## Concepts - **Rigidity in Space**: a spinning gyro tends to remain fixed in the plane it was spinning when mounted on gimbals so the aircraft can rotate around it. - **Precession**: When you apply a force to a spinning gyro, it reacts 90 degrees later in the direction of rotation—not where you pushed. # Pitot-Static Instruments - Airspeed Indicator: - An airspeed indicator measures the difference between **ram (dynamic) pressure** from the pitot tube and **static pressure**from the static port. That pressure difference increases as airspeed increases. Inside the instrument, this differential pressure expands or contracts a diaphragm, which moves the needle to display indicated airspeed - **Instrument Check:** Airspeed indicator should read zero during taxi (unless facing a strong wind). During takeoff, ensure airspeed comes alive, or abort. - Altimeter - An altimeter measures altitude by sensing changes in **static pressure**. As the airplane climbs and static pressure decreases, a sealed aneroid wafer inside the instrument expands, moving the needles to indicate a higher altitude. As the airplane descends and static pressure increases, the wafer compresses, and the needles move to indicate a lower altitude. - **Instrument Check**: Set the altimeter to current altimeter setting - must indicate within 75 feet of actual elevation of that location for IFR flight. - Vertical Speed Indicator - A vertical speed indicator measures the rate of change of static pressure using a diaphragm connected directly to the static source and a surrounding case that receives static pressure through a calibrated leak. During a climb, static pressure decreases rapidly inside the diaphragm while the case pressure lags behind due to the restricted leak, creating a pressure difference that moves the needle to indicate a climb. During a descent, static pressure increases inside the diaphragm faster than in the case, reversing the pressure difference and moving the needle to indicate a descent. - VSI displays rate and trend information. It can lag up to 6 to 9 seconds to stabilize on accurate vertical speed indication. - Instrument Check: Before engine starts, if not displaying zero, record the delta and use that as new zero. VSI is not required for IFR flight but very useful! ## Pitot Blockage - **Pitot blocked, drain hole open:** Airspeed drops to **zero**. - **Pitot blocked, drain hole also blocked:** Airspeed indicator acts like an **altimeter**: - Climb → airspeed **increases** - Descent → airspeed **decreases** - **Static Port blocked:** - ASI will read inaccurate because pitot pressure is now being compared to trapped static pressure. - As altitude increases: static pressure outside decreases, but the instrument still uses higher trapped static pressure → **ASI reads lower than actual**. - As altitude decreases: outside static pressure increases, but instrument still uses lower trapped static pressure → **ASI reads higher than actual**. - Altimeter will read the altitude of the static pressure trapped in the line. - VSI will read 0 with the static air trapped in the line and the calibrated leak equalizing the pressure. # Gyroscopic Instruments - Attitude Indicator - Horizontal gyro with 2 gimbals - Self-erecting, has pendulous vanes that take advantage of precession to open the vane 90 degrees offset to auto-correct - Errors - After you roll out of a steep 180 degree turn, attitude indicator shows turn in opposite direction and slight climb. (less than 5 degree bank and 1 bar width of pitch error). The pendulous vanes uses gravity as vertical reference and in a steep coordinated turn, it aligns with the load factor (centripetal + centrifugal). It corrects after leveling off. - Acceleration and deceleration can also induce precession errors: acceleration causes the horizon bar to move down indicating climb, deceleration horizon bar moves up indicating descent. See Somatogravic illusion. - Heading Indicator - Vertical gyro with gears to turn the card accordingly. - Errors - Due to aircraft performing turns, precession can cause heading to drift from correct setting and as a result this instrument must be periodically checked against the magnetic compass and reset approximately 15 mins. - Turn coordinator (electrical typically) - Provides indications for standard rate turn - 3 degrees per second; 360 in 2 mins. - **Std Rate Bank = (TAS / 10) + 5** - Turn and Slip Coordinator: vertical gyro, uses precesion - Turn coordinator: canted gyro; allows to sense both rate of roll and rate of turn. - Inclinometer - ball moves freely in a sealed tube filled with fluid. - Slip: rate of turn is to slow for angle of bank; ball moves to inside of turn (think rudder is pointed to the inside of the turn while the nose is pointed outside of the turn) - Skid: rate of turn is too much for the angle of bank, ball moves to the outside. (rudder is pointed out of the turn while nose is pointed inwards). Skids generate higher than normal centrifugal force; load factor is increased. - Instrument Checks - Check ammeter for positive charging rate on engine start - Listen for unsual noise from gyros, expect gyros to achieve full rotation speed approx. 5 mins. - Taxi turns should show turns in the correct direction and ball in inclinometer to swing to the outside of the turn # Magnetic Compass - Errors - Variation: - Angular difference between true and magnetic north pole - All courses on IFR charts are published in magnetic. Only winds aloft are in true so need to convert. - Deviation - Error due to magnetic interference with metal components and magnetic fields from avionics, etc. After adjustments, corrections are recorded on compass correction card. - Dip - Cause of errors that matter during flight - Magnets in compass align with magnetic field, so farther north, they tend to dip more compared to equator - Turning Errors: - Northerly turning error occur when you turn to and from heading of north or south, prevalent in latitudes away from equator. - Compass LEADS from turning away from SOUTH. Compass LAGS when turning away from NORTH. No effect on E or W. - North - From East (090 → 360): `[TARGET HDG] [+ LAT] [+ HALF STD TURN RATE]] // Stop turn right of 360` - From West (270 → 360): `[TARGET HDG] [- LAT] [- HALF STD TURN RATE]] // Stop turn left of 360` - South - From East (090 → 180): `[TARGET HDG] [+ LAT] [- HALF STD TURN RATE]] // Stop turn after crossing HDG due to lag` - From West (270 → 180): `[TARGET HDG] [- LAT] [+ HALF STD TURN RATE]] // Stop turn after crossing HDG due to lag` - ANDS: greatest on E or W headings, if you accelerate, compass turns to north. If you decelerate, it turns to the south. - Instrument Checks - Compass is full of liquid and during taxi, ensure it swings freely and indicates known headings # Types of Airspeed - **Indicated Airspeed (IAS)**: What the airspeed indicator reads directly from the pitot-static system. No corrections applied. - **Calibrated Airspeed (CAS):** IAS corrected for instrument and position error. - **Equivalent Airspeed (EAS):** CAS corrected for compressibility effects (important at high speeds / high altitudes). Airspeeds above 200 KIAS and altitudes above 20,000 feet, air is compressed in front of an aircraft as it passes through air and causes abnormally high airspeed indication. EAS is lower than CAS. - **True Airspeed (TAS):** EAS or CASE corrected for air density (temperature and altitude). Represents your **actual speed through the air mass**. - **Vne** – Never exceed speed - **Vno** – Maximum structural cruising speed - **Va** – Maneuvering speed - **Vfe** – Maximum flap extended speed - **Vlo** – Maximum landing gear operating speed - **Vle** – Maximum landing gear extended speed - **Vlo** – Maximum gear extension or retraction speed - **Vs** – Stall speed in clean configuration - **Vso** – Stall speed in landing configuration - **Vx** – Best angle of climb - **Vy** – Best rate of climb - **Vg** – Best glide speed - **Va decreases** as weight decreases. - **White arc** → flap operating range - **Green arc** → normal operating range - **Yellow arc** → caution range (smooth air only) - **Red line** → Vne # Types of Altitudes - **Indicated Altitude** – What the altimeter reads with the current altimeter setting applied. Above FL180 - set altimeter to 29.92. Below FL180, set altimeter to current reported setting of a station within 100 nm of aircraft. - Calibrated Altitude: indicated altitude corrected to compensate for instrument error. - **Pressure Altitude** – Indicated altitude corrected to standard pressure (29.92 inHg). Used for performance calculations. Vertical distance above a theoretical plane - standard datum plane. - **Density Altitude** – Pressure altitude corrected for non-standard temperature. Indicates aircraft performance capability. - **True Altitude** – Actual height above mean sea level (MSL). - **Absolute Altitude** – Height above the terrain (AGL). - **Field Elevation** – Airport elevation above MSL. # Changes in temperature, humidity and pressure **Concepts** - Altimeter is a pressure to altitude translator; you set the base, it detects the pressure, calculates the difference and shows the altitude. - If you set the wrong base - you get the wrong altitude. - Think of atmosphere as layered pressure gradients - levels of same pressure stacked on top of each other - Altimeter is calibrated to standard atmospheric conditions (29.92"). As airplane moves up, it detects the difference and indicates the altitude. - If conditions are not standard, the whole stack, all pressure gradients, move up or down. **Pressure Effects** - Uneven heating of the Earth’s surface creates temperature variations. - Temperature differences cause air density changes. - Density differences lead to pressure differences across large regions. - **If pressure decreases, and you don't change altimeter** - Pressure gradients sink towards the ground - You will continue to follow the same pressure gradient as you fly - You will be closer to the terrain - YIKES - "When going high to low - look out below" - **If pressure increases, and you don't change altimeter** - Pressure gradients rise towards the sky - You will be higher than intended - think being higher than what ATC intended or in the same level as someone else when you shouldn't be - "When going low to high - clear the sky" **Temperature Effects** - Cold air is more dense, so the pressure gradients contract - Warm air is less dense, so the pressure gradients expand - Because of less spacing they are packed tighter so layers are lower, or closer to the ground - Altimeter is unaware so assumes standard spacing - So in cold temperature, your true altitude will be lower than indicated altitude even with correct altimeter setting. - When warm, true altitude will be higher than indicated altitude. **Humidity Effects - Humid is less dense than dry air - Affects aircraft performance and not altimeter accuracy - Humidity means more water vapor in atmosphere - water vapor is lighter than dry air - Moist air displaces dry air and results in less dense air - Humid air may feel heavy but physically its less dense - Less dense air means less oxygen for combustion - Less dense air means less air molecules to grab leading to less thurst for the propeller - Less dense air means wings need to move faster to produce the same lift - long takeoff roll, poor climb performance. # Electronic Flight Displays - Consists of - PFD Primary Flight Display - MFD - Multi-function Display - Backup Instruments - Powered by - AHRS - Attitude and Heading Reference System - Magnetometer: instead of suspended magnet, uses a flux valve or flux gate - electronic means of sensing magnetic force lines. - Powered by MEMS gyro: tiny quartz discs that move back and forth in one plane sensed by electrical circuits translating changes into signals that AHRS uses to determine attitude and rates of rotation - Ring Laser Gyro: uses light to detect rotation and changes in attitude. - Errors: displays red X over affected instrument - ADC - Air Data Computer - Pitot tude, static source and OAT provide information to the ADC to determine appropriate ratings for airspeed indicator, altimeter and VSI. - Also provides true airspeed and OAT on the PFD