PERFORMANCE AND LIMITATIONS

Performance and Limitations Lesson by wifiCFI


Performance and Limitations (PHAK C11)

The performance or operational information section of the Aircraft Flight Manual/Pilot’s Operating Handbook (AFM/ POH) contains the operating data for the aircraft; that is, the data pertaining to takeoff, climb, range, endurance, descent, and landing. 

The use of this data in flying operations is mandatory for safe and efficient operation. 

Considerable knowledge and familiarity of the aircraft can be gained by studying this material.

Types of Altitude (PHAK C11) The 5 types of Altitude: Indicated Altitude

True Altitude

Absolute Altitude

Pressure Altitude

Density Altitude

Indicated Altitude: The altitude read off the Altimeter in flight.

What do the different needles indicate?

What is the short fat needle?

Altitude in thousands of feet

What is the long skinny needle?

Altitude in hundreds of feet

What is the skinny needle with the triangle on the end?

Altitude in tens of thousands of feet

True Altitude:

The height above mean sea level (MSL).

Absolute Altitude:

The height above the ground (AGL).

Pressure Altitude:

Pressure altitude is the height above the standard datum plane (SDP). The aircraft altimeter is essentially a sensitive barometer calibrated to indicate altitude in the standard atmosphere.

If the altimeter is set for 29.92 "Hg SDP, the altitude indicated is the pressure altitude.

The SDP is a theoretical level at which the pressure of the atmosphere is 29.92 "Hg and the weight of air is 14.7 psi. 

As atmospheric pressure changes, the SDP may be below, at, or above sea level. 

Pressure altitude is important as a basis for determining aircraft performance, as well as for assigning flight levels to aircraft operating at above 18,000 feet.


How to figure Pressure Altitude:

Method 1:

Set the aircraft’s altimeter to 29.92 and the indicated altitude will be Pressure Altitude

Method 2: 

Minus the current altimeter setting from 29.92 (SDP). 

Times that number by 1000

Add that number to field elevation

Example:

Current Altimeter setting of 30.12

29.92 – 30.12 = -0.20

-0.20 x 1000 = -200

Field elevation = 4473 ft

4473 – 200 = 4273 Pressure Altitude

Method 3:

Use the chart located in PHAK Chapter 11.

Density Altitude:

The more appropriate term for correlating aerodynamic performance in the nonstandard atmosphere is density altitude, the altitude in the standard atmosphere corresponding to a particular value of air density.

Density altitude is pressure altitude corrected for nonstandard temperature. 

As the density of the air increases (lower density altitude), aircraft performance increases. 

Conversely, as air density decreases (higher density altitude), aircraft performance decreases. 

A decrease in air density means a high density altitude; an increase in air density means a lower density altitude.

It is essentially the altitude the aircraft is performing at, regardless of its actual altitude.

Density altitude is used in calculating aircraft performance.

Example:

Aircraft’s actual altitude above sea level = 4473ft MSL

Density Altitude for the day = 7,000ft

The aircraft will perform as if it were at 7,000ft NOT its actual altitude of 4473ft.

Effects of Temperature on Density Altitude:

Density altitude is determined by first finding pressure altitude and then correcting this altitude for nonstandard temperature variations.

Hotter temperatures = Higher Density Altitude

Worse aircraft performance

Cooler temperatures = Lower Density Altitude

Better aircraft performance

Effects of Moisture on Density Altitude:

Water vapor is lighter than air; consequently, moist air is lighter than dry air. 

Therefore, as the water content of the air increases, the air becomes less dense, increasing density altitude and decreasing performance.

The opposite is also true.

Effects of Pressure on Density Altitude:

Density is directly proportional to pressure. 

If the pressure is doubled, the density is doubled, and if the pressure is lowered, so is the density. 

This statement is true only at a constant temperature.

The 4 types of Airspeed:

Indicated Airspeed

Calibrated Airspeed

True Airspeed

Groundspeed

Indicated Airspeed: The airspeed read directly off the airspeed indicator. What is indicated airspeed used for? Speed limits

Speed restrictions from ATC

The aircraft’s Vspeeds

Airspeed Markings What does the bottom of the white arc represent?

Vso (stall speed in the landing configuration)

What does the bottom of the green arc represent?

Vs (stall speed in the clean configuration)

What does the top of the white arc represent?

Vfe (max flap extension speed)

What does the top of the green arc represent?

Vno (normal operating range)

What does the yellow arc represent?

Caution range

What does the red line represent?

Vne (never exceed speed)

Calibrated Airspeed:

It is IAS (Indicated Airspeed) corrected for pitot-tube installation.

Airspeed indications are most accurate when the pitot-tube points directly into the airflow.

However, at varying angles of attack, the pitot-tube’s position varies in relation to the relative wind.

This causes small indication errors that are published, by the manufacturer, as Calibrated Airspeed.

True Airspeed:

The speed of the aircraft relative to the airmass in which it is flying.

True Airspeed increases 2% per thousand feet of Density Altitude increase.

Why does True Airspeed Increase with Altitude?

Because of the decrease in drag at higher altitudes.

The air at higher altitudes is less dense than the air at lower alitudes.

Hence, the decrease in aircraft drag.

Groundspeed:

The aircraft’s speed across the ground.

Groundspeed is True Airspeed corrected for wind direction and velocity

What is groundspeed used for?

Time and distance calculations

How do I calculate my aircraft’s groundspeed?

Use of the E6B

Vspeed Definitions:

Vso = power off stalling speed in the landing configuration

Vs1 = power off stalling speed in a specified configuration

Vy = best rate of climb speed

Maximum increase in altitude per unit of time

Vx = best angle of climb speed

Maximum increase in altitude per horizontal distance unit

Vle = max speed the aircraft can be operated with the landing gear extended

Vlo = max speed at which the landing gear can be safely extended or retracted

Vfe = max permissable speed with the wing flaps extended to a specified degree

Va = design maneuvering speed (see “Principles of Flight” lesson for more info)

Vno = maximum structural cruising speed

Vne = the speed that should never be exceeded in flight

Performance (PHAK C11)

Performance is a term used to describe the ability of an aircraft to accomplish certain things that make it useful for certain purposes.

The primary factors most affected by performance are: 

Takeoff and landing distance

Rate of climb

Ceiling

Payload

Range

Speed

Maneuverability

Stability

Fuel economy

Interpolation:

Not all of the information on the charts is easily extracted.

Some charts require interpolation to find the information for specific flight conditions.

Interpolating information means that by taking the known information, a pilot can compute intermediate information. 

However, pilots sometimes round off values from charts to a more conservative figure. 

Using values that reflect slightly more adverse conditions provides a reasonable estimate of performance information and gives a slight margin of safety.

Airspeed Calibration Chart:

Cover and explain for your aircraft (POH Chapter 5).

Stall Speed Chart:

Cover and explain for your aircraft (POH Chapter 5).

Takeoff Distance Chart:

Cover and explain for your aircraft (POH Chapter 5).

Temperature Conversion Chart:

Cover and explain for your aircraft (POH Chapter 5).

Cruise Performance Chart:

Cover and explain for your aircraft (POH Chapter 5).

The Cruise Performance Chart gives pilot’s a lot of information.  Such as: % Brake Horsepower

True Airspeed

Fuel Burn in Gallons Per Hour (GPH)

To correctly use the Cruise Performance Chart, one must know: The aircraft’s cruising altitude

Temperature at the cruising altitude

How that temperature deviates from standard temperature at the chosen cruise altitude

Cruise RPM setting

The aircraft’s Recommended Leaning Procedure

Range Profile Chart:

Cover and explain for your aircraft (POH Chapter 5).

The Range Profile Chart shows the distance our aircraft can fly, at a specified altitude, in Nautical Miles.

This chart also accounts for a 45 minute fuel reserve.

To use this chart properly a pilot must know:

Their cruising altitude

%BHP in cruise at the chosen altitude

Performance Charts (POH C5)


Endurance Profile Chart:

Cover and explain for your aircraft (POH Chapter 5).

The Endurance Profile Chart is ran the same way as the Range Profile Chart.

Instead of NM range, the Endurance Chart gives us distance in Hours.

Crosswind Component Chart:

To properly run the Crosswind Component Chart the pilot must first:

Know the wind direction and velocity

Know the takeoff runway

Compute the angular difference between the wind direction and the takeoff runway

Example

Wind direction = 250

Wind velocity = 20kts

Takeoff Runway = 210

Angular difference = 40

Answer

Crosswind Component = 13kts

Headwind Component = 15kts

FAA Sources Used in this Lesson

Pilot’s Handbook of Aeronautical Knowledge (PHAK) Chapter 11

Pilots Operating Handbook (POH)


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