Page 17 - June 2015 Volume 9, Number 6

Thus, brake deice uses “raw,” unregulated P3 air. The temperature of this air depends on both the OAT and, more importantly, the speed of the compressor: N1 or Ng speed.
To use brake deice properly, a few facts must be recognized. First, there is a limitation that tells us not to use it if OAT is above 15° C. Of course, in that warm of a situation brake freezing is of no concern. However, if used, wheel well temperatures can exceed a comfortable, safe value.
Second, one is almost assured of creating undesirably high wheel well temperatures if the system remains on too long with the gear retracted. That is why there is a timer in the control circuit that shuts off the system 10 minutes after the gear is up. The POH directions tell you to turn off the brake deice switch yourself if the system has not automatically terminated operation – evidenced by the proper annunciator light(s) extinguishing – 10 minutes after “Gear Up.”
Third, the bleed air that is tapped off of the Instrument Air line and directed to the brake assembly robs some of the normal “Little P3” flow into the other systems. This results in both interesting and critically important considerations.
At the “interest” level is the fact that at last 85% N1 is required to have enough air to operate both the brake deice option and the wing boots. Furthermore, on the ground, High Idle must be used if brake deice is to be effective. The temperature of the bleed air at Low Idle may not be sufficient to guarantee good brake deicing.
Also, don’t be surprised to see a momentary illumination of left, right or both Bleed Air Fail annunciators when using boots and brake deice together since pneumatic pressure can drop so low as to activate the low pressure sensing switches attached to the failure warning tubes.
At the “critical” level is the effect of brake deice usage on Rudder Boost. The Rudder Boost system on 200s and F90s operates considerably different than that installed on the 300-series, so I need to discuss this in two separate presentations. First, for the 300 and 350 ...
In these airplanes, Rudder Boost is a mandatory, no- go system since without it the worst-case, engine-out situation requires the pilot to use more rudder force than the certification rules allow. The force applied by the Rudder Boost system comes from the same servo motor that the autopilot system uses for yaw damping. The force varies, depending upon the magnitude of the power differential between the two engines. This power difference is sensed by propeller torque transducers on the 350s but by raw, Little P3 pressure in the 300s. That leads to a problem.
With brake deice activated, the P3 pressure sensor for the good engine feels less pressure than it should
JUNE 2015
KING AIR MAGAZINE • 15