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Learning hub · Instrumentation · Lesson 2

Airspeed markings, engine instruments and the compass

The coloured arcs on the airspeed indicator are a summary of the flight manual, and the engine gauges tell you how the engine feels. The compass is old technology, but you need to know why it is not always right.

About 12 minutes, plus the quiz. Part of Instrumentation. It builds on Pitot-static and gyro instruments.

1. The airspeed indicator markings

Light aeroplanes mark their speed limits on the dial. These are the standard arcs.

MarkingRangeMeaning
White arcVs0 to VfeFlap operating range
Green arcVs1 to VnoNormal operating range
Yellow arcVno to VneCaution: smooth air only
Red lineVneNever exceed

The definitions, in plain words:

  • Vs0: the stalling speed (or the minimum steady flight speed) in the landing configuration, flaps and gear down.
  • Vs1: the stalling speed in a specified configuration, usually clean.
  • Vfe: the highest speed with the flaps extended.
  • Vno: the maximum structural cruising speed. Above it, only in smooth air.
  • Vne: the never-exceed speed.
  • Va: the design manoeuvring speed. Below it, a full, abrupt movement of one control should stall the wing before the structure is overloaded. It does not protect against combined or repeated control movements. It is quoted for the maximum mass and is lower at a lower mass, and it is not marked on the dial.
  • Vx: the speed for the best angle of climb, the most height in the shortest distance. Vy: the speed for the best rate of climb, the most height in the shortest time. Vx is slower than Vy.

The values come from the flight manual (POH), and the manual rules if it differs from anything you remember.

2. Engine instruments

  • RPM (tachometer): engine speed. With a fixed-pitch propeller it is your power indicator. Watch for the red line.
  • Oil pressure: should rise soon after start. If it does not, stop the engine, as the manual says. Low pressure in flight or a slow fall in pressure is a serious sign.
  • Oil temperature: should be in the green. High temperature with falling pressure suggests a loss of oil.
  • CHT (cylinder head temperature) and EGT (exhaust gas temperature): found in better-equipped aeroplanes. They help you manage the mixture and warn of overheating.
  • Fuel gauges: only indications. Add your own record of fuel and time.

Look at the trends, not one number. A temperature that creeps up while the pressure falls means more than either one alone. It is normal for the oil temperature to rise slowly after start.

3. The magnetic compass: variation, deviation and dip

The compass has a magnet on a card floating in liquid, which damps its swings. It points to magnetic north, not true north, and it has other errors. Turning and acceleration errors (UNOS and ANDS) are in the earlier lesson. Here are the other three.

Variation is the angle between true north and magnetic north. It depends on where you are on Earth and changes slowly over the years. It is printed on charts. Deviation is the error caused by magnetism in the aeroplane itself: metal, wiring, radios. It depends on the heading. It is measured in a compass swing and written on a deviation card near the compass.

To convert, remember the order True, Variation, Magnetic, Deviation, Compass. Going from true to compass: east variation is subtracted and west is added (“variation east, magnetic least”). The same applies to deviation, going from magnetic to compass.

Worked example (invented numbers): true heading 090°, variation 5°E, deviation 3°W. Magnetic = 090 − 5 = 085°. Compass = 085 + 3 = 088°.

Dip is the downward slope of the magnetic field. It is small near the equator and large near the poles. The card must be free to tilt, and dip is what makes the turning and acceleration errors; the compass is also less useful close to the magnetic poles. Keep phones, torches and headsets away from it, as they may add deviation.

Training material only. Use the markings, limits and deviation card for your own aeroplane, and the flight manual for engine limits.

4. Try it yourself

Work each one out on paper first, then open the answer to compare. The numbers are invented.

  1. An invented aeroplane has Vs0 40 kt, Vs1 48 kt, Vfe 85 kt, Vno 110 kt and Vne 140 kt. Give the range of the white, green and yellow arcs and the position of the red line. Are you allowed to extend the flap at 90 kt?
    Show the answer
    1. White arc: 40 to 85 kt. Green arc: 48 to 110 kt. Yellow arc: 110 to 140 kt. Red line: 140 kt.
    2. 90 kt is above Vfe (85 kt), outside the white arc.
    3. So no: slow down below 85 kt first.
  2. Your true heading is 270°. The variation is 6°W and the deviation card says 2°E for this heading. What do you steer on the compass?
    Show the answer
    1. West variation is added: magnetic = 270 + 6 = 276°.
    2. East deviation is subtracted: compass = 276 − 2 = 274°.
    3. Steer 274° on the compass.
  3. In cruise the oil temperature creeps up and the oil pressure falls slowly over ten minutes. Both are still just in the green. What should you think and do?
    Show the answer
    1. Two trends together point to a possible oil leak or loss, even though neither is outside its limit yet.
    2. Check the other gauges. Follow the flight manual’s procedure for abnormal oil readings.
    3. Do not wait for a red line. Plan to land at the nearest suitable aerodrome and tell air traffic services if needed.

5. Watch

The coloured arcs and the V-speeds marked on the airspeed indicator.

Open on YouTube if the player does not load. Not made by VectorEFB; check what you learn against your flying school’s material and your instructor.

More to watch

Variation and deviation of the magnetic compass.

Open on YouTube if the player does not load. Not made by VectorEFB.

What the EGT and CHT gauges show and how pilots use them. This video is American, so wording and some rules differ from European (ICAO/EASA) practice.

Open on YouTube if the player does not load. Not made by VectorEFB.

6. Check yourself

Five original practice questions. Answer them, press Check answers and read each explanation.

1. What does the white arc on the airspeed indicator show?

The white arc runs from Vs0 to Vfe, where the flaps may be extended.

2. What is Vno?

Vno is the top of the green arc. Above it, fly only in smooth air.

3. Which is the best angle of climb speed?

Vx gives the most height in the shortest distance. Vy gives the most height in the shortest time.

4. What is compass deviation?

Variation is the difference between true and magnetic north. Deviation is caused by the aeroplane and is shown on the deviation card.

5. True heading 180°, variation 4°E. What is the magnetic heading?

East variation is subtracted when going from true to magnetic: 180 − 4 = 176°.

Next: Drag, stability and turning flight. Back to the learning hub.

For training and situational awareness only. Not certified for navigation. Always use official charts, the AIP, NOTAMs and a proper weather briefing for real flights.