Chapter 8: Flight Instruments
When a pilot understands how each instrument works and recognizes when an instrument is malfunctioning, he or she can safely utilize the instruments to their fullest potential.
Pitot-Static Flight Instruments
The pitot-static system utilizes combined static air pressure and dynamic air pressure. Static pressure — also known as ambient pressure or barometric pressure — is always present whether an aircraft is moving or at rest. Dynamic pressure is created by wind, or by the motion of the aircraft through the air.
The pitot tube measures the total combined pressures when an aircraft moves through the air. A small opening at the front allows the total pressure to enter the pressure chamber. The airspeed indicator (ASI) utilizes the pitot tube, in addition to a static pressure source. The static pressure is canceled so that the ASI only dynamic pressure is indicated.
The static chamber is vented through a static port (or multiple static ports) to the free undisturbed air on the side(s) of the aircraft. Pressure moves freely in and out of the pitot-static instruments.
Some aircraft have an alternate static source, for use if the primary static source becomes blocked. The alternate static source is often inside the cabin. Note that, due to the venturi effect of the air flowing around the fuselage, air pressure inside the cabin is lower than exterior pressure. If the alternate static source is engaged:
The operating handbook will detail variations if the alternate static source is engaged. If an alternate static source is not available, breaking the glass face of the VSI will introduce static pressure and render the instrument inoperative. The VSI is recommended in this case, since it is the least-critical static-source instrument.
The altimeter measures the height of an aircraft above a given pressure level. It contains sealed aneroid wafers evacuated to an internal pressure of 29.92 Hg". The wafers expand (lower pressure) and collapse (higher pressure) with changes to static pressure, which moves the hands on the face of the instrument.
Three needles indicate tens, thousands, and ten-thousands of feet. A crosshatch flag appears on some altimeters when flight operations are below 10,000 feet. Corrected barometric pressure is input via a dial and read in the Kollsman window (barometric pressure setting window). The corrected pressure will indicate height above sea level.
Pressure and temperature are variable, both over distances and over time. If corrected pressure is not input to the altimeter, aircraft flying from an area of high pressure to low pressure will arrive at a lower altitude than indicated ("High to low, look out below."). For this reason, the altimeter is frequently adjusted for corrected pressure during flight. Conversely, flight into an area of higher pressure with an uncorrected altimeter will result in an indicated altitude lower than the actual altitude ("Low to high, look to the sky").
Many altimeters have an operational range of 28.00 to 31.00 Hg". Aircraft in extreme high pressure will indicate an altitude lower than the actual altitude. Flight operations are not recommended in extreme low pressure, since the indicated altitude will be higher than the actual altitude, which is potentially dangerous.
Cold air is denser than warm air. Unless the altimeter is corrected for non-standard temperature, colder air will cause the airplane to fly at an altitude is lower than the altimeter indication. Thus, when flying into a cooler air mass while maintaining a constant indicated altitude, true altitude will be lower. This can create a specific risk regarding terrain clearance. As with air pressure, a change in temperature means "High to low (or hot to cold), look out below." A flight to warmer weather will cause an uncorrected altimeter to indicate an altitude higher than the actual altitude.
The altimeter setting broadcast by ground stations is the station pressure corrected to mean sea level. It does not account for irregularities at higher levels. High, mountainous terrain also can cause the altimeter to indicate altitudes 1,000 feet (or more) higher than actual altitude. Flight planning over mountainous areas should always include additional altitude for safety.
When the actual pressure is lower than what is set in the altimeter window, the actual altitude of the aircraft is lower than what is indicated on the altimeter. One inch of pressure is equal to approximately 1,000 feet of altitude. Therefore, an uncorrected altimeter that is 0.25 Hg" higher than the correct pressure will indicate an altitude 250 feet higher than the correct altitude.
"Altitude" normally means height above sea level, which is used for traffic separation. However, there are various types of altitude:
The density of the air affects how much power a naturally aspirated engine produces, as well how efficient the airfoils are. Fewer air molecules — lower pressure, caused by higher altitudes and/or warmer air — results in reduced aircraft performance.
If an altimeter is off more than 75 feet from the surveyed field elevation, when the local altimeter setting is input, it should be referred for service.
Vertical Speed Indicator (VSI)
The vertical speed indicator is a differential pressure instrument. A diaphragm is connected to the static source, while the casing includes a calibrated leak. Case pressure thus remains higher or lower than static pressure for a period of time, causing an indicated pressure differential during climb or descent. When the indication is stabilized, the rate of altitude change can be determined.
The VSI displays two types of information:
The time period from the initial change in the rate of climb or descent, until the VSI displays an accurate indication of the new rate, is called the lag. If pitch attitude is constant, the needle will stabilize after 6-9 seconds.
The VSI should indicate a near-zero reading prior to leaving the ramp and just before takeoff. If the VSI indicates anything other than zero, that indication can be referenced as the zero mark.
Airspeed Indicator (ASI)
The airspeed indicator is a differential pressure gauge that measures and promptly indicates the difference between pitot (impact/dynamic pressure) and static pressure. In flight (or any headwind), the pressure on the pitot line becomes greater than the pressure in the static lines, which is then indicated by the needle on the face of the instrument. The readout can be calibrated in miles per hour, knots, or both.
Static pressure is introduced into the instrument casing, while dynamic pressure from the pitot tube is introduced into the diaphragm.
There are multiple types of airspeeds:
On general aviation aircraft, the airspeed indicator includes color markings, as required by the FAA:
Additional airspeed limitations include:
Prior to take off, wind may cause the airspeed indicator to indicate a speed greater than zero. On rollout, confirm the airspeed indicator is functional ("Airspeed is alive.").
Blockage of the pitot tube and/or static port can be caused by moisture (including ice), dirt, or insects. The pitot tube may become blocked during flight due to visible moisture. Some aircraft may be equipped with pitot heat for flight in visible moisture.
If the pitot tube becomes blocked and its associated drain hole remains clear, the remaining pressure drops to ambient air pressure and the ASI reduces to zero, since there is no differential between static and dynamic pressure.
If both the pitot tube opening and the drain hole should become clogged simultaneously, then the pressure in the pitot tube is trapped, and airspeed changes do not appear on the instrument.
If the aircraft descends while the pitot system is obstructed, the pressure in the pitot system remains constant. As the descent is made, static pressure increases against the diaphragm, causing it to compress, and resulting in an indication of decreased airspeed. In a climb, static pressure decreases, the diaphragm to expands, and indicated airspeed increases.
If the static system becomes blocked but the pitot tube remains clear, the ASI will continue to operate, but it will indicate lower than the actual airspeed when the aircraft is operated above the altitude where the static ports became blocked. In this case, the trapped static pressure is higher than normal for that altitude. If the aircraft descends, the static pressure increases on the pitot side showing an increase on the ASI. (This scenario presumes the aircraft does not change airspeed.)
A blockage of the static system causes the altimeter to freeze at the altitude where the blockage occurred, due to trapped static pressure.
A blocked static system causes the VSI to produce a continuous zero indication.
Opening the alternate static source introduces static pressure from the flight deck into the system. Refer to the operating handbook for expected variations when using an alternate static source.
Electronic Flight Display (EFD)
Refer to the Pilot's Handbook of Aeronautical Knowledge, Chapter 8.
Gyroscopic Flight Instruments
The most common instruments containing gyroscopes are the turn coordinator, heading indicator, and the attitude indicator. Any spinning object exhibits gyroscopic properties. The two fundamental properties of gyroscopic action are rigidity in space and precession.
"Rigidity in space" refers to the principle that a gyroscope remains in a fixed position in the plane in which it is spinning. A bicycle wheel is unstable and easily maneuvered at low speed. At high speed, the wheel is stable and resistant to change. A spinning gyroscope mounted on gimbal wheels remains in the plane in which it was originally spinning.
"Precession" is the tilting or turning of a gyro in response to a deflective force. The reaction to this force occurs at a point 90 degrees in the direction of rotation. This allows the gyro to determine a rate of turn by sensing the amount of pressure created by a change in direction.
Precession can cause a freely spinning gyro to become displaced from its intended plane of rotation through bearing friction and other factors.
Most aircraft have at least two sources of power for gyroscopic instruments to ensure at least one source of bank information is available if one power source fails. In most aircraft, vacuum, pressure, or electrical systems power the gyroscopic instruments. A vacuum or pressure system spins the gyro by drawing a stream of air against the rotor vanes to spin the rotor at high speed. If the vacuum pressure drops below the normal operating range, the gyroscopic instruments will become unstable and inaccurate. Cross-checking instruments during flight is recommended.
Turn Indicators & Inclinometer
The two types of turn indicators are turn-and-slip indicators and turn coordinators.
The turn-and-slip indicator only shows the rate of turn in degrees per second. A turn needle shows the direction and rate of turn.
The turn coordinator senses both rate of roll and rate of turn. A rapid roll rate causes the miniature aircraft to bank more steeply than a slow roll rate. A standard-rate turn is defined as a turn rate of three degrees per second. A standard-rate turn can be established and maintained by aligning the wing of the miniature aircraft with the turn index. Note that the turn coordinator indicates the rate and direction of turn. The turn coordinator does not display a specific angle of bank.
The inclinometer — or "ball" on the face of the turn indicator — is used to depict aircraft yaw. In coordinated flight, the ball is centered. If aerodynamic forces are unbalanced, the ball moves away from the center of the tube. In a slip, the rate of turn is too slow for the angle of bank, and the ball moves to the inside of the turn. In a skid, the rate of turn is too great for the angle of bank, and the ball moves to the outside of the turn.
To center the ball, apply rudder pressure on the side to which the ball is deflected. Use the simple rule, "step on the ball," to remember which rudder pedal to press. Varying the angle of bank can also help restore coordinated flight from a slip or skid. To correct for a slip, decrease bank and/or increase the rate of turn. To correct for a skid, increase the bank and/or decrease the rate of turn.
The attitude indicator, with its miniature aircraft and horizon bar, displays a picture of the attitude of the aircraft. The relationship of the miniature aircraft to the horizon bar is the same as the relationship of the real aircraft to the actual horizon. The instrument gives an instantaneous indication of even the smallest changes in attitude.
The gyro in the attitude indicator is mounted in a horizontal plane and depends upon rigidity in space for its operation. The horizon bar represents the true horizon. This bar is fixed to the gyro and remains in a horizontal plane as the aircraft is pitched or banked about its lateral or longitudinal axis, indicating the attitude of the aircraft relative to the true horizon.
The gyro spins in the horizontal plane and resists deflection of the rotational path. Since the gyro relies on rigidity in space, the aircraft (and occupants) actually rotate around the spinning gyro. Normally, the miniature aircraft is adjusted so that the wings overlap the horizon bar when the aircraft is in straight-and level cruising flight.
The banking scale is used only to control the degree of desired bank. .
The heading indicator is a mechanical compass. It is unaffected by the forces that make the magnetic compass difficult to interpret. The aircraft (and occupants) rotate around the rotating gyro, which is rigid in space.
A heading indicator displays headings based on a 360-degree azimuth, with the final zero omitted. For example, "6" represents 060 degrees, while "21" indicates 210 degrees. The adjustment knob is used to align the heading indicator with the magnetic compass.
The heading indicator drifts, due to precession caused by friction, as well as the age of the instrument and quality of its maintenance. Drift also is caused by the Earth's rotation, which happens at a rate of 15 degrees per hour. Because the gyro cannot track the rotation of the earth, it can drift by as much as 15 degrees per hour (discounting friction).
A horizontal situation indicator (HSI) receives a magnetic north reference from a magnetic slaving transmitter and generally needs no adjustment. The magnetic slaving transmitter is called a magnetometer.
Advanced systems include the Attitude and Heading Reference System (AHRS), the flux gate compass system, and the remote indicating compass. See the Pilot's Handbook of Aeronautical Knowledge, Chapter 8, for more information.
Any magnet that is free to rotate will align with the Earth's lines of flux, which leave the surface of the earth at the north pole and emerge at the south pole.
An aircraft magnetic compass has two small magnets attached to a metal float sealed inside a bowl of clear compass fluid. A card is wrapped around the float and viewed through a glass window with a lubber line across it. The buoyancy of the float takes most of the weight off of the pivot, and the fluid damps the oscillation of the float and card. While the card can pivot and tilt, compass indications are erratic and unpredictable at steep bank angles.
The magnetic compass can be confusing to read because of its backward setup. For example, when flying a heading of 360, a heading of 030 (northeast) is to the pilot's right. On the compass, a heading of 330 (northwest) appears to the right on the card. This is because the compass is a stationary object, while the aircraft (and occupants) rotate around it. If the pilot turns right, to a heading of 030, the compass card will appear to move from the left to right, and then stop at "3". It's important to remember that non-centered numbers on a compass do not offer a prompt on which direction to turn in order to acquire a new heading. (To change headings using a compass, the desired heading must be "dragged" to the lubber line.)
The magnetic compass is affected by several kinds of compass errors, which must be compensated for by the pilot, both in flight planning and during flight .
The magnetic North Pole to which the magnetic compass points is not collocated with the geographic North Pole. The difference between true north and magnetic north is called variation in aerial navigation (in surveying and land navigation, it's called "declination"). Isogonic lines on aviation charts identify the amount of magnetic variation in that region. The agonic line has no variation with true north, and can be found centrally in North America.
Variation can be to the east or to the west. When variation is east, pilots subtract the amount of variation from their true course ("East is least"). If the variation is west, pilots add the variation to their true course ("West is best").
Within an aircraft, flowing electrical current, magnetized parts, and conflict with the Earth's magnetic field create a compass error called deviation, which varies based on aircraft heading.
In order to compensate for deviation, an AMT will "swing the compass" with the aircraft positioned on a compass rose. A compensator assembly can then be adjusted at every 30 degrees of heading. Any remaining errors are then recorded on a compass correction card. Pilots use the values recorded on the correction card to calculate compass headings for flight planning.
The Earth's magnetic field runs parallel to its surface at the Magnetic Equator. Away from this location, the earth's magnetic field has a vertical component, known as magnetic dip or the dip angle. This increases in an downward direction toward the Magnetic North Pole, and upward toward the Magnetic South Pole. The compass disregards this force by only rotating in a horizontal plane. However, close to the magnetic poles, the horizontal component of the Earth's field is too small to align the compass, and it becomes unusable.
North and south turning errors are caused by magnetic dip, i.e. dip effect, when the aircraft is banked and the compass is not restricted to the horizontal plane. The acronym NOSE is often used to recall the nature of these compass errors and the pilot's corrections:
The magnetic dip and the forces of inertia cause acceleration error and deceleration error when accelerating and decelerating on easterly and westerly headings. The aft end of the compass card is tilted upward when accelerating and downward when decelerating during changes of airspeed. The acronym ANDS is often used to recall the nature of these compass errors and the pilot's corrections:
Oscillation is a combination of all of the errors previously mentioned and results in fluctuation of the compass card in relation to the actual heading direction of the aircraft.
The vertical card magnetic compass eliminates some of the errors and confusion encountered with the magnetic compass. The dial is rotated by a set of gears from the shaft-mounted magnet.
The outside air temperature gauge (OAT) is a simple and effective device mounted so that the sensing element is exposed to the outside air. An accurate air temperature provides the pilot with useful information about temperature lapse rate with altitude change.
Compass-error questions on knowledge tests can be challenging, because they require clear answers to questions that must be visualized. And you may not remember what a compass does in various scenarios, or understand how to parse a question's specific phrasing.
There are two types of compass-error questions: NOSE and ANDS.
North Opposite, South Exceed (NOSE)
For the errors when the aircraft is on a north or south heading, start with a single paradigm: The compass gets slow and sticky while the aircraft is banked through northerly headings, while it's fast and loose when the aircraft is banked through southerly headings. This is easily observed in the airplane or a flight simulator.
Why is this happening? When the aircraft is banked, the compass no longer is restricted to a horizontal plane. There is a vertical component to the compass in a bank, and that vertical component is north-seeking. Thus, the compass has a bias toward magnetic north, just as it does in straight-and-level, unaccelerated flight.
Imagine while you are banking the airplane that you are trying to "catch" the compass with the aircraft's accurate heading, as determined by the gyroscopically stabilized heading indicator. Can you catch something that is slow and sticky? No problem. You'll always overtake the compass when banking through northerly headings.
However, the opposite is true with southerly headings — the compass cannot be caught. It's running away, and it's too fast. It wants to show you where north is.
Therefore, when turning left or right from a southerly heading, the compass will turn at a faster rate than is actually occurring. "South Exceed."
If rolling out on a southerly heading, start the rollout after the compass indication passes south, because it's running away from you. It's farther ahead of the aircraft's actual heading. The variable can be identified by your current latitude. If flying at a latitude of 40 degrees, you can expect that the compass is 40 degrees ahead of your accurate heading. This rollout should be done "by a number of degrees approximately equal to the latitude minus the normal rollout lead" (there's the answer to one test question right there).
When making turns from a northerly heading, the compass is so slow and sticky that it initially will present a turn in the opposite direction. "North Opposite." Again, it wants to show you where north is.
A turn to a northerly heading should be stopped prior to the compass reaching the desired heading, because it's easy to catch up with a compass, and overtake it, on northerly headings. The compass gets sticky on those headings because it's where it wants to be — and it's reluctant to leave.
Accelerate North, Decelerate South (ANDS)
The ANDS questions are the easier of the two types. If the aircraft accelerates on an east or west heading, the compass will show a turn to the north (Accelerate North). If the aircraft decelerates on this same east or west heading, the compass will show a turn to the south (Decelerate South). Simple enough.
Why is this happening? The compass is slightly tilted when the aircraft is accelerating or decelerating. Again, the compass wants to show you where north is, and when it's no longer restricted to a horizontal plane it will indicate a turn.
Commercial Pilot & Flight Instructor Test Questions
The pitot system provides impact pressure for which instrument? Airspeed indicator.
Which statement is true about the effect of temperature changes on the indications of a sensitive altimeter? Colder-than-standard temperatures will place the aircraft lower than the altimeter indicates.
— "High to low, look out below" applies to both pressure and temperature.
What is true altitude? The vertical distance of the aircraft above sea level.
— Also referred to as "MSL" (mean sea level) and actual altitude. Airport, terrain, and obstacle elevations on charts are stated in true altitude.
What is absolute altitude? The vertical distance of the aircraft above the surface.
What is pressure altitude? The altitude indicated when the barometric pressure scale is set to 29.92.
— Another possible correct answer is "The airplane's height above the standard datum plane."
Under what condition is indicated altitude the same as true altitude? When at sea level under standard conditions.
— In standard conditions, pressure, indicated, and density altitude are equal.
If the static pressure tubes are broken inside a pressurized cabin during a high-altitude flight, the altimeter probably would indicate lower than actual flight altitude.
— This is true at high-altitude flight, if the altimeter senses the cabin altitude, which would be at lower pressure (if pressurized).
Pitot-static system errors are generally the greatest in which range of airspeed? Low airspeed.
— High angles of attack do not permit dynamic pressure to directly strike the opening of the pitot tube.
If a pitot tube is clogged, which instrument would be affected? Airspeed indicator.
The pitot tube provides impact pressure for which instrument? Airspeed indicator.
If both the ram-air input and drain hole of the pitot system are blocked, what airspeed indication can be expected? No variation in airspeed during level flight, even if large power changes are made.
— This can be considered a "closed system," where no information can be captured in the pitot tube, resulting in no information. If the ram-air input is blocked but the drain hole is not blocked, the system is partially closed and the airspeed indicator will behave like an altimeter. The drain hole essentially becomes another static port.
What airspeed indicator marking identifies the maximum structural cruising speed of an aircraft? Upper limit of the green arc.
— "Maximum structural cruising speed" (Vno) should not be confused with "never exceed" speed (Vne), which is the red line on the airspeed indicator.
Which airspeed is identified by color coding on an airspeed indicator? Maximum structural cruising speed.
— For this question, the distractors are "Design maneuvering speed" and "Maximum gear operation or extended speed."
What is an important airspeed limitation not color coded on airspeed indications? Maneuvering speed.
During power-off stalls with flaps full down, the stall occurs and the pointer on the airspeed indicator shows a speed less than the minimum limit of the white arc on the indicator. This is most probably due to installation error in the pitot-static system.
— Ram-air does not strike the pitot-tube directly when flying at high angles of attack.
A possible result of using the emergency alternate source of static pressure inside the cabin of an unpressurized airplane is the altimeter may indicate an altitude higher than the actual altitude being flown.
— Air flowing around the aircraft speeds up (Bernoulli's principle), and thus air entering the cabin is slightly lower pressure than that of the surrounding air.
What does the lower limit of the white arc on an airspeed indicator represent? Power-off stall speed in a landing configuration.
What does the lower limit of the green arc on an airspeed indicator represent? Power-off stall speed in a specified configuration.
— "Specified" can be though of as "Cruise" configuration in most small aircraft. Consult the operating handbook for specifics.
Which instrument would be affected by excessively low pressure in the airplane's vacuum system? Heading indicator.
— For this question, the distractors are "Pressure altimeter" and "Airspeed Indicator."
What is V2 speed? Takeoff safety speed.
Deviation error of the magnetic compass is caused by certain metals and electrical systems within the aircraft..
Which statement is true about magnetic deviation of a compass? Deviation varies for different headings of the same aircraft.
In the Northern Hemisphere, a magnetic compass will normally indicate a turn toward the north when an aircraft is accelerated while on an east or west heading.
In the Northern Hemisphere, if an aircraft is accelerated or decelerated, the magnetic compass will normally indicate correctly when on a north or south heading.
— Magnetic dip affects the compass when the aircraft is accelerated or decelerated on an east or west heading. "ANDS," accelerate north, decelerate south.
In the Northern Hemisphere, which would be correct about starting the rollout from a turn using a magnetic compass? Start the rollout after the compass indication passes south by a number of degrees approximately equal to the latitude minus the normal rollout lead.
— When turning in a southerly direction, the forces are such that the compass float assembly lags rather than leads. The aircraft should be allowed to pass the desired heading prior to stopping the turn. "South exceed," which is to say the turn should exceed the compass heading.
What should be the indication on the magnetic compass as you roll into a standard-rate turn to the right from a south heading in the Northern Hemisphere? The compass will indicate a turn to the right, but at a faster rate than is actually occurring.
— The compass will immediately indicate a turn to the right because of the northernly turning error. During the first part of the turn, it will show a faster rate of turn than you are actually making. A northerly turn should be stopped prior to arrival at the desired heading.
What indication should be observed on a turn coordinator during a right turn while taxiing? The miniature aircraft will show a turn to the right and the ball moves to the left.
— The ball moving opposite turn is the more obvious part of the two-part answer.
What indications should you observe on the turn-and-slip indicator during taxi? The ball moves freely opposite the turn, and the needle deflects in the direction of the turn.
Which statement is true regarding preheating of an aircraft during cold-weather operations? The flight deck, as well as the engine, should be preheated.