Up close and personal with DC-7 tankers: retardant drops pt 1

The tank system used in the DC-7 is an Aero Union multi-door tank system with eight doors where each of the eight tanks holds 375 gallons.Up until now when I have written about retardant tank systems in use by firefighting aircraft (e.g. the Erickson AirCrane and the AT-802), I have written about constant flow tank systems with either one variable single door or multiple doors. As I understand it, both of these systems are controlled by computers. Where the PIC has only to set the desired coverage level and possible the quantity of retardant to be dropped. These systems are easier to use, but come at a price. According to Larry Kraus, an Aero Union constant flow variable single door system for the DC-7 would cost at least$250,000 (perhaps more), representing a significant hunk of change out of Butler's budget.

The panel shown in this photo is located on the co-pilots side of the cockpit. The red light over the drop arming switch marked "off" is constantly on until the tank is armed, in which case it goes off. By the way, this switch is configured the same way across all of Butler's DC-7 tankers. Moving on, the switch on the left side of the drop panel marked "Flow/High/Low has been disabled as they no longer use the flow feature.

 

The dial in the photo above is called an intervelometer. Larry uses this dial to set how many tanker doors open at once. In this photo, it is set to safe, meaning that the doors will not open, moving clockwise:

LFT is the four doors on the left opening at once,
RT is the four doors on the right, and
ALL is the salvo of all eight doors opening at once

Continuing clockwise, to open double doors at a time, the dial is set to 1-2, and to open a single door at a time, Larry sets the dial to 1.

Larry provided an interesting piece of history about the intervelometer: "As far as the intervelometer, it was a surplus military item, as were a number of parts in the Aero Union 8 door tank. It originally was used for selecting the firing sequence for air to ground rockets on fighter bombers."

 We are not done yet, there are two more dials Larry uses, one sets the interval between door openings in tenths of second when pilot-in-command (Larry) presses the drop button that is located on the top left side of the yoke. The second dial sets the total number of doors to open. Larry explains the settings in this picture:

"In this picture,they are set to 0.4 seconds and 8 doors. At 130 kts. and 150 ft. above the terrain, with the intervelometer set for single doors, I would get approximately a coverage Level 6, or 6 gallons of retardant every 100 square feet (a 10 ft by 10 ft area) on the ground."

On Friday, I'll be writing more about retardant drops, so stay tuned!