1. If a flight is made from an area of low pressure into an area of high pressure without the altimeter setting being adjusted, the altimeter will indicate
A. the actual altitude above sea level.
B. higher than the actual altitude above sea level.
C. lower than the actual altitude above sea level.

2. If a flight is made from an area of high pressure into an area of lower pressure without the altimeter setting being adjusted, the altimeter will indicate
A. lower than the actual altitude above sea level.
B. higher than the actual altitude above sea level.
C. the actual altitude above sea level.

If you answered B an A, respectively, then you are also having some of the same confusion about this topic that many other students experience. Let’s start with the knowledge required to answer these questions.

It is easy to maintain a consistent height above ground if the barometric pressure and temperature remain constant, but this is rarely the case. The pressure and temperature can change between takeoff and landing on a local flight and even more drastically when flying cross country between areas of varying pressure and temperature. If these changes are not taken into consideration, flight becomes dangerous.

For example, when flying from an area of high pressure to an area of low pressure without adjusting the altimeter, a constant indicated altitude will remain but the aircraft’s actual altitude above ground level will be lower than indicated. Conversely, when flying from an area of low pressure to an area of high pressure the aircraft’s actual altitude above ground level will be higher than the indicated altitude on the altimeter. The image below is a good visual depiction of flying from an area of high pressure to an area of low pressure and the resulting altitude of the aircraft above the ground level if the altimeter is not adjusted. 

The reason so many students answer these questions incorrectly is not so much because they don’t understand the principle knowledge, but rather because they are not understanding what the question is asking. Both questions are asking what the ALTIMETER will indicate, not what the aircraft’s actual altitude is relative to the ground.

The correct answer to question 1 is C: the altimeter will indicate lower than the actual altitude above sea level. When flying from a low pressure area into a high pressure area the aircraft’s altitude will slowly increase while the altimeter remains constant; therefore, the ALTIMETER is indicating a LOWER altitude then what the aircraft is actually flying.

Vice versa, the correct answer to question 2 is B: the altimeter will indicate higher than the actual altitude above sea level. When flying from a high pressure area into a low pressure area, the aircraft’s altitude will slowly decrease while the altimeter remains constant therefore the ALTIMETER is indicating a HIGHER altitude then what the aircraft is actually flying.

Tricky questions yes, but also a very important piece of knowledge to fully understand. You should now be able to correctly answer these on your knowledge test as well as remember to adjust the altimeter when flying between different air pressure systems.

The post CFI Brief: Altimeter Pressure Errors first appeared on Learn To Fly.

The compass consists of a float, which is free to turn on a hardened steel pivot that rides in a glass bearing. There are two small bar magnets attached to the bottom of the float, and a calibrated card is mounted around the float. The float assembly rides in a bowl of compass fluid, which is a highly refined kerosene-type liquid. The calibrated card is visible to the pilot through the glass front of the bowl, and the direction the aircraft is headed is read on the card opposite the vertical lubber line just behind the glass. The magnetic compass is subject to several errors and limitations:

Variation—This is the error caused by the compass pointing toward the magnetic north pole, while the aeronautical charts are oriented to the geographic north pole. Variation is not affected by changes in heading, but it does change with the location on the earth’s surface. Aeronautical charts show the amount of variation correction to be applied.

Deviation—This is the error caused by local magnetic fields produced by certain metals and the electrical systems within the aircraft. Deviation error is corrected for by “swinging” the compass. The aircraft is aligned with the directional marks on a compass rose on the airport, and the small magnets inside the compass housing are rotated to minimize the error between the compass reading and the direction of the mark with which the aircraft is aligned. Corrections are made on the four cardinal headings, and the errors are read every 30°. A compass correction card is made for each specific aircraft and installed near the compass to show the pilot the compass heading to fly for each magnetic heading. This error can be exasperated by pilot actions—be careful not to put metal items up on the dashboard (like a kneeboard) as the additional metal can affect the compass deviation error.

Magnetic dip error—This error is caused by the compass magnets pointing downward as they align with the earth’s magnetic field. This downward pointing is caused by the vertical component of the field, and is greatest near the magnetic poles.

Northerly turning error—This error is caused by the vertical component of the earth’s magnetic field. When flying in the northern hemisphere, on a northerly heading, and banking in either direction to start a turn, the vertical component of the magnetic field pulls on the north-seeking end of the magnets and rotates the compass card in the direction opposite that of the turn being started. When flying on a southerly heading, and banking in either direction to start a turn, the vertical component pulling on the magnets rotates the card in the same direction as the turn is being made. The card moves in the correct direction, but at a rate greater than the actual rate of turn.

Southerly turning error—When turning in a southerly direction, the forces are such that the compass float assembly lags rather than leads. The result is a false southerly turn indication. The compass card, or float assembly, should be allowed to pass the desired heading prior to stopping the turn. As with the northerly error, this error is amplified with the proximity to either pole. To correct this lagging error, the aircraft should be allowed to pass the desired heading prior to stopping the turn. The same rule of 15° plus half of the latitude applies here (i.e., if the airplane is being operated in a position around the 30° of latitude, the turn should be stopped 15° + 15° + 30° after passing the desired heading).

Acceleration error—This error is caused by the center of gravity of the compass float being below its pivot. When the aircraft is flying in the northern hemisphere on an easterly or westerly heading and accelerates, the rear end of the float tips upward, and the magnetic pull on the compass magnets causes the card to rotate and indicate a turn toward the north. When the aircraft decelerates on an easterly or westerly heading, the rear end of the float dips down and the magnet is pulled in the direction that rotates the card to indicate a turn toward the south. This acceleration error does not occur when accelerating or decelerating on a northerly or southerly heading.

When making turns by reference to the magnetic compass, these corrections should be made:

1. If you are on a northerly heading and start a turn to the east or west, the indication of the compass lags or shows a turn in the opposite direction.
2. If you are on a southerly heading and start a turn to the east or west, the indication of the compass leads the turn, showing a greater amount of turn than is actually being made.
3. The amount of lead or lag depends upon the latitude at which you are flying. For all practical purposes, the lead or lag is equal to approximately 1° for each degree of latitude.
4. When rolling out of a turn, using a coordinated bank. Start the rollout before the desired heading is reached by an amount that is equal to approximately one-half of the bank angle being used.

Two very useful acronyms to help you remember a couple of the compass areas are UNOS and ANDS:

UNOS: Under Shoot North and Overshoot South. This is used to help you remember northerly turning errors.

ANDS: Accelerate North Decelerate South. Discussed above, is an easy way to remember acceleration errors.

The post CFI Brief: Magnetic Compass first appeared on Learn To Fly.

The magnetic compass is the primary indicator of direction in most airplanes. It is, however, difficult to read in turbulence and subject to acceleration and turning errors, making it a difficult instrument to fly by accurately. The heading indicator (HI) is a gyroscopic instrument that you should keep aligned with the magnetic compass in flight. Although it takes its directional reference from the compass, it is not subject to the same acceleration and turning errors. This makes accurate turns and a constant heading possible.

There are mechanical factors present in the HI (mainly friction) that will cause it to drift off its original alignment with magnetic north because of gyroscopic precession. This is called mechanical drift. In addition, because the airplane is flying over a rotating earth, a line in space from the airplane to north will steadily change. This causes apparent drift. Both mechanical and apparent drift can be corrected by simply realigning the HI with the magnetic compass periodically, as described below.

You should check the power source of the HI prior to flight and, when taxiing, check the correct turn indications on the HI (“turning right, heading increases—turning left, heading decreases”). The HI has a slaving knob that enables the pilot to realign the HI with the magnetic compass, correcting for both mechanical drift and apparent drift. This should be done every 10 or 15 minutes. Some older heading indicators have to be uncaged after realigning with the magnetic compass. Advanced airplanes have HI gyros that are aligned automatically.

To manually align the heading indicator with the magnetic compass:

• choose a reference point directly ahead of the airplane, aim for it and fly steadily straight-and-level;
• keep the nose precisely on the reference point, and then read the magnetic compass heading (when the compass is steady);
• maintain the airplane’s heading toward the reference point and then refer to the HI, adjusting its reading (if necessary) to that taken from the magnetic compass; and
• check that the airplane has remained steadily heading toward the reference point during the operation (if not, repeat the procedure).

The post Flight Instruments: The Heading Indicator and Magnetic Compass first appeared on Learn To Fly.

The purpose of an angle of attack (AOA) indicator is to give the pilot better situational awareness pertaining to the aerodynamic health of the airfoil. This can also be referred to as stall margin awareness. More simply explained, it is the margin that exists between the current AOA that the airfoil is operating at, and the AOA at which the airfoil will stall (critical AOA).

Speed by itself is not a reliable parameter to avoid a stall. An airplane can stall at any speed. Angle of attack is a better parameter to use to avoid a stall. For a given configuration, the airplane always stalls at the same AOA, referred to as the critical AOA. This critical AOA does not change with:

• Weight
• Bank Angle
• Temperature
• Density Altitude
• Center of Gravity

An AOA indicator can have several benefits when installed in General Aviation aircraft, not the least of which is increased situational awareness. Without an AOA indicator, the AOA is “invisible” to pilots. These devices measure several parameters simultaneously and determine the current AOA providing a visual image to the pilot of the current AOA along with representations of the proximity to the critical AOA. These devices can give a visual representation of the energy management state of the airplane. The energy state of an airplane is the balance between airspeed, altitude, drag, and thrust and represents how efficiently the airfoil is operating.

The post Flight Instruments: Angle of Attack Indicators first appeared on Learn To Fly.

In instrument conditions, when the natural horizon cannot be seen, pitch attitude and bank angle information is still available to the pilot in the cockpit from the attitude indicator. The pitch attitude changes against the natural horizon are reproduced in miniature on the attitude indicator.

In straight-and-level flight, for instance, the wings of the miniature airplane should appear against the horizon line, while in a climb they should appear one or two bar widths above it.

In a turn, the wing bars of the miniature airplane will bank along with the real airplane, while the horizon line remains horizontal. The center dot of the miniature airplane represents the airplane’s nose position relative to the horizon. Today, there are a variety of attitude indicators you might see. Some are referred to as a primary flight display (PFD). In any display, the principles remain the same: the center dot or center point’s position relative to the horizon indicates a climb or descent.

Scanning the instruments with your eyes, interpreting their indications and applying this information is a vital skill to develop if you are to become a good instrument pilot. Power is selected with the throttle, and can be checked (if required) on the power indicator. Pitch attitude and bank angle are selected using the control column, with frequent reference to the attitude indicator. With both correct power and attitude set, the airplane will perform as expected. The attitude indicator and the power indicator, because they are used when controlling the airplane, are known as the control instruments. The actual performance of the airplane, once its power and attitude have been set, can be cross-checked on what are known as the performance instruments—the altimeter for altitude, the airspeed indicator for airspeed, the heading indicator for direction, and so on.

• in holding patterns (which, for example, may be racetrack patterns with legs of 1 or 2 minutes duration);
• in timed turns (a 180° change of heading at standard-rate of 3° per second taking 60 seconds); and
• to measure time after passing certain radio fixes during instrument approaches (at 90 knots groundspeed, for instance, it would take 2 minutes to travel the 3 NM from a particular fix to the published missed approach point).

Another area on the instrument panel contains the navigation instruments, which indicate the position of the airplane relative to selected navigation facilities. These NAVAIDs will be considered in detail later in your training, but the main ones are:

• VHF omni range (VOR) cockpit indicator, which indicates the airplane’s position relative to a selected course to or from the VOR ground station;
• automatic direction finder (ADF), which has a needle that points to a nondirectional beacon (NDB); and
• distance measuring equipment (DME) or VORTAC, which indicates the slant distance in nautical miles to the selected ground station.

Instrument scanning is an art that will develop naturally during your training, especially when you know what to look for. The main scan to develop initially is that of the six basic flight instruments, concentrating on the AI and radiating out to the others as required. Then as you move on to en route instrument flying, the navigation instruments will be introduced. Having scanned the instruments, interpreted the message that they contain, built up a picture of where the airplane is and where it is going, you can now control it in a meaningful way.

The post IFR: Instrument Scanning first appeared on Learn To Fly.

Unlike the turn needle, the turn coordinator is designed so that it reflects roll rate as well as turn rate. Neither instrument indicates bank angle. Bank angle, turn rate, and airspeed are interrelated, as shown in the figure below. For a given bank angle, the rate of turn increases as the airspeed decreases. Consider a light trainer and a jet, both banked 20°: the trainer would complete a 360° turn in a much shorter time than the jet and with a much smaller radius. If both airplanes maintained a 3° per second turn rate, they would both complete the circle at the same time—but the jet would be at an extreme bank angle. The bank angle for a 3° per second turn is approximately 15 percent of the true airspeed, so the trainer at 80 knots would bank 12°, while the jet at 400 knots would have to bank 60°.

The ball indicates the quality of the turn, with respect to rudder-aileron coordination. The force that causes an airplane to turn is the horizontal component of lift, which is opposed by centrifugal force. If the rate of turn is too great for the angle of bank, centrifugal force is greater than the horizontal component of lift, and the ball rolls toward the outside of the turn. This is termed a “skidding” turn, and either a steeper bank angle (increasing horizontal component) or less rudder pressure on the inside of the turn (reduced centrifugal force) will return the ball to the center. The reverse situation has the ball falling to the inside of the turn in a “slip,” caused by too little centrifugal force and too much horizontal component.

Less bank angle or more inside rudder will return the ball to the center when slipping. A rule of thumb is to “step on the ball”—apply pressure to the rudder pedal on the side of the instrument that the ball is on.

The table below illustrates how various elements of a turn are affected if either bank angle or airspeed is kept constant. For example, with a constant bank angle, an increase in airspeed will decrease the rate of turn while increasing the turn radius; the load factor would not be affected.

Glass cockpit displays do not have the familiar ball, but combine it with the triangular turn index at the top of the heading indicator in the form of a line parallel to the base of the triangle; if the line slides away from the triangle, indicating lack of coordination, just “step on it” with rudder pressure to move it to its proper location.

The post Flight Instruments: The Turn and Slip Indicator first appeared on Learn To Fly.

The aircraft heading displayed on the rotating azimuth card under the upper lubber line in the figure is 330°. The course indicating arrowhead that is shown is set to 300°. The tail of the course indicating arrow indicates the reciprocal, or 120°.

The course deviation bar operates with a VOR/LOC navigation receiver to indicate either left or right deviations from the course that is selected with the course indicating arrow. It moves left or right to indicate deviation from the centerline in the same manner that the angular movement of a conventional VOR/LOC needle indicates deviation from course.

The desired course is selected by rotating the course indicating arrow in relation to the azimuth card by means of the course set knob. This gives the pilot a pictorial presentation. The fixed aircraft symbol and the course deviation bar display the aircraft relative to the selected course as though the pilot was above the aircraft looking down.

The TO/FROM indicator is a triangular-shaped pointer. When this indicator points to the head of the course arrow, it indicates that the course selected, if properly intercepted and flown, will take the aircraft TO the selected facility, and vice versa.

The glide slope deviation pointer indicates the relationship of the aircraft to the glide slope. When the pointer is below the center position, the aircraft is above the glide slope, and an increased rate of descent is required.

To orient where the aircraft is in relation to the facility, first determine which radial is selected (look at the arrowhead). Next, determine whether the aircraft is flying to or away from the station (look at the TO/FROM indicator) to find which hemisphere the aircraft is in. Next, determine how far from the selected course the aircraft is (look at the deviation bar) to find which quadrant the aircraft is in. Finally, consider the aircraft heading (under the lubber line) to determine the aircraft’s position within the quadrant.

Think you got it? Try to answer these two knowledge test questions from the instrument knowledge exam to see if you understand how the HSI works.

1. (Refer to Figures 98 and 99.) To which aircraft position does HSI presentation ‘D’ correspond?
A—4
B—15
C—17

2. (Refer to Figures 98 and 99.) To which aircraft position does HSI presentation ‘A’ correspond?
A—1
B—8
C—11

The post CFI Brief: Horizontal Situation Indicator (HSI) first appeared on Learn To Fly.

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 actually rotates around the spinning gyro. An adjustment knob is provided with which the pilot may move the miniature aircraft up or down to align the miniature aircraft with the horizon bar to suit the pilot’s line of vision. 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 pitch and bank limits depend upon the make and model of the instrument. Limits in the banking plane are usually from 100° to 110°, and the pitch limits are usually from 60° to 70°. If either limit is exceeded, the instrument will tumble or spill and will give incorrect indications until realigned. A number of modern attitude indicators do not tumble.

Every pilot should be able to interpret the banking scale illustrated below. Most banking scale indicators on the top of the instrument move in the same direction from that in which the aircraft is actually banked. Some other models move in the opposite direction from that in which the aircraft is actually banked. This may confuse the pilot if the indicator is used to determine the direction of bank. This scale should be used only to control the degree of desired bank. The relationship of the miniature aircraft to the horizon bar should be used for an indication of the direction of bank.

On Thursday, our CFI will share his favorite flight instrument and some related FAA Knowledge Exam questions you might encounter on your own test!

Want to get new Learn to Fly Blog posts delivered straight to your inbox the moment they’re published? Sign up to our new email subscription service on top of the sidebar to the right! You’ll only get new Learn to Fly Blog posts and we won’t share your email address with anyone ever. Thanks for reading the Learn to Fly Blog!

The post Flight Instruments: Attitude Indicator first appeared on Learn To Fly.

91.171 VOR equipment check for IFR operations.
(a) No person may operate a civil aircraft under IFR using the VOR system of radio navigation unless the VOR equipment of that aircraft—
(1) Is maintained, checked, and inspected under an approved procedure; or
(2) Has been operationally checked within the preceding 30 days, and was found to be within the limits of the permissible indicated bearing error set forth in paragraph (b) or (c) of this section.

VOR accuracy may be checked by means of a VOR Test Facility (VOT), ground or airborne checkpoints, or by checking dual VORs against each other. A VOT location and frequency can be found in the Airport/Facility Directory (A/FD). To use the VOT, tune to the appropriate frequency and center the CDI. The omni-bearing selector should read 0° with a FROM indication, or 180° with a TO indication. The allowable error is ±4°. VOR receiver checkpoints are listed in the A/FD. With the appropriate frequency tuned and the OBS set to the published certified radial, the CDI should center with a FROM indication when the aircraft is over the designated check point. Allowable accuracy is ±4° for a ground check, and ±6° for an airborne check. If the aircraft is equipped with dual VORs, they may be checked against each other. The maximum permissible variation when tuned to the same VOR is ±4°.

The pilot must log the results of the VOR accuracy test in the aircraft logbook or other record. The log must include the date, place, bearing error, if any, and a signature. Most rental aircraft keep a separate log in a dispatch binder or in the aircraft for pilots to easily determine when the last inspection was conducted.

The post CFI Brief: Checking the accuracy of your VOR first appeared on Learn To Fly.

Attitude Indicator
The rigidity in space principle makes the gyroscope an excellent “artificial horizon” around which the attitude indicator (and the airplane) pivot.

When viewing the attitude indicator, the direction of bank is determined by the relationship of the miniature airplane to the horizon bar. The miniature airplane may be moved up or down from the horizon with an adjustment knob. Normally, the miniature airplane will be adjusted so that the wings overlap the horizon bar whenever the airplane is in straight-and-level flight.

Turn Coordinator
The turn coordinator (also using the principle of the gyroscope) uses a miniature airplane to provide information concerning rate of roll and rate of turn. As the airplane enters a turn, movement of the miniature aircraft indicates rate of roll. When the bank is held constant, rate of turn is indicated. Simultaneously, the quality of turn, or movement about the yaw axis, is indicated by the ball of the inclinometer.

Heading Indicator
The heading indicator is a gyroscopic instrument designed to avoid many of the errors inherent in a magnetic compass. However, the heading indicator does suffer from precession, caused mainly by bearing friction. Because of this precessional error, the heading indicator must periodically be realigned with the magnetic compass during straight-and-level, unaccelerated flight.

Below I have included some questions as they relate to gyroscopic flight instruments, these are very similar to what you can expect to see on your private pilot knowledge exam as well as your instrument rating knowledge exam. I will go ahead and post the answers in the comments section Monday.

1. One characteristic that a properly functioning gyro depends upon for operation is the
A—ability to resist precession 90° to any applied force.
B—resistance to deflection of the spinning wheel or disc.
C—deflecting force developed from the angular velocity of the spinning wheel.

2. (Refer to Figure 5.) A turn coordinator provides an indication of the
A—movement of the aircraft about the yaw and roll axis.
B—angle of bank up to but not exceeding 30°.
C—attitude of the aircraft with reference to the longitudinal axis.

The post CFI Brief: Gyroscopic Flight Instrument Questions first appeared on Learn To Fly.