instrument lessons #2 (pitot statitoc and vacuume system)

Pitot Static System

• Crucial for IFR (Instrument Flight Rules) flying; expect questions about it during orals.
• Essential to understand its function, failure modes, and troubleshooting.

Components of the Pitot Static System

• Pitot tube: Measures dynamic pressure.
• Static port(s): Measure static pressure; located on the side of the airplane.
• Instruments: Airspeed indicator, altimeter, and vertical speed indicator (VSI).

System Overview

• Pitot tube connected only to the airspeed indicator.
• Static port connected to airspeed indicator, altimeter, and VSI.
• The pitot-static system provides data to these three instruments.

Function of Each Component

• Pitot Tube: Provides ram air (dynamic pressure) to the airspeed indicator.
• Static Port: Provides static pressure to the airspeed indicator, altimeter, and VSI.

Airspeed Indicator

• Determines airspeed by measuring the difference between static pressure (from the static port) and dynamic pressure (ram air from the pitot tube).
• Connected to both the pitot tube and static port.
• Contains a diaphragm that expands or contracts based on ram air pressure.
  • As airspeed increases, ram air increases, the diaphragm expands, and the airspeed indicator displays a higher speed.
  • As airspeed decreases, ram air decreases, the diaphragm contracts, and the airspeed indicator displays a lower speed.

Altimeter

• Uses an aneroid wafer, which is sealed with a pressure of 29.92 inches of mercury (standard pressure setting).
• Static air from the static port enters the casing.
• Altitude is determined by comparing the pressure inside the sealed wafer to the static pressure in the instrument casing.
  • As the aircraft climbs, outside pressure decreases, the wafer expands, and the altimeter indicates an increase in altitude.
  • As the aircraft descends, outside pressure increases, the wafer contracts, and the altimeter indicates a decrease in altitude.

Vertical Speed Indicator (VSI)

• Compares static pressure to a change in static pressure.
• Two lines of static pressure are connected to the VSI.
• One line provides instant static pressure from the static port to the diaphragm.
• The other line measures changes in static pressure through a calibrated leak.
• The calibrated leak causes a slight time delay, which is why the VSI is a secondary instrument.
  • When climbing, the diaphragm contracts instantly due to lower static pressure. The calibrated leak slowly lets air out, showing a vertical speed increase.
  • When descending, the diaphragm expands instantly due to higher static pressure. The calibrated leak slowly allows air in, showing a vertical speed decrease.

Clogs and Malfunctions of the Pitot Static System

• Clogs often occur due to ice accumulation or obstructions like bugs.
• A clogged pitot tube affects only the airspeed indicator.
• A clogged static port affects the airspeed indicator, altimeter, and VSI.

Clogged Pitot Tube

• The airspeed indicator will act like an altimeter.
• As the aircraft climbs (lower pressure), the airspeed indicator will increase.
• As the aircraft descends (higher pressure), the airspeed indicator will decrease.

Clogged Static Port

• Airspeed Indicator: Displays erroneous information.
  • If the static port is clogged at e.g., 5,000 feet, the airspeed indicator will use that pressure as a reference.
  • Climbing to a higher altitude will result in a lower-than-normal airspeed reading.
  • Descending to a lower altitude will result in a higher-than-normal airspeed reading.
• Altimeter: Freezes at the altitude where the clog occurred because the pressure in the casing doesn't change.
• VSI: Drops to a zero indication and freezes because there's no change in static pressure.

Troubleshooting

• Clogged Pitot Tube: Use pitot heat to melt any ice blockage.
• Clogged Static Port: Open the alternate static source.

Alternate Static Source

• Draws static air from inside the aircraft cabin.
• Cabin air is not completely static due to air movement, resulting in lower pressure than outside.
• With the alternate static source, expect a slight increase in indicated airspeed, altitude, and momentary change in vertical speed.
• Pilots should close windows, vents, and turn off windshield defrost/heat to make cabin air as static as possible.

Effects of Using Alternate Static Source

• Slight increase in airspeed due to the lower pressure.
• Slight increase in altitude due to the lower pressure.
• VSI will momentarily show an increase in vertical speed, then return to normal.

Vacuum System

• Powers the attitude indicator and heading indicator (HSI).
• In older aircraft, the turn coordinator may also be connected to the vacuum system.

Components and Operation

• Vacuum pump: Connected to the aircraft engine; sucks air through the system.
• Air filter: Removes contaminants from the air.
• Suction gauge: Measures the amount of suction.
• Relief valve: Prevents over-pressurization.
• Airflow Cycle: Air enters through the air filter, passes through the vacuum pump, flows to the attitude and heading indicators, goes through a relief valve, and vents overboard.

Gyroscope Operation

• Air passing through the attitude and heading indicators spins up gyroscopes within the instruments.
• Gyroscopes operate based on two principles: rigidity in space and precession.
• Rigidity in Space: A gyroscope resists changes to its orientation.
• Aircraft rotates and pitches around the gyroscopes.

Gyroscopic Precession

• When a force is applied to a gyroscope, the resulting force is felt 90 degrees ahead in the direction of rotation.
• If the gyroscope is spinning and a force is applied at the front, the force will be felt 90 degrees from that point.

Gyroscope Limitations

• Gyroscopes can tumble when excessive forces are applied.
  • Attitude indicator: May tumble with a pitch attitude of 60 to 70 degrees up or down.
  • Heading indicator: Excessive bank angles can cause tumbling.
  • Turn coordinator: Excessive bank angles can cause tumbling.