SILVER IODIDE AIRBORNE GENERATORS
Generators for use on aircraft burn the acetone solution of
silver iodide and sodium iodide. Acetone is highly inflammable
and it is necessary to handle it with care in appropriately designed
apparatus. The same type of equipment as designed by C.S.I.R.O.
is used in all our current experiments and drought relief operations.
The solution is carried in a tank in the fuselage and fed by a
pressure pump to burners under the wings, where it is atomised
and ignited by spark plugs. The consumption is two gallons per
hour per burner, corresponding to 550 grams of silver iodide,
and producing 1017 ice nuclei per hour active at -17° C.
2. THE EQUIPMENT
A photograph of a complete cloud-seeding unit is shown in
Fig. 1. Electrical and
mechanical circuit diagrams appear in Figs. 2 and 3 (not shown).
(a) Solution Tank.
This is fastened to the floor inside the aircraft cabin and consists
of a 12-gallon stainless steel tank. D.C.A. require the tank to
take pressures up to 5 P.S.I. to withstand any pressurisation
with changing altitude in the event of a blockage in the vent
lines. This tank is completely surrounded by a sealed tank vented
to the outside air. This is a precaution to prevent solution of
acetone fumes entering the cabin in the event of a leak in the
main silver iodide tank.
Fitted to this outer tank is the instrument panel, solution filter,
main supply cock and fuel gauge, as shown in Fig. 1.
These are 28-volt 2-amp. direct-current stainless steel gear pumps
capable of delivering the correct amount of silver iodide solution
to the burners at a pressure of 25 P.S.I. They are operated through
circuit breakers from the aircraft supply. A pressure indicator
lamp lights when there is pressure in the fuel lines.
High-energy ignition units, called "banger boxes", (they
make an intermittent banging sound when firing), are used to ignite
the solution in the burners. As they can give a lethal shock,
care must be taken to ensure that the power is off and the high-tension
lead short-circuited to discharge any residual energy before any
routine maintenance is contemplated.
(d) Silver Iodide Nuclei Generators.
Each unit consists of a streamlined pod containing a inconel metal
flame chamber, swirl jet atomiser, thermocouple flame indicator
and spark plug. The pods are usually located on pylons approximately
12 inches below the wings, outboard of the engines and are connected
to the solution tank via copper pipes fitted in the mainplane.
(e) Flame Indicators.
In some cases mirrors are fitted in the rear of the cabin for
visual inspection of the pods in flight. When ignited, about 1
inch of visible flame in daylight can be observed from the rear
of the flame tube. A thermocouple switch located on the pod, however,
extinguishes an amber "tell-tale" lamp on the instrument
panel when the burner is up to temperature. The hottest section
of the burner runs at 1100° C (approx.).
3. PRE-FLIGHT GROUND CHECKS
The C.S.O. should check the seeding equipment pre-flight in
the following sequence:-
(a) Be sure the aircraft is facing into wind. (Acetone attacks
the paintwork of aircraft). This will ensure that the solution
will blow away from the aircraft, when checking the pumps.
(b) Read your fuel gauge and see you have sufficient silver iodide
solution in the tank for your requirements. If you have to refuel,
remember to take all the necessary precautions against static
electricity and spillage.
(c) Check the solution tank vent lines and ram pressure inlet
which are located outside the fuselage. Wasps often block these
lines with mud nests.
(d) Inspect the burner pods. A removable plate, with a circular
hole (the size of hole depending on the type of aircraft) which
acts as a ram air restrictor, is located in the front section
of the pod. Make sure this is firmly in position.
(e) Apply power to the burner control panel; switch on the ignition
units, one at a time. If normal, you will hear them fire approximately
once a second. This must always be the first electrical operation.
If there is any acetone-silver iodide solution left in the burner
pod from pump testing, you will have a fire. Never try to light
the burners on the ground.
4. IN FLIGHT - START
(a) Master switch on.
(b) Open solution cock.
(c) Pumps on - speed control on high.
(d) When pressure lamps light - ignite one burner at a time.
(e) Look in mirrors to see if solution has ignited or watch for
the amber thermocouple lamp to extinguish.
(f) For correct flow rate of 2 galls./hr., turn the pump speed
control back until the pressure lamp just goes out or, using the
mirror, adjust until you can see (in daylight) approximately 1
inch of visible flame in the burner.
5. IN FLIGHT - CLOSE DOWN
(a) Switch off fuel pumps.
(b) Wait until thermocouple "tell-tale" lamps illuminate.
(c) Master switch off.
(d) Turn off solution. Failure to close the cock on solution tank
will result in loss of silver iodide. In flight, the solution
tends to syphon out through the pumps and spray nozzles.
6. ON LANDING
Always clean your flame chambers in the pods immediately after
flight. Silver slag tends to build up in the flame chamber after
the burners have been operating in excess of 30 minutes. Removal
of slag is best accomplished by hosing the flame chambers out
with cold water. If no hose is available, immediate use of a bucket
of water and a wire brush will save hours of work the next day.
Equipment - Pre-flight Ground Check
facing into wind.
gauge - if refuelling necessary take precautions for static
lines and ram pressure inlet.
air restrictor firm.
power to control panel and switch on ignition units one at
on main solution cock.
on fuel pumps and observe solution spray leaving swirl jets.
solution lines for leaks, with pumps running.
off pumps, master switch and close solution cock.
Equipment - Fault Finding
power - check aircraft main switch and tank circuit breaker.
spark - "banger box" u/s or check ignition circuit
running but no line pressure - pump u/s or tank solution
line pressure low:
(i) air leak in system - tighten line couplings.
(ii) blocked filter - clean filter.
(iii) blocked vent line - remove blockage.
line pressure correct but no solution to flame chamber
- blocked swirl jet, remove and clean.
or sticky fuel gauge - replace.
flight, all systems operate but no flame - speed to high
or no restrictor (IAS should be 130k).
equipment and procedures described in this document, although
generic in nature, were applicable to the cloud-seeding
operation by Beagle 206S-2 VH-UNL in 1969. Tony Wills, who
crewed the aircraft as Cloud Seeding Officer (CSO), has
no recollection of mirrors being fitted to the Beagle.
A.A. Tapp (1968) Equipment and Servicing, in 'Fourth Course
of Instruction in Cloud-Seeding Techniques', July 22- August2,
1968; pp27a- 27b; CSIRO, Division of Radiophysics, Sydney.
(via Tony Wills)