Module 2 · Instruments and Systems · Lesson 1 of 7
Lesson 2.1The Pitot-Static System
Three instruments read air pressure. Know which pressure each one uses and you can work out any blockage.
The pitot tube faces into the wind and collects impact (ram) pressure. The static port sits flush on the side of the airplane and senses ambient (static) pressure. The airspeed indicator uses both. The altimeter and vertical speed indicator use static pressure only.
Airspeed Indicator
A diaphragm gets ram pressure inside and static pressure outside. The difference is dynamic pressure, shown as airspeed.
Altimeter
Sealed aneroid wafers expand and contract with static pressure. A linkage turns that into altitude.
Vertical Speed Indicator
Static pressure goes straight into a diaphragm and through a calibrated leak around it. It shows the trend right away; the rate lags a few seconds behind (IFH p. 5-8).
Dynamic pressure (airspeed) = impact pressure − static pressure.
Everything about pitot-static failures follows from this line. If one side of the subtraction gets stuck, the answer goes wrong in a way you can predict.
Pitot-Static Failure Simulator
Interactive · F02Block a port in flight, then climb or descend to see each instrument react. The behavior follows IFH pp. 5-2–5-3. The size of each error is simplified for teaching; the alternate static error in your airplane is in its POH.
What Each Blockage Does
| Blockage | Airspeed | Altimeter | VSI |
|---|---|---|---|
| Pitot inlet blocked, drain open | Drops to zero | Normal | Normal |
| Pitot inlet and drain both blocked | Acts like an altimeter: reads higher in a climb, lower in a descent | Normal | Normal |
| Static port blocked | Correct only at the blockage altitude; reads low above it, high below it | Freezes at the blockage altitude | Freezes on zero |
| Alternate static selected | Reads a little fast | Reads a little high | Shows a climb for a moment |
The pitot tube is the only thing feeding ram air, so a pitot blockage affects the airspeed indicator only. If you suspect ice in or on the pitot tube, turn on pitot heat. A static blockage affects all three instruments.
With a blocked static port, use the alternate static source. If the airplane is unpressurized and has no alternate source, one last option is to break the VSI glass. The VSI is not required for instrument flight, and opening it gives the altimeter and airspeed indicator static pressure from the cabin. Expect the VSI to read in reverse, and expect other errors too. Know your airplane's own procedure before you consider it.
In a glass cockpit, an air data computer (ADC) takes in pitot, static and outside temperature and computes airspeed, true airspeed, vertical speed and altitude. The same blockages corrupt the same outputs, because the computer is fed by the same ports.
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Module 2 · Instruments and Systems · Lesson 2 of 7
Lesson 2.2The Altimeter and Types of Altitude
The altimeter is a barometer. It is only as right as the setting in its window and the temperature outside.
The altimeter shows height above a pressure level, using a standard lapse rate of 1,000 feet per inch of mercury. The knob on a sensitive altimeter sets the current altimeter setting into the Kollsman window, in inches of mercury, millibars or both.
- Below 18,000 feet MSL in the U.S., keep the altimeter set to a station within 100 NM.
- At and above 18,000 feet (the transition altitude), set 29.92 and fly flight levels.
- On the instrument taxi check, the altimeter set to the local setting should show the surveyed field elevation within ±75 feet.
High to Low, Hot to Cold
Interactive · F03The pointers are labeled the way the IFH errata corrects them: the long pointer is 100 feet, the short wide pointer is 1,000 feet, and the thin pointer with the triangle is 10,000 feet. The pressure error uses 1,000 feet per inch (IFH p. 5-4; AIM 7-2-3b). The temperature effect is a simplified model to show direction; for real cold-weather corrections use the ICAO Cold Temperature Error Table (IFH p. 5-6; AIM 7-3-1, TBL 7-3-1). The IPH (p. 4-33) calls it the AIM 7-2-3 table; in the current AIM it is TBL 7-3-1.
Fly from high pressure to low pressure without updating the setting and the altimeter reads higher than you are. Flying from hot air to cold air causes the same error. Either way, you are lower than the altimeter says.
Types of Altitude
| Altitude | What it is | Setting |
|---|---|---|
| Indicated | What the dial shows when set to the local setting | QNH |
| Pressure | Height above the standard 29.92 in Hg plane; used at and above 18,000 feet in the U.S. | QNE |
| Density | Pressure altitude corrected for nonstandard temperature; used for performance | — |
| True | Actual height above mean sea level | — |
| Absolute | Height above the terrain directly below you (AGL), read from a radar altimeter. Not the same as QFE, which sets the altimeter to read zero on one airfield and is mostly used outside the U.S. | Radar altimeter |
QNH is pressure measured at the airport and corrected to sea level, so on the field the altimeter shows field elevation. QNE is always 29.92 in Hg (1013.2 hPa). QFE is mostly used outside the U.S., and transition levels differ from country to country. The IPH also reminds you that pressure altimeters have mechanical, elastic, temperature and installation errors.
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Module 2 · Instruments and Systems · Lesson 3 of 7
Lesson 2.3Gyroscopic Instruments
A spinning gyro resists being moved. The attitude and heading indicators use that; the turn indicator uses the way a gyro reacts when it is pushed.
A gyroscope has two properties: rigidity in space and precession. The attitude indicator and heading indicator rely on rigidity. Turn indicators rely on precession.
Inside the Attitude and Heading Indicators
Animation · F04Errors are drawn larger than life so you can see them. In a real attitude indicator they are small.
Attitude Indicator
Shows pitch and bank. Normally vacuum-driven in GA airplanes, electric in others. Older units have tumble limits. Should show the correct attitude within 5 minutes of engine start.
Heading Indicator
Shows changes in heading but cannot sense heading itself. Set it to the magnetic compass. Some are slaved to a flux gate and sync on their own. Normally vacuum-driven in GA airplanes.
Turn Indicators
A turn coordinator shows rate of turn and rate of roll. A turn-and-slip indicator shows rate of turn only.
Accelerate and it shows a slight pitch up. Decelerate and it shows a slight pitch down. After a 180° turn it can show a slight turn in the opposite direction. Cross-check it against the other instruments.
On the Taxi Check
- Turn coordinator: wings level and ball centered when not turning. In a turn it shows the turn in the correct direction, and the ball moves to the outside of the turn.
- Attitude indicator: correct pitch, and bank within ±5°, within 5 minutes of engine start (vacuum).
- Heading indicator: set, and showing correct headings as you turn.
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Module 2 · Instruments and Systems · Lesson 4 of 7
Lesson 2.4Magnetic Compass Errors
The compass needs no power and no vacuum, which makes it your last heading reference. It is accurate only in straight, level, unaccelerated flight.
Remember the compass errors and limitations with DV MONA.
- DDeviation: error from the airplane's own magnetic fields. The deviation card shows the corrections.
- VVariation: the difference between true north and magnetic north.
- MMagnetic dip.
- OOscillation: the card swings in turbulence and with rough control.
- NNorth and south turning errors. Northern hemisphere: UNOS, undershoot north, overshoot south.
- AAcceleration errors. Northern hemisphere: ANDS, accelerate north, decelerate south.
Watch the Compass Lie
Animation · F05Drawn from the text, not from IFH Figure 5-20: the IFH errata withdraws that figure as inaccurate. The compass is shown as a heading strip, and the error size is exaggerated so the direction is easy to see.
Turning to north, roll out early. Turning to south, roll out late.
On east or west headings, accelerating makes the compass show a turn toward north, and decelerating shows a turn toward south. On north or south headings these errors go away.
The taxi check for the compass: it swings freely, it is full of fluid, it shows known headings, and the deviation card is installed.
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Module 2 · Instruments and Systems · Lesson 5 of 7
Lesson 2.5Glass Displays, ADS-B and Angle of Attack
Electronic displays replace the spinning and flexing parts with sensors and computers. Angle of attack shows a margin the airspeed indicator cannot.
The Parts of a Glass Cockpit
| System | What it does |
|---|---|
| AHRS | Attitude and heading reference system. Gives more accurate, reliable attitude and heading than separate gyros. Modern units are solid state, with no moving parts. |
| ADC | Air data computer. Takes pitot, static and outside temperature and computes airspeed, true airspeed, vertical speed and altitude. |
| PFD | Primary flight display: attitude, altitude, airspeed, vertical speed and heading, with rate tapes. |
| MFD | Multi-function display: moving map, system status, weather and traffic. Can back up the PFD. |
| Flight director | Command bars over the attitude display that guide you to a selected heading, course or vertical speed. |
| FMS | Flight management system. Takes in several sensors, picks the best navigation source and guides the autopilot and flight director. |
On the taxi check in a glass cockpit, look over the PFD, MFD and engine display for red X's, messages, warnings and odd symbols.
Transponder and ADS-B
ADS-B Out automatically broadcasts the airplane's position and other data to ATC ground stations and to nearby aircraft with ADS-B In. Since January 1, 2020, aircraft in certain controlled U.S. airspace must run ADS-B Out at all times. GNSS is not required by rule as the position source, but it is currently the only source that meets the rule's performance requirements. A Mode C transponder and ADS-B Out are required in Class A, B and C airspace; within 30 NM of a Class B primary airport (the Mode C veil); above the ceiling and within the lateral limits of Class B or C, up to 10,000 feet MSL; in Class E at and above 10,000 feet MSL in the 48 contiguous states, except at or below 2,500 feet AGL; and in Class E over the Gulf of Mexico at and above 3,000 feet MSL within 12 NM of the coast. In Class A, ADS-B Out must be 1090ES (§91.215(b); §91.225; AIM 4-1-20). Two codes to know: 7600 for radio failure and 7700 for emergency.
Angle of Attack
Loss of control is the number one root cause of fatal accidents in general and commercial aviation, and about half of the fatal accidents in the maneuvering phase involve a stall or spin. An AOA indicator shows your stall margin: how far the wing is from its critical angle of attack.
The published stall speed is a 1G number. It is valid only in unaccelerated (1G), coordinated flight, at one weight, usually max gross. The critical angle of attack does not change with weight, bank angle, temperature, density altitude or CG. An airplane can stall at any speed, but in a given configuration it always stalls at the same angle of attack.
Airspeed Moves; The Critical Angle Stays Put
Interactive · F06Example airplane with a published 1G stall speed of 50 KIAS at max gross weight. The AOA scale and the stall speeds in a turn are worked out from that one number with standard load-factor math, to show the idea in the addendum's Figure 1. Your airplane's numbers are in its POH.
- How the unit was calibrated
- Probes or vanes that are not heated
- The type of indicator
- Flap setting
- Wing contamination
The manufacturer publishes the limits for each installation. Get training on the specific unit before you rely on it.
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Module 2 · Instruments and Systems · Lesson 6 of 7
Lesson 2.6IFR Airworthiness and Inoperative Equipment
Before you file, prove the airplane is legal: the right documents, the right inspections, the right equipment, and a plan for anything that is broken.
Documents on Board: ARROW-PEC
- AAirworthiness certificate
- RRegistration certificate
- RRadio station license (flights outside the U.S.)
- OOperating limitations and information (AFM)
- WWeight and balance data for this airplane
- PPlacards
- EExternal data plate
- CCompass deviation card
Inspections for IFR: AVIATES
- AAirworthiness Directive inspections
- VVOR check every 30 days (§91.171(a)(2))
- IInspections: annual every 12 calendar months; 100-hour if carrying a person for hire or giving flight instruction for hire in an airplane the instructor provides (§91.409(b))
- AAltimeter, encoder and static system every 24 calendar months (§91.411(a)(1))
- TTransponder every 24 calendar months (§91.413(a))
- EELT every 12 calendar months (§91.207(d))
- SSupplemental Type Certificate inspections
Equipment for IFR: GRAB CARD
For IFR, §91.205(d) requires all the VFR-day equipment (plus VFR-night equipment at night) and these:
- GGenerator or alternator
- RRadios: two-way communication and navigation equipment suitable for the route
- AAltimeter, sensitive and adjustable for barometric pressure
- BBall (slip-skid indicator)
- CClock showing hours, minutes and seconds, with a sweep-second pointer or digital display, installed in the airplane
- AAttitude indicator
- RRate-of-turn indicator
- DDirectional gyro (heading indicator)
At and above FL240, if you navigate by VOR, you also need DME or RNAV.
Something Is Broken. Can You Go?
Interactive · F07The 14 CFR 91.213(d) path. The examples assume the item is not on the type certificate's required list or the equipment list, and no AD applies.
Instrument Taxi Check
ChecklistThe generic check follows the IFH instrument preflight (IFH pp. 5-36–5-38). Use your airplane's checklist for the real thing. Tap an item to tick it.
The IPH adds that an IFR preflight should pay close attention to antennas, static wicks, ice protection equipment, the pitot tube and the static ports. Broken or missing static wicks should be replaced before an instrument flight: precipitation static can cause anything from a squeal in the headset to a complete loss of VHF communication and wrong compass readings.
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Module 2 · Instruments and Systems · Lesson 7 of 7
Lesson 2.7Ice Protection and Automation
Ice protection buys time to get out of the ice. Automation lowers workload only if you know what mode it is in.
Kinds of Ice
Clear, Rime, Mixed
Needs visible moisture and an airframe below freezing. Clear ice is the most dangerous: heavy, hard, and formed from large supercooled drops near freezing (about −10 °C to 0 °C). Rime is rough and white, from small drops at colder temperatures.
Pitot and Static
Structural ice on sensors such as the pitot tube and static ports. This is how ice becomes an instrument failure.
Intake and Carburetor
Ice that cuts off engine air. Carburetor ice is typical at −7 °C to 21 °C with relative humidity above 80%.
Icing Risk Check
InteractiveThresholds from IPH p. A-2 (anti-ice before visible moisture at +5 °C or colder), AWH p. 20-3 (structural ice) and IFH p. 4-13 / AWH p. 20-7 (carburetor ice). OAT stands in for airframe temperature here.
If Ice Starts to Build
Ice protection reduces available power, and if you turn it on after enough ice has built up it may not remove it all. It never lets you stay in icing indefinitely. At the first sign of ice, act to get out of it:
- Climb or descend to significantly colder air.
- Climb or descend to air above freezing.
- Fly to an area clear of visible moisture.
- Turn toward an area of known non-icing conditions.
If none of these is possible, consider landing at the nearest suitable airport. Consider a controllability check in the landing configuration, and be careful with configuration changes. At night, you may need a flashlight to see ice on the wings, gear and tail. How your airplane's anti-ice or deice equipment works, and its limits, are in its POH or AFM.
Automation and Autopilot Modes
You control the airplane through the autopilot by choosing pitch and roll modes. Mode names vary by manufacturer, so the AFM is the reference for your system. Brief how you will use the autopilot before every approach.
| Mode | What it does |
|---|---|
| ALT HOLD | Holds the current altitude until you pick another pitch mode. |
| ALT ACQ | Captures a preselected altitude, then changes to ALT HOLD. |
| V/S | Climbs or descends at a selected vertical speed. |
| LVL CHG / FL CHG | Climbs with maximum climb thrust or descends with the throttles at idle, on airplanes with autothrottles. |
| VNAV | Follows a programmed vertical path. |
| LOC / NAV | Tracks a localizer, VOR course or RNAV course. |
| APP | Tracks the localizer and arms glideslope capture. Selecting it after leveling at the FAF altitude makes sure you capture the glideslope from below, not a false glideslope. |
| TO/GA | Takeoff and go-around pitch guidance. |
If you are flying on autopilot, disengage altitude hold and speed hold. They make the airplane maneuver more and add structural stress. Hold a level attitude and let airspeed and altitude move. Turn on anti-ice and deice equipment, because icing can be rapid.
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Module 2 · Instruments and Systems · Knowledge Check
Knowledge CheckModule 2 Knowledge Check
Fourteen questions in the style of the oral exam, each tagged with its ACS element. Pass mark: 80% (12 of 14).