MD26-28 400Hz inverter - not very reliable

It means that a federal bureaucrat has decided that the design and manufacturing system meet FAA standards, and thus the part can be sold as an aircraft component.

Since the FAA standards are a bad joke, it doesn't mean the design and manufacturing system are actually adequate.

The best (but not legal) solution would be to save the box, and to have someone with a clue build you a real inverter (it's not hard) and then seal the box up again.

Michael

What's mostly involved is spending a bunch of money. The approval comes from the MIDO, but they won't even talk to you unless you have a thick sheaf of documents generated by a DER. In theory, this is someone who knows what he is doing. In practice, it's a cushy retirement gig for FAA bureaucrats.

You would like to think that the PMA would assure some level of quality and reliability, but all it assures is a nice retirement income for an 'engineer' who was never employable in the private sector.

Michael

I've had two of the above fail in under 3 years. They drive the KI229 inverter.

I found details of this unit at

I've taken the last one to bits. It was potted in silicone rubber so it took an extra 5 minutes... For $400, you get components worth about $10, forming a primitive circuit comprising of a transformer, two power transistors and a few bits costing pennies.

It states that it is "PMA approved" - what does this mean and what is required to get such an approval?

Both units developed an intermittent, vibration-related, fault (no

400Hz output) which is curious since they are potted and not a lot can move about.

Peter.

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"Tim Shoppa" wrote

It occassionally fails when switch on on the ground (no correlation with OAT, humidity, or whether the voltage is 24V or 28V) and then suddenly sorts itself out a few minutes after takeoff.

The components comprise of a big ferrite toroid, two TIP31 transistors, one 3uF 100V metallised foil cap, a choke, and a few resistors. Pretty obviously it is a self-oscillating design (the transformer primary has a number of taps); not something I would have chosen for load stability, and in fact the frequency varies almost directly with the supply voltage.

Peter.

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"Michael" wrote

I can build one allright. I just wondered what is involved in getting such an approval for making something like this.

Peter.

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I read in sci.electronics.design that Michael wrote (in ) about 'MD26-28 400Hz inverter - not very reliable', on Fri, 18 Feb

2005:

What are MIDO, DER and PMA?

Mass of Indolent Doddering Oldsters?

Don't EVER Retire?

Paradise for Mumbling Ancients?

--
Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

Hello Peter,

It sometimes surprises me how often equipment fails even though it is certified. Like vacuum pumps, for example. There has got to be a way to improve.

The only way to find out is if you can simulate the failure on the ground, preferably in your workshop, and then immediately probe the components. In this case most likely the transistors. Toroids can also fail but that would require a stiff mechanical jolt. I have seen some that looked ok but upon test showed a reduced AL value. Usually hair cracks.

Regards, Joerg

Morons, Idiots, Dolts & Oafs

Doddering Elitist Retards!

Pathetic & Moronic Accountants?

--
Beware of those who suffer from delusions of adequacy!

Michael A. Terrell
Central Florida

Manufacturing Inspection District Office Designated Engineering Representative Parts Manufacturing Authorization

Michael

Had a couple of these inverters powering the AC sync-type fuel flow indicators in a couple of Beech 18's. The third aircraft had the same fuel flow lash-up, but was powered by a 6.5 lb motor-verter.

Only had issues with one of the solid state inverters, am thinking it was an older version of the same unit. I'm assuming the construction "standard" has changed a little. I could get to the parts inside it, but they were truly potted in place.

It was by no means a high-tech unit, but the construction at that time was decent.

One of my all-time favorites is a Honeywell/Grimes 70-0196-1 "solid state flasher unit". 28v 1.0-5.5 amp input. About the size of a turn signal/warning light flasher if you potted it into a neat little anodized aluminum box (hint hint) and ran a couple of wires out the top. Used to make the red light bulb on the top of a Chieftain/Cheyenne blink.

Current list price-wait for it-$1035.61, but available at your local Aviall branch for only $880.27 (order it online, they'll throw in the shipping).

Mark

Usually solid state stuff should be pretty dependable, but silicon devices don't like reversed polarity spikes. These can happen if the master contactor ever stutters when the starter is engaged. I'm surprised there isn't more transient protection in avionics and solid state regulators.

I realize that this particular failure sounded like a vibration problem, but a few more isolating and shunting diodes in our electronics would seem to help reliability a lot.

Hello nrp,

And some ferrites which can be awesome protection for the electronics in case you get into that surprise thunderstorm.

When debugging EMI in a composite structure back in the 80's I was mighty disappointed with the EMI susceptibility of some NAV/COM gear.

Regards, Joerg