Function faa_rule_density_altitude_ft 

pub fn faa_rule_density_altitude_ft(
    station_pressure_hpa: f64,
    temperature_f: f64,
) -> f64

Density altitude by the FAA rule of thumb, feet: NWS pressure altitude plus 120 ft for every °C the station is away from ISA temperature at that pressure altitude. Humidity is not an input.

DA_ft = PA_ft + (120 * 5/9) * (T_F - ISA_T_F(PA))      ISA_T_F(PA) = 59 - 3.57 * PA_ft/1000

This is the density altitude the engine has always reported — pdf_dope_card::calculate_density_altitude delegates to it — and the one resolve_atmosphere_for_density_altitude, the density-altitude ENTRY mode, inverts exactly.

It stays humidity-free for compatibility, not out of necessity. Every card printed to date shows this figure, and the entry mode that inverts it has shipped since MBA-1366 taking a temperature but no humidity (the run’s humidity is applied separately, afterwards). A humidity-aware forward could be inverted just as well given the run’s humidity, the way temperature already is; it would change every printed card and what an entered density altitude resolves to.

It is not the National Weather Service’s density altitude either, despite the NWS pressure altitude inside it: the NWS calculator uses virtual temperature, so it includes humidity and tracks density_matched_altitude_m to within a few meters.

And it is an approximation. The rule is a straight line where the real dependence curves, and 120 ft/°C is slightly steeper than the exact slope at sea level (118.6 ft/°C for an ideal gas, 119.0 for the CIPM density density_matched_altitude_m uses), so it drifts from the exact figure as the temperature moves away from standard FOR THE PRESSURE ALTITUDE: about +22 m at 30 °C and 1013.25 hPa in completely dry air. See density_matched_altitude_m for the exact, humidity-inclusive figure, and why the bridge reports both.