Understand how a pitot static system works with aircraft instruments, their potential errors, and airspeeds relationships.

What did Daniel Bernoulli have to do with flight? Nothing

Learn to recognize pitot static instruments or they will trick you when the malfunction.

What 3 instruments are a part of the pitot static system? (draw the 3 pitot static instruments)

• Airspeed (ram and static)      VSI (static +calibrated leak) Altimeter (static)

 Draw pitot tube: ram air and 2 static holes—connect to all instruments

How many drain holes does the DA40 have? ZERO

Draw Alternate static source attached to the static line

AIRSPEEDS –Instruments simply measure air pressure e.g. Tire gauge.

Rule of thumb: TAS increases about 2% of the CAS per thousand feet of altitude
Why? Temperature and pressure decrease air density, therefore the aircraft must go faster to get the same amount of lift.

Drag decreases squared to the increased altitude (draw graph)

Problem: 10,000 ft altitude, 100 KIAS, GS: 110 knots.  Headwind or Tailwind?

TAS=120 knots (2% x 10(thousands of feet) = 20%; 20% of 100 =20; 100 + 20 = 120)

• GS:      Speed over the ground (TAS corrected for wind)

Errors: What happens if?  Icing

Relative to same TAS	Airspeed Indicates	VSI Indicates	Altimeter Indicates
When altitude is higher/lower than altitude when frozen	Higher	Lower	Higher	Lower	Higher	Lower
Ram Air blocked	Faster	Slower	Same	Same	Same	Same
Static port blocked	Slower	Faster	Same	Same	Same	Same
Ram & Static blocked	Same	Same	Same	Same	Same	Same
Alt Air Open	Faster	Faster	Same	Same	Higher	Higher

Alternate Static source

Open=All Instrument Indications Increase

Draw airplane below

Pressure corrected for altitude	Ambient Air 0 knots	Ambient Air 110 knots	Source Measured air
Pitot Mast	29”Hg	29”Hg around pitot mast	29”Hg =static
Alternate Static	29”Hg	-20”Hg around cockpit	-3”Hg =less than static

Bernoulli’s principle: increase in the speed of the fluid creates decrease in pressure

Daniel Bernoulli

• Cockpit pressure changes with open windows or vents.

Conclusion and Evaluation: Understanding how the instruments work and how physical attributes are measured can help us interpret data correctly and fly safer.

Resources

Definitions:

Atmospheric pressure: the weight of the air molecules; usually measured in inches of mercury(“Hg) by a barometer

Standard Level Sea Pressure: 29.92 “Hg (also in millibars at 1,013.2 mb)

Altimeter: An aneroid barometer adjustable to the local atmospheric pressure setting (corrected for altitude of reporting station) to indicate altitude in feet:  we adjust our altimeter because we fly through air with different pressures

Pitot Static System: The system in the airplane that allows the altimeter to get a correct air pressure reading.

Pressure Altitude: altitude corrected for non-standard pressure—shown on altimeter when set to a sea level pressure of 29.92”

Density Altitude: Pressure altitude corrected for non-standard temperature

Atmospheric Stability: The atmosphere’s ability to resist vertical motion

Standard Lapse Rate: the rate at which temperature decreases with an increase in altitude.  The average lapse rate of dry air is about 3°C/1,000feet, but because moisture is generally in the air, the average is closer to 2°C/1,000feet.

Altimeter setting: atmospheric pressure corrected for altitude

VSI: vertical speed indicator

Airspeeds

IAS: Indicated Airspeed is what is shown on your airspeed indicator. Indicated Airspeed: airplane performance speed (ram air corrected for altitude)

CAS: Calibrated Airspeed: Indicated Airspeed corrected for angle of attack (pitot installation error) (less air requires a higher angle of attack to maintain level flight)

TAS: True Airspeed: the actual speed the aircraft is moving through the air (CAS corrected for non-standard temperature & pressure)

GS: Ground Speed: Speed over the ground

Atmospheric pressure:

PHAK 11-3  Unequal heating of the Earth’s surface creates circulations patterns, causes changes in air density, and causes changes in air pressure or the force exerted by the weight of air molecules.

The actual pressure at a given place and time differs with altitude, temperature, and density of the air.

All local barometric pressure reading are converted to a sea level pressure to provide a standard for records and reports.  To achieve this, each station converts its barometric pressure by adding approximately 1:Hg for every 1,000 feet of elevation.  For example, a station at 5,000 feet above sea level, with a reading of 24.92”Hg, reports a sea level pressure reading of 29.92 “Hg.  Using common sea level pressure readings helps ensure aircraft altimeters are set correctly, based on the current pressure readings.

Relationships:

1” mercury = 1,000 feet

PHAK 7-2

PHAK 7-14

PHAK 10-3