4 arcminutes (an arcminute is a sixtieth of a degree), and nobody commands it — you command a heading, and the turn is what holding that heading means on a sphere. “I am not turning” is right about the controls; “you are turning” is right about the path. The arbiter is a gyro, which holds against space rather than against a reference quietly rotating underneath it.

If that still feels like a definitional trick, close the circuit. Go right round a parallel and come back facing the way you started; something rotated through a full turn, and the only question is how much of it you supplied:

The globe column is 360° × sin φ and the gap is the curvature enclosed — the share of the turning the planet does for you. At the pole you circle a point and supply all of it; at the equator the planet supplies all of it and the models disagree by a full revolution. Which makes his route sharper than he argued: the lateral Coriolis deflection — the sideways drift a moving body shows on a rotating Earth — vanishes there too, where at 45°N a wings-level aircraft’s track curves at 21.3°/hr from Coriolis alone.

4 · The island of pilots, and what a flight would show

The chapter closes on pilots who have never seen a globe, protesting that they never point the nose down and never adjust for spin (p. 122). His own p. 119 analysis answers the first half: the pendulous vanes erect the attitude indicator to local vertical continuously; on a globe local vertical rotates at exactly V÷R as you travel; an aircraft flown level against that indicator follows the curve automatically. The two jets he names carry no vane-erected instrument — a G800 or Global 8000 computes attitude in a laser inertial reference, so the modern “level” reference is itself part of the Earth model (§1).

The two directions are asymmetric in the flat model’s disfavour. Vertically the aircraft is held on the curve: a surface of constant pressure wraps the planet, so fly a Euclidean straight line at constant pressure altitude and you climb off it and settle back. Horizontally there is no aerodynamic mechanism at all — to turn you bank. So he picked the right quantity: the flat model’s requirement is a commanded action, a persistent one-directional bank held for fourteen hours and reversing on the return leg. The globe needs the curve that cannot be avoided; the flat model needs the one that cannot happen by itself. Instrument by instrument at the equator, eastbound:

The heading row is the subtle one and the place a quick rebuttal goes wrong. On the flat map “north” means toward the centre, so flying the equator circle keeps the pole exactly abeam — off the wingtip — and the heading indicator sits on 090° — the same reading a globe gives. Magnetic heading alone cannot separate the models. Each model on its own bookkeeping: an inertially-derived heading reads a clean 090° only where the constants in the mechanisation match the world being flown over (§1). True heading throughout: magnetic declination, the compass’s offset from true north, shifts from about 4°W at Quito to near zero at Pontianak, identically under either model.

Which suggests a cleaner version of the experiment

Take the steering out of the loop: roll-attitude hold at zero bank, no heading or lateral-navigation reference, wind logged. The globe says you stay on the line. The flat map says 25 miles off in the first hour, 100 in two, 391 in four — southward, and southward on the return leg too. The chapter predicts “opposite adjustments on the return trip,” but flown straight, a flat equator curves away in the same direction whichever way you travel; it is the required bank that reverses, not the departure. Crosswind separates cleanly in the same log: wind drift accumulates linearly as drift angle against heading, while the flat departure grows as the square of distance and moves the pole-referenced heading itself. A hundred miles is a GPS readout, not an arcminute.

5 · His altitude test, answered on its own terms

The book also proposes a mechanism and states a test. The aether is reworked as a magnetic-toroidal field which “propagates as a vortex, with a rotation rate that varies based on vertical position along the Z-axis”, and which “increases in velocity as one moves down the Z-axis toward the ground” (pp. 186–187). The supporting data is a table of fibre-optic gyroscope readings attributed to Bob Knodel:

Printed immediately beneath it: “I could not find the formal document with their exact measurements. We need multiple teams to perform this to see if the results are true.” That is the right thing to say about data you cannot source, so we argue with the design, not the numbers. The test: compare the measured drift at many locations along the same line of latitude and at different heights; “if the values vary greatly, then the claim is falsified.”

His own table is the demonstration. The three stations span three quarters of a degree of latitude, two of them identical to the printed precision. Across that span a rotating Earth allows the vertical component a total range of 0.15°/hr, from 9.43 to 9.59. The disagreement reported is “more than a full degree per hour” (p. 185) — some seven times wider than everything latitude has available at those sites. Whatever produced it, a rotating Earth does not predict it and cannot be convicted by it. Altitude is a real discriminator in principle — his vortex predicts a spread there and a rotating Earth predicts none — but the weakest available, because a spread is exactly what a misbehaving instrument, a bad site or a sloppy method also produces. Latitude is where the two accounts differ by something no instrument fault imitates: a large signal, varying as a known function, reversing sign at a known place. He picked the variable where winning proves least.

The gyroscope in Behind the Curve

The table above follows up the most widely seen gyroscope result in this dispute. In the 2018 documentary Behind the Curve, Knodel’s team set a ring-laser gyroscope on a bench to test for Earth rotation and found a drift of 15° an hour — the rate the globe assigns to the Earth’s spin, measured by flat-Earth researchers on their own instrument, the same class of instrument this page says aligns at every gate. The book’s position (p. 185) is that the documentary stopped the story halfway, before the altitude work; take that at face value and it changes nothing here. The globe’s answer to the 15°/h is the one §1 has already given: a rotating Earth predicts exactly that reading, predicts it to vary from site to site as the sine of the latitude, and predicts it to reverse sign across the equator (§7); a stationary Earth predicts zero. And whether the 15°/h is the Earth turning or a medium drifting — the question the altitude test was built to reopen — is settled by the instruments with no light in them, which read the same rotation.

6 · The follow-up experiment, which is the better one

The chapter proposes a second flight, almost in passing: “start from the bottom of India, Africa, or Australia and fly a route that perfectly follows magnetic South.” That is the sharper test of the two, because southbound the models disagree not about a rate but about a topology: on a globe those tracks converge to a point and cross to the other side; on any pole-centred flat map they spread apart without limit.

What discriminates is any leg with a substantial east–west component, where the map’s circumference has to stretch:

The last is the one to keep. Novolazarevskaya sits at 11.8°E and Progress at 76.4°E, both around 70°S — an east–west hop between two research stations, flown by Basler BT-67 in a working day on the DROMLAN network. On a pole-centred flat map those stations lie on a circle some 69,500 miles around, and the hop becomes 11,800 miles: a fortnight’s flying for an aircraft with under 2,000 miles of range. It is flown routinely, so the map is wrong by a factor of eight. The transpolar version has been flown commercially: Pan Am Flight 50, 28–30 October 1977, a 747SP, San Francisco over the North Pole to London, London to Cape Town, Cape Town to Auckland across the South Pole, and home in 54 hours 7 minutes. The great circle between those two cities peaks at 74°S; the flight went over the pole itself, about 7,550 miles against the 747SP’s range of roughly 7,650. On the flat map the same cities are 17,000 miles apart — beyond any airliner ever built, and beyond that 747SP by a factor of more than two.

7 · The route that carries the most, which is not this one

The instinct behind the chapter’s choice is right: pick the route where the models disagree most. What it picks for is the turn rate — and a rate is the one thing a flat map can always answer, because the map has a free scale and rescaling it produces a different rate; §2 is that problem in miniature. The quantity a scale cannot touch is one that changes sign. Three models are on the table, and only one is not a constant:

The disc is not his model — §1 covers a stationary Earth — but it is the strongest rotating flat model anyone offers, and it predicts the same reading everywhere, since on a disc the spin axis is normal to the surface at every point. So the discriminator was never “does the gyro read something” — the question his aether reply at pp. 183–187 is built to absorb — but whether the reading varies, and whether it reverses. Which makes the useful route a north–south one that crosses the equator. What a gyro sees on real scheduled services, against the transatlantic leg for comparison:

And the measurement needs almost nothing: orientation and position at the start, orientation and position at the end, and the elapsed time. No steering log, no continuous recording, no charter. The cost is in the last column: Atlanta–Santiago is essentially a meridian, on which a flat plane and a globe both hold the heading constant, so its curvature discriminator reads zero. North–south buys the rotation law and throws away the geometry; east–west does the reverse — an argument for a closed circuit with one leg of each.

What the reading has to survive. If the number comes from the inertial reference’s computed output, the box was handed sin φ before it reported sin φ, and the test is softer than it looks. Two ways out, and the second needs no instrument nobody will lend you: take the raw body rates rather than the computed heading; or fly the circuit in both directions, because the geometric term reverses with circuit direction and the rotation term does not. The two separate by symmetry alone, with no constant taken on trust from a manufacturer.

Margin is not the difficulty. A navigation-grade gyro’s bias stability sits at 0.003–0.010°/hr — the real floor, because a bias does not average away. The London–Cape Town signal is 20.17°/hr: three orders of magnitude clear, consistent with the 840× figure §1 gives for the same city.

Which means the quantity this section is about changes sign inside a single downloadable file. Latitude runs −8.07° to +39.73°, so 15.041 sin φ runs −2.11 to +9.61°/hr, and integrated across the flight the platform has to account for 39.2° of rotation that a flat Earth does not have. Against a heading specification of 0.02°. We parsed the file rather than trusting its header; what the check found is in the method notes.

8 · What would settle it

• 1. A gyro on a table. A navigation-grade gyro, stationary, measures earth rate about the local vertical: 15.04°/hr × sin φ. Take the same instrument to two latitudes and the change is predicted in advance. No aircraft, no charter, no tracking data anyone has to trust.
• 2. The wings-level equatorial leg of §4. Zero bank, no lateral reference, position and wind logged. Either the departure grows as the square of distance, or it does not happen.
• 3. And the full route, if it is ever funded. Worth flying for the public dataset alone. The number to record is accumulated inertial yaw against the local frame: 0.0° on the globe, 75.9° on the flat map’s own best layout.

Statute miles south of the parallel you started on, at 560 mph in still air; the first four columns are the globe. On the equator the aeroplane holds latitude 0.0000° and heading 090° for as long as you leave it alone. Begin the identical run at 45°N and the same untouched aeroplane is 577 miles south of its parallel after four hours, with its heading swung from 090° to 118° — and nobody moved a control. Computed in wings_level_drift.py; the simulator should reproduce the globe columns to within its own wind and trim noise. The flat column is the azimuthal-equidistant layout §2 and §6 use, from the same script; its 391 miles at the equator is the figure quoted in §4, arrived at independently here.

The simulator is a globe by construction — X-Plane’s developer documentation warns that “true north is only the same as the negative Z axis for the point 0,0,0” and that the vertical “divergence increases as you go away from the reference point” — so flying it proves nothing about the shape of the Earth. What it tests is this page. §4 asserts that a globe behaves this way; a flight simulator is an independent implementation of the same model, written by people with no stake in this argument, and it lets anyone check the claim in an evening. It also forecloses the reply that simulators quietly model a flat Earth with the globe as a skin. Read down the flat column: on a pole-centred flat map a parallel is a circle and a straight line is not, so a hands-off aeroplane leaves its parallel at every latitude, and away from the equator the two models merely disagree by an amount, which can always be argued about. The top row cannot be. A sphere has exactly one line that is both a parallel and a great circle, and an aeroplane left alone on it stays on it forever. A flat plane has no such line at any radius.

Sources & further reading

• Levi Miller, Globe Deconstruction? — prerelease draft, 2026. Chapter “The Equator Flight Data Challenge,” pp. 117–123; pendulous vanes at p. 119; the route table at p. 121; the island of pilots at p. 122. The aether, vortex and Knodel material is at pp. 183–187, in the run headed “Evidence of Earth spin?”, with the degree-per-hour unit stated in prose at p. 185. The same challenge appears as a question card at p. 46, printed as “Q4” (the draft's numbering runs Q1, Q2, Q4, Q4, Q5) and indexed as Q3 on this site's contents page to keep twelve questions in order.
• Recorder specifications: 14 CFR Appendix M to Part 121 (roll, pitch and heading at 0.5° resolution, ±2° accuracy, 1 Hz) and 14 CFR 121.344.
• Public-land survey practice: Standard Field Tables and Trigonometric Formulas (US General Land Office / BLM) — Table 12, azimuths of the tangent to the parallel; Table 13, offsets from the tangent; Table 14, offsets from the secant. Also the Manual of Surveying Instructions, 1973 on running true parallels and the 24-mile standard-parallel interval.
• Pan Am Flight 50, 28–30 October 1977, Boeing 747SP N533PA Clipper New Horizons, San Francisco–London–Cape Town–Auckland–San Francisco over both poles in 54h 07m: Airways. Intra-Antarctic feeder legs are flown by the DROMLAN network between Novolazarevskaya, Progress and the other coastal stations.
• Inertial reference alignment: ARINC 704A, the characteristic defining the airborne IRS — sold rather than published, and we have not read it. D. Titterton & J. Weston, Strapdown Inertial Navigation Technology (2nd edn, IET, 2004), Ch. 10 §10.1 ff., “Inertial Navigation System Alignment”, whose opening distinguishes “self-alignment, using gyrocompassing techniques” from alignment against a master reference. Chapter and section verified against a licensed excerpt; the alignment equations sit behind the publisher's paywall.
• The 737 alignment limit, because §1 leans on it. Operator summaries of the limitations section, a 737-800 limitations study guide (Smith & Allen, 1999–2004), and pilots quoting their manuals on the 737 operational-latitude thread and the general IRS alignment thread. The study guide and a line-pilot quotation of an operator's FCOM (with 737-matching exception bands) both give 78° 15′; a competing 78° 14.75′ traces to the latitude of Longyearbyen and appears alongside a 747-400 FCOM quotation giving an alignment band of 70° 12.0′ to 78° 15.0′. All of these are people quoting a manual; no Boeing document was opened, and whether the limit lives in the AFM or the FCOM varies by operator, so “flight manual” is deliberately loose. The separate 737 maximum operating latitude near 82°, with its longitude-band exceptions, is set by magnetic variation tables, not the cosine; the A330 FCOM states that handover directly: true heading above 82°N, above 73°N between 90°W and 120°W, above 60°S.
• Mechanical predecessors: Delco Electronics, Carousel IV-A Inertial Navigation System Operations Manual — gyrocompass alignment on a gimballed spinning-mass platform, parked only, degrading above 80° latitude, in an era ending well before GPS reached initial operational capability in 1993. Ring-laser retrofit of 747-300 fleets: Litton / Northrop Grumman LTN-92 announcements, a replacement for earlier “spinning wheel” systems in the same ARINC 561 installation. The announcements do not name the outgoing system, so the page says spinning-mass rather than Carousel.
• Polar alignment: Wang et al., “On inertial navigation and attitude initialization in polar areas”, Satellite Navigation (2020) — gravity and the rotation vector become parallel at the pole; GNSS aiding is the substitute. High-latitude operations guidance (a pilot-authored compilation of regulatory material): northern control area navigation is by long-range inertial and GPS-based FMS “referenced to True North only.”
• §7 — the flight data. Hskping_DC8_20160817_R1.ict from NASA's ATom archive: ICARTT plain text, 6.4 MB, no authentication; its header names the LN-251 as the source of position and attitude and gives 0.02° on heading, pitch and roll. What we checked in it is in the method notes.
• §7 — what is not obtainable. Uncompensated body rates are defined by NSF/NCAR's RAF processing algorithms as BROLLR, BPITCHR and BYAWR, “the raw output of the IRU … rate gyro” at 50 Hz and 0.0039°/s, and a published production file header shows all three in an archived C-130 file, from two inertial units. They are not clickable: the data API returns “Missing API Key”, the OPeNDAP route redirects to a login, and the surviving form is an email order queue. Even if obtained, “raw” means “uncorrected by NCAR” — about the file, not the physics — and whether ARINC 429 labels 326/327/330 carry Earth rate or have it removed by the internal mechanisation is stated in no public document we could reach (ARINC 704A defines it and is sold). Hence the both-directions symmetry test in §7, which does not depend on the answer. And Earth rate is 0.004178°/s against a transmitted resolution of 0.0039°/s: the entire signal is about one least-significant bit.
• §7 — the airliner corpus. NASA's DASHlink sample flight data, one regional-jet type, 35 tail numbers, 186 parameters at 1–16 Hz, records TH (true heading) and MH (magnetic) separately, plus LATP, LONP, TRK, DA, PTCH and ROLL, and no angular rates of any kind. Parameter names from the data dictionary in the alacer/flightdata conversion scripts — a single source, whose preamble records NASA's administrator saying no authoritative variable description exists. We have not downloaded a flight; the smallest unit is an ~800 MB per-tail archive. No parameter carries a GPS name, so the position is IRS- or FMS-derived by inference from the naming, not by documentation.
• The simulator check. X-Plane’s Data Output writes selected quantities to data.txt at a chosen rate; its flight-data-recorder format defines named columns including time secon, lon degre, lat degre, ptch deg, roll deg and hdng TRUE, built so real recorder data can be replayed. Microsoft Flight Simulator has no native equivalent and relies on third-party recorders through SimConnect. The quoted warnings about true north and the local vertical are from X-Plane’s coordinate-system documentation for plugin authors, where they are programming caveats rather than claims about the world.
• Companion pages: Does the Earth Spin, or Does the Sky? for gyrocompasses, gyrotheodolites and the Coriolis correction; One Spin, Written Across Every Latitude for the 15.04 sin φ result measured independently.