Safety Update Concerning Brake Master Cylinders in Dual Applications

July 2023

Recent testing has revealed the potentially hazardous condition of a locked brake caliper, when using Bob's brake master cylinders in two-pilot installations. This problem only relates to installations using four master cylinders in one airplane, but it requires immediate attention.

In the installation where the problem occurred, fluid is routed from a reservoir to the pilot's master cylinder's top port, then from the bottom port of the pilot's master cylinder to the top of the copilot's master cylinder. The output at the bottom of the copilot's master cylinder is routed to the brake caliper.

In this installation, if the copilot applies and holds brake pressure while the pilot applies pressure, the copilot's cylinder can act as a check valve and lock the brake. If the brake fluid routing is different from this case, such as if the reservoir first feeds the copilot's cylinders, then the problem would be reversed. The master cylinders are run in series, and the "middle" set is the problem.

The Gerdes cylinders use a higher internal parts count and complexity that Bob was motivated to improve on with his design. In an installation where only one pilot uses the brakes, these cylinders are perfectly safe as-is. Give thought to possible future modifications to add a second set of brakes, perhaps making a note in the aircraft's documentation.

Bob is working on a design solution but for now, if you have an installation with dual pedals, here are the recommended mitigations before further flight:

First choice: Gerdes Cylinders in the Middle Remove the middle master cylinders and replace them with Gerdes cylinders. This will be the least intrusive but will require sourcing a pair of master cylinders. Unfortunately one of the motivators in Bob's redesign was the scarcity of the Gerdes cylinders.

Second choice: Single Pilot Brakes Remove the middle master cylinders and reroute the brake lines to bypass the copilot's brakes. This will require removal of the brake pedals from the copilot's rudder pedals, and possibly creating some new lines. Route the reservoir output to the top of the master cylinder, and route the bottom of the master cylinder to the caliper.

It may also be possible to operate the airplane safely with dual cylinders in place- we operated this way for several months before encountering the hazard. But carefully consider the risk of copilot brake application, even unintentional. It would be prudent to not allow copilot interaction with the rudder pedals at all, until one of the two choices above can be completed.

For airplanes still under construction, it will be best to wait for Bob's design solution unless you are very close to the first flight.

If you have any questions please reach out to Bob by phone at 540-473-3661.