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.

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

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.

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.

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.

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.

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.

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.

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

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.

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.

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.

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.