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. On this part of the site 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.

Performance Factor, Weight

The weight of an aircraft 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 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.

Performance Factor, Air Density

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.

International Standard Atmosphere
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 30 hPa per 30 feet at lower altitudes (1" per 1000 feet), temperature drop of 1.98 C per 1000 feet (300 m) up to 36000 feet.

Density Altitude

Density Altitude Chart Density altitude is the combined effect of the factors mentioned above. It is defined as height in the standard atmosphere. 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 above (or below) the standard atmosphere.

For a larger density altitude chart click here or on the small image. Usage: first set a horizontal line representing the temperature, then draw pressure altitude and drop down vertical to obtain density altitude.

When taking off at a density altitude above ISA sea level, you still will see the same indicated airspeed. But because air density is lower the true airspeed will be higher and thus ground speed is higher. To get to the same indicated airspeed with the same engine power you will need more runway.
Note: Aircraft indicated stall speed is always the same when density altitude is higher or lower. Density altitude influences the true airspeed / ground speed and not the indicated airspeed.

Try this one day (only do this when you have adequate runway available): To simulate a takeoff from 3000 feet field elevation, ie full throttle (30" manifold pressure) open the throttle to maximum of 27" manifold pressure and note how much more runway you will need and the difference in acceleration of the aircraft.

Normally aspirated engines

When your engine doesn't have a turbo- or supercharger 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).

The effects of density altitude on takeoff and landing are:

• Takeoff distance is increased one percent for every 100 feet of aerodrome pressure above sea level, landing distance one percent for every 400 feet.
• Takeoff distance is increased one percent for every 1 C above the standard temperature for the aerodrome elevation
• Rate of climb and angle of climb are reduced, as is 50 feet obstacle clearance distance after takeoff

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.