There was a time when portable watches used to be two inches in diameter and as much as three-quarters of an inch thick. These watches were kept in the pocket and taken out only when you needed to know the time.

We all know how these pocket watches evolved, became slimmer and turned into wrist watches. The display gauges in the aircraft cockpit underwent a near identical transformation except that; on the evolution time-scale, they evolved nearer the electronic revolution so instead of becoming slimmer versions of their earlier self, they turned into slim line digital gauges.

Naturally, the sensor that actually picked up the aircraft engine data also got transformed and began transmitting data in electrical pulses that could be interpreted by the smart electronics circuits inside the slimline gauges.

In next to no time the integrated circuits got so miniaturized that an entire circuit was fitted inside a single chip 2 x 1 cm in size which came to be called the IC chip (short for integrated circuit). During this same period, onboard memory was developed which totally transformed the way data was handled and interpreted because data could now be temporarily stored, recalled and used – almost in the same way as your calculator memory.

Next came a permanent memory module – the Random Access Memory (RAM). Thanks to this new development, instead of data being lost when the aircraft engines were turned off, critical engine data could be stored and retrieved on a semi-permanent basis.

Along with the miniaturization of the innards of the slimline gauges in aircraft, the front end i.e. the display itself underwent a change and the humble analog dial was replaced with a Liquid Crystal Display (LCD). So now the pilot could see the actual digits. This was also the time the analog wrist watch began to get replaced with a digital wrist watch. This was also the time when electronic pagers began to make their appearances in the market.

The next big development occurred when the memory modules was linked to the faceplate of the Slimline Instruments which totally revolutionized and enhanced the value and usefulness of the slimline gauge. The pilot could now directly interact with the gauge by feeding in alarm triggers in the form of upper and lower limits. For example, minimum temperature and maximum temperature. These figures would be stored in the memory module and constantly compared with the incoming temperature data.

So, if the temperature fell below the minimum figure or rose above the maximum figure, an audio-visual alarm could be triggered. This meant the pilot(s) no longer needed to keep monitoring the gauges and instead, could actually enjoy the flight.

Each slimline gauge usually displayed one piece of information. So, you had a slimline gauge for Oil Temp Probes, OAT, RPM, Voltage, Manifold Pressure, OIL Pressure and so forth.

As of today, individual sensors pick up the data directly from the engine and transmit it as a voltage via wires to the individual slim line gauge for which it is meant. The electronic board and IC circuit onboard the slim line gauge interprets the incoming voltage, converts it into a meaningful number and displays it on the screen. The display was bright enough to be seen even with the sun directly behind the pilot.

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