PPL Written Test – Airplane Instruments

In the Northern Hemisphere, a magnetic compass will normally indicate a turn toward the north if an airplane is accelerated while on an east or west heading.

During flight, the magnetic compass indications can be considered accurate only when in straight-and-level, unaccelerated flight. During acceleration, deceleration, or turns, the compass card will dip and cause false readings.

Magnetic fields produced by metals and electrical accessories in the airplane disturb the compass needle and produce errors. These errors are referred to as compass deviation.

Acceleration and deceleration errors on magnetic compasses do not occur when on a north or south heading in the Northern Hemisphere. They occur on east and west headings.

Due to the northerly turn error in the Northern Hemisphere, a magnetic compass will initially indicate a turn toward the west if a right (east) turn is entered from a north heading.

In the Northern Hemisphere, a magnetic compass will normally indicate a turn toward the south if an airplane is decelerated while on an east or west heading.

In the Northern Hemisphere, a magnetic compass normally initially indicates a turn toward the east if a left (west) turn is entered from a north heading.

The pitot system provides impact pressure, or ram pressure, for only the airspeed indicator.

The pitot-static system is a source of pressure for the altimeter, vertical-speed indicator, and airspeed indicator. The pitot tube is connected directly to the airspeed indicator and provides impact pressure for it alone. Thus, if the pitot tube becomes clogged, only the airspeed indicator will become inoperative.

The pitot-static system is a source of air pressure for the operation of the altimeter, airspeed indicator, and vertical speed indicator. Thus, if the pitot and outside static vents become clogged, all of these instruments will be affected.

The pitot-static system is a source of air pressure for the operation of the airspeed indicator, altimeter, and vertical speed indicator. Thus, if the static vents become clogged, all three instruments will become inoperative.

The red line on an airspeed indicator indicates the maximum speed at which the airplane can be operated in smooth air, which should never be exceeded intentionally. This speed is known as the never-exceed speed.

The maneuvering speed of an airplane is an important airspeed limitation not color-coded on the airspeed indicator. It is found in the airplane manual ( Pilot's Operating Handbook) or placarded in the cockpit. Maneuvering speed is the maximum speed at which full deflection of the airplane controls can be made without incurring structural damage. Maneuvering speed or less should be held in turbulent air to prevent structural damage due to excessive loads.

The caution range is indicated by the yellow arc on the airspeed indicator. Operation within this range is safe only in smooth air. The airspeed indicator in Fig. 4 indicates the caution range from 165 to 208 MPH.

The maximum speed at which the airplane can be operated in smooth air is indicated by the red radial line. The airspeed indicator in Fig. 4 indicates the red line is at 208 MPH.

The full flap operating range is indicated by the white arc on the airspeed indicator. The airspeed indicator in Fig. 4 indicates the full flap operating range is from 60 to 100 MPH.

The never-exceed speed is indicated by a red line and is found at the upper limit of the yellow arc. Operating above this speed may result in structural damage.

The lower airspeed limit of the green arc indicates the power-off stalling speed in a specified configuration. "Specified configuration" refers to flaps up and landing gear retracted.

The maximum flaps-extended speed is indicated by the upper limit of the white arc. This is the highest air speed at which a pilot should extend full flaps. At higher airspeeds, severe strain or structural failure could result. The upper limit of the white arc on the airspeed indicator shown in Fig. 4 indicates 100 MPH.

The normal flap operating range is indicated by the white arc. The power-off stall speed with flaps extended is at the lower limit of the arc, and the maximum speed at which flaps can be extended without damage to them is the upper limit of the arc.

The lower limit of the white arc indicates the power-off stalling speed with wing flaps and landing gear in the landing position.

The maximum structural cruising speed is the maximum speed for normal operation and is indicated as the upper limit of the green arc on an airspeed indicator. The upper limit of the green arc on the airspeed indicator shown in Fig. 4 indicates 165 MPH.

Altimeter 2 indicates 14,500 ft. because the shortest needle is between the 1 and the 2, indicating about 15,000 ft; the middle needle is between 4 and 5, indicating 4,500 ft; and the long needle is on 5, indicating 500 ft., i.e., 14,500 ft.

The altimeter has three needles. The short needle indicates 10,000-ft. intervals, the middle-length needle indicates 1,000-ft. intervals, and the long needle indicates 100-ft. intervals. In altimeter 1, the shortest needle is on 1, which indicates about 10,000 ft. The middle-length needle indicates half-way between zero and 1, which is 500 ft. This is confirmed by the longest needle on 5, indicating 500 ft., i.e., 10,500 ft.

Altimeter 3 indicates 9,500 ft. because the shortest needle is near 1 (i.e., about 10,000 ft.), the middle needle is between 9 and the 0, indicating between 9,000 and 10,000 ft., and the long needle is on 5, indicating 500 ft.

Altimeters 1 and 2 indicate over 10,000 ft. because 1 indicates 10,500 ft. and 2 indicates 14,500 ft. The short needle on 3 points just below 1, i.e., below 10,000 ft.

Absolute altitude is altitude above the surface, i.e., AGL.

True altitude is the actual altitude above mean sea level, i.e., MSL.

Density altitude is the pressure altitude corrected for nonstandard temperature.

Pressure altitude and density altitude are the same when temperature is standard.

Indicated altitude (what you read on your altimeter) approximates the true altitude (distance above mean sea level) when standard conditions exist and your altimeter is properly calibrated.

Pressure altitude equals true altitude when standard atmospheric conditions (29.92" Hg and 15C at sea level) exist.

Pressure altitude is the airplane's height above the standard datum plane of 29.92" Hg. If the altimeter is set to 29.92" Hg, the indicated altitude is the pressure altitude.

Altimeter setting is the value to which the scale of the pressure altimeter is set so that the altimeter indicates true altitude at field elevation.

When increasing the altimeter setting from 29.15 to 29.85, the indicated altitude increases by 700 ft. The altimeter-indicated altitude moves in the same direction as the altimeter setting and changes about 1,000 ft. for every change of 1" Hg in the altimeter setting.

Atmospheric pressure decreases approximately 1" of mercury for every 1,000 ft. of altitude gained. As an altimeter setting is changed, the change in altitude indication changes the same way (i.e., approximately 1,000 ft. for every 1" change in altimeter setting) and in the same direction (i.e., lowering the altimeter setting lowers the altitude reading). Thus, changing from 30.11 to 29.96 is a decrease of .15 in., or 150 ft. (.15 x 1,000 ft.) lower.

When an altimeter setting is at a lower value than the correct setting, the altimeter is indicating less than it should and thus would be showing lower than the actual altitude above sea level.

When flying from higher pressure to lower pressure without adjusting your altimeter, the altimeter will indicate a higher than actual altitude. As you adjust an altimeter barometric setting lower, the altimeter indicates lower.

In air that is warmer than standard temperature, the airplane will be higher than the altimeter indicates. Said another way, the altimeter will indicate a lower altitude than actually flown.

The airplane will be lower than the altimeter indicates when flying in air that is colder than standard temperature. Remember that altimeter readings are adjusted for changes in barometric pressure but not for changes in temperature. When one flies from warmer to cold air and keeps a constant indicated altitude at a
constant altimeter setting, the plane has actually descended.

On warm days, the atmospheric pressure levels are higher than on cold days. Your altimeter will indicate a lower than true altitude. Remember, "low to high, clear the sky."

The horizon bar (marked as B) on Fig. 7 represents the true horizon. This bar is fixed to the gyro and remains on a horizontal plane as the airplane is pitched or banked about its lateral or longitudinal axis, indicating the attitude of the airplane relative to the true horizon. An adjustment knob is provided, with which the pilot may move the miniature airplane (marked as C) up or down to align the miniature airplane with the horizontal bar to suit the pilot's line of vision.

The direction of bank on the attitude indicator (AI) is indicated by the relationship of the miniature airplane to the deflecting horizon bar. The miniature airplane's relative position to the horizon indicates its attitude: nose high, nose low, left bank, right bank. As you look at the attitude indicator, you see your airplane as it is positioned with respect to the actual horizon. The attitude indicator in Fig. 7 indicates a level right turn.

There really are no yaw and roll axes, i.e., an airplane yaws about its vertical axis and rolls about its longitudinal axis. However, this is the best answer since the turn coordinator does indicate the roll and yaw movement of the airplane. The movement of the miniature airplane is proportional to the roll rate of the airplane. When the roll rate is reduced to zero (i.e., when the bank is held constant) the instrument provides an indication of the rate of turn.

Due to gyroscopic precession, directional gyros must be periodically realigned with a magnetic compass. Friction is the major cause of its drifting from the correct heading.