Density Altitude and Drone Performance: Why Hot, High and Humid Air Matters

Density altitude answers one question: how high does the air feel to your aircraft? The FAA defines it as the altitude in the standard atmosphere that corresponds to a particular air density (FAA Remote Pilot Study Guide, p. 21). When it is high, the air is thin and performance drops; when it is low, the air is dense and performance improves (FAA Remote Pilot Study Guide, p. 21). Two of the knowledge areas on the Part 107 test cover this: the effects of weather on performance, and determining the performance of a small unmanned aircraft (14 CFR 107.73(c) and (h)).

Three altitudes, one idea

Everything starts from a reference day, the International Standard Atmosphere. At sea level it is 15 °C (59 °F) with the barometer at 29.92 inches of mercury, and most aircraft performance data is built on it (FAA Remote Pilot Study Guide, p. 22). Going up through it, temperature drops by roughly 2 °C (3.5 °F) per 1,000 feet (FAA PHAK, p. 11-2).

• Pressure altitude is the reading on an altimeter set to 29.92, which places the sensed pressure at its height in the standard atmosphere (FAA PHAK, p. 11-3).
• Density altitude takes pressure altitude and corrects it for a temperature that isn't standard (FAA PHAK, p. 11-3).
• Field elevation equals the density altitude in standard atmospheric conditions, but conditions are rarely standard (Aviation Weather Handbook, sec. 8.4.1.5).

Why it matters: wherever the aircraft really is, it flies with the performance it would have at the density altitude (FAA PHAK, p. 11-4).

What pushes it up

In short, heat, humidity, high ground and low pressure push density altitude up, and their opposites bring it down (FAA Remote Pilot Study Guide, p. 21).

• Heat. Warming air at a constant pressure makes it less dense (FAA Remote Pilot Study Guide, p. 21). The Aviation Weather Handbook names temperature as the most important factor: hot days bring high density altitudes, cold days lower ones (Aviation Weather Handbook, sec. 8.4.1.5).
• Height and low pressure. Hold the temperature steady and density moves in step with pressure: twice the pressure, twice the density (FAA Remote Pilot Study Guide, p. 21). Higher up, the air is cooler, which adds density, but the pressure is lower, which takes it away. The pressure drop is usually the stronger effect, so expect thinner air the higher you go (FAA Remote Pilot Study Guide, p. 21).
• Humidity. Moisture lightens the air, because water vapor is lighter than dry air, so the more humid it is, the higher the density altitude (FAA Remote Pilot Study Guide, p. 22). Humidity usually isn't counted as an essential part of the calculation, though it still plays a part (FAA Remote Pilot Study Guide, p. 22). No chart or rule of thumb covers it, so simply expect weaker performance on humid days (FAA PHAK, p. 11-5).

The humidity point can seem backward, but the PHAK spells it out: warm, moist air must be less dense than cold, dry air (FAA PHAK, p. 11-5).

Why thin air costs a drone

Less dense air takes away three things, according to the PHAK: engine power, since an engine breathes in less air; propeller thrust, since propellers work less efficiently; and lift, since thin air pushes less on wings and other airfoils (FAA PHAK, p. 11-2). Only the first is about engines. Propellers and wings lose out whatever turns them, so an electric drone is affected too.

Climbing is where you notice it. A climb needs thrust or power to spare, and higher up the aircraft needs more power just as less becomes available (FAA Remote Pilot Study Guide, p. 22). Payload adds to the problem: a heavier aircraft needs more angle of attack, so more drag and less spare thrust for the climb (FAA Remote Pilot Study Guide, p. 22).

Fixed-wing drones have one more trap: the airspeed indicator won't warn you. The usual indicated airspeeds now mean a faster true airspeed and groundspeed, so takeoff and landing need more room and the climb rate drops (Aviation Weather Handbook, sec. 8.4.1.5).

How far from standard are you?

A quick first check is to compare the temperature with the standard temperature for your pressure altitude. The PHAK's own example shows why it matters: with the altimeter at 29.92 reading 5,000 feet and the air 20 °C warmer than standard, the density altitude comes out above 7,000 feet (FAA PHAK, p. 11-4).

Try it on a job of your own. Say a survey site sits at a pressure altitude of 3,000 feet. The standard temperature there is about 15 − (3 × 2) = 9 °C (FAA PHAK, p. 11-2).

• At 7 a.m. it is 8 °C, 1 °C below standard. Cooler air is denser (FAA Remote Pilot Study Guide, p. 21), and density altitude is pressure altitude corrected for the temperature (FAA PHAK, p. 11-3), so it comes out a little below 3,000 feet.
• At 3 p.m. it is 31 °C, 22 °C above standard. Now the density altitude is above 3,000 feet, and the drone flies with the performance of that higher altitude (FAA PHAK, p. 11-4). The PHAK example shows the size of the effect: 20 °C above standard added more than 2,000 feet at a 5,000-foot pressure altitude.

The next section shows how to get an exact figure. The plan doesn't need one: the morning flight gets denser air, more thrust margin and a better climb.

Planning a hot or high job

• Lighten the load. The published maximum takeoff weight isn't a promise for every day. The study guide says a hot, high or humid site may mean taking weight off before you fly (FAA Remote Pilot Study Guide, p. 29).
• Allow for a slower climb (Aviation Weather Handbook, sec. 8.4.1.5), and give obstacles more room.
• Keep more battery in hand. Range is one of the performance factors the FAA lists (FAA Remote Pilot Study Guide, p. 22), and before every flight Part 107 has you confirm the aircraft has the power to last as long as you plan to fly (14 CFR 107.49(d)).
• Know your aircraft's limits. Look for the maximum weights, the high and low temperature limits and the highest operating altitude; FAA guidance encourages manufacturers to provide them (AC 107-2A, para. 8.12.2.5.1). With no manufacturer figures, FAA guidance suggests starting from performance data that people flying the same model have already worked out and published (AC 107-2A, para. B.1).
• Fly early when you can. Low temperatures go with a low density altitude (FAA Remote Pilot Study Guide, p. 21).

Where to find density altitude

• Airport broadcasts. Listen to the ASOS or AWOS by phone or radio: it adds density altitude once that is over 1,000 feet higher than the airport's elevation. An ATIS may add a "check density altitude" advisory (Aviation Weather Handbook, Table 3-27, p. 3-36).
• Work it out. Read the pressure altitude from an altimeter set to 29.92, note the outside air temperature, and let a flight computer do the rest (Aviation Weather Handbook, sec. 8.4.1.5). The PHAK prints a density altitude chart that does the same (FAA PHAK, p. 11-4).

Common misreadings

• "Humid air is heavy." Moist air is lighter than dry air and raises density altitude.
• "High density altitude means dense air." It means the opposite: thin air and poorer performance.
• "My site is at 3,000 feet, so that's my density altitude." That holds in standard conditions, which are rare.
• "It's electric, so it doesn't matter." Propellers and wings still lose out in thin air.
• "I checked this morning." A hot afternoon can change the answer, so check again.

Practice the idea

Density altitude comes up in both the Part 107 weather questions and the loading and performance questions, and each topic has free samples with explanations. The key numbers cheat sheet collects the limits worth memorizing, and the free 20-question practice test shows where you stand. Part 107 Complete ($39, one-time) adds full lessons on performance and weather, a METAR and TAF trainer, and timed mock exams. To see how weight and turns stack on top of thin air, read drone weight and balance and load factor.

Questions people ask

What is density altitude in simple terms?

It is the height in the standard atmosphere where the air would be as dense as the air around you. Wherever your aircraft really is, it flies with the performance it would have at that height (FAA Remote Pilot Study Guide, p. 21; PHAK, p. 11-4).

Does humidity raise or lower density altitude?

It raises it, because moist air is less dense than dry air. It usually isn't counted as an essential factor, but it still plays a part (FAA Remote Pilot Study Guide, p. 22).

Does density altitude matter for an electric drone?

Yes. Thin air reduces propeller thrust and the lift from airfoils, even with no engine taking in air, and high density altitude may call for less weight before flight (PHAK, p. 11-2; FAA Remote Pilot Study Guide, p. 29).

Which factor affects density altitude most?

The Aviation Weather Handbook calls temperature the most important factor. Hot days bring high density altitudes and cold days lower ones (Aviation Weather Handbook, sec. 8.4.1.5).