Flying Notes

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Aircraft Flight Instruments ​

This page covers the portion of Task A. Aircraft Flight Instruments and Navigation Equipment from the FAA-S-8081-9E Flight Instructor Instrument Practical Test Standards pertaining to aircraft flight instruments.

Pitot-Static System ​

Pitot static system and instruments. FAA-H-8083-25B Pilot's Handbook of Aeronautical Knowledge Chapter 8: Flight Instruments Figure 8-1.

Altimeter ​

  • The indicated altitude is correct, however, only when the sea level barometric pressure is standard (29.92 inHg), the sea level free air temperature is standard (15 °C or 59 °F), and the pressure and temperature decrease at a standard rate with an increase in altitude.
  • Can adjust the altimeter for nonstandard pressure but not temperature
    • True altitude thus varies with temperature
    • Higher temps means true altitude is higher than indicated and lower temps means true altitude is lower than indicated
    • Recall "hot-to-cold look out below"
    • Beware obstacle clearance especially when flying in colder temps
  • Mental model: altimeter setting provides true datum at ground level of the reporting station. Then consider the pressure gradient (which decreases with altitude). Compared to the standard pressure gradient, the pressure gradient in colder more dense air will decrease more quickly, and in hot air the pressure gradient will decrease less quickly.
Altimeter. FAA-H-8083-25B Pilot's Handbook of Aeronautical Knowledge Chapter 8: Flight Instruments Figure 8-2.

Preflight Check ​

  • Reads within 75 feet of field elevation when set to local altimeter setting

Vertical Speed Indicator (VSI) ​

  • Not required equipment for VFR or IFR flight
  • Uses static pressure only
Vertical Speed Indicator (VSI). FAA-H-8083-25B Pilot's Handbook of Aeronautical Knowledge Chapter 8: Flight Instruments Figure 8-5.

Preflight Check ​

  • Should indicate 0 when on the ground
  • If it indicates something other than 0, this value can be used as 0 and then interpret changes from that value
  • Should see a small momentary deviation from 0 when changing to alternate static source

Airspeed Indicator (ASI) ​

  • Requires pitot and static pressure
  • Required for day VFR operation
  • Lower limits of green and white arc are power-off stall speeds
  • Other speeds not here are, for example, Va, Vg, and VLE
  • The same behavior described by "hot-to-cold look out below" for the altimeter applies to the airspeed indicator as well - when flying to a warmer area, for example, true airspeed will increase (given a constant power setting and true altitude)
Airspeed indicator. FAA-H-8083-25B Pilot's Handbook of Aeronautical Knowledge Chapter 8: Flight Instruments Figure 8-7.
Airspeed indicator markings. FAA-H-8083-25B Pilot's Handbook of Aeronautical Knowledge Chapter 8: Flight Instruments Figure 8-8.

Preflight Check ​

  • Should read 0 when stationary and while taxiing

Failure modes ​

  • Alternate static source
    • Due to the venturi effect of the air flowing around the fuselage, the air pressure inside the flight deck is lower than the exterior pressure.
Pitot static system failure modes and instrument indications. Reference unknown.

Attitude Indicator ​

  • Also known as artificial horizon
  • Principles
    • Rigidity in space
    • Precession
  • Electrically or vacuum powered
    • What is in POH might have changed and need to consult supplements
    • For example, the Garmin G5 attitude indicator uses solid state (electrically powered) gyro instead of vacuum powered.
  • Due to friction, gyroscopic instruments can drift and need to be reset periodically (for example heading indicator to compass)
    • For example, check and reset every 15 minutes
  • Certain gyroscopic instruments have specific pitch and bank limits that induce a tumble of the gyro.
  • Modern systems such as G5 or G1000 use solid-state devices instead of spinning gyros to determine the aircraft attitude
Attitude indicator. FAA-H-8083-25B Pilot's Handbook of Aeronautical Knowledge Chapter 8: Flight Instruments Figure 8-23.

Preflight Check ​

  • During taxi check
    • Erect
    • Less than 5 degree lean

Heading Indicator / Horizontal Situation Indicator / Radio Magnetic Indicator ​

  • Heading indicator
    • Also known as directional gyro
    • Usually powered by the aircraft vacuum system
    • Gyroscopic rigidity keeps them fixed in space, but need to be periodically set to the compass
    • Errors in compass make it hard to use to turn to specific headings, especially in turbulent air - heading indicator helps with this
    • Some heading indicators referred to as horizontal situation indicators (HSI) receive a magnetic north reference from a magnetic slaving transmitter and generally need no adjustment.
      • See also: remote indicating compass
  • Radio magnetic indicator (RMI)
  • Modern systems use an Altitude Heading Reference System (AHRS) to determine the aircrafts heading that incorporate a magnetometer in them
Heading indicator. FAA-H-8083-25B Pilot's Handbook of Aeronautical Knowledge Chapter 8: Flight Instruments Figure 8-25.

Magnetic Compass ​

Note: much of the tendencies described below are specific to the northern hemisphere and the opposite behavior would occur in the southern hemisphere.

  • Variation
    • The difference between true and magnetic directions
    • More on magnetic variation below
  • Deviation
    • Created by local magnetic field around aircraft
  • Dip
    • Magnetic field isn't in the plane of the aircraft, so it tends to pull compass down
    • Only really an issue near magnetic poles, and minimum at magnetic equator
  • Oscillation
    • Combination of all other errors
  • Acceleration / Turning
    • These are related to, or manifestations of dip errors
    • Recall ANDS/UNOS
    • Accelerate North Decelerate South
      • This acronym indicates the error we would see - that is during accelerations on an easterly or westerly heading, we would see the compass falsely turn towards north when accelerating and falsely turn towards south when decelerating.
      • Causes errors when accelerating and decelerating on easterly and westerly headings
      • Acceleration causes an indication toward north, deceleration causes an indication toward south
    • Undershoot North Overshoot South
      • When turning from a northerly heading the compass will undershoot and initially show a turn in the opposite direction - undershooting the initial heading change
      • When turning from a southerly heading, the compass will overshoot and initially show a turn in the correct direction that is greater than the actual initial heading change
      • e.g. a northerly turn should be stopped prior to arrival at the desired heading
Compass dip (unknown reference)
Vertical card compass. FAA-H-8083-15B Instrument Flying Handbook Chapter 5: Flight Instruments Figure 5-22.

The explanation in FAA-H-8083-25B Pilot's Handbook of Aeronautical Knowledge regarding compass dip-correction weights doesn't make sense. It states:

This is done by lowering the center of gravity below the pivot point and making the assembly heavy enough that the vertical component of the magnetic force is too weak to tilt it significantly out of the horizontal plane.

But the next sentence after it states:

Because the dip angle is of no navigational interest, the compass is made so that it can rotate only in the horizontal plane.

So if the compass can only rotate in the horizontal (x−y) plane of the aircraft, it's not clear why the location of the compass card CG in the z direction is of any consequence. Also not that this explanation of compass design in FAA-H-8083-25B Pilot's Handbook of Aeronautical Knowledge is different than in older FAA handbooks. For example, in a previous FAA-H-8083-15A Instrument Flying Handbook from 2008 on page 3-14 and in a previous FAA-H-8083-25A Pilot's Handbook of Aeronautical Knowledge also from 2008 on page 7-25, the same paragraph appears:

In acceleration error, the dip-correction weight causes the end of the float and card marked N (the south-seeking end) to be heavier than the opposite end. When the aircraft is flying at a constant speed on a heading of east or west, the float and card is level. The effects of magnetic dip and the weight are approximately equal. If the aircraft accelerates on a heading of east [Figure 3-21], the inertia of the weight holds its end of the float back and the card rotates toward north. As soon as the speed of the aircraft stabilizes, the card swings back to its east indication. If, while flying on this easterly heading, the aircraft decelerates, the inertia causes the weight to move ahead and the card rotates toward south until the speed again stabilizes.

It's also not reasonable to expect that compass technology has changed in the 8 years between the current and former edition of the PHAK. But more importantly, there is no physical basis for the explanation in the current PHAK. Also browsing Aircraft Spruce, the compasses require selection of northern or southern hemisphere, indicating presumably the asymmetric dip-correction weight.

There seem to be other compass types, and different ways to accommodate dip error without a weight on one of the compass "seeking" ends. However, for the purposes of understanding compass errors, the mental model of a weight opposite the "seeking" end is probably the best.

Magnetic Variation ​

  • The E/W variation on isogonic lines indicates where you would end up relative to true north if you followed your compass north.
  • Thinking back to private pilot flight planning with the flight plan form and manually charting on a sectional chart, we want to navigate based on true directions. If magnetic north and true north were collocated, life would be good. They are not, so we need to correct for magnetic directions so that we can navigate by true directions.
  • Note: Figure 45 in the FAA-CT-8080-5H Instructor Knowledge Test Supplement shows the sectional chart around Corpus Christi TX and easterly magnetic variation, in case that is helpful to remember anything on the test.
  • By convention, magnetic variation is positive when magnetic north is east of true north, and negative when it is to the west.
    • In other words, if we are trying to fly to true north from Corpus Christi (which indicates an easterly magnetic variation) using our compass, we would end up east of true north. This is how to think about easterly / westerly magnetic variation - it's where you would end up relative to true north if you followed your compass north.
    • So, in this case we would need a more westerly heading to actually end up at true north.
  • TC +W/-E Var = MC +R/-L WCA = MH (this is what we do on flight plan form)
    • Remember for the test it is subtract E and L and add W and R when going from TC to MC
    • It is the opposite when going the other way
  • When measuring true course on a line drawn on a sectional chart, measure the course angle at the midpoint of the line
  • The K identifier in front of airports is for ICAO recognized airports that are located in the contiguous United States
Notice easterly magnetic variation on isogonic line. This means that if you took off from Corpus Christi and pointed your aircraft to compass north, you would end up east of true north. CFI test supplement CT-8080-5H.
Isogonic lines. FAA-H-8083-25C Pilot's Handbook of Aeronautical Knowledge Figure 8-33.

Turn-and-Slip Indicator / Turn Coordinator ​

  • Turn coordinator
    • Roll rate (initially)
    • Rate of turn (after it stabilizes)
    • Quality of turn
    • Contains inclinometer
      • The ball
      • "step on the ball" to coordinate flight
  • Turn and slip indicator
    • Rate of turn
    • Quality of turn
  • Turing taxi should indicate direction of turn and ball should go to outside of turn
Turn indicators. FAA-H-8083-25B Pilot's Handbook of Aeronautical Knowledge Chapter 8: Flight Instruments Figure 8-21.

Electrical System ​

  • Most planes 14 or 28 volt (DC) systems
  • Alternator or generator supplies power
    • Alternator provides more power than a generator at a lower RPM
    • One of the basic differences between a generator and an alternator is the number of magnetic poles used to produce the electricity. Generators normally have 2 or 4 poles, while alternators have between 8 and 14 poles. The larger number of poles allows an alternator to produce its electrical power at a lower engine RPM than a generator.
  • Batteries store power
  • Receptacle to connect plane to external power
    • Good for cold weather starting
  • Bus bar as common point for powered connections
  • Fuses / breakers
  • Ammeter
    • Positive is charging, negative is discharging
Master switch. FAA-H-8083-25B Pilot's Handbook of Aeronautical Knowledge Chapter 7: Aircraft Systems Figure 7-33.
Ammeter and loadmeter. FAA-H-8083-25B Pilot's Handbook of Aeronautical Knowledge Chapter 7: Aircraft Systems Figure 7-35.
Starting circuit. FAA-H-8083-25B Pilot's Handbook of Aeronautical Knowledge Chapter 7: Aircraft Systems Figure 7-20.
Electrical system. FAA-H-8083-25B Pilot's Handbook of Aeronautical Knowledge Chapter 7: Aircraft Systems Figure 7-34.

Vacuum System ​

  • The vacuum system applies a vacuum to instruments that require it spin gyros, as an alternative to electrically powered gyros.
  • Attitude and heading indicators are common instruments to use vacuum driven gyros.
C172M Vacuum system diagram. Note the heading indicator and attitude indicator are powered by the vacuum system.
Typical vacuum system. FAA-H-8083-25B Pilot's Handbook of Aeronautical Knowledge Chapter 8: Flight Instruments Figure 8-20.

Primary Flight Display (PFD) ​

PFDs provide increased situational awareness (SA) to the pilot by replacing the traditional six instruments used for instrument flight with an easy-to-scan display that provides the horizon, airspeed, altitude, vertical speed, trend, trim, rate of turn among other key relevant indications.

G1000 Primary Flight Display (PFD). Cessna Support.

Multifunction Display (MFD) ​

G1000 MFD with engine instrument display. High Performance Aviation

Flight Management System (FMS) ​

  • A Flight Management System (FMS) automates the tasks of managing the onboard navigation systems
  • A FMS is not a navigation system in itself
  • Aggregates sources of position data to determine best actual position
  • Simplifies entering of of waypoint information
    • Instead of latitude/longitude coordinates use a database of predefined waypoints
  • An FMS uses an electronic database of worldwide navigational data including navigation aids, airways and intersections, Standard Instrument Departures (SIDs), STARs, and Instrument Approach Procedures (IAPs) together with pilot input through a CDU to create a flight plan.
  • The FMS provides outputs to several aircraft systems including desired track, bearing and distance to the active waypoint, lateral course deviation and related data to the flight guidance system for the HSI displays, and roll steering command for the autopilot/flight director system.
  • This allows outputs from the FMS to command the airplane where to go and when and how to turn.

Electronic Engine Instrument Display ​

  • Tachometer
  • Oil pressure
  • Oil temperature
  • EGT
  • Vacuum
  • More
G1000 MFD with engine instrument display. High Performance Aviation.

References ​