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The beginning scenes of the movie "San Andreas" partly take place in a rescue helicopter. One of the panel gauges read "GAS PROD" above it. I googled it but didn't find much. Can someone explain what it does, and how it measures what's it measures? Thanks!

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The instrument shows the gas generator rpm in the engine. It think you are referring to this scene (screenshot from youtube):

Gas Prod

The Bell 412 in the movie uses a pair of Pratt & Whitney Canada PT6T-3BE Twin-Pac turboshafts, which are free turbine engines. The turboshafts used in helicopters consist of,

  • Compressor

  • Combustion chamber

  • turbine

    and other components.

The gas producer (from which 'GAS PROD' came) forms part of the turbine. The gas generator forms the first stage that runs the compressor and makes the engine self sustaining. It also runs the oil and fuel pumps. As it is a free turbine, the gas generator stage is not mechanically connected to the next (power turbine) stage. From faasafety.gov:

The turbine section consists of a series of turbine wheels that are used to drive the compressor section and the rotor system. The first stage, which is usually referred to as the gas producer or N1 may consist of one or more turbine wheels. This stage drives the components necessary to complete the turbine cycle making the engine self-sustaining. Common components driven by the N1 stage are the compressor, oil pump, and fuel pump. The second stage, which may also consist of one or more wheels, is dedicated to driving the main rotor system and accessories from the engine gearbox. This is referred to as the power turbine (N2 or Nr).

In some cases, NG is used in place of N1. The gas generator speed is measured in the PT6 engines using a tachometer-generator mounted on the engine.

From United turbine training Manual:

The tachometer-generator produces an electric current which is used in conjunction with a tachometer to indicate gas generator RPM. The tachometer-generator drive and mount pad are located at the 5 o’clock position on the accessory gearbox and is driven form the internal scavenge pump.

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    $\begingroup$ Except in this case, the gauge measures the intermediate turbine temperature. $\endgroup$ – Simon Dec 7 '15 at 12:26
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    $\begingroup$ @aeroalias Look at the left-most gauge, it says FUEL QTY on the panel and Fuel Quantity on the gauge. The center and right-most gauge in the picture are labeled ITT on the panel and ITT in the gauge. The label GAS PROD goes with the gauges not shown above the top of the picture. This answer seems to correctly answer the question "what does a gas prod gauge measure", but the image really adds a lot of confusion; partly because the OP's question asks "what does gauge with GAS PROD above it measure", which in this case is ITT. $\endgroup$ – nexus_2006 Jul 3 '16 at 19:57
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    $\begingroup$ You have captured the wrong part of the instrument panel. The gage you are showing is the ITT gage, while the Gas Prod is just above it, but you cropped it in your picture. Go back to the picture, and crop it so that "Gas prod" is under the gage. that will show the gas prod gage. (As you can see, "ITT" is under the ITT gage that you are showing in the picture.) $\endgroup$ – KorvinStarmast Nov 11 '18 at 22:28
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That's an Inter Turbine Temperature gauge - measures the temperature of the gas exiting the high pressure gas core turbine and entering the low pressure power turbine.

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The short version is that in turbine helicopters they have a gas generator gauge. Referred to as the "N1" gauge.
It's critical you monitor that gauge when starting, so that you don't attempt to start the turbine when it's internal temperature is too hot.

:)

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