Source: https://guthlabs.ai/docs/instruments/pressure-altitude-density-altitude/mechanism/

instrument mechanism / verified-clean

# Pressure Altitude / Density Altitude mechanism

The inspectable method record for Pressure Altitude / Density Altitude, kept separate from implementation and qualification.

Status: **VERIFIED_CLEAN**Lifecycle: **verified-clean**

## Model scope

The formulas are recognizable aviation approximations and the non-certified-for-flight-planning label is appropriate. The instrument needs explicit input units, an explicit definition that ISA temperature is evaluated at pressure altitude, and a defined ISA lapse-rate/table and valid operating range before release.

- The pressure-altitude formula is an aviation approximation using 1000 ft per inHg; it is not the exact standard-atmosphere pressure-altitude equation, especially for large altimeter-setting deviations.

- Density altitude requires ISA temperature at the pressure altitude, but the specification does not explicitly state that ISA_temp must be evaluated at PA.

- The ISA temperature source, lapse rate, altitude range, and interpolation method are not defined; the stated data need is insufficient as an implementation specification.

## Units and conventions

- field_elev and all altitude outputs are in ft.

- altimeter is in inches of mercury (inHg); if supplied in hPa, it must first be converted, approximately hPa / 33.8639.

- OAT and ISA_temp are in degrees Celsius.

- The coefficient 120 has units of ft per degree Celsius.

- For the test vectors, ISA temperature is approximated as ISA_temp = 15 - 1.98*(PA/1000), in degrees Celsius, within the troposphere.

## Inspectable vector

One of 4 source-bound review vectors:

````
{
  "inputs": {
    "field_elevation_ft": 0,
    "altimeter_setting_inHg": 29.92,
    "OAT_C": 15
  },
  "expected_outputs": {
    "pressure_altitude_ft": 0,
    "density_altitude_ft": 0
  },
  "working": "PA = 0 + (29.92 - 29.92)*1000 = 0 + 0 = 0 ft.\nISA_temp = 15 - 1.98*(0/1000) = 15.0 C.\nDA = 0 + 120*(15.0 - 15.0) = 0 + 120*0 = 0 ft."
}
````

## Failure boundary

**BREAK_CLAIMED** · SELF_FLAGGED

The extreme-magnitude vector produces Infinity for both outputs in plain JavaScript arithmetic. The instrument should reject or flag such inputs rather than expose non-finite altitudes. The supplied specification has no actual denominator-based undefined-result trap and no explicitly defined multi-regime formula branch.

## Other records

[01**Overview**](<https://guthlabs.ai/docs/instruments/pressure-altitude-density-altitude/>)[03**Implementation**](<https://guthlabs.ai/docs/instruments/pressure-altitude-density-altitude/implementation/>)[04**Qualification**](<https://guthlabs.ai/docs/instruments/pressure-altitude-density-altitude/qualification/>)

## Sources and evidence

These pointers bind the page to evidence. They do not grant authority or collapse verification into deployment.

- `VERIFIED_LEDGER.json`

- `evidence-tranche4/tranche4-results.json`

- `BOUNDARY_HAZARD_CATALOG.json`