12/15/2008 Ask Mike! Archive Page
Although we have discussed the dynamic brake relay in the 24-volt Bonanzas and Barons before; this time, we expand on it.

Good Afternoon, Jack;

These 24-volt aircraft have a very fast gear cycle time, so the dynamic brake relay was applied.  Basically, when the limit switch is reached, the power is removed from that cycle winding of the motor, and the other side is grounded, resulting in a generator, that counter-acts the EMF and stops the motor immediately.  No physical brake, but electric (dynamic).

Rarely is the dynamic brake relay, itself, a cause of over-run of the landing gear.  Most often, it is a problem with the motor brushes as poor contact with the commutator will not necessarily interrupt the cycle (motor energized), but interfere with the ground (of the generator action).  On an in-service difficulty situation, the most likely cause is either the grease seal has allowed grease (contamination) to the commutator, or the brushes are worn sufficiently to relax the spring tension holding them to the commutator.  When the motor brushes are changed as a matter of preventative maintenance, the motor should be disassembled enough to assure all residue (from the worn brushes) has been removed from the area as this can act as an insulator (contamination) to the electrical circuit.  Then, after reassembly (and before reinstallation), the motor needs to be run-in, both directions, to assure the brushes are properly seated.  If they are not, then this can also result in the over-run (again, due to lack of good contact to the commutator).  As a side note, during inspection, if I note the brushes are worn and need replacing, I perform a very critical look at the motor.  Generally, I send the motor off for repair instead of performing a field repair.  (We do the actuators in-house, but not the motors).

I am going on the assumption you have the inspection, maintenance, and rigging procedure available to you, as published by Beech in the current Shop / Maintenance Manual.  The one I have open is 36-590001-9B2, rev dated 10/1/08, applicable to (among others ) F33A, CE 772 & after.

As you are aware, Beech manuals leave a lot to be desired; for example, they tell you to change the motor brushes at 500 hours, but don't give much detail.  (Personal opinion:  500 hours is way too often, unless the aircraft is a flight school type, with multiple cycles of the gear per hour.)  However, you really need to pay attention to the caution note after step (h) of the rigging procedures (32-00-00-201).  If, in finding the dynamic braking action is erratic, and the actuator has contacted it's internal stops, it should be removed for complete disassembly, inspection, and repair as required, as the referenced "serious damage" may not be readily apparent.

When the repairs you do are complete, set the limit switch actuators to stop the gear very early, and confirm your over-travel is consistent (dynamic braking action) before you set the limit switches at the recommended positions (5/8 - 3/4 of a turn before hitting the internal stop).  Note:  the 5/8 to 3/4 of a turn over-travel is for the newer (white) actuators.  The older (gray or green) actuators use a different measurement; consult the shop manual for specifics.

Try to salvage your weekend, you have some work ahead of you on Monday.

Gear Green,
Mike