The Various Factors Contributing To The Aircraft’s Performance
Most aircraft accidents occur during the takeoff and landing phase of the flight. Collisions with obstacles during climb out, runway overruns on landing occur too often.We will take a look at the various factors contributing to the performance of the aircraft in this part of the flight. Hopefully we help the pilot ensuring safe operation during these phases of the flight as the rules require that of the pilot in command.
Here we take a look on two performance factors, weight and air density. Weight is something we can do about but air density depends on more factors as we will see.The weight of an airplane directly influences the stall speed. A rule of thumb is that 10% increase in weight equals 5% increase in stall speed. And this has its effect on the performance of the aircraft.
Normally, liftoff speed is 15% above stall speed. Thus if weight increases, liftoff speed increases. As a result more time is needed (slower acceleration) to get to that liftoff speed and you will use more runway at the same time. General rule is that a 10% increase in weight means 20% more runway needed for takeoff.
During landing the same effects apply. A heavier commercial aircraft has a higher approach speed (stall speed is higher) and therefore needs more runway to stop. Rule of thumb: 10% more weight means 10% more runway needed when landing.When air density decreases both engine and aerodynamic performance decreases. The reason being is that air molecules are further apart from each other (thus less air per m3/ft3).
A number of factors (altitude/pressure, temperature and humidity) influence the air density. A higher altitude, higher temperature and high humidity all have one result: they lower the density of the air. And as a result of that they lower aircraft performance.
A standard atmosphere has been established to enable comparison of aircraft performance, calibration of instruments (altimeters, transponders encoders, etc) and meteorological purposes. There are a number of standards developed: sea level pressure is 1013.25 hPa (29.92 inHg), a temperature of 15 °C, pressure lapse rate is 1 hPa per 27 feet at lower altitudes (1″ per 1000 feet), temperature drop of 1.98 °C per 1000 feet (300 m) up to 36000 feet.
The performance of your aircraft depends on air density, which has a direct effect on lift, drag, engine performance and the propeller. When air density decreases, aircraft performance decreases. Rule of thumb: Density altitude can be calculated by taking pressure altitude and adding (or subtracting) 120 feet for each 1 °C difference above (or below) the standard atmosphere at that altitude.
When its not a turbo-prop aircraft it will also suffer from the less dense air. Each intake stroke (which is by volume) will contain less air molecules and thus less power can be developed by the engine. Propeller (and wing) efficiency is also reduced at higher density altitudes (for fixed and controllable types).
High density altitudes are most commonly found at high elevation airports in combination with high ambient temperatures. When there is also a low atmospheric pressure system this will accentuate the effect even more. Taking off in these conditions can be dangerous, make sure to check all related performance charts for your aircraft.
