Density altitude explained: why high, hot and humid days hurt performance

What density altitude is

Air density decides performance. Thinner air means the engine takes in less air and makes less power, the propeller has less air to push and makes less thrust, and the wing has less air to deflect, so it needs a higher true airspeed to make the same lift.

Three things make air thinner: altitude, heat and humidity. Density altitude folds them into one number: the altitude in the standard atmosphere that has the same density as the air you are actually flying in. High density altitude means poor performance.

Working it out: pressure altitude first

Start with pressure altitude, the height above the standard pressure level of 29.92 inches of mercury. The quickest way to read it is to set 29.92 in the altimeter's window. Otherwise, correct field elevation by about 1,000 feet for every inch the altimeter setting differs from 29.92: add when the setting is below 29.92, subtract when it is above.

Then correct for temperature. The standard atmosphere is 15°C at sea level and cools about 2°C for every 1,000 feet. The warmer the actual air is than standard at that altitude, the higher the density altitude. Performance charts and flight computers do this step precisely; a common rule of thumb adds roughly 120 feet for every degree Celsius above standard.

• Example: a field at a pressure altitude of 5,000 feet, where standard temperature is about 5°C.
• On a 30°C afternoon the air is 25°C warmer than standard.
• 25 × 120 feet adds about 3,000 feet: a density altitude of roughly 8,000 feet.

Humidity, the factor charts leave out

Water vapor is lighter than the dry air it displaces, so humid air is less dense. Most performance charts do not include humidity, which means that on a hot, humid day the airplane will usually do somewhat worse than the chart predicts. Plan for it rather than trusting the chart to the foot.

What it does to the airplane

At high density altitude a normally aspirated trainer needs a much longer takeoff run, climbs weakly, and flies a higher true airspeed on approach and landing at the same indicated airspeeds, so it covers more ground and needs more runway to stop.

Climb suffers most of all, because climb depends on excess power, and high density altitude takes power away. A climb gradient that clears trees easily on a cool morning may not clear them at all on a hot afternoon.

• Longer takeoff ground roll and longer distance to clear a 50-foot obstacle.
• Lower rate and angle of climb.
• Higher true airspeed and groundspeed at the same indicated airspeed.
• Longer landing roll.

How the test asks it

Expect questions that ask which conditions give the highest density altitude (high, hot and humid), what density altitude does to takeoff distance or climb, and how to find pressure altitude from an altimeter setting. Chart questions give a pressure altitude and temperature and ask you to read takeoff distance or climb rate; read the chart's notes, and when a value falls between rows, interpolate or take the less favorable one.