The Six-Pack: the Six Basic Instruments

The FAA handbook (Pilot’s Handbook of Aeronautical Knowledge) speaks of the “six pack” of instruments, which a glass cockpit (a cockpit with electronic displays) now shows on one screen. Here it means the six basic instruments in the table below.

The six instruments

Instrument	Shows	Data source
Airspeed indicator (ASI)	indicated airspeed, from the difference between pitot pressure and static pressure	pitot pressure and static pressure
Attitude indicator	a picture of the aircraft’s attitude relative to the horizon	gyroscope
Altimeter	height above the pressure level you have set	static pressure
Turn coordinator	rate of roll and rate of turn; the ball shows coordination	gyroscope
Heading indicator	the heading; it drifts and is aligned with the magnetic compass	gyroscope
Vertical speed indicator (VSI)	climb, descent or level flight, in feet per minute	static pressure

Pitot pressure and static pressure

The pitot-static system uses the static air pressure and the dynamic pressure that arises from the aircraft’s motion through the air. The airspeed indicator, altimeter and vertical speed indicator work from these pressures. Only the airspeed indicator uses the pitot tube. It takes in the total pressure, the sum of dynamic and static pressure. The airspeed indicator also applies the static pressure to the opposite side, so the two static pressures cancel out and the dynamic pressure is left to be indicated. The altimeter and vertical speed indicator use static pressure alone.

Altimeter

The altimeter measures the air pressure where it is and shows it as a height in feet. It shows the correct height only after you have set the pressure in its setting window (Kollsman window). This setting corrects only for pressure differences, not for temperature. In air colder than standard temperature, your aircraft is lower than the altimeter shows.

Vertical speed indicator

The vertical speed indicator needs time: if the pitch attitude stays constant after a change, the needle settles after 6 to 9 seconds. This delay is called lag.

Gyroscopic instruments

The most common gyroscopic instruments are the turn coordinator, heading indicator and attitude indicator. A gyroscope stays in the plane in which it spins (rigidity in space). Depending on the aircraft, it is driven by vacuum, pressure or electricity. If the vacuum falls below the normal range, the gyroscopic instruments can become unstable and inaccurate.

Turn coordinator

In the turn coordinator the gyro is canted. It therefore senses rate of roll and rate of turn, but it does not show a bank angle. A standard-rate turn is 3° per second. The display includes an inclinometer, a liquid-filled curved tube with a ball inside. If you keep the ball centred, the flight stays coordinated.

Heading indicator

The heading indicator drifts because friction displaces it from its set position. In addition, the Earth turns 15° in one hour, so it can be off by up to 15° per hour. A heading indicator without automatic north-seeking (a “free” gyro) needs regular correction: according to the FAA handbook, you check it about every 15 minutes and align it with the magnetic compass.

When an instrument reads wrongly

Errors almost always point to a blocked pitot tube, blocked static ports or both. Causes include moisture (including ice), dirt and insects. According to the FAA handbook, you check before flight that the pitot cover is removed and that both openings on the pitot tube are clear.

If the pitot tube is blocked and the drain hole is clear, the airspeed indicator falls to zero. A blocked static port affects three instruments: the airspeed indicator reads inaccurately, the altimeter freezes at the altitude where the blockage occurred, and the vertical speed indicator shows zero continuously.

Some aircraft have an alternate static source. With it, the altimeter reads slightly high and the airspeed indicator reads too much. Look up the error for your aircraft in the flight manual.

What the EASA syllabus covers

The EASA syllabus for the PPL(A) theory covers, under “Instrumentation”, the altimeter, vertical speed indicator, airspeed indicator, magnetic compass and gyroscopic instruments. For the turn coordinator it uses the term “Turn and bank indicator”. The FAA handbook distinguishes the “turn-and-slip indicator” from the “turn coordinator”.