Tutorial

2(f). Atmosphere

So how do temperature, pressure and density affect an aircraft and its performance?

The effects of temperature changes on an aircraft's altitude:

• When air is warmer than average the airplane will be higher than the altimeter indicates
• When air is colder than average the airplane will be lower than the altimeter indicates
• When temperature lowers en route, the airplane is lower than the altimeter indicates
• When temperature rises en route, the airplane is higher than the altimeter indicates

Pilots can determine the correct pressure altitude from the on-board altimeter by setting the altimeter at the standard altitude reading of 29.92 inches (of mercury at sea level). The altimeter will then indicate the pressure altitude at which the aircraft is flying.

The effects of pressure changes on an aircraft's altitude.

• Flying from a high pressure area to a low pressure area without adjusting the altimeter while maintaining a constant indicated altitude would result in a loss of true altitude
• Flying from a low pressure area to a high pressure area without adjusting the altimeter while maintaining a constant indicated altitude would result in a gain of true altitude

Density altitude is perhaps the most critical to an airplane's performance during takeoff and landing. Results can be disastrous if the density altitude is incorrectly computed. Density altitude is a comparison between the air density at your aircraft's current altitude to the standard atmosphere where the air density is the same. Temperature, pressure and humidity determine air density. Pilots differentiate between high-density altitude and low-density altitude in terms of the performance of an airplane. Let's say that at an airplane's present flight location the day is hot. That means that the air has become thinner (fewer molecules in the air). When that hot location is compared to the standard atmosphere its density is the same as if the aircraft were located at a much higher altitude. That means that the airplane at its current location will act as if it is flying through air that is at a higher altitude. That means the airplane is flying in high-density altitude conditions. Now let's say that the present aircraft location is in very cold air. The air has now become heavier than before (more molecules in the air). When that cold location is compared to the standard atmosphere its density is the same as if the aircraft were located at a much lower altitude. That means that the airplane at its current flight location will act as if it is flying through air that is at a lower altitude or low-density altitude conditions. It is crucial for a pilot to know the density altitude of the airport at which takeoff and landings are planned. The pilot also needs to know the runway's length and the height of structures, trees and land immediately following the runway, so as to ascertain clearance at the climb rate. Knowing the performance specifications of one's airplane is also important. After computing all the necessary altitudes, the pilot needs to know if the airplane can perform safely under all these conditions.

Here are some of the effects of density changes on an aircraft's performance.

• High density altitude conditions reduce an airplane's performance because:
  • the engine is now taking in less air to support combustion so power is reduced;
  • the propellers have less air to move in than under normal conditions so thrust is reduced;
  • a jet engine has less mass of gases exiting the exhaust end so thrust is reduced;
  • there are less molecules in the air, the lighter air exerts less force on the wings which result in reduced lift;
  • reduced thrust and lift means more runway length is needed for takeoff and more clearance area at the end of the runway is needed because of a reduced climb rate.
• Low-density altitude conditions increase an airplane's performance because of:
  • greater thrust than normal due to a greater number of molecules in the air with which propellers and jet engines can interact;
  • greater lift force as heavier air exerts more force on the wings;
  • faster speed and faster climb rate as thrust and lift are increased.