The Six Basic Flight Instruments Explained: How Each Works and Fails

Look at the panel of a small training aircraft like a Cessna 172 and you will see six round dials arranged in two rows. Pilots call them the "six pack". Together they tell the pilot how fast the aircraft is flying, how high it is, whether it is climbing, which way it is pointing and whether it is turning. This guide explains each of the six basic flight instruments in plain language, how they work, and the errors student pilots are expected to know, using the FAA's Pilot's Handbook of Aeronautical Knowledge as the main source.

This is neutral, educational information, not flight training. For Indian student pilots, aircraft instruments are part of the DGCA CPL ground subjects (Technical General) in CAR Section 7, Series B, Part IV. Always check the current syllabus on dgca.gov.in.

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The six pack at a glance

The FAA's Pilot's Handbook of Aeronautical Knowledge, Chapter 8 (Flight Instruments) groups the six basic instruments into two families:

• Pitot-static instruments, worked by air pressure: the airspeed indicator, the altimeter and the vertical speed indicator.
• Gyroscopic instruments, worked by spinning gyros: the attitude indicator, the heading indicator and the turn coordinator.

In most panels the top row is airspeed, attitude and altitude, and the bottom row is turn coordinator, heading and vertical speed. The attitude indicator sits in the centre because it is the instrument pilots look at most.

The pitot-static system

Two kinds of pressure feed the first three instruments. The pitot tube, usually under a wing, faces into the airflow and picks up ram (impact) pressure. The static port, usually a small hole on the side of the fuselage, senses the still air pressure around the aircraft, called static pressure. The airspeed indicator uses both. The altimeter and vertical speed indicator use static pressure only.

1. Airspeed indicator (ASI)

The ASI compares pitot pressure with static pressure. The difference grows as the aircraft flies faster, and a diaphragm inside turns that difference into a speed, usually in knots. The dial carries colour markings that matter in every exam:

• White arc: the flap operating range, from the stall speed in landing configuration up to the maximum flap extended speed.
• Green arc: the normal operating range.
• Yellow arc: the caution range, to be flown only in smooth air.
• Red line: the never-exceed speed (V_NE).

The speed the dial shows is indicated airspeed. Corrected for instrument and position errors it becomes calibrated airspeed, and corrected for altitude and temperature it becomes true airspeed, the actual speed through the air.

2. Altimeter

The altimeter is an aneroid barometer: sealed metal wafers expand as static pressure falls during a climb, and gears turn that into feet. It is only accurate when the correct pressure setting (such as QNH) is dialled in. Our full guide on altimeter settings, QNH and QFE covers this in detail.

3. Vertical speed indicator (VSI)

The VSI shows the rate of climb or descent in feet per minute. Inside, a diaphragm receives static pressure directly, while the case around it receives static pressure through a small calibrated leak. During a climb or descent the two pressures differ for a moment, and the needle shows that difference. The FAA notes that the VSI shows the trend (up or down) straight away, but the exact rate takes about 6 to 9 seconds to settle.

When the pitot-static system is blocked

Blockages are a favourite exam topic. The FAA handbook describes them like this:

• Pitot tube blocked (for example by ice or an insect): if the drain hole stays open, the ASI drops to zero. If the drain hole is also blocked, the trapped pressure makes the ASI act like an altimeter, showing a higher speed as the aircraft climbs and a lower one as it descends. The altimeter and VSI are not affected.
• Static port blocked: the altimeter freezes at the altitude where the blockage happened, the VSI reads zero, and the ASI becomes wrong, reading too low in a climb and too high in a descent.
• Alternate static source: many aircraft have a backup static source inside the cabin. Cabin pressure is usually a little lower than outside, so the altimeter and ASI read slightly high, and the VSI briefly shows a climb when it is selected.

This is why pilots check that the pitot cover is removed and the static ports are clear during the pre-flight walk-around, and why many aircraft have pitot heat.

The gyroscopic instruments

A spinning gyro has two properties that make these instruments work. Rigidity in space means it keeps pointing the same way however the aircraft moves around it. Precession means that when a force is applied to it, the gyro reacts 90 degrees further along in its direction of spin. In many light aircraft the attitude and heading indicators are driven by a vacuum (suction) system, while the turn coordinator is electric, so one failure does not take out all three.

4. Attitude indicator

Often called the artificial horizon, it shows the aircraft's pitch (nose up or down) and bank (wings tilted) against a horizon line, using rigidity in space. A small aircraft symbol in the middle is compared with the horizon bar, and marks around the top show bank angle. It is the only instrument that gives an immediate picture of both pitch and bank, which makes it central when flying in cloud.

5. Heading indicator

Also called the directional gyro, it shows which way the aircraft is pointing. A magnetic compass swings and gives errors in turns and when speeding up or slowing down, so the heading indicator gives a steady reading instead. It has no magnetic sense of its own, though, and slowly drifts because of precession and friction. The FAA advises aligning it with the magnetic compass about every 15 minutes in straight and level, unaccelerated flight.

6. Turn coordinator

The turn coordinator uses precession to show the rate of roll and then the rate of turn. When the small aircraft symbol lines up with the mark, the aircraft is in a standard-rate turn of 3 degrees per second, which is a full 360 degrees in 2 minutes. Some older aircraft have a turn-and-slip indicator instead, which shows rate of turn only.

Below it sits the inclinometer, a ball in a curved glass tube. If the ball is centred, the turn is coordinated. If it is off-centre, the aircraft is slipping or skidding, and the pilot uses rudder to "step on the ball", meaning press the rudder pedal on the side the ball has moved to.

What about glass cockpits?

Many modern trainers and every airliner show the same information on screens. A primary flight display (PFD) puts airspeed, attitude, altitude, vertical speed and heading on one screen, fed by an air data computer and an attitude and heading reference system instead of separate dials. The FAA handbook covers these too. The principles are the same, so learning the six pack still makes the glass displays easier to understand, and most panels keep backup instruments for failures.

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Frequently asked questions

What are the six basic flight instruments?

The airspeed indicator, altimeter and vertical speed indicator (pitot-static), and the attitude indicator, heading indicator and turn coordinator (gyroscopic).

Which instruments use the pitot tube?

Only the airspeed indicator uses pitot pressure. It compares it with static pressure. The altimeter and vertical speed indicator use static pressure alone.

What happens if the static port is blocked?

The altimeter freezes, the vertical speed indicator shows zero, and the airspeed indicator reads too low in a climb and too high in a descent.

What is a standard-rate turn?

A turn at 3 degrees per second, so a full circle takes 2 minutes. The turn coordinator has a mark that shows it.

Why does the heading indicator need resetting?

It has no magnetic reference and slowly drifts because of precession, so pilots align it with the magnetic compass about every 15 minutes in steady, level flight.

Are flight instruments asked in DGCA exams?

Aircraft instruments are part of the DGCA CPL ground syllabus (Technical General) in CAR Section 7, Series B, Part IV. Exact questions vary, so study from the current official syllabus.

Wrap-up

The six pack splits neatly in two. Three instruments read air pressure through the pitot tube and static port, so blockages make them lie in predictable ways. Three use spinning gyros, relying on rigidity in space and precession. Learn what each one shows, how it works and how it fails, and you have the base for both your DGCA Technical General preparation and your first lessons in the cockpit.

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Related: Altimeter settings: QNH and QFE · How VOR navigation works · How heavy aircraft fly · DGCA exams explained

General educational information only, not flight training. Instrument explanations follow the FAA Pilot's Handbook of Aeronautical Knowledge (Chapter 8, Flight Instruments), checked on 21 October 2026. Panels differ between aircraft, so always follow the aircraft's flight manual, your instructor and the current DGCA syllabus (dgca.gov.in).