Skip to content

Readback checking

Available now.

What it does

Pilot readbacks are now actually checked against what ATC issued, instead of always being accepted. Miss a required item and you get corrected:

negative, climb and maintain five thousand

read back hold short instructions

Get it right, and you get the standard confirmation, restating anything you were only warned about leaving out (not required, but worth a reminder):

readback correct, departure frequency one one niner point two

What's checked, and how strictly

xatc.phraseology.readback.check_readback(issued, kind, pilot_text) compares a pilot's transmission against the Clearance ATC actually issued, for nine kinds of instruction: ifr_clearance, taxi, taxi_in, takeoff, altitude, heading, frequency, direct, approach. Each kind has its own required-vs-warning breakdown -- for example, an IFR clearance readback must include the altitude and squawk code, but the clearance limit and departure frequency are only warned about if missing, not rejected outright. A taxi readback must include a hold-short for every other runway crossed on the way -- holding short of the assigned departure runway itself (the one you're taxiing to) is only a warning if left out, and so is the runway assignment and the full taxi route themselves, since real pilots often trim those from a long taxi readback. That warning becomes required, though, the moment a taxi readback reads back nothing at all -- a bare "wilco" isn't a readback, so it can't lean on the runway being merely a warning to get away with saying nothing. A taxi_in readback (Ground's taxi-to-parking instruction, once you're on the ground after landing -- see Arrival) requires every runway it crosses on the way to the stand to be read back, the same all-required treatment hold-shorts get on the way out; miss one and you get "read back runway crossing". A direct readback requires the fix you were cleared direct to (see En-route requests). An approach readback -- the approach clearance itself, see Arrival -- requires both the runway and, when a vector was needed to intercept, the assigned heading.

The SID is a special case: leaving it out is only a warning, but naming a different one is treated as wrong regardless -- read back "Bangr Nine departure" when you were actually cleared via the Summa Two, and you get corrected ("negative, Summa Two departure"), the same as getting an altitude or squawk wrong.

The checker tolerates real ASR quirks rather than demanding an exact transcript: common homophones (tree/fife/niner/won/fower for digits), abbreviated forms ("one six left" without the word "runway", "maintain five" for 5,000 ft, a frequency without the leading "one", "cleared as filed"), and digit words that only mean a number right after another digit ("two to fower" meaning 2-to-4, not "two two four").

How it's wired into the engine

Issuing an IFR clearance or a taxi clearance now sets a pending readback that has to be satisfied before the flight can actually advance -- the phase change (to CLEARANCE or TAXI_OUT) doesn't happen until the readback is correct. Altitude and frequency instructions are also readback-checked, but don't gate a phase change on their own. A takeoff clearance is deliberately not readback-checked.

Under normal or checkride strictness, getting it wrong keeps the clearance pending -- ATC doesn't just move on, but it also doesn't automatically repeat the whole clearance verbatim; the correction only restates what was missing or wrong. If you reach the hold-short point with a taxi readback still outstanding, Ground proactively asks for it once, rather than letting things fall through to a plain "say again" at the runway.

Under relaxed strictness (the same conformance monitor setting that scales the ground/airborne rules), a readback problem no longer holds anything up: ATC still restates what was wrong or missing -- "negative, climb and maintain five thousand, squawk six six six two" -- but accepts the readback and the flight advances anyway, so a garbled ASR transcript can't get a pilot stuck in a loop. The wording differs slightly for a missing item in this mode: it states the actual value outright ("hold short of runway one six left") instead of asking you to read it back again.

Readback marks in the panel

Every transcript entry the engine ran a readback check against shows one of three marks (rendered by the radio panel's own JS, not spoken): ✓ readback (correct), ✗ readback (wrong or incomplete), or • no readback expected -- a readback or a plain "roger" sent when nothing was actually pending on that position, so there was nothing to check it against (mainly takeoff clearances, and anything outside the nine kinds above).

Only the ifr_clearance kind actually gets ATC to speak "readback correct" out loud. Every other kind -- taxi, taxi_in, takeoff, altitude, heading, frequency, direct, approach -- is silent when the readback is correct, the same way a real controller doesn't bother repeating "readback correct" for every taxi instruction or altitude confirmation. The panel's ✓ mark is how you actually know a readback landed for those; ATC itself only speaks up when something's missing or wrong, or for that one first IFR clearance.

Limitations

  • Only the nine kinds above are checked; nothing else (e.g. a wrong-frequency acknowledgment) is readback-verified.
  • Takeoff clearances are exempt by design, not an oversight -- a real "cleared for takeoff" readback is short enough that a full check adds little value over what the runway-incursion rule in the conformance monitor already catches.

Verification

Unit tests cover the ASR normalization rules and each readback kind's required/warning split directly, plus extensive coverage threaded through the engine's own test suite -- confirming a wrong or incomplete readback actually blocks the phase advance under normal/checkride strictness, a correct one advances it and restates any warned-about omission, relaxed strictness accepts a bad readback while still restating the problem (with the missing-item wording difference), a wrong SID is treated as wrong rather than just a missing warning, and the hold-short reminder fires when a taxi readback was never corrected.

Every readback the engine checks is also recorded for the post-flight debrief, which now has a dedicated Readbacks section.