By Robert N. Rossier, EAA 472091
This piece originally ran in Robert’s Stick and Rudder column in the June 2022 issue of EAA Sport Aviation magazine.
A few months ago, while checking conditions for an upcoming flight, I was astounded at the density altitude. According to the automated surface observing system, it was -3,400 feet. That’s right — 3,400 feet below sea level. Today is a totally different story. The chill of winter has been replaced by a steamy, sunbaked summer day, resulting in a density altitude of 2,200 feet. Through the change of seasons, we’ve seen a 5,600-foot swing in density altitude, all while our field elevation has remained constant at 83 feet MSL. The effect is somewhat stunning. Since that winter morning, my takeoff distance has increased by roughly 50 percent, and my climb rate has easily taken a 250 foot per minute hit. Clearing that FAA 50-foot obstacle at the end of the runway now takes an easy 500 feet more than it did in frozen February.
Perhaps more concerning when flying a light twin, performance on a single engine can be especially dismal when density altitude begins to soar. Where we might be able to climb or at least maintain altitude on a single engine under normal conditions, bumping up our density altitude by a few thousand feet might mean the best we can do on a single engine is a downhill ride. For an IFR flight, we might not be able to maintain the minimum en route altitude or the minimum obstacle clearance altitude unless both engines are running.
Understanding Density Altitude
The textbook definition of density altitude is pressure altitude corrected for nonstandard temperature. But we can also think of it simply as performance altitude. The aircraft will perform as if it is at the density altitude, rather than the actual altitude or field elevation. Even if our altimeter tells us we’re at 2,000 feet, if the density altitude is 4,000 feet, the aircraft will perform as if it was at 4,000 feet.
Remember that the temperature and pressure altitude are just two factors in determining density altitude. Since water vapor is less dense than air, high humidity also drives up density altitude. These sultry summer days can provide an unexpected challenge unless we’re factoring density altitude into our performance and planning data.
While we may revel in the warmth of summer, we should be careful to measure the effects of warmer conditions on our aircraft performance. Aerodynamic lift is greatly influenced by the density of the air. As the air becomes thinner (higher density altitude), we need to increase our velocity through the air to create the same amount of lift. As density altitude increases, our true airspeed increases.
Thinner air also translates to reduced power for normally aspirated (nonturbocharged) engines. Thinner air means less air and oxygen are sucked into the cylinders with every intake stroke, and that means less power is developed on the power stroke. Every thousand feet increase in density translates to a 3 percent reduction in an engine’s power.
But it isn’t just our wings and engines that suffer from reduced performance. As airfoils, our propellers create less thrust as the density of the air decreases. Reduced propeller efficiency combined with reduced engine power is of particular concern during takeoff, resulting in longer takeoff runs and reduced climb rates. And since we’re flying faster at high density altitude, our climb gradient is also diminished by increased density altitude.
The good news is that we can count on the airspeed indicator to tell us when we’ve achieved flying speed. Although we’ll be traveling over the ground at a higher speed when taking off at a high density altitude, the airspeed indicator will still provide such critical information as stall speed, flap speeds, gear extension speed, and maneuvering speed.
Estimating Density Altitude
We can often learn the current density altitude simply by listening to the automatic terminal information service or the automated weather systems available at airports across the country. But even if we don’t have that information, we can estimate the density altitude if we know the field elevation and the current temperature. Charts in our pilot’s operating handbook (POH) or flight planning software can be used to calculate the density altitude. Failing that, we can make an estimate based on a rule of thumb.
To estimate density altitude, add 600 feet to field elevation for every 10 degrees Fahrenheit higher than standard temperature. For an 89-degree day at sea level (30 degrees hotter than standard), we can estimate the density altitude at 1,800 feet (600 x 3). As you may recall, standard temperature at sea level is 59 Fahrenheit, and it decreases 3.5 degrees Fahrenheit for every 1,000 feet. For an airport at 3,000 feet MSL, standard temperature is slightly less than 49 degrees Fahrenheit, so for an 89 degree day, the density altitude increases by 2,400 feet (600 x 4) to 5,400 feet.
Evaluating Performance Changes
The more critical aspect of flight planning for high density situations is determining the effect that density altitude will have on our takeoff and landing performance. While we might be tempted to apply some rules of thumb, the best answer is to refer to the performance charts in our POH.
Different manufacturers provide their performance data in different forms. Some use tables; some use graphs. Most manufacturers provide adjustments based on pressure altitude with temperature corrections, or with adjustments based on density altitude. Whatever aircraft we fly, and whatever form is used, we owe it to ourselves to become familiar and proficient in calculating performance for takeoff, climb, cruise, and landing. When the data shows we’re nearing the limits, we still have choices. We may be able to reduce our weight by removing baggage, fuel, or passengers in order to achieve the needed performance, or we can wait until the high temperatures subside, and fly then.
One effect overlooked by unwary pilots is that climb speeds (VX and VY) change with density altitude. Our best angle of climb speed (VX) increases and our best rate of climb speed (VY) decreases as density altitude increases. The effect is usually just a few knots, but even that can be critical, so be sure to review the POH.
Beyond knowing how to determine takeoff and landing performance for high density altitude, we may need to adjust some of our technique to ensure we get the best performance available.
One consideration of which we should be aware is that when operating at high density altitude (typically in excess of 3,000 feet), we may need to lean the mixture at maximum power to obtain the best performance for takeoff. If using an EGT, lean the mixture at full power to reach maximum temperature, and then enrichen until the temperature decreases by 75 degrees Fahrenheit. For an aircraft not equipped with an EGT, lean the mixture at full throttle until the rpm peaks and begins to drop. Then enrichen as far as possible while still maintaining peak rpm. These are of course guidelines, and we should always abide by the manufacturer’s operating procedures.
Some fuel-injected aircraft have a fuel flow meter calibrated for different altitudes, which can also be used for leaning the engine for takeoff at high density altitude. Use the fuel flow settings provided in the POH for the prevailing conditions.
Although the aircraft performance was stellar last winter, I don’t miss the cold — not even a little bit. But we should all pay attention to prevailing conditions, and consider all the factors that density altitude impacts in our performance and planning.
Robert N. Rossier, EAA 472091, has been flying for more than 40 years and has worked as a flight instructor, commercial pilot, chief pilot, and FAA flight check airman.