Menu Sign In Contact FAQ
Welcome to our forums

Alternator replacement

That is the FAA (or EASA for that matter) position on replacing an alternator with a different type?

For example the TB20 model is the ALU-8521.

Plane Power do their PMA equivalent – the AL24-70 which they claim weighs 9lb 12.8oz, against 12lb 2oz for the ALU-8521. That is a saving of 1.1kg. PP also state “replaces LW-14324 alternators on Lycoming 320, 360, 540, and 720 engines” and the 14324 is the original TB20 alternator. And their applicability list confirms this. What they don’t have is an STC for a TB20 but does that mean anything?

Surely a PMA is sufficient?

FAR 43 Appendix A uses the words like “basic change to the electrical system” which this change cannot possibly trigger. So this should not be a Major Alteration.

However some people believe that without an STC you cannot change such a component.

PP do have an STCd version (STC) which even has its own voltage regulator but the TB20 is not on the airframe list in the STC.

Any views?

Shoreham EGKA, United Kingdom

I think the alternator is considered part of the airframe, not the engine, and that your airframe needs to be on their PMA or STC list. If not listed, under FAA regs you’d need a field approval. My belief in that is not 100% certain My litmus test is to ask which maintenance logbook (engine or airframe) you’d use to record the installation.

If that is the case, the alternator manufacturer might be able add the Socata to the list if provided with data to support applicability. If that’s not possible, the PMA or STC list of almost identical installations on other airframes would support the approval.

PS Edited to add that if the Socata airframe maintenance manual does list the alternator by manufacturer LW-14324 part number, I think you’d have a plausible argument for installing the replacement under the PMA.

Last Edited by Silvaire at 04 Feb 16:18

Another alternator, with a huge PMA applicability list, is this 150 amp Plane power one.

It weighs “12.5lb” versus “12lb 2oz” for the standard Hartzell type ALU-8521. That is well below the level at which a new W&B schedule needs to be done.

One would hope that a 150A alternator is more sturdy than the standard 80A one. The extra current should be needed less and less with modern avionics – for example the maximum current drawn by my TB20, with the recently fitted LED lights, during normal flight with everything on, is only about 25A, increasing briefly to about 50A during landing gear and flap operations.

Shoreham EGKA, United Kingdom

Aircraft electrical system will be limited by existing wiring and output breaker. ALT-FLX is capable of up to 100amps at 14v and 150amps at 28v.

The strangest thing with all these alternators both to replace lower output alternators and generators is that the higher output alternators uses the same wiring AND the same alternator / generator fuse if installed. If your battery was quite weak and have some power on, you will pop the existing CB as it overloads that CB / overload the excisting wirng.

Alltough allowed in accordance with most installation instructions it would make sense to upgrade the wiring and circuit breaker / amp meter as well


I have certainly found that when changing a Gill battery for a Concorde one, the initial charging current (after the engine is started, and when the ALT breaker is pushed in) is maybe 2x higher. That’s because the Concorde has a lower internal resistance – apart from having the same Ah capacity it’s a bit like having a battery which is about 2x bigger in all respects.

How long this condition lasts for depends on how much charge was in the battery. Normally it is only a few seconds but it can be a minute or two if the battery was a bit low.

The current is about 25-30A. The old Gill would take maybe 15A. This is all within the circuit breaker rating which is 60A.

It’s an interesting Q whether a “bigger” alternator will actually output more current if nothing else on the aircraft has changed. I don’t think it should, because the voltage regulator is (or should be) still set up to make it output 28.0V. The equipment will thus draw the same current, and the battery charging current should be the same as before unless the old alternator was so weak that its output collapsed down from the 28.0 purely due to battery charging.

The only reason I am looking at this alternator is to improve in-flight reliability. This is pretty relevant on a single alternator aircraft

They make impressive claims regarding a much lower internal operating temperature rise over ambient – here.

Last Edited by Peter at 08 Jul 05:55
Shoreham EGKA, United Kingdom

I have found these wind mill generators really good, even while there constructions looks a bit weak. My experiances with these are better than the vacuum pump driver alternators for example. None of which I have seen where able to deliver the output as claimed on the test bench.

True on the internal resistance. Lower internal resistance is better on all respects. The output of the alternator does depend on the regulator and RPM. Full output will NOT be available at low RPM.

The current will not change if the load hasn’t changed and the voltage is the same. Depending on all conditions such as internal resistance of the battery, state of charge, other loads, RPM etc, this alternator could put out more than the wiring and circuit breaker is designed for. Therefore it would not be a good idea.

If the output would never exceed the 60A it is quite useless to replace it with a higher current version, as you don’t use it anyway. And these older equipement have service / maintenance documentation available. Most of the new equipment is said to have same TBO as the engine, but in practice it will fail regulary before TBO, without the possibility to have it repaired.

A second full size alternator would be best option, a wind driven one second. Upgrading to a higher size would still give you only one alternator.
A backup battery is also a good alternative, especially in combination with a good voltage warning light, such as in the TB as standard, so you could reduce load as soon as possible.


For most pilots, losing 1.1kg of their body weight would be much cheaper, doesn’t require an STC, and will help the pilot retain their medical :-)

Andreas IOM

Absolutely agree! I am 3kg off my peak of about 2 years ago… I would not do this to save weight (even though an alternator is way out in the front of the CofG) and actually this latest one is very slightly heavier.

Shoreham EGKA, United Kingdom

…What they don’t have is an STC for a TB20 but does that mean anything?

Surely a PMA is sufficient?

Why would you need an STC for a PMA replacement part? AIUI It is the same as a like-for-like replacement if your aircraft is listed in the PMA approval….

EGPD / OMDW / YPJT, United Kingdom

If it’s PMAd then sure, you don’t need an STC. You can just install it directly.

Loads of bits used in GA are PMAd. EASA threatened to stop this a few years ago but they have evidently not succeeded.

There may be a different rule for AOC aircraft… I don’t know. But then they would have loads of downtime.

Shoreham EGKA, United Kingdom
11 Posts
Sign in to add your message

Back to Top