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I have always had a deep appreciation for technology and the future and I encourage that technology should move us forward whether that be in aviation terms or otherwise but one question has been on my mind for a very long time and that is why we have computerized gauges and not the old fashioned ones with the gauge hand behind glass. In the event that a plane loses power, wouldn't it be better to have physical gauges because if the plane loses power, those critical gauges on a screen will disappear. Is it just me or does it not make a huge difference? I suppose an old-fashioned glass gauge will stop too, just the same as a glass cockpit gauge?

Thanks. This is a question I have always wanted to ask someone who has the expertise to answer it.

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    $\begingroup$ Thats a really long way to ask the question, "Are steam gauges more reliable than glass panels?" $\endgroup$ – abelenky Mar 21 '18 at 14:27
  • $\begingroup$ Very good question! I have a feeling the way this developed was something like: „Hey, look, using digital displays we can condense the instruments, make them cheaper and easier to repair, and lose the flight engineer. Let’s add an independent power supply to make sure at least some data is displayed for enough time to land in case everything fails, so we’re not worse off than on those old-fashioned steam gauges we‘re ditching ...“ $\endgroup$ – Cpt Reynolds Mar 21 '18 at 14:54
  • $\begingroup$ Steam gauge instruments on jets still require electrical power obviously for radio nav instruments, but also to keep the gyroscopes spinning. $\endgroup$ – user71659 Mar 21 '18 at 15:38
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    $\begingroup$ @jamesqf: Unsure if you're pedantic or just not familiar with the term: Round, analog gauges are frequently called "steam" gauges for their similarity to actual steam-gauges; not because there is any actual steam involved (there isn't). $\endgroup$ – abelenky Mar 21 '18 at 19:30
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    $\begingroup$ @abelenky: No, I'm not familiar with the term, and I doubt many people are. If you mean analog gauges, why don't you say analog gauges? It's only one extra letter :-) $\endgroup$ – jamesqf Mar 22 '18 at 5:44
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Most aircraft using electronic displays have double (and sometimes triple) redundant power supplies to keep the electronics alive in cases of power failure. The mechanical gauge panels of the not so distant past did not have these same levels of redundancy built in.

Beyond power considerations, glass panels also contain features like weather radar, collision avoidance, and navigation that further increase the reliability and safety of the system as a whole.

That said, it's fair to say that glass panels are more reliable. The funny thing is, critical mechanical gauges are still included on most panels as last resort backups.

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    $\begingroup$ Many mechanical gauges do not require power (altimeter, airspeed indicator, vertical speed indicator) and those that do (attitude indicator) often have a backup in larger aircraft. $\endgroup$ – Steve Kuo Mar 21 '18 at 15:43
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    $\begingroup$ @SteveKuo On jets, mechanical VSIs are largely gone due to TCAS. Primary altitude and airspeed are really electronic because an air data computer does calibration. They were ADC-based since at least the 727. $\endgroup$ – user71659 Mar 21 '18 at 16:07
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    $\begingroup$ @user71659: I'm really trying to understand: "Vertical Speed Indicators are largely gone due to Traffic Collision Avoidance Systems"?? Those two don't seem to be related? Are you referring to some other acronym? $\endgroup$ – abelenky Mar 21 '18 at 18:05
  • $\begingroup$ Yea, I think the disappearance of mechanical VSI's has very little to do with TCAS. It's more likely that they were eliminated simply because the vertical speed is shown right next to the speed tape on just about every glass panel out there. Even systems without TCAS display vertical speed in this way, thus there is no need to crowd the instrument panel with a mechanical VSI. $\endgroup$ – Geoff Mar 21 '18 at 20:55
  • $\begingroup$ @abelenky My statement is correct as is. Just look at picture of a 757/767 cockpit. TCAS requires a red zone/green zone indicator on the VSI (called the RA display), and a traffic display (TA display). Most retrofits integrated this into a single digital VSI because it was the easiest. The alternative retrofit was a needle VSI with LED scale and traffic being displayed on the radar display, but that requires significant mods to the radar side (ability to draw symbols, override). $\endgroup$ – user71659 Mar 21 '18 at 21:06
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Most integrated flight displays – the so-called glass cockpit – contain multiple redundancies both on the forms of computer power, and electrical power sources to keep them operating in the event of an emergency or other electrical problems.

As an example, I obtained my multi engine add-on in a Diamond DA-42 TwinStar airplane equipped with a Garmin G1000 glass cockpit. The aircraft’s electrical system uses a primary 24 V battery which is supplies power to all the electrical buses in the aircraft. It is also charged by two 24V, 60 amp alternators, one on each engine, which supply power to their own electrical bus and the battery bus. The main electrical busses supply power to the avionics power bus and both alternators can feed this bus as well. In the event of a total electrical failure to both the battery and alternator power buses, there is an emergency battery available to power the avionics for at least 30 minutes of continued operation.

Many aircraft carry separate back up EFIS displays, each with its own emergency power supply for additional redundancy.

If you compare this with the typical systems an twin aircraft with a conventional cockpit, most of the gyroscopic instruments are powered by a pair of vacuum pumps, one on each engine. The turn and bank indicator is powered by the aircraft’s electrical system. Lose both of the vacuum pumps, and you’re on partial panel.

In comparing these two systems you would have to look at the probability of an all out failure for both systems. The new glass cockpits have been just as reliable, if not more reliable, then conventional steam gauges and far more functional in the event of a failure.

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  • $\begingroup$ Everyone is concentrating on the power supply side, but it's not the whole story. The display itself, as a unit, may fail. And it doesn't have to be the screen: failure of a single panel button or knob may block certain functionality, or even many of them. This is inherently more dangerous than a failure of any one steam gauge, but this is, of course, offset by always having at least two glass displays, each being able to take the most important functions of both. $\endgroup$ – Zeus Mar 22 '18 at 3:36
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Glass panels are more reliable and accurate in every way over steam gauges.

When it comes to gauges, the most accurate and reliable ones are found in industrial applications like steam plants or ships because weight was not an important factor.

On an application like an airplane or a car, weight is an important consideration. Obviously gauges on an airplane are accurate, but not nearly as accurate as they are in other applications. A 50 year-old altimeter might be accurate within 10 feet, which is fine, just a little jarring when landing.

With the advancement of technology, glass panels have a definite weight advantage over gauges and can have a precision, accuracy and reliability beyond a need to display. It depends on the sensor (does anyone need their air speed to the seventh decimal point?). Glass panels can self-test to determine the accuracy of the readings and alert you to the need to replace a sensor, but a gauge cannot do that.

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    $\begingroup$ 0.000 000 5 knots is about 0.000 26 mm/s. I very, very, very strongly doubt that even top of the line aviation instruments are that accurate. $\endgroup$ – a CVn Mar 21 '18 at 16:12
  • $\begingroup$ @MichaelKjörling - What you are describing is precision, not accuracy. They can be confusing at times, however. $\endgroup$ – ymb1 Mar 21 '18 at 16:44
  • $\begingroup$ @MichaelKjörling I am describing both. Electronic gauges can be accurate and precise. But the precision is not of much benefit as you point out. It's just a capability beyond an analog gauge. Perhaps I need to rewrite the answer, since it's been edited once. $\endgroup$ – gwally Mar 21 '18 at 17:17
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    $\begingroup$ @gwally - only one typo was corrected, you can still edit it. $\endgroup$ – ymb1 Mar 21 '18 at 17:42
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    $\begingroup$ @ymb1 Just for clarification since it's been edited now: my comment was in response to gwally's discussion on accuracy and the (rhetorical) question "does anyone need their air speed to the seventh decimal point". It's not meaningful to display (say) an airspeed indication to seven decimal places if the instrument is not that accurate (by which I mean what your linked Wikipedia section refers to as "trueness") in the first place, even if the decimals were somehow useful. It's like how people convert a quantity from one unit to another and in the process go from three significant digits to ten. $\endgroup$ – a CVn Mar 21 '18 at 20:21

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