Density Altitude and the Effect of Altitude on Aircraft Performance

A discussion of density altitude on aircraft performance is natural for someone learning to fly in Colorado. Being blessed with more than 300 Visual Meteorological Condition (VMC) days a year and visibility which often stretches more than a hundred nautical miles over the front range of the Rocky Mountains is both awe-inspiring and often times humbling, particularly for pilots who learned to fly closer to sea level. I'd like to clarify a few definitions first in my discussion of why density altitude is such an important environmental factor which affects pilots of anything from the illustrious J-3 Cub to jets such as the Embraer 145 and especially helicopters, such as my personal favorite steed, the HH-60 Blackhawk (seen below in our Ft. Carson training area near the base of Pikes Peak, approximately 6500 feet above sea level.)

Definitions (FAA, 2016):

• Pressure Altitude - The height above the standard datum plane. Essentially, this is the aircraft altitude above mean sea level based on a standard pressure setting of 29.92. 
• Density Altitude - Pressure Altitude corrected for nonstandard temperature. According to the FAA and ICAO, a standard atmosphere is based on a temperature of 15 degrees Celsius at a Pressure Altitude of 0 feet and a standard decrease in temperature of 2 degrees Celsius per thousand feet of altitude above sea level. 
• International Standard Atmosphere (ISA) - ISA is the baseline temperature at a given altitude based on the above-mentioned standard lapse rate. So at a pressure altitude of 2000 feet, the standard temperature would 11 degrees Celsius, or ISA + 0. At jet altitudes such as 35,000 feet, the standard temperature would be -54.3 degrees Celsius. See below Chart from the PHAK

A temperature above ISA will result in a high density altitude than pressure altitude. Practically speaking, this means a aircraft will perform as if it is at a higher altitude than the altimeter would read. Density altitude can be calculated using an E6B flight computer if you're old like me, or often times can be calculated in the aircraft Flight Management System or GPS functions menu. But why do we care about this theoretical number?

Effects of Altitude on Aircraft Performance

• Higher than standard temperature results in lower than standard air density- that is, hot air is less dense than cold air. 
• Air density affects the aircraft in two significant areas, the wings and the engines. 
• Less dense air creates less lift over a wing (or propeller)
• Less Dense air is more difficult to combust, resulting in a decrease in horsepower or thrust

As Lift and Thrust are the two factors required for an aircraft to fly, a higher density altitude degrades both of these. Significantly, higher density altitudes require longer runways and reduce climb rate in airplanes. Helicopters face a different dilemma, in that a helicopters rotors are required to maintain a constant speed to create lift. Less lift from the rotor system requires more power from the engines, which are forced to operate at a lower output because of the less dense air.

Example:

• A Citation Ultra business jet (seen above) at sea level on a standard day would require approximately 3400 feet of runway to take off at its max takeoff weigh of 16,300 pounds. 
• The same aircraft at the same weight would require 5130 feet to takeoff at an airport 6,000 feet above sea level on a 20 degree Celsius day. 
• After takeoff the same aircraft would take only 16 minutes to climb to an altitude of 35,000 feet from sea level on standard day
• The aircraft departing from 6,000 feet on a 20 degree Celsius day would take 20 minutes to reach 35,000 despite having a 6,000 foot head start. 

So What?

• Piston Aircraft typically reach their absolute ceiling, the altitude at which the aircraft can no longer climb at all, between 11,000 and 14,000 feet on a standard day. 
• In the above example of 6000 feet and 20 degrees Celsius, the density altitude at take off is already 7912 feet, and this would be considered a cool summer day in Denver. By midsummer, when temperatures easily reach 30+ Celsius, the Density Altitude approaches 9-10,000 feet and there is almost no surplus performance to allow an aircraft to climb over obstacles, such as those Rocky Mountains John Denver told us all about.

How Do We Counteract Density Altitude?

• Training and preparation are key to high density altitude flying. From piston aircraft to jets, a thorough preflight plan is required to account for reduced performance on take off, in climb, and for the always-expected go-around. Options include reducing weight by carrying fewer passengers or less fuel and operating earlier in the day before temperatures increase.
• In areas of high terrain such as mountain passes, it is recommended that aircraft clear all passes by a minimum of 2500 feet and avoid flying in winds greater than 20 knots, as the downdrafts created on the leeward side of mountains in winds greater than this will usually exceed the aircraft's performance ability to overcome.  
• In turbine aircraft, higher Density Altitudes usually result in a higher operating temperature (less air, more fuel, higher temperatures) and so preflight planning must take these operating limitations into account, particularly in the event of an engine failure. 

The 

References
Federal Aviation Administration (2016) Pilot's Handbook of Aeronautical Knowledge. FAA Flight Standards Service

Flight Safety Inc (1999) Citation V Ultra Operating Manual - Rev 4. 

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