# Rta Time Control > Use when you must compute the required time of arrival (RTA) function of a flight management system: estimate the arrival time at a downstream waypoint from the remaining distance and ground speed, derive the speed adjustment needed to satisfy the RTA time constraint, check the constraint against the achievable arrival window set by the minimum and maximum cruise Mach bounds, and return the Mach command, the predicted time error and the feasibility verdict for the FMS time control. Produces the ETA, time error, required ground speed and Mach command, achievable window and remaining time error. Trigger: required time of arrival, RTA time constraint, speed adjustment, arrival window, time error, 4D trajectory, FMS time control, Mach command. - Skill: `ashfordeou/rta-time-control` (Agent Skill, multi-file: 3 files) - Install (CLI): `npx skillmds@latest add ashfordeou/rta-time-control` - Raw SKILL.md: https://api.skillmd.com/api/skills/ashfordeou/rta-time-control/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Coding & Dev Tools - License: Apache-2.0 - Author: ashfordeOU (https://skillmd.com/u/ashfordeou) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/ashfordeou/rta-time-control --- # RTA Time Control (avionics/flight-management/rta-time-control) Use when the task is the required-time-of-arrival (RTA) control function of a flight management system: predicting the time of arrival at a downstream waypoint, computing the speed adjustment that lands the aircraft at the waypoint at the RTA time constraint, and judging whether the constraint lies inside the achievable arrival window set by the minimum and maximum cruise Mach. This leaf implements the deterministic speed-command law in pure Python, stdlib only. It pairs with avionics/flight-management/ performance-computation for the speed selection context, avionics/flight-management/vertical-navigation for the vertical profile, and avionics/flight-management/flight-planning for the route structure that carries the leg. ## Domain quick reference - Ground speed law: GS = TAS + V_wind, with the along-track wind V_wind positive for a tailwind. TAS and GS in m/s. - Estimated time of arrival: eta = d_rem / GS, a flight duration from the prediction time t_now, with d_rem the remaining distance. - Time error: e = (t_now + eta) - t_RTA, positive when the aircraft is late against the RTA time constraint; the constraint is met when |e| <= TIME_TOL_S (5 s). - Required ground speed: GS_req = d_rem / (t_RTA - t_now), the constant ground speed that arrives exactly at the RTA time. - Required Mach: M_req = (GS_req - V_wind) / a, with the ISA speed of sound a = sqrt(GAMMA * R * T), T = 288.15 - 0.0065 h in the troposphere, GAMMA = 1.4, R = 287.05 J/(kg K). - Achievable window: the cruise Mach envelope [M_min, M_max] maps to ground speeds GS_min = M_min * a + V_wind and GS_max = M_max * a + V_wind, so arrivals can only fall in [d_rem / GS_max, d_rem / GS_min] relative to t_now. - Command law: met within tolerance, hold the current speed; M_req inside the envelope, command M_req (feasible); M_req above M_max or below M_min, command the nearer bound (unfeasible) and report the remaining time error of the best achievable arrival. - The speed command is a Mach number; a calibrated-airspeed presentation of the same command is a display-layer conversion outside this leaf. - Units are SI throughout: m, m/s, s. DO-178C frames the software context for the FMS function; the relations above are standard engineering methodology, summary-only. ## Workflow 1. Fix the leg state: remaining_distance_m, ground_speed_m_s, wind_along_m_s, rta_time_s on the same clock as t_now_s (default 0), altitude_m, and the cruise envelope mach_min, mach_max. 2. Get the current prediction: eta_s for the ETA duration and time_error_s for the sign and size of the RTA time error. 3. Size the window: achievable_window returns gs_min, gs_max, eta_min_s and eta_max_s; if rta_time_s falls outside it, the constraint cannot be met at any cruise speed. 4. Run the command law: rta_speed_command(inputs) decides hold, feasible-speed-change or bound-command and returns the full verdict dict with required_gs_m_s, required_mach, command_mach, feasible, predicted_eta_s, remaining_error_s and the verdict string. 5. Convert speeds with tas_from_ground_speed and mach_from_tas when a side check on TAS or Mach is needed, and isa_speed_of_sound for the local speed of sound. 6. Confirm the deterministic checks with the contract test scripts/test_rta_time_control.py. ## Worked example Leg remaining 450000 m (about 243 NM), current ground speed 250 m/s, wind along +15 m/s (tailwind), altitude 10668 m (module speed of sound a = 296.53 m/s; the spec display rounds it to 296.51), Mach envelope [0.72, 0.84], t_now = 0. - ETA: eta_s(450000, 250) = 1800 s. - Case 1, RTA 1900 s: time_error_s(1800, 1900, 0) = -100 s (early, must slow). required_gs = 450000/1900 = 236.84 m/s; required TAS = 236.84 - 15 = 221.84 m/s; required Mach = 221.84/296.53 = 0.7481 (spec anchor 0.74818). Inside the envelope: command_mach 0.7481, feasible True, predicted_eta_s 1900 s, remaining_error_s 0, verdict rta-feasible. - Case 2, RTA 1200 s: required_gs = 450000/1200 = 375 m/s, required Mach 1.214, above mach_max 0.84: command_mach 0.84, feasible False; ground speed at Mach 0.84 = 0.84 * 296.53 + 15 = 264.09 m/s; best eta = 450000/264.09 = 1703.97 s (spec anchor 1704.1); remaining_error_s = 1703.97 - 1200 = +503.97 s late (spec anchor 504.1), verdict rta-unfeasible. - Case 3, RTA 1800 s (equals the ETA): |error| 0 <= 5 s, hold the current speed; command_mach = (250 - 15)/296.53 = 0.7925 (spec anchor 0.79257). - Window: eta_min_s = 450000/264.09 = 1703.97 s (fastest) to eta_max_s = 450000/228.50 = 1969.33 s (slowest, spec anchor 1969.4). An RTA of 2000 s falls beyond eta_max_s: command mach_min 0.72, feasible False, remaining_error_s = 1969.33 - 2000 = -30.67 s (still early), verdict rta-unfeasible. ## Verification - Confirm eta_s(450000, 250) returns 1800 s exactly. - Confirm required_gs for RTA 1900 s is 236.84 m/s within 0.01 and the required Mach for the 221.84 m/s TAS is within 1e-4 of 0.74818. - Confirm the window eta bounds land within 0.5 s of 1704.1 s and 1969.4 s and that gs = M * a + wind holds for both envelope bounds. - Confirm the hold branch keeps the current-speed Mach 0.7925 command whenever |time error| <= 5 s and the feasible branch reports zero remaining error with predicted_eta_s equal to the RTA time. - Confirm the unfeasible fast case (RTA 1200 s) reports remaining_error_s +504.1 s within 0.5 and the verdict rta-unfeasible. - Confirm every negative distance, non-positive ground speed, RTA time at or before t_now, non-positive mach_min, inverted envelope, negative altitude and headwind above the minimum-cruise TAS raises ValueError. - Run the contract test offline: python3 scripts/test_rta_time_control.py (42 tests, deterministic). ## Related leaves - avionics/flight-management/performance-computation: the cruise speed selection context that supplies the speed the RTA function adjusts. - avionics/flight-management/vertical-navigation: the descent path that carries the aircraft to the constrained waypoint. - avionics/flight-management/flight-planning: the route structure whose legs carry the remaining-distance input. - avionics/flight-management/lateral-navigation: the lateral guidance the time-based operation runs alongside. ## Pitfalls - Dropping the wind from the Mach conversion: GS = TAS + V_wind with a tailwind positive, so required Mach is (GS_req - V_wind) / a - converting the 236.84 m/s required ground speed directly to Mach without removing the +15 m/s tailwind commands roughly 0.05 Mach high instead of the correct 0.7481. - Flipping the time-error sign: e = (t_now + eta) - t_RTA is positive when the aircraft is LATE, so an RTA of 1200 s against an 1800 s ETA means speed up, while -100 s (early) means slow down - swap the sign and every command drives the aircraft the wrong way. - Commanding M_req outside the envelope: the command law clamps to the nearer of mach_min and mach_max and reports the unfeasible verdict with the remaining error of the best achievable arrival - an RTA of 1200 s needs Mach 1.214 but the aircraft delivers Mach 0.84 and arrives +503.97 s late, so the report must carry that residual, not a silent clamp. - Ignoring the achievable window: arrivals can only fall in [d_rem / GS_max, d_rem / GS_min], so an RTA of 2000 s beyond eta_max_s 1969.33 s is unmeetable at any cruise speed - check the window before computing a required speed, and still report the best-case remaining error of -30.67 s (early). - Holding speed outside the tolerance: the hold branch applies only when |e| <= TIME_TOL_S (5 s); outside it the command is M_req inside the envelope, never the current speed, however close the arrival looks. - Using the wrong speed of sound or presentation: a is sqrt(GAMMA * R * T) at the cruise altitude (296.53 m/s at 10668 m, not the sea-level value), and the command is a Mach number - the calibrated-airspeed presentation of the same command is a display-layer conversion outside this leaf. ## Behavior contract (gate 3) Run the deterministic contract test (stdlib unittest, offline): python3 scripts/test_rta_time_control.py The test covers the ISA speed of sound at sea level, cruise altitude and in the isothermal stratosphere, the ETA division, the time error sign convention for early, late and on-time arrivals, the required ground speed anchor, the TAS from ground speed and wind with headwind rejection, the Mach conversion anchors and round trip, the achievable window values with internal consistency, the feasible slow-down command, the hold branch on time and within tolerance, the unfeasible fast and slow cases with their remaining error values, the nonzero t_now clock handling, the verdict strings and the ValueError rejection of non-physical inputs. ## Compliance - Standards referenced, not reproduced: DO-178C is the software considerations context for FMS functions and is cited reference-only; the RTA relations above are standard engineering methodology, summary-only per standards-map.yaml. No proprietary FMS documentation is reproduced. - compliance: STANDARDS-REF, gated: false.