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Density Altitude Lab

How Density Altitude Changes Performance

Set the field elevation, altimeter setting and temperature. The lab shows the density altitude and its effect on a Cessna 172S's takeoff, climb and landing. Performance numbers come from the 172S NAV III POH (172SPHBUS-04), Section 5.

Example conditions
1,384
29.82
43 109 °F
Takeoff weight (lb)POH chart

Distances assume a short-field technique, a paved dry runway and zero wind. Landing uses the 2,550 lb chart, the only landing weight in the POH.

Density altitude
4,930 ft
The airplane performs as if it were 3,546 ft higher than the field.
Pressure altitude
1,484 ft
Field + (29.92 − setting) × 1,000
Standard temperature at this altitude
12.0 °C
31.0 °C warmer than standard
Air density
86%
of sea-level standard
Rule of thumb
5,200 ft
PA + 120 × (OAT − standard)
From Field Elevation to Density Altitude
Altimeter setting+100 ftCorrects for pressure different from 29.92 inHg
Temperature+3,446 ftCorrects for temperature different from standard

The airplane performs as if it were 3,546 ft higher than the field.

Performance in Selected Conditions

Compared with the same airplane at sea level on a standard day (15 °C, 29.92).

Takeoff ground roll
1,346 ft
+40% worse
Sea-level standardNow
Outside the POH chart: extrapolated
Takeoff over 50 ft
2,288 ft
+40% worse
Sea-level standardNow
Outside the POH chart: extrapolated
Rate of climb
554 fpm
−24% worse
Sea-level standardNow
Landing over 50 ft
1,479 ft
+11% worse
Sea-level standardNow
Outside the POH chart: extrapolated
Takeoff over 50 ft
2,288 / 9,300
Landing over 50 ft
1,479 / 9,300

Fits with a 50% margin.

Solid bar: POH distance. Hatched: the 50% safety margin many schools add. Dark tick: end of the runway.

Density Altitude and Aircraft Performance

Temperature and Density Altitude

Each line is one pressure altitude. The dashed line is a standard day. The ring marks the selected conditions.

Takeoff Distance vs. Density Altitude

Takeoff over a 50 ft obstacle at 2,550 lb, from the POH chart, one line per pressure altitude.

Rate of Climb vs. Density Altitude

Rate of climb from the POH climb table, read at density altitude. The ring marks the selected conditions.

What Thins the Air

Higher altitude, higher temperature and higher humidity all lower air density. The engine makes less power and the propeller less thrust. The wing still flies at the same indicated airspeed, but that takes a higher true airspeed, so the takeoff and landing rolls get longer.

About 120 Feet per Degree

Each 1 °C above standard adds roughly 120 ft of density altitude. Standard temperature starts at 15 °C at sea level and drops about 2 °C per 1,000 ft.

The POH Assumes Dry Air

The charts don’t account for humidity. Humid air is less dense, so on a muggy day expect slightly worse performance than the chart shows.

POH Data Behind the Takeoff Numbers
Short-field takeoff at 2,550 lb (Figure 5-5): ground roll / total over 50 ft. Highlighted cells bracket the selected conditions.
Press. alt (ft)0 °C10 °C20 °C30 °C40 °C
Sea level860 / 1,465925 / 1,575995 / 1,6901,070 / 1,8101,150 / 1,945
1,000940 / 1,6001,010 / 1,7201,090 / 1,8501,170 / 1,9901,260 / 2,135
2,0001,025 / 1,7551,110 / 1,8901,195 / 2,0351,285 / 2,1901,380 / 2,355
3,0001,125 / 1,9251,215 / 2,0801,310 / 2,2401,410 / 2,4201,515 / 2,605
4,0001,235 / 2,1201,335 / 2,2951,440 / 2,4801,550 / 2,6851,660 / 2,880
5,0001,355 / 2,3451,465 / 2,5451,585 / 2,7551,705 / 2,9751,825 / 3,205
6,0001,495 / 2,6051,615 / 2,8301,745 / 3,0751,875 / 3,3202,010 / 3,585
7,0001,645 / 2,9101,785 / 3,1701,920 / 3,4402,065 / 3,7302,215 / 4,045
8,0001,820 / 3,2651,970 / 3,5752,120 / 3,8802,280 / 4,2252,450 / 4,615
Data: Cessna 172S NAV III POH 172SPHBUS-04, Figure 5-5 (short-field takeoff), Figure 5-7 (climb), Figure 5-11 (short-field landing). Density altitude from the ICAO standard atmosphere.Rate of climb is the POH standard-day table read at density altitude, an approximation. A training aid for ground school, not for flight planning.