Airplane Performance #

Need-to-know Speeds #

• IAS (Indicated Airspeed): speed measured by onboard instruments
• CAS (Calibrated Airspeed): IAS corrected for instrument errors and other factors
• TAS (True Airspeed): CAS corrected for altitude and temp changes

Not used: ~* EAS (Equivalent Airspeed): The ASI reading corrected for position, for instrument error and for adiabatic ompressible flow for the particular altitude. Equivilant to CAS at sea level in standard atmosphere.~

• Ground Speed: Speed relative to the surface of the earth

List of speeds typically needed for calculations. Located usually in the POH.

Important standardized values #

• Standard atmosphere: 29.92" of mercury to 15c at sea level
• Temperature: -2c for every 1,000 feet to 35,000ft

General #

• The AFM/POH in planes are not standardized and some data is in tabular form rather than graphs.
• As plane ages, the performance degrades and thus do not take chart literally
• Each aircraft has different performance numbers and different ways of computing them, so you must check the performance chart in the POH.

As air is less dense, it reduces:

• Power, engine takes in less air
• Thrust, propeller is in thinner air
• Lift, air exerts less force on airfoils

Pressure Altitude #

• Pressure altitude is the height above the standard datum plane (SDP).
• Aircraft altimeter essentially a sensitive barometer
• The SDP is the theoretical level which the pressure of the atmosphere is 29.92" Hg and weight of air is 14.7 psi.

Determining the pressure altitude #

Can be determined by 3 methods:

• By setting the barometric scale of the altimeter to 29.92"Hg and reading indicated altitude
• Applying a correction factor to the indicated altitude according to the reported “atimeter setting”
• Flight computer

Density Altitude #

• Remember Density Altitude is the altitude that the airplane “thinks” it’s at based on the density of the surrounding air.
• Used to calculate aircraft performance
• Using a flight computer, density altitude can be computed by inputting the pressure altitude and outside air temperature at flight level.

• Koch Chart can be used to determine the increase in take-off distance and initial rate of climb.

If you do not have a koch chart, you can use this rule of thumb:

• Add 10% to the normal take-off ground roll and to the distance to clear a 50ft obstacle for every 1,000ft increase in DA up to DA of 3,000ft. Above 3k, add 20%.

Example:

• Airport at sea level, 29.92Hg and 15c
• Take off would be 1,000ft
• At 25c and same Hg, the DA would now be 1,000ft.
• For increase of 1,000ft add 10%, thus take-off roll would be 1,100ft.

Humidity #

• POH usually are based on perfectly dry air, in reality never completely dry
• Humidity (relative humidity) refers to amount of water vapor contained in the atmosphere.
• As water content in air increases, air becomes less dense, increasing density altitude and decreasing performance
• Higher temperature, greater amount of water vapor air can hold
• No rule of thumb or chart to compute effects of humidity on density altitude, but must be taken into consideration. Expect decrease in overall performance in high humid situations.

Stall speeds #

• 1.3 Vso is used to get the “final” approach speed
• Should ONLY be used on short final

Take-off distance #

Example take-off chart from POH:

• Usually as a chart in the POH.
• Grass runways increase take-off distance (up to 15%) due to grass..
• Hydroplaning is a problem with wet grass

Wind is a big factor:

Climb performance #

There is a “max” (100% throttle / climbing) and a cruising speed that uses the necessary power. For example, a aircraft would use all of 200hp of the engine to climb, but 130hp for cruise.

• Max ROC (rate of climb Vx): Largest amount of climb over distance. Standard departure.
• Max AOC (angle of climb Vy): To get over an object

Service ceiling #

Power control #

RPM #

• Rule of thumb is 100 rpm, or one inch of manifold pressure, procues approx. 5 kt change in airspeed.

Attiude Plus Power Equals Performance #

• An aircraft’s performance is the product of attitude and power. Performance is expressed in terms of airspeed, altitude, rate of climb / descent, or other criteria.
• If either attitude of power is changed, a change of performance will result.

Examples #

Check out the examples from “Aircraft Performance” chapter 11 of the FAA manual.