RA

Compass Issues: Deviation and Magnetic Dip

  • Deviation: This is the difference between the heading you want to fly (what you steer) and what your magnetic compass actually shows. It's caused by things like electrical currents and metal parts inside the airplane itself. Think of it like your airplane's own magnetic quirks.

    • For example, to fly exactly east (090 degrees), you might actually have to steer 095 degrees due to deviation.

    • Deviation Card: Every airplane has a special card that tells you how much to adjust your steering for different headings to correct for deviation. This card lists the required steering for every 30 degrees.

    • Why it happens: The electrical systems and magnetic materials in the airplane mess with the compass.

    • Keep your phone away from the compass; it can create magnetic interference and give you false readings.

  • Variation between aircrafts: Every airplane has its own unique deviation, and it can even change a little day by day. Even small errors (1-4 degrees) can make a big difference on long flights.

  • Instructor's Note: Don't worry too much about "RDO on/off" for exams; it's a minor detail that usually causes only a 1-3 degree difference.

  • Tip: Always consider deviation when planning your flights, especially on longer trips where small errors add up.

Magnetic Dip and Compass Errors

  • Magnetic Dip: The Earth's magnetic field pulls your compass needle downwards, towards the magnetic North Pole. This downward pull causes your compass to act weird (lag or swing) during turns and when you change speed, especially closer to the North or South Poles.

  • Location Matters: The closer you are to the poles, the stronger the dip and the bigger these errors become. At the Equator, there's very little dip.

  • Compensation: A small weight on the compass aims to balance this dip, but it doesn't fix it completely.

  • Big Idea: Magnetic dip is the main reason for Acceleration (ANDS) and Turning (UNOS) errors in your magnetic compass.

  • When is the compass accurate? Only when you're flying straight and level, without speeding up, slowing down, or turning. Any bumps (turbulence), speed changes, or turns will introduce errors.

  • How it behaves when turning:

    • North/South (0°/180°): When turning towards or away from North, the compass tends to lag (show you're turning less than you actually are). Turning right from a North heading might even make it look like you're turning left initially.

    • South-heading turns: The compass might jump ahead and then settle back.

    • East/West: These headings usually have fewer turning errors.

  • Mnemonics (Memory Helpers):

    • UNOS: Undershoot North, Overshoot South. This means when you're turning to a North heading from East or West, you'll need to stop your turn before hitting North. When turning to South, you'll need to continue your turn past South.

    • ANDS: Accelerate North, Decelerate South. If you're flying East or West and suddenly speed up, the compass will momentarily swing towards North. If you slow down, it will swing towards South.

  • Latitude Factor: These errors get worse the further you are from the Equator. For example, at 15° North, the error might be twice as big. Higher latitudes mean bigger errors.

  • Practical Tip: Expect minor wiggles in your compass when flying normally. Plan for larger errors on longer flights or at higher latitudes.

Turbulence Effects on the Compass

  • Magnetic dip is the main cause of errors when you accelerate or turn.

  • Oscillation: When you hit turbulence, the compass needle will start to swing back and forth (oscillate) as the magnet bounces around.

  • Compass Design: The compass is often sealed in liquid (like kerosene). Turbulence makes this liquid and the magnet inside it less stable.

  • What it means for you: In bumpy air, your magnetic compass can give you unreliable readings that lag or swing wildly. Always double-check with your heading indicator (a different instrument) in turbulence.

The Pitot-Static System: Pitot Tube, Static Port, and Alternate Static Source

  • Your instruments get information from two main sources:

    • Pitot tube: This measures the air rushing into it (dynamic pressure). It's used by the Airspeed Indicator (ASI).

    • Static port: This measures the calm, outside air pressure (ambient static pressure). It's used by the ASI, Altimeter, and Vertical Speed Indicator (VSI).

  • Pitot Tube Details:

    • It has an opening facing forward that measures the ram air pressure. There's also a second hole inside that helps the ASI figure out the difference in pressure.

    • You'll usually find it on the wing or fuselage, sometimes on a mast (a stick).

    • If it gets blocked (by ice, dirt, or insects), your airspeed readings will become wrong or freeze up.

  • Static Port Details:

    • It measures the still air pressure outside the plane. It's located on a smooth, undisturbed part of the plane, often near the front left side.

    • Alternate Static Source: If your main static port gets blocked, you can switch to an alternate source, usually inside the cockpit. Beware: using the alternate source causes all your instruments that rely on it (Altimeter, VSI, ASI) to show incorrect readings (e.g., altimeter will read higher).

    • If the static port is blocked, your ASI, Altimeter, and VSI will all be affected, but in different ways depending on what kind of blockage it is.

  • Pressure and Altitude:

    • As you climb higher, the air pressure outside decreases (static pressure drops).

    • Lower pressure and colder temperatures affect how your plane performs and how instruments read.

  • Standard Atmosphere (Aviation Basics):

    • Standard sea-level pressure: P ext{SL} = 29.92 \text{ inHg (or} \approx 1013.25 \text{ hPa)}.

    • Standard sea-level temperature: T ext{SL} = 15°\text{C (or } 59 °\text{F)}.

    • European units: Often use 1013.25 \text{ hPa}, which is the same as 29.92 \text{ inHg}.

    • ATC (Air Traffic Control) gives you a local altimeter setting called QNH. For international flights, you might hear 1013.25 \text{ hPa} (QNE) or QNH.

  • Standard Atmosphere Model (general rules):

    • Altitude: As you go up, pressure drops.

    • Temperature Lapse Rate: Temperature usually drops about 2°\text{C} for every 1000 feet you climb.

    • Pressure Lapse: Near sea level, pressure drops roughly 1 \text{ inch of Hg} for every 1000 feet (a rough estimate).

  • Altimeter and Static Pressure: The altimeter uses static pressure to show your height. The altimeter setting (in the Kollsman window) adjusts it to show your height above a specific reference pressure (usually sea level).

  • Alternate Static Source Notes: If you use this, your altimeter will typically read higher than actual. Your VSI and ASI might also briefly show changes before settling.

Airspeed Indicators: Types of Airspeed and How They Work

  • How Airspeed Indicators Work: They measure the difference between the ram air pressure (from the pitot tube) and the static air pressure (from the static port).

    • Bigger difference = Higher indicated airspeed (IAS).

    • Blocked Pitot Tube: The ASI will freeze or give unreliable readings.

    • Blocked Static Port: All instruments using static pressure are affected (ASI, Altimeter, VSI). The ASI will react as if the airplane is stopped or if the pressure is totally different.

  • Types of Airspeed:

    • Indicated Airspeed (IAS): This is what your airspeed indicator directly shows you. It's based on the air pressure difference.

    • Calibrated Airspeed (CAS): This is your IAS corrected for any errors in the instrument itself or how it's installed on the plane.

    • True Airspeed (TAS): This is your CAS corrected for altitude and non-standard temperatures. It's your actual speed through the air.

  • Why TAS is higher than IAS at altitude: As you climb higher, the air gets thinner (less dense). To get the same pressure reading for the ASI, you have to fly faster through this thinner air. So, for the same IAS, your TAS will be much higher at altitude.

  • Ground Speed vs. Airspeed:

    • Ground Speed (GS): Your TAS adjusted for wind. If you have a tailwind, your GS will be faster than your TAS. If you have a headwind, your GS will be slower than your TAS.

  • The E6B Tool: This is a classic circular slide rule used to calculate things like speed, distance, and time. Modern planes have computers that do this, but the E6B is still a key tool to learn.

  • Density and Performance: Thinner air (lower density) at altitude means a higher TAS for the same IAS. This also affects how your engines and wings work.

Airspeed Indicator: Arcs and V Speeds

  • The airspeed indicator has colored arcs that show important speed ranges:

    • White Arc: This is your flap operating range, used for landing and approaches.

    • Green Arc: Your normal operating range.

    • Yellow Arc: The caution range. Only fly in this range when the air is smooth.

    • Red Line: VNE - Never Exceed Speed. Flying faster than this speed can damage the aircraft.

  • Stall and Flap Speeds (important for landing/takeoff):

    • Vs1: Stall speed with flaps and landing gear up (plane is