Weather - Nonstandard Pressure | Training Blog

How do you correct an altimeter for nonstandard pressure? How inaccurate is an altimeter that is using an incorrect barometric pressure setting?

Cover image: AI-generated editorial illustration based on the instrument panel of GCAC Cessna 172K N84455.

An altimeter does not measure height directly. It senses atmospheric pressure and converts that pressure into an altitude indication. When that pressure changes, the setting in the altimeter has to change with it.
 
What Altimeter Setting to use?
Correct for nonstandard pressure by setting the current reported altimeter setting in the Kollsman window.
​The Kollsman window is the small pressure-setting window on a conventional altimeter. Turning the adjustment knob moves the barometric scale and changes the altitude indication.

​Below 18,000 feet MSL, 14 CFR § 91.121 requires using either:

1. The current reported altimeter setting of a station along the route and within 100 nautical miles of the aircraft. Depending on the operation, that information may come from ATC, ATIS, AWOS, ASOS, or another appropriate reporting station.
2. For aircraft without radios or a means to procure the altimeter setting from an appropriate station, you can set the altimeter using the elevation of the departure airport.

At or above 18,000 feet MSL, use the standard setting of 29.92 inHg.
​
The Club's earlier standard-pressure lesson explains why 29.92 inHg is the standard-atmosphere reference. It is not the correct local setting for ordinary flight below 18,000 feet MSL.

The adjustment knob moves the pressure scale shown in the altimeter setting window. Source: FAA Pilot's Handbook of Aeronautical Knowledge, Chapter 8, Figure 8-2, p. 8-3.

Estimate the Pressure-Setting Error
For a quick estimate, 1.00 inHg of setting error is approximately 1,000 feet of altitude error. Therefore:
0.10 inHg ≈ 100 feet
0.25 inHg ≈ 250 feet
0.50 inHg ≈ 500 feet
 
If the altimeter is set to 29.95 but the current setting is 29.70, the setting is 0.25 inHg too high. The altimeter will indicate approximately 250 feet too high, so the airplane is approximately 250 feet lower than indicated.

For the pressure-setting error alone, use this method:

1. Setting in the window - current setting = pressure difference
2. Pressure difference × 1,000 feet ≈ indicated altitude error
3. Approximate pressure-corrected altitude = indicated altitude - signed error

 
Using this week’s example:

1. 29.95 - 29.70 = +0.25 inHg
2. +0.25 × 1,000 = +250 feet of indicated error
3. If the altimeter indicates 3,000 feet, the pressure-only estimate is approximately 2,750 feet

The positive error means the altimeter is reading high. If the setting in the window were lower than the current pressure, the error would reverse and the airplane would be higher than indicated.
 
This is an approximation for understanding the size and direction of a pressure-setting error. It is not a substitute for setting the altimeter correctly.

​This section is summed up neatly by one of our favorite aviation mnemonics:
“Going from high to low, watch out below!”

At the same indicated 3,500 feet, flying from higher pressure toward lower pressure without updating the setting leaves the airplane lower than indicated. GCAC instructional adaptation of FAA Aviation Weather Handbook, Chapter 8, Figure 8-13, p. 8-12.

Pressure and Temperature Are Different Errors
Adjusting the Kollsman window compensates for nonstandard pressure. It does not compensate for nonstandard temperature.
 
Cold air can place the airplane lower than indicated even when the current pressure setting is in the window. Our nonstandard-temperature lesson explains that separate effect.
 
That distinction matters: the quick pressure calculation estimates one source of error, not every difference between indicated and true altitude.

Make It a Cockpit Habit 
Before takeoff, set the current reported pressure and compare the altimeter indication with the known field elevation. During flight, listen for updated settings and put them in the window promptly.

Standard checklists included with all Club aircraft include an altimeter setting check as part of the Taxi Checklist. Additionally, the Run-Up checklist includes a sanity check to verify that the indicated altitude matches the known field altitude.
 
Club members can also practice reading and adjusting a conventional altimeter with the interactive altimeter simulator on GCAC's Aviation E-Teaching Tools page.
 
Common Mix-Ups

• Treating 29.92 inHg as the normal local setting below 18,000 feet MSL
• Reversing the error direction when pressure falls
• Calculating the difference but leaving the old setting in the window
• Assuming a current pressure setting also corrects for nonstandard temperature

 
In a Nutshell
Set the appropriate current pressure in the Kollsman window. If you need to estimate the consequence of a wrong setting, use approximately 1,000 feet per 1.00 inHg.
 
With 29.95 set instead of 29.70, the altimeter reads approximately 250 feet high, placing the airplane approximately 250 feet lower than indicated. Then remember that temperature, instrument, and installation errors are separate considerations.
 
FAA Sources and References
1.  14 CFR § 91.121, Altimeter settings.
2.  FAA Aeronautical Information Manual, Chapter 7, Section 2, paragraphs 7-2-1 through 7-2-3.
3.  FAA Pilot's Handbook of Aeronautical Knowledge, FAA-H-8083-25C, Chapter 8, pp. 8-3 through 8-6 and Figure 8-2.
4.  FAA Aviation Weather Handbook, FAA-H-8083-28B, Chapter 8, pp. 8-10 through 8-12 and Figure 8-13.

Next Week
Airport Operations - Non-Towered Airport Traffic Pattern
 
What is the recommended entry into a non-towered airport pattern when approaching from the opposite side of the pattern?
 
Arriving from the side opposite the traffic pattern takes more planning than simply aiming for the downwind. Next week, we'll compare the FAA's entry options and look at how altitude, traffic scanning, descent location, and local airport context fit together before the airplane joins the pattern.