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What does a negative pressure-altitude result mean in the model?
A field at 120 ft with an altimeter of 30.36 inHg gives a pressure altitude of −320 ft; keep the minus sign when density altitude is computed.
Why the sign can turn negative
Crabwise uses the FAA handbook rule PA = elevation + (29.92 − altimeter) × 1000. When the setting is above 29.92 inHg the bracket is negative and the correction is subtracted. At a low field on a high-pressure day the result can fall below zero. A negative pressure altitude is a point in the standard atmosphere where pressure is higher than at standard sea level, not a height below the ground.
The app accepts the setting in inHg or hPa and converts hPa to inHg before applying the rule. The negative value then flows unchanged into the ISA and density-altitude steps.
Worked example
An entered elevation of 0 ft and a 30.42 inHg setting give −500 ft in the simple adjustment. This is not a measurement of terrain clearance.
- Signed pressure altitude (ft)
- -500
Keep the signed value
- Enter the field elevation in feet.
- Enter the altimeter setting and check the inHg or hPa selector.
- Read the pressure altitude with its sign.
- Carry the same signed value into the density-altitude calculation.
The Pro step-by-step view shows the substitution 120 + (29.92 − 30.36) × 1000 beside the result, so the setting that turned the bracket negative stays visible.
A cold, high-pressure morning
Elevation 120 ft, altimeter 30.36 inHg: PA = 120 + (29.92 − 30.36) × 1000 = −320 ft. With an OAT of 5°C, the ISA temperature at −320 ft is 15 − 1.98 × (−0.32) = 15.6336°C, so the deviation is −10.6336°C. Density altitude comes out at about −1610.671123 ft.
- Pressure altitude (ft)
- −320
- ISA temperature (°C)
- 15.6336
- ISA deviation (°C)
- −10.6336
- Density altitude (ft)
- −1610.671123
If the same field reports 1030 hPa instead, the app converts it to about 30.415883 inHg. Pressure altitude is then about −375.882564 ft, the ISA temperature about 15.744247°C, and with the same 5°C OAT the density altitude is about −1680.298583 ft. A higher setting pushes both values further below zero.
Common mistakes
- Replacing −320 with 0 before the density-altitude step.
- Typing a setting in hPa while the selector shows inHg.
- Reading a negative density altitude as a terrain or obstacle figure.
- Dropping the minus sign when copying the pressure altitude into a log or another tool.
Scope
The rule is the handbook approximation, and the model's domain starts at −2000 ft. The FAA Pilot's Handbook below explains pressure altitude.
Educational calculation only. Check current aircraft limits, weather, performance and reserve requirements with approved information.