Page 20 - June 2015 Volume 9, Number 6

– since air is escaping out of the brake deice manifold – and hence the system applies less rudder force than it should. Therefore, no takeoff or go-around should be initiated with brake deice in use since the pilot would not have the proper help in applying rudder force.
“So, we’ll tell the pilot to turn off brake deice on runway lineup and on final approach, right?” says Mr. Beechcraft engineer. “No!” responds Mr. FAA certifier. “Someday a pilot will forget that important step. You need to make it automatic and fool-proof.”
In response to that directive, Beech made a system change in the model 300 that deactivates brake deice whenever the Autofeather system completes its arming process ... as both power levers are well-advanced. So if the crew does not turn off brake deice themselves when taking the runway, it will deactivate automatically when the Autofeather annunciators illuminate. Once the landing gear is retracted and a safe altitude is reached after departure, the Autofeather switch must be turned off before brake deice may be used to warm the main wheel wells to melt any ice that has accumulated there. Likewise, when we need to accomplish our once-a-day exercise of the brake deice valves, even in the summer, we must assure that the Autofeather annunciators are not on ... by turning the Autofeather switch off or by retarding power levers. Just remember to reposition the switch back to Arm when the deice valve exercise is finished.
It is a very nice, simple, improvement that the 350 measures differential power via torque, not via P3 pressure. Although with brake deice on, slightly more N1 and ITT will be necessary to create the same torque, the yawing tendency relates to torque differential directly, so brake deice has no effect on the necessary rudder assist that is delivered. Therefore, there is no autofeather- related shutoff of the brake deice system in 350s.
In the case of the 200- and F90-series, Rudder Boost is not a no-go system. (None of the 100-series airplanes have a Rudder Boost system at all.) Although it is standard equipment, it is provided not by FAA directive, but simply to make the airplane somewhat easier to handle in one-engine-inoperative operations. It therefore follows that the brake deice system’s effect on Rudder Boost should be understood, but even if it impacts Rudder Boost negatively, it is not a critical concern.
First, with brake deice robbing air from the engines’ compressors, ITT may be about 20° C higher at a given power setting. If the torque value found on the “Minimum Takeoff Power” graph in the POH cannot be achieved with this additional ITT, then brake deice must be turned off so that Minimum Takeoff Power can be reached.
Second, in this series of King Airs, unlike in the 300 and 350, Rudder Boost is an all-or-nothing proposition, not varying the force based on the difference in engine
18 • KING AIR MAGAZINE
power. The sensor that initiates the rudder boosting force is a “Delta P” switch tied in to the left and right Little P3 lines, looking at raw, unregulated P3 pressure. With Brake Deice on, since some of that air is being diverted overboard, the Delta P switch will see less of a difference so rudder force will not be applied until more power is added on the “good” side.
Realizing that brake deice usage during takeoff has these negatives associated with it, and knowing that a brake assembly is not going to freeze up while the wheel is turning, I strongly suggest that you always turn off Brake Deice as part of your Runway Lineup procedure on any model King Air you fly.
The actual valve that opens to direct P3 to the brake assembly is a bit of an oddity. Instead of being a simple solenoid valve similar to the Instrument Air Shutoff valve aft of the firewall, this valve is electrically activated but pneumatically operated. The solenoid opens a port that permits P3 to provide the force that actually moves the valve. This means that the valve does not move unless the engine is operating, supplying raw Little P3 pressure.
Long ago, it was found that these valves were prone to not operating when they should: Either not opening when turned on or not closing when turned off. Analysis of the situation convinced Beech that regular exercise was necessary for the valve to remain relatively trouble- free. Now, all of the POH supplements direct us to cycle the valves once each day, regardless of OAT. I encourage you to incorporate that into your checklist procedures as you begin the descent into your home base airport on the last leg of the day. In that manner, if a valve fails to close, (1) it won’t be subject to too high temperatures for too long since you are using descent and approach power settings and (2) the engine will soon be shut down at the hangar, eliminating the concern about hot bleed air continuing to flow.
The F90-, 100- and 200-series contain a single, green advisory annunciator that illuminates to indicate that power is being sent to the brake deice valves. In no way, however, does that annunciator actually confirm proper valve operation. One or both valves could fail to function and yet the light is still there. Hence, when doing your once-a-day, in-flight test, do not merely observe the annunciator. Make certain that both sides show a minor drop in torque and a minor increase in ITT when brake deice is selected on and, perhaps even more important, make sure they return to normal when the system is turned off. You will also observe a small fluctuation in cabin altitude as the bleed air supply is slightly affected during the test.
The 300s and 350s contain separate, left and right, brake deice advisory annunciators and these are triggered by actual valve movement to the open position. Nice! However, I still encourage you to monitor torque and ITT when you do your in-flight test.
JUNE 2015