Characteristics and General Definitions

1. Units & Standard Conversions

Pilots must move fluently between metric, imperial and aviation-specific units. The values below are quoted exactly from the source and must be memorised — examiners test the precise figures.

2. Introduction — Common Instrument Problems

Pilots receive information about the aircraft's state — speed, altitude, position and attitude — through instruments and displays. Regardless of vintage, every instrumentation system must contend with four general characteristics:

• Range — the span of values it can display.
• Resolution — the smallest change it can show.
• Accuracy — how closely the indication matches truth.
• Reliability — consistency of correct indication over time.

3. Measuring Range versus Accuracy

It is often necessary to show a large operating range yet still indicate accurately across the whole range. For example, an airliner limited to a maximum permitted airspeed of 350 knots might use an instrument designed to display up to 380 or 400 knots. But safety-critical speeds must be read to the nearest knot. If the entire range is squeezed onto a single revolution, each one-knot division becomes too small to read accurately — that is the range-versus-accuracy conflict.

Circular scale — linear

A simple indicator showing values over a range of 0 to 30 units uses evenly spaced graduations; the required accuracy governs the spacing.

Circular scale — non-linear

Some instruments must show changes more accurately at certain parts of the scale. A rate-of-climb indicator uses a logarithmic scale so that low rates of climb/descent are spread out and easily read, while high rates are compressed.

4. High Range / Long Scale Displays

5. Ergonomy & Standard Panel Layouts

Ergonomy (human engineering) is the science of the relationship between people and machines. For instruments it means designing displays unlikely to be misread and arranging them so interpretation is easy and correct.

From the "basic six" to the "basic T"

The flying instruments were first arranged as the basic six; other instruments were scattered to suit the manufacturer. Developments then led to the standard basic T.

6. Analogue versus Digital Presentation

Presentation can be analogue (a pointer on a dial) or digital (a row of numbers). With a 3-pointer analogue altimeter, an altitude such as 24 020 ft is harder to absorb at a glance than a digital readout. Digital numbers are easier to read accurately.

7. Electronic (Glass) Displays

Traditionally instruments lived on the instrument panel. With modern electronic displays, the displays remain on the flight deck where the crew can see and operate them, but the computing and power units are located remotely — usually in a separate compartment called the Avionics Bay or the Electrics and Electronics (E&E) Bay.

flowchart LR
  S[Sensors / Probes] --> C[Computing units
in Avionics / E&E Bay]
  P[Power units] --> C
  C --> D[Flight-deck displays
CRT / LCD glass screens]
  D --> Crew([Flight Crew])

8. Readability & Parallax

A readable instrument is designed around an eye reference point — the anticipated position of the pilot's eye in normal viewing. Where an index/reference mark sits in front of a scale, the eye, index and scale must all be in line.

9. Coloured Arcs & Colour Standardization

Conventional (non-electronic) instruments

Electronic displays — CS-25 standardization

Practice Questions & Detailed Answers

Master Reference Tables

All numerical values in this chapter

Memory aids

Answer key