Factors Affecting Performance
Performance of the aircraft depends on the density of the air in which it flies.
Factors affecting air density are:
Standard Atmosphere Definition
The International Standards Association (ISA) has defined a Standard Atmosphere as:
The standard atmosphere definition provides a means for instrument and aircraft manufacturers to specify the performance of their products in a uniform way. This definition was arrived at by studying the average sea level pressure and temperature over a number of years, seasons, and locations around the world.
Seldom will an aircraft be in standard atmosphere conditions. In order to define performance of an instrument or an aircraft in a non-standard atmosphere, conversions must be applied to adjust the readings or performance numbers to agree with the standard atmosphere. This adjustment is called Density Altitude, and will be more fully defined later in this section.
Effects of Nonstandard Air Density
Air Density decreases:
With lower air density:
This results in:
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.
Density Altitude CalculationsDensity Altitude can be found in two ways
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.
Pressure Altitude can be determined two ways.
Airport Altitude = 2367 ft
Altimeter Setting = 30.40 In. Hg
Conversion Factor= -440 feet (from table )
Pressure Altitude = Airport Altitude + Conversion Factor =2367+(- 440) = 1927
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:
· Use E6B Flight Computer (see E6B instruction book)
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.The red line on the chart is a Standard Atmosphere Temperature line.
Performance charts provided by the manufacturer are based on Standard Atmosphere. Therefore you must adjust your current situation (barometric pressure and temperature) to Standard Atmosphere. This is done by calculating your Density Altitude, then using this Density Altitude as the altitude in the manufacturers performance table when interpreting the performance table data.
Aircraft Performance Charts
Aircraft Performance Charts state performance figures in standard atmosphere conditions.
You should consult the manufacturers Pilot Operating Handbook for the aircraft to be flown for takeoff performance tables or graphs.
Takeoff performance is influenced by several factors.
· Adverse conditions
1. High density altitude (high altitude runway, low pressure, high temperature)
2. Runway conditions - mud, soft field, slush, snow, tall grass, rough surface, uphill
3. Tailwind (downwind takeoff)
4. High gross weight or overload
5. High Humidity
· Favorable conditions
6. Low density altitude (low altitude runway, low temperature, high pressure)
7. Downhill runway
9. Low weight
10. Low Humidity
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.
Wind direction and velocity significantly affect takeoff distance. A direct headwind will greatest provide takeoff assist. A 90° crosswind will give no assistance in takeoff. A tailwind component significantly increases the takeoff roll.Gross weight affects takeoff performance.
Increased gross weight:
· Requires a higher takeoff speed in order to achieve sufficient lift.
· Results in reduced acceleration due to greater inertia.
· Increases rolling friction , further reducing acceleration.
Gusting or strong crosswinds require that the aircraft be held on the ground until definite liftoff can be achieved. Once liftoff has occurred, sufficient speed is needed to prevent settling back onto the runway. If the landing gear contacts the runway when in a sideways drift, undue stress is placed on the landing gear.
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.
1. Flaps up, full throttle, mixture leaned above 3000 feet for smooth operation.
2. Fuel Used includes, warm-up and takeoff allowance.
3. For hot weather, decrease rate of climb 20 ft/min for each 10°F above standard day for the particular altitude.
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:
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.
· Higher than Standard Temperature
· High Humidity
· Lower than Standard Pressure
· Heavy Weight
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 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.
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.