Up close with the AT-802 with the help of Queen Bee: FRDS-I Gate System

Here is some information that I obtained from the Air Tractor website about the first generation Air Tractor Fire Retardant Delivery System (FRDS), a constant flow retardant delivery system, used by Queen Bee. Air Tractor is now into the second generation of the FRDS.

Introduction:
The Air Tractor Fire Retardant Delivery System, or FRDS, is an electrically controlled, hydraulically actuated constant flow airtanker door system. System operational parameters allow multiple constant flow drop capability at coverage levels between 0.5 and 4.0 gcf, with a maximum flow, or "full coverage" rate approaching coverage level 6.
Operation:
The system is operated by an onboard PLC (Programmable Logic Computer) which receives input from a pilot interface panel in the cockpit, the drop trigger, and various internal sensors. During a typical mission, the pilot will select the amount to drop (in 50 gallon increments) the desired coverage level (in 0.5 gcf increments) and ground speed. He then will arm the system and close the drop trigger when needed. The PLC will control the drop using sensors for door angle, flow rate, and acceleration to meter the retardant flow, closing automatically when the preset amount has been dropped. The system incorporates a "learning" algorithm to compensate for errors caused by wear, material viscosity, etc., making the coverage level self calibrating. In addition to the full automatic drop mode described above, a manual automatic mode is also available through the PLC. In this mode the PLC retains control of the flow rate to ensure accurate coverage level while the length of line, or drop quantity, is controlled by the pilot by simply holding the drop trigger closed for the desired distance. This mode is useful in operational situations such as when the pilot is instructed to "Put a line from the road to the rocks." The requested line can be dropped with minimum waste, saving the remaining
retardant for other targets.
Redundancy:
A redundant PLC is installed which provides for backup electronic control in a simplified manual mode. This system provides for continued operation in the event of a failure of the primary PLC unit. A mechanically operated pneumatic emergency jettison system is also installed which utilizes compressed air from a reservoir to force the doors open in the event of an in-flight emergency.
Components:
The FRDS consists of dual, centerline mounted, longitudinally hinged, overcentering type doors driven by a rotary type hydraulic actuator powered by a 3000psi electric hydraulic system with an accumulator. The system is controlled by a PLC via electric hydraulic controls and sensors for hydraulic pressure, door angle, door lock, acceleration, and tank quantity. Operation is controlled with a pilot interface panel in the cockpit. An 18 gallon class A foam tank, electric gauge, and automatic foam injector pump and controller are also mounted.

Obtained from the Air Tractor website on February 6, 2009. Note: The link that I used in last February no longer active, probably because Air Tractor is now into the second generation FRDS. Thanks to my friends at Queen Bee for the photos!