Aircraft Performance

Factors Affecting Performance

Factors affecting air density are:

1. Barometric pressure
2. Altitude
3. Temperature
4. Humidity

Standard Atmosphere Definition

The International Standards Association (ISA) has defined a Standard Atmosphere as:

• Sea Level Barometric Pressure of 29.92 inches of Mercury (in. Hg)
• Sea Level Temperature of 15° Celsius (15° C or 59° F)
• Relative humidity of 0 %
• Standard temperature lapse rate of 2° C per 1000 feet altitude
• Standard pressure lapse rate of 1 in. Hg per 1000 feet altitude
• A standard decrease in density as altitude increases

Effects of Nonstandard Air Density

Air Density decreases:

• With Air Temperature Increase
• With Altitude Increase
• With Humidity Increase
• With Barometric Pressure Decrease

With lower air density:

• The engine develops less power.
• The propeller produces less thrust.
• The wings produce less lift.

This results in:

• Longer takeoff run
• Poorer climb performance
• Longer landing distance

Density Altitude

Density altitude is a way of relating the density of the air you are in compared to the standard atmosphere. Three atmospheres are illustrated. The Standard Atmosphere (29.92 in. Hg and 15 degrees Celsius) in middle shown in gray. A less dense atmosphere (A ) (lower pressure and/or Higher Temperature) is shown on the right in red. A more dense atmosphere (B) (higher pressure and/or Colder Temperature) is illustrated on the left in blue.

If you are at an actual (true) altitude at location A in atmosphere (A) (the red atmosphere on the right), you will have to go to altitude (A') in the Standard Atmosphere to find the same air density. This altitude in the standard atmosphere at (A') is called the DENSITY ALTITUDE. Similarly, if you are at atmosphere (B) (colder or high pressure shown as blue on the left) the air will be more dense than standard. Therefore you will have to go down to a lower actual altitude in the standard atmosphere at (B') to find the equivalent air density. This equivalent altitude in the Standard Atmosphere is the DENSITY ALTITUDE.

Density Altitude Calculations

• Using conversion charts
• Using the E6B Flight Computer

Density Altitude calculation is a 2 step process.

Pressure Altitude adjusts for pressure difference between your air and standard atmosphere. The question is “What would your altimeter read if you were in a standard atmosphere at your current actual altitude?” This altitude is called PRESSURE ALTITUDE.

• In the aircraft, adjust your altimeter setting to 29.92 in. Hg (standard pressure), and read the altitude value shown by the altimeter needles. Or...
• Apply a correction factor from a Pressure Altitude Correction Table as shown below.
  

NOTE: If your barometric pressure is not shown in the table (say a value such as 30.35) you will have to interpolate to get the correct pressure altitude adjustment.

Step 2. Find Density Altitude

Density Altitude uses Pressure Altitude as a basis, and adds in a correction factor for nonstandard temperature.

Calculate Density Altitude using:
1. PRESSURE ALTITUDE and
2. Outside Air Temperature (OAT)

· Use E6B Flight Computer (see E6B instruction book)
· Use Density Altitude Chart like the one shown below.


For Example: If you found the Pressure Altitude, doing either of the steps cited above, to be 4000 feet, and the outside Air Temperature (OAT) is 16° , do the following on the chart to find Density Altitude.

Locate 16° C on bottom scale. Go vertically up to intersect the 4000 foot Pressure Altitude slanted line (blue line). Go left horizontally (blue line) to read Density Altitude = 5000 feet from the left side scale. You have now adjusted for the difference from standard temperature by using the chart.

Aircraft Performance Charts

Aircraft Performance Charts state performance figures in standard atmosphere conditions.

Takeoff Performance

Takeoff performance data shown in the manufacturers' charts indicates the minimum runway requirements necessary for successful takeoff. Any factor that adversely affects the takeoff distance must be taken into account to insure safe operation. Consider that the listed minimum distance is for standard atmospheric conditions, ideal runway and wind conditions.

Glide performance is the distance that the aircraft will glide with an inoperative engine. The best distance is attained by gilding at an angle of attack that provides the maximum lift/drag ratio (L/Dmax).

In the event that the engine becomes inoperative, it is important to establish the maximum glide airspeed as quickly as possible. This will permit the maximum radius of emergency landing options. While gliding toward a suitable landing area, effort should be made to identify the cause of the failure. If time permits, an engine restart should be attempted as shown in the start-up check list.

The Pilot Operating Handbook will contain a Climb Performance chart or Table similar to the one below for a given aircraft. Note that 4 different tables are provided. (Sea Level, 5000 ft, 10,000 ft and 15,000 ft). Note that these altitudes are PRESSURE ALTITUDES and the respective temperatures are Standard Temperatures for those altitudes. In other words, the values are given for standard Density Altitudes.

Example:
Given: Gross weight 2000 lb: Pressure Alt. 5000 ft: Temperature 61° F.

The rate of climb is 610 at 5000 feet pressure altitude and standard temperature of 41° F. Since the temperature is 20° F higher that the standard 41°, subtract 40 feet per minute from the 610, to get a rate of climb = 610 - 40 = 570 ft/min.

Climb performance depends on the aircraft’s reserve power or thrust. Reserve power is the available power above that required to maintain level flight at a given airspeed. If an aircraft requires only 120 horsepower for a given cruise, and the engine is capable of delivering 180 hp., then the reserve horsepower available for climb is 60 hp.

Two airspeeds are important to the climb performance. These are:

• Vx Best Angle of Climb
• Vy Best Rate of Climb
  

These V-speeds are defined in the POH. The Best Angle of Climb produces the greatest altitude in a given distance. The principal use of Best Angle of Climb is for clearing obstacles on takeoff The Best Rate of Climb produces the greatest altitude over a given period of time. It is predominately used as climb to cruise altitude.

Many of the same factors that affect takeoff and cruise performance also affect climb performance.

Heavy weight requires a higher angle of attack to develop adequate lift. The increased drag results in poorer climb performance. It takes longer to attain cruise altitude and requires the engine to develop full power for a longer period of time.

Consult the POH for Climb Performance data for the aircraft to be flown.

The cruise performance can be specified two ways.
· Maximum Range
· Maximum Endurance

Maximum Range is the distance that an aircraft can fly at a given power setting. It requires maximum speed versus fuel flow. Maximum Duration is the maximum time the aircraft can fly. This requires that the flight condition must provide for a minimum of fuel flow.

Takeoffs and landings under significant cross wind conditions can be dangerous and should be avoided. Crosswinds can be so strong that the sideways drift cannot be sufficiently overcome by using a “side slip” into the wind to compensate for the wind drift. Excessive side load on the landing gear can cause gear failure or an upset aircraft. The Maximum Crosswind Component for the aircraft will be listed in the POH. The maximum crosswind is usually about 20% of the landing configuration stall speed. The diagram above can be used to calculate the headwind and crosswind components. For most light aircraft, the maximum tested crosswind component is in the 12 to 15 knot range. In the chart, the numbers around the periphery of the chart mark the degrees difference between the wind and the runway heading (magenta lines). The radial lines are are in 5° increments with numbers on each 10° line.

For example, with a wind of 150° at 30 kt and landing on runway 12 (120°), the degrees of crosswind will be 150° - 120° = 30°. Locate the 30° radial line out from the lower left of the graph. This is the differential between the wind direction and the runway heading. Follow the 30° radial line (magenta) to the 30kt wind arc (blue). A vertical line (blue) from this intersection will be the crosswind component of 15 kts. This is the same as if you had a wind of 15 kts directly from the side.

If you plot a horizontal (blue) line, you will see that your headwind component is 26 kts. This is the same effect as if you had a direct headwind of 26 kts.

The minimum landing distance is attained by landing at the minimum safe speed which allows sufficient margin above the stall speed for satisfactory control and go-around capability. Gross weight and headwind are important considerations in determining minimum landing distance.

Excessive airspeed above that recommended in the POH will significantly increase landing distance. High density altitude increases landing distance. As a rule of thumb, the increase in landing distance is about 3.5% for each 1,000 feet in density altitude.

A number of factors affect braking. A wet, icy or snow covered runway will appreciably decrease braking ability. In crosswinds or gusty conditions, higher than normal approach speed will improve controllability, but will require longer rollout to stop. A down-sloping runway also increases stopping distance.

Braking immediately after touchdown is ineffective because the wings are still producing lift. The pilot should use the natural aerodynamic drag as much as possible to slow the aircraft. Maintain up-elevator to a high angle of attack as long as possible. The nose of the aircraft will settle naturally as airspeed is dissipated. Therefore it is not necessary (and is unwise) to force the nosewheel hard onto the runway.

After touchdown, hold up-elevators during braking to reduce the load on the nosewheel. Avoid severe braking to minimize stress on the nose gear and scrubbing of rubber from the main gear tires.

Gross weight affects stopping ability. Heavy loads and high touchdown speeds result in greater forward momentum, and require significantly more runway than normal. The most critical conditions for landing performance result from some combination of high gross weight , high density altitude and unfavorable wind conditions. These conditions produce the greatest landing distance and require the greatest dissipation of energy by the brakes.