50% of the participants in our poll found density altitude as their chief concern during the summer months, so let’s break it down.
A Case Study: A Commercial Pilot and the Unsuspecting Danger of Density Altitude
[The following are excerpts from the NTSB Final Report for accident number WPR17FA171.]
On July 29, 2017, about 1420 Pacific daylight time, a Cessna 172N airplane, N5381J, was substantially damaged after it collided with mountainous terrain shortly after departure from Big Bear City Airport (L35), Big Bear, California. The commercial pilot and passenger were fatally injured.
According to airport personnel, the pilot and his girlfriend arrived at L35 in the accident airplane the day before the accident. After they deplaned, the pilot proposed to his girlfriend on the airport ramp. The following day, an airport surveillance camera captured the airplane depart runway 08 normally and begin a climb. According to a witness located near the departure end of the runway, the airplane came into view about 100 ft above ground level. He stated that the airplane did not gain any altitude as it entered a nose-high attitude that he described as “hanging on the prop.” Both he and his wife, who was with him at the time, yelled to the airplane to “lower the nose.” The engine sounded smooth and continuous. As the airplane reached a park adjacent to the departure end of the runway, it turned to the crosswind leg of the airport traffic pattern momentarily before turning 90° to a tight downwind leg. The witness remarked that both turns appeared coordinated; however, the airplane maintained a nose-high pitch attitude. The airplane then sank slowly and the wings began to rock back and forth before the airplane disappeared behind the trees.
Another witness, located near the crosswind leg of the runway 08 traffic pattern, observed an airplane fly low toward the south over him. The airplane began a turn to the right, then disappeared over the back side of an adjacent hill. The witness reported that the airplane’s engine sounded continuous.
The 1615 recorded weather observation at L35 included wind from 090° at 7 knots, 10 statute miles visibility, scattered clouds at 8,500 ft, temperature 25°C (77° F), dew point 5°C (41° F), and an altimeter setting of 30.33 inches of mercury.
An NTSB weather study showed a density altitude of 9,138.9 ft msl about the time of the accident. The pressure altitude at the time of the accident was 6,362.2 ft.
L35 was located at an elevation of 6,752 ft above mean sea level and was equipped with one asphalt runway in a 08/26 configuration. At the end of each runway are digital signs that show the current density altitude. The density altitude is also announced over the airport’s automatic weather observation system.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: The pilot’s exceedance of the airplane’s critical angle of attack during takeoff in high density altitude conditions, which resulted in an aerodynamic stall, loss of control, and subsequent impact with terrain. [Emphasis added.]
This is a sad and unfortunate case study. While we cannot go back in time to stop this fateful flight from happening, we can learn from it and hopefully keep ourselves and passengers from encountering the same dangers of density altitude.
What Is Density Altitude?
Density altitude is defined as pressure altitude corrected for nonstandard temperature. This means that density altitude expresses the inverse relationship between pressure altitude and temperature into a quantitative altitude that pilots can understand and relate to.
- For example, Perry, Florida, has an altitude of 42 feet MSL. On June 16, 2022, the temperature was 35°C degrees Celsius. The barometric pressure was 30.10 in. of Hg. The pressure altitude (which is what the indicated altitude would be if the altimeter was set to 29.92 in. of Hg) was -138 feet with a density altitude of 2,264 feet.
- This means an airplane taking off from Perry, FL would perform as if it was really 2,264 feet MSL instead of 42 feet MSL.
- A laymen’s analogy would be that density altitude is the altitude equivalent of the heat index, where the altitude provided is the altitude that your body (or airplane) would feel.
How Does This Inverse Relationship Work?
When temperature at a given altitude is higher than standard temperature, the density altitude is higher. The opposite is also true: when temperature at a given altitude is lower than standard temperature, the density altitude is lower.
Why Do Pilots Care About an Inverse Relationship?
As the density altitude increases, the air becomes less dense. Thus, in high density altitude situations, there are fewer air molecules. Consequently, in high density altitude situations, your aircraft will perform worse, with less climb performance and acceleration.
Why Is Performance Worse at High Density Altitudes?
The higher the density altitude, the more lift, thrust, and power are reduced.
- Lift is reduced because less-dense air exerts less force on airfoils.
- Thrust is reduced because a propeller is less efficient in less-dense air.
- Power is reduced because the engine has less air to make power.
How Humidity Contributes to a Higher Density Altitude
Humidity also plays a role in causing a higher density altitude. Moist air is less dense than dry air. Higher temperatures allow air to hold more water vapor. During the summer, hot days are often accompanied by higher humidity, which exacerbates the issue and causes density altitude to be much higher than true altitude (MSL) or pressure altitude (altimeter set to 29.92 in. Hg).
At What Altitudes Should I Begin to Worry about Density Altitude?
ALL ALTITUDES. Many pilots believe density altitude only comes into play in the mountains. While the higher the absolute altitude, the more density altitude may come into play, density altitude should be evaluated at every altitude, especially in the summer.
- For example, on July 16, 2022, Key West, FL (KSGL), is mere feet from the Florida Straight. With an absolute altitude of 10 feet, it had a density altitude of 1,643 feet!
How to Determine Density Altitude
KEYW Field Elevation = 3 feet Temp. 31 °C A30.09
DA (ft) = Pressure Altitude (feet) + 120 x (OAT (°C) – ISA (°C))
DA = -167 feet + 120 x (31 °C – 15 °C)
DA = -167 feet + 120 x (16 °C)
DA = -167 feet + 120 x 16 °C
DA = -167 + 1,920
1st Step after Knowing Pressure Altitude
Use your AFM to determine the performance effects of density altitude on your ground roll for both takeoff and landing. If there are obstacles, use the AFM to determine the aircraft’s ability to clear the obstacles safely. As a safety precaution, consider adding more roll to your takeoff or landing than your AFM requires so you can increase margin for safety. An AFM is required for aircraft certification and provides specific performance criteria. Therefore, your takeoff and landing roll will most likely benefit from more distance than the AFM requires.
Mitigation Strategies
- Flying in the mornings or evenings, when it’s cool, allows you to fly when the density altitude will most likely be the lowest.
- Less weight is also best when flying, but especially so in high density altitude situations. Not too dissimilar to a pilot hauling heavy luggage from the car to the plane, the less your plane has to haul on a hot and humid day, the happier the plane is going to be.
- The fuel mixture may need to be set less rich to maximize your engine’s performance. As always, consult your POH for the proper procedures specific to your aircraft.
- Always use the entire runway and do not be embarrassed to back taxi on the runway to get every extra foot of runway length. Always add to the margin of safety.
- Whenever you determine your takeoff and landing roll, always expect that you’ll need more distance. The more margin of safety you’ve added, the better.
What Should I Have Learned?
Density altitude has contributed to fatalities. When the temperature and humidity are high, which is typical in the summer months, density altitude will be higher. During preflight planning, use your AFM to determine what your predicted takeoff and landing roll is. Add to this roll to provide a wider margin for safety. Flying during cooler times of the day and adjusting your fuel mixture accordingly can help mitigate some of the effects of higher density altitudes encountered during the summer months.