Saturday, August 09, 2008

Bendix D3000 magneto redundancy

John, I purchased and read your magneto book, which I found informative and interesting. After reading the book, I do have two questions I'm wondering if you could answer given your experience with magnetos.

1. Our Cardinal has the dual magneto, although I am considering a conversion to a non-D at the upcoming overhaul. My question regards redundancy: what internal elements are common to the two magnetos within the D housing?

Do they only share a common drive shaft (and gear), or are any other electromechanical components shared - e.g., cam, magnets, etc.? Do you know of any source where I could see internal photos or mechanical or cutaway drawings of the D3000 magneto so I could better understand its design?

Yes, the maintenance/overhaul manual for this magneto. About the only parts that are truly redundant is the coil, points, and capacitor.

The D3000 magneto is a good magneto, easy to work on and reliable. Like any device it has its limitations. Follow the maintenance/overhaul manual, service bulletins and airworthiness directives. 

The following three areas should be well understood, respected and maintained, as all have caused fatal aircraft accidents:

  • Impulse coupling spring (part number 10-51324)
  • Hold-down clamps
  • Cam retaining screw

    Impulse Coupling Spring:
    Magneto Impulse coupling spring

    Breakage retards timing causing complete loss of engine power. Cessna 172N N738BC ditched at sea with 2 fatalities. On any magneto the impulse area sometimes gets rusty from condensation. The impulse spring gets tiny rust pits that create stress corrosion cracking. There is no warning - it just breaks. With two magnetos you lose timing but you can turn the bad magneto off; with the D3000 magneto the broken spring retards the timing on both magnetos and you lose power.

    Follow TCM's instructions and recommendations to the letter in regards to inspecting and replacing the impulse spring. Personally, if I lived in a corrosive area I would replace it every year.

    Hold-down Clamps:
    See my article at

    Cam Retaining Screw: "Everything hangs on this connection"

    Here is a copy of an email I received some time ago from Germany:

    A pilot came to an aircraft workshop with the problem,that the engine did not start well and did not reach more the 2200 RPM. (Cessna 170 N engine Lyc.O-320-H2AD SNR L-8408-76T)
The workshop made -a test run on ground:

the engine rpm drop was 120-130 rpm.-
The different pressure in the cylinders was 1.)80/78; 2.)80/79; 3.)80/78; 4.)80/77.-
The intake tube of the Cylinder No 3 was leaky -it was renewed.-
The timing of the ignitions examined and adjusted.-
The air intake filter was dirty and cleaned -
The following ground check did show no problems: rpm drop 70-80 rpm max rpm 2320 rpm.

After this the aircraft made a take off and crashed immediately in the ground, as the engine lost power

The investigation did show, that the screw (Fig 1-14 D-3000 Magneto manual ) was loose and so the cam breaker could turn on the cone.
Magneto cam retaining screw
D-2000, D-3000 points and cam

The problem here is that the mechanic did not order a new cam screw and re-used the old one. You loosen this screw when you adjust internal timing. This screw is a self-locking screw that uses a nylon patch on the threads. The locking effectiveness is poor if re-used. Continental says to replace it with a new one. Follow the factory instructions - accept no deviation or alternative methods of compliance.

Continental (Bendix) has addressed each one of these areas in their maintenance manual. Personally, if the mechanic did not have the maintenance manual for the D3000 in hand I would not let him touch the magneto.

There is an interesting discussion of this screw and the problems associated with installing it that I highly recommend be read by anyone working on this magneto.,%20G-EKMW%2011-06.pdf

Important information regarding distributor gears

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