An aircraft's 'six-pack'

THE "six-pack" in aviation refers to six essential flight instruments in an aircraft cockpit: the attitude indicator (AI), heading indicator (HI), turn co-ordinator, airspeed indicator (ASI), altimeter and vertical speed indicator (VSI).

In the early days of aviation, pilots navigated from one point to another using rivers, roadways, hills and other landmarks that could be viewed from the air.

This limited the pilot to fly by visual flight rules (VFR) only. This is a set of regulations under which a pilot operates an aircraft in weather conditions known as visual meteorological conditions (VMC) in which VFR flights are permitted once the pilot has a very clear view of the surrounding environment including terrain.

However, during night time and periods of adverse weather with cloud ceilings, pilots could not fly as they had no visual reference to the ground and could not use visual means to avoid obstructions and other aircraft in flight.

The flight instruments provide the pilot with situational awareness data about the aircraft's position such as attitude, altitude, airspeed, vertical speed, heading and rate of turn.

Safety was vastly improved as pilots could fly the aircraft in level flight, and make turns, without reference to outside the aircraft such as the horizon.

When pilots fly using instruments only, it is called instrument flight rules (IFR).

Flying by VFR requires an airspeed indicator, an altimeter and a compass or other suitable magnetic direction indicator.

Flying by IFR additionally requires an attitude indicator, a heading indicator, a rate of turn indicator, an adjustable altimeter and a vertical speed indicator.

Instrument meteorological conditions are weather conditions that require pilots to fly primarily by reference to flight instruments, and therefore under IFR, as opposed to flying by outside visual references under VFR. Typically, this means flying in a cloud or poor weather, where little or nothing can be seen or recognised when looking out of the cockpit window.

Flights under IMC also require radio navigation instruments to navigate along the aircraft's planned route from the departure point to the arrival point.

If the weather is less than VMC, pilots are required to use IFR, and operation of the aircraft will be primarily through referencing the instruments rather than visual reference.

When operation of an aircraft under VFR is not safe because the visual cues outside the aircraft are obscured by weather, IFR must be used.

This permits an aircraft to operate in instrument meteorological conditions, which is essentially any weather condition less than VMC, but in which aircraft can still operate safely.

The following gives a brief description of the "six-pack" instruments:

• The attitude indicator (AI): Also known as an artificial horizon is an instrument that shows the aircraft's position in space in relation to the horizon.

It is a primary instrument for flight under IMC. The AI informs the pilot of the aircraft's orientation relative to the earth's horizon and gives an immediate indication of the smallest orientation change.

The miniature aircraft and horizon bar mimic the relationship of the aircraft relative to the actual horizon. From this the pilot can tell whether the wings are level and if the aircraft nose is pointing above or below the horizon. Attitude is usually displayed in the degrees.

• Altimeter: Altitude is determined based on the measurement of atmospheric pressure. The greater the altitude, the lower the pressure and vice versa.

When a barometer is supplied with a non-linear calibration to indicate altitude, the instrument is a type of altimeter called a pressure altimeter or barometric altimeter.

In aircraft, a barometer altimeter measures the atmospheric pressure from a static port outside the aircraft.

Air pressure decreases with an increase in altitude at approximately one inch of mercury per 1,000 feet.

The barometric altimeter is calibrated to show the pressure directly as an altitude above mean sea level, in accordance with a mathematical model atmosphere defined by the International Standard Atmosphere (ISA).

Modern aircraft use a "sensitive altimeter" in which the sea-level reference pressure can be adjusted with a setting knob.

The reference pressure, in inches of mercury in Canada and the US, and hectopascals (previously millibars) elsewhere, is displayed in the small Kollsman window on the face of the aircraft altimeter. This is necessary since sea level reference atmospheric pressure at a given location varies over time with temperature and the movement of pressure systems in the atmosphere.

• The airspeed indicator (ASI): This displays the aircraft's speed in knots relative to the surrounding air. A speed of 70 knots is 70 nautical miles per hour.

The airspeed indicator works by measuring the ram-air pressure in the aircraft's pitot tube relative to the ambient static pressure.

The indicated airspeed (IAS) must be corrected for non-standard pressure and temperature in order to obtain the true airspeed (TAS).

The instrument is colour coded to indicate important airspeeds such as the stall speed, never-exceed airspeed or safe flap operation speeds.

• The turn co-ordinator: Also known as turn and slip indicator is essentially two aircraft flight instruments in one device.

One indicates the rate of turn or the rate of change in the aircraft's heading.

The other part indicates whether the aircraft is in co-ordinated flight, showing the slip or skid of the turn.

The slip indicator is actually an inclinometer that at rest displays the angle of the aircraft's transverse axis with respect to horizontal, and in motion displays this angle as modified by the acceleration of the aircraft. The most commonly used units are degrees per second or minutes per turn.

• The heading indicator: Also known as the directional gyro displays the aircraft's heading in compass points in relation to magnetic north when set with a compass.

Bearing friction causes drift errors from precession of the gyro which must be periodically corrected by calibrating the instrument to the magnetic compass.

• An aircraft vertical speed indicator (VSI): Also known as a rate-of-climb indicator or variometer, displays the aircraft's rate of climb or descent, typically in feet per minute. Pilots use the VSI to monitor an aircraft's rate of climb or descent during various flight phases like takeoff, landing and cruising, ensuring a smooth and efficient flight path.

Today’s jet aircraft are highly automated with high-precision instruments that allow the aircraft to operate safely in all weather conditions and fly the most optimum routes.