Ground School Class 3

Altimeter Gauge

Altimeter
Attached to the static air port, a sealed bladder inside the altimeter gauge inflates and deflates with changes in air pressure. The changes in size are transfered via a series of gears and levers to drive the gauge needle. A small dial and window allows for the setting of the current barometric pressure so the instrument reads the correct distance to the ground.

There are several types of altitude:

Indicated - altitude above sea level.
Pressure - 29.92 inches of mercury is the standard datum.
Density - pressure corrected for non-standard temp.
True (MSL) - Above Mean Sea Level.
Absolute (AGL) - Above Ground Level.

The allowable altimeter error for ILR flight is 75ft. Any greater error and the aircraft is no longer legal to fly.

The altimeter needs adjustment every so often (every 100 miles or so) to account for the changes in atmospheric pressure. This is especially important when traversing from high pressure to low pressure or from high to low temperature. As the pressure or temp drops the un-adjusted altimeter starts to show an increase in altitude, causing a correction that is descending rather than maintaining level flight.

There are a few ways to make the altimeter adjustments. The radio is the most common - as you pass near different airports, you can listen in to their ASOS or other automated weather broadcasts that will state the current barometric readings for the area.

Vertical Speed Indicator

Vertical Speed Indicator
Also connected to the static port, the VSI indicates the aircrafts rate of climb or descent in feet per minute. The trend is always accurate; if the needle shows accent, you're climbing, if ti shows descent, you're descending. However the rate indicated has some lag and takes a few seconds to settle down and provide and accurate reading.

Gyroscopic Instruments
These instruments use gyroscopes and the principle of rigidity in space, as the aircraft essentially rotates around the instruments. Gyros are either electric or vacuum powered.

Attitude Indicator

Attitude Indicator
The attitude indicator is also known as the eight ball. It show the pitch and roll of the aircraft in a single reading. The attitude indicator uses a vacuum powered gyroscope.

Directional Gyroscope

Directional Gyro
The directional gyro is just a stabilized compass. It needs to be adjusted via the magnetic compass every 15 minutes in order to remain accurate.

Turn Coordinator

Turn Coordinator
The turn coordinator uses the combination of an electric gyroscope and gravity to help the pilot make good coordinated turns using just the right amount of rudder.

"Whiskey Compass"

Magnetic Instruments
This means the regular compass or "whiskey" compass. It's just a ball floating in liquid, so the compass is hard to read in flight due to it's bouncing around all the time. The instrument has mass and is affected by acceleration, deceleration when traveling east or west and leads or lags turns when traveling north or south. The compass reads magnetic north and all navigation charts are based on true north. Our sectional charts provide the information to calculate a heading correction. An isogonic line follows the curve of the earths magnetic field. The difference between the isogonic line reading and true north is used to correct your navigation heading.

Forces of Flight
There are four forces of flight:

1. Lift
2. Thrust
3. Weight (Gravity)
4. Drag

When in level, non-accelerated flight, all four forces are in equilibrium.

Lift & Wings

Lift is generated by the wings. The wing elements are the leading edge, trailing edge, camber and the chord-line. The chord-line is an imaginary line drawn straight between the center of the leading edge and the center of the trailing edge. The angle of incidence is the angle that the wings are attached to the fuselage. The angle of attack is the angle between the chord-line and the relative wind. The wing will always stall at the same angle of attack, regardless of air speed.