Firstly, I'm not even sure if there's only one alternator connected to the engine in a C172-like aircraft. But if that is true, then why don't manufacturers connect multiple alternators to a single? What are the draw-backs of doing so, other than a waste of generated power?

I just want to know whether it is possible - and preferably without any major draw-backs.

Without too much increase in weight and complexity, one more alternator could together produce twice the electricity - this might actually be primarily useful for large air-liners relying more on electricity than hydraulics, like the A380 and the 787 Dreamliner. So, again, why not?

  • $\begingroup$ @mins - Let's assume that there are two 14V alternators. Can't they individually be connected to the battery and simultaneously charge the battery without any sort of connection between each other? $\endgroup$ – ClobberXD Oct 2 '17 at 7:23
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    $\begingroup$ Nothing keeps them from doing it, the SR-20 and SR-22 have dual alternators, as do many designs which have large electrical requirements. $\endgroup$ – GdD Oct 2 '17 at 10:19
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    $\begingroup$ It does use mechanical power from the engine @AnandS , there's no way to get around it. How much I couldn't say. $\endgroup$ – GdD Oct 2 '17 at 10:56
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    $\begingroup$ It would be a tangible value, but it would appear in increased fuel burn rather than a slower turning engine since we can adjust engine speed via propeller pitch or throttle position. To find the mechanical power draw, you would look at the power output of the alternator divided by the alternator production efficiency. Big picture, you probably wouldn't notice much decrease in aircraft performance, but there would be a measurable drop in engine performance $\endgroup$ – ANDY-S Oct 4 '17 at 0:36
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    $\begingroup$ @ANDY-S Speaking as an electrical engineer: Alternators can be made highly efficient across a range of power ratings. And efficiency is only one design parameter. There is no reason that a small alternator cannot be more efficient than a large one - so that Two small ones can also be more efficient than a large one. $\endgroup$ – Russell McMahon Nov 25 '19 at 1:54

You almost answered the question yourself.

Having more than one alternator would introduce more complexity not only mechanically but also in the electric system (even if they work correctly). Alternators scale really good so that a bigger alternator generates more electric power then two smaller of the same weight.

So the only reason to do that on a smaller air plane is to have a backup if one of them fails.

This brings up two questions:

  1. How often do alternators fail?
  2. what are the consequences of a failed alternator?

For the first question I have a (non representative) experience: At my airfield 6 SEP plane are are posted. In 25 years I'm flying there we had 1 failed alternator.

For the second question we have to look in the flight instructions and the planes manuals. For smaller planes like those you mentioned they state that you should turn off some electric devices and land on the next reachable airfield. That means there is no immediate risk when an alternator fails.

The conclusion is that we simply don't need more than one alternator in a SEP plane.

  • $\begingroup$ That's a good point - I understand that there is no need for multiple alternators, but is it possible? Let's assume that the necessity exists... $\endgroup$ – ClobberXD Oct 2 '17 at 7:19
  • $\begingroup$ @AnandS Yes, it would be physically possible to get an engine to run multiple alternators. $\endgroup$ – Dan Oct 2 '17 at 7:22
  • $\begingroup$ Does the engine's output power reduce if multiple alternators are spinning off of it? Is the reduce in power negligible or drastic? $\endgroup$ – ClobberXD Oct 2 '17 at 7:25
  • $\begingroup$ @AnandS this highly depends on how the alternator fails. If it simply shuts off electrically the engines power output is (slightly) better than with the alternator. If it is blocked mechanically the engines output is reduced. $\endgroup$ – Timothy Truckle Oct 2 '17 at 8:10
  • $\begingroup$ Alternators can be made highly efficient across a range of power ratings. And efficiency is only one design parameter. There is no reason that a small alternator cannot be more efficient than a large one - so that Two small ones can also be more efficient than a large one. $\endgroup$ – Russell McMahon Nov 25 '19 at 1:52

It is possible and I have done it, with payloads which draw more than the aircraft can provide.

Need an STC, and in my case about $12K engineering costs, for a Cessna 210.

I have also stuck extra starter/generators on aircraft like the Caravan 208B, and in that case there was some concern with the torque on the accessory drive for the 300A SG with a second 200A SG.

The additional power was needed for payloads which were active sensors and had fairly high power needs for both the sensors and the targeting and processing equipment.


Point to add, backup alternators (smaller units)on cirrus aircraft are required to provide backup power to display systems since engine indications on the later generation aircraft are only on electronic displays (although the displays have compacted views on remaining units in the event of failure of the display) meaning it does not constitute a dire emergency if the main alternator goes belly up (rare but still happens).


Definitely possible. Here is one that is offered


Measuring 4.6" wide and 6" deep, and weighing only 5.75 lbs., the BC410-H will clear the tachometer cable and oil filter on stock Lycoming engines.


The bottom of the page discusses how it is spline driven (vs a belt) and fits on a vacuum pad.

I only see it listed for Experimental/Homebuilt aircraft, so that could be an issue getting it approved for a Certificated C-172.

Some of us have pulled their entire vacuum system and gone to battery backed electronic Attitude Indicator/Direction Gyro, such as a pair of Garmin G5s; 4 hours should be enough time to get clear of clouds and make a VFR landing somewhere.


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