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The 120-ft-per-degree rule is a quick estimate, not a dependable substitute for a density-altitude calculation or an aircraft’s performance charts. A TypeScript model can calculate density altitude from pressure and temperature inputs, but more numerical precision does not make it an aircraft takeoff-performance calculator. For flight planning, use the applicable aircraft AFM/POH and its performance charts.
What density altitude tells you
Density altitude describes air density in terms of the altitude in the standard atmosphere at which that density would occur. It is an indicator of aircraft performance, not a height reference to use in place of pressure altitude, true altitude, or absolute altitude. The FAA’s Aeronautical Information Manual explains that higher temperature and humidity raise density altitude, and that high density altitude reduces aircraft performance.
Thinner air affects both engine and aerodynamic performance. The FAA identifies reduced horsepower and propeller efficiency, increased true airspeed for a given indicated airspeed, longer takeoff and landing distances, and reduced climb rate as effects of high density altitude. Its illustrative example says an average small airplane needing 1,000 feet for takeoff at sea level under standard conditions would need approximately 2,000 feet at an operational altitude of 5,000 feet. That is an example, not a universal aircraft-specific prediction.
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The rule estimates density altitude by adding 120 feet for each degree Celsius that the outside-air temperature is above the International Standard Atmosphere (ISA) temperature at the pressure altitude:
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estimated density altitude = pressure altitude + 120 × (outside-air temperature − ISA temperature)
The temperature difference is in degrees Celsius; the result is in feet. For a temperature below ISA, the temperature-deviation term is negative. This arithmetic is convenient to do mentally, but it is a linear rule of thumb. It does not calculate air density from atmospheric relationships, and it does not provide aircraft-specific takeoff, climb, or obstacle-clearance performance.
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What the TypeScript example calculates
A DEV Community article published October 1, 2025, presents a pure TypeScript function that accepts pressure altitude in feet and outside-air temperature in Celsius. It calculates ISA temperature, temperature deviation, pressure and temperature ratios, and a density ratio; it then derives a density-altitude value using a standard-atmosphere relationship and compares that result with the 120-ft estimate. The article labels its model as based on ICAO Doc 7488: Building Deterministic Aviation Physics in TypeScript.
The article’s listed examples report that the linear estimate differs from its calculation by 68 feet for sea-level summer conditions, 218 feet for Denver (KDEN), 249 feet for Leadville (KLXV), and 306 feet for Death Valley (L06). These are figures reported by the article, not FAA figures; they have not been independently verified here. They illustrate why a simple linear estimate and an atmospheric calculation can produce different outputs in selected scenarios, but they do not prove that the rule always underestimates or fails at a particular altitude.
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What “deterministic” needs to mean in practice
A function with fixed inputs and no external state is easier to reason about and test. But the article’s description of its implementation as deterministic does not establish identical results across JavaScript engines, runtime versions, hardware, or math-library implementations. A cross-platform guarantee would require a defined execution environment and tests that demonstrate the claimed consistency.
A production implementation should document its supported input range, units, invalid-input behavior, rounding policy, and atmospheric model. The article excerpt specifies a standard-troposphere pressure-ratio calculation for altitudes up to 36,089 feet. Implementations should also make clear whether they model humidity or atmospheric layers outside that range; those limits should not be left implicit.
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Why a density-altitude answer is not a runway answer
Density altitude is one input to aircraft performance planning. Actual takeoff and climb performance also depends on the aircraft and its configuration, weight, runway surface and length, wind, and obstacles. The FAA’s Aircraft Performance & Calculations fact sheet emphasizes using the aircraft’s performance information and accounting for runway conditions and obstructions. A generic calculator, even one returning a precise density-altitude value, cannot supply approved aircraft-specific takeoff distance or obstacle-clearance data.
The FAA fact sheet gives separate rules of thumb for takeoff-distance changes: add 15% per 1,000 feet of density-altitude increase for fixed-pitch propeller aircraft up to 8,000 feet, or 12% per 1,000 feet for constant-speed propeller aircraft up to 6,000 feet. These are fact-sheet approximations, not approved performance values for a particular aircraft. The same fact sheet points readers to the aircraft owner’s manual or POH and the associated performance charts for planning.
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Which method to use
| Method | Useful for | Limit |
|---|---|---|
| 120-ft-per-degree estimate | A quick mental approximation from pressure altitude and temperature deviation from ISA. | Linear rule of thumb; not an aircraft-performance calculation. |
| Atmospheric density-altitude calculation | Estimating density altitude from stated atmospheric inputs using a defined model. | Model assumptions and input limits matter; it does not replace aircraft-specific performance data. |
| Aircraft AFM/POH performance charts | Planning for the specific aircraft and conditions covered by its documentation. | Requires the correct aircraft documentation and accurate inputs for the planned conditions. |
For further general study, the FAA’s Pilot’s Handbook of Aeronautical Knowledge is a useful resource. It does not replace the relevant aircraft’s AFM/POH.
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