MD26-28 400Hz inverter - not very reliable

It states that it is "PMA approved" - what does this mean and what is

required to get such an approval?

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

Hello Peter,

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



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.

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.



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

http://www.analogconsultants.com

On 18 Feb 2005 09:15:12 -0800, "Michael"
wrote:

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

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,

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.



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

http://www.analogconsultants.com