Hot Air

Hot Air

Hot air rises.

We see evidence of this as we see steam rising from a hot liquid, in the operation of a hot air balloon, and in flames rising from a fuel source. The reason for this is that as a gas or liquid raises in temperature it becomes less-dense and will rise above cooler surroundings.

The increase in temperature causes the molecules of the medium to vibrate more quickly, and a vibrating molecule needs more room to move than does a still molecule. (Actually, a molecule will have some movement until the temperature drops to absolute zero — 460◦ below zero Fahrenheit). As a wing of an airplane must operate in the air, the density of the air, the actual distance between the molecules, has a direct relationship to how that wing will perform.

 

Aircraft performance is based on atmospheric conditions that are known as a “standard.” That “standard” at sea level is 59◦F or 15◦ Celsius, and an atmospheric pressure of 29.92 inches of mercury. The predicted performance of the aircraft shown in the pilot operation manual is based on these conditions. Therefore a predicted take off roll of 800 feet shown in the performance charts of an aircraft must be adjusted if the conditions are above or below the “standard” conditions.

A pilot is not usually concerned if the conditions are cooler than standard, because in that case the aircraft will perform better in the denser air of the cooler day…however the pilot must take notice if the temperature is substantially warmer than standard. In the case of a warmer than standard day, the aircraft performance will suffer due to the less-dense air. The wing, the propeller, and the engine will not achieve optimal performance, as the molecules of air are farther apart than in cooler conditions, and the wing and propeller must be supported by the molecules of air.

 

The engine of the airplane will suffer in performance, as it cannot get the same amount of air to mix with the fuel during warmer conditions. Some engines are able to compensate for this by additional accessories, such as intakes that draw cooler air from outside the engine compartment, or by using a turbocharger or supercharger that compresses the intake air before it reaches the cylinder.

The pilot-in-command must compute takeoff and landing distances due to the effects of less-dense air. An airplane that would use 800 feet of runway to get airborne on a standard day may use twice or three times as much runway on a very hot day. These factors can severely affect the ability of an aircraft to get off the ground.

As an example, the altitude of Santa Ynez Airport is 671 feet above sea level. If the temperature during the day rises to 100◦, then the density altitude, the altitude of the airport adjusted for the higher than standard temperature, is nearly 3,400 feet. The pilot must consult his operating manual to determine how many feet of runway the airplane will require in those conditions.

Most aircraft will perform adequately at a density altitude of 3,400 feet, and there will be sufficient runway for a safe takeoff. However if the airport is at a higher altitude such as in Colorado, New Mexico, or Utah, a careful consultation of the manual must be made to determine if a safe takeoff can be made.

 

One of the highest airports in California is at Lake Tahoe. At 6,264 feet above sea level, this airport is a favorite with pilots and tourists for the beautiful surroundings and resorts available. During the summer, it would not be unusual for air temperatures to reach 85◦F…a nice day. But a pilot with a non-turbocharged aircraft must make some careful calculations prior to attempting a takeoff from Tahoe. A calculation would show that the density altitude of nearly 9,300 feet would decrease performance to the point that if loaded to maximum weight, the aircraft may not have enough runway (8,500 feet at Tahoe) to accelerate to take off speed and then make a safe climb.

At this high density altitude, the engine is unable to produce its rated power, and in the less-dense air the aircraft could be in serious trouble as it nears flying speed and is only able to get off the ground a few feet…a condition known as ground effect allows the wing to gain some lift within a few feet of the ground, but the wing will not fly effectively above the ground effect zone.

 

The wing of the airplane will only fly when it achieves a sufficient airspeed to produce lift. At a high density altitude, the actual speed (true airspeed) of the aircraft must be much higher as the less-dense air will have less effect on the wing, therefore producing a lower indicated airspeed. Not only is this difference noticeable in takeoffs but also in landing rolls. As the true airspeed is substantially higher, the groundspeed will also be higher during the landing phase. A pilot must also compute the amount of runway needed for landing to determine if enough runway is available for a safe landing.

The highest airport in the U.S. is at Leadville, Colorado; 9,927 feet above sea level. An 85-degree Fahrenheit day in Leadville would calculate to nearly 13,700 feet density altitude. Many piston engine aircraft would be unable to achieve flying speed at that density altitude and an unsafe condition would exist for takeoff. A prudent pilot would wait until a cooler part of the day or may decrease the weight of the aircraft by offloading fuel or cargo to achieve a weight that would allow a safe takeoff.