Evaluation of Long Term Retardant Drop Patterns from Air Tractor 802 Amphibious Float and Wheel Equipped Aircraft

Wally McCulloch

February 2006


Air Tractor (AT) 802 airtankers have been utilized in firebombing operations in Canada for the past 10 years. Initially, all AT 802 aircraft were equipped with wheels, and delivered long term fire retardants onto fires. In 2003, an AT 802 equipped with amphibious floats was introduced into aerial firefighting operations in British Columbia. This aircraft was utilized for approximately 80 hours of operational firebombing. In 2004, the first amphibious float equipped AT 802 was tested in Alberta (Figure 1). Starting in 2005, Alberta contracted for a group of three amphibious float equipped AT 802 aircraft to add to its diverse operational fleet. The AT 802 on amphibious floats is known in the industry as the Fire Boss.

The float equipped aircraft are amphibious and can land at air tanker bases to load long term retardant or scoop off the nearest suitable body of water and deliver water or foam onto the fire. An onboard injection system is included to inject foam concentrate into the water load.

Alberta Sustainable Resource Development (SRD) and British Columbia Forest Protection Branch Air Attack Officers working with the AT 802 on amphibious floats observed that the drop patterns of long term fire retardant out of these aircraft were different compared to those of the AT 802 aircraft on wheels. The retardant dropped from the AT 802 on floats appeared to hold together better as it exited the aircraft and delivered a shorter and narrower pattern on the ground. This tighter pattern may mean the retardant coverage on the ground is heavier than that selected for the drop.

Figure 1. Amphibious float AT 802 (FIREBOSS) (left) and wheeled AT 802 (right).

Alberta SRD Fire Protection Division asked FERIC to conduct preliminary tests to determine if the AT 802 on amphibious floats did in fact provide a different or improved pattern and to also determine if the pattern resulted in improved or increased retardant coverage on the ground. These tests were carried out August 25, 2005 in the Pincher Creek area.


The objectives of this evaluation were to:

1. Determine any differences in long term retardant drop characteristics between the AT 802 on amphibious floats and the AT 802 on wheels.

2. Determine any additional testing required if differences were found between drops from the two aircraft during the ground evaluation.


On July 22, 2005, Wally McCulloch of FERIC met with Wally Born and Buck Dryer of the Alberta SRD Fire Protection Division to determine the scope of the testing and evaluation procedures for the drop tests. At this meeting, it was agreed that the first step in the evaluation process would be a preliminary drop test in the Pincher Creek area in southwestern Alberta where the AT 802 fleet was based.

The drops would be evaluated by experienced personnel on the ground to determine if any differences in drop characteristics or ground coverage were apparent between the aircraft. If significant differences were found, further testing may be undertaken to determine actual retardant coverage levels for each aircraft or to document differences in coverage levels on the ground.

In these tests, both aircraft would be loaded with the same volume of long term retardant and would carry out the drops at the same air speed and altitude. Weather readings would be recorded by the Air Attack Officer in the bird dog aircraft at the time of each drop.

Each AT 802 aircraft on wheels normally carries 3024 litres (800 US gallons) of long term retardant. On its first sortie, the AT 802 amphibious float aircraft carries approximately 2420 litres (640 US gallons). The reduction in retardant capacity is due to the added weight of the floats. As it burns off fuel, the amphibious AT 802 can increase its load.

Prior to the retardant drops, Wally Born conducted a briefing with all personnel directly involved with the drop tests. The briefing confirmed that two AT 802 aircraft, Tankers 684 (wheels) and 682 (amphibious floats) would participate in the tests. Each aircraft would be loaded with 2420 litres of unthickened long term fire retardant mixed at a ratio 4.75:1 (water to retardant concentrate), and each would do two drops. The drop configuration would be “All at coverage level 4.” This configuration calls for the entire retardant load being released at coverage level 4. This level results in retardant ground coverage of 4 US gallons per 100 square feet.

Sean McNeil of Alberta SRD Fire protection Division would ride in a helicopter to video and photograph the test drops.

The drops were carried out on August 25, 2005. The evaluation team consisted of Wally McCulloch of FERIC, Wally Born and Sean McNeil of Alberta SRD Fire Protection Division, and Rick Pedersen of Conair Group Inc. Wally McCulloch, Wally Born, and Rick Pedersen carried out the evaluations at the drop site. All personnel directly involved with the evaluation have extensive aviation and retardant experience. Sean McNeil observed and photographed the drops and assisted with the evaluation on the completion of the four drops. Jorge Kaffenberger, a FERIC intern student, was also on site and took photos of the drops and assisted with the drop measurements. As well, two additional employees of Conair were on the drop site.

The test site was a large flat grassy area on the east side of the road and a mid-density pine stand on the west side. One drop was made by each of the aircraft onto the grassy area and one drop from each in the timbered area.


The test drops were completed and evaluated on August 25. All drops were made north to south into the wind. The first two drops were in the open field on the east side of the road, followed by two drops into a pine stand on the west side.

The evaluation team determined where the effective coverage started and stopped for each drop and measured the length between those two points through the centre of the drop. The width was measured at the mid point of each drop.

Drop 1:  Tanker 684  Wheels                                    Time:14:26  

Weather: Temperature: 16˚C,  Relative Humidity: 54%,  Wind SW 10-15: km/h

The drop was easily visible from the air. This drop had some drift due to the wind, and had spotty coverage at the tail end. The load was affected by the wind which held it up and caused some disruption in the pattern. Retardant coverage was good throughout the drop.

Drop Length: 110 metres      Drop Width: 28 metres

Drop 2: Tanker 682  Amphibious Floats                   Time: 14:32   

Weather: Temperature: 16˚C, Relative Humidity: 54%, Wind: SW10-15 km/h

This drop was south of the first one and also into wind. It was held up by the wind, with some drift into a gully at the north end of the drop. Coverage was good to excellent throughout the effective area of the drop.

Drop Length: 101 metres      Drop Width: 20.5 metres

Figure 2 shows the retardant drop patterns for drops 1 and 2.

Drop 3:  Tanker 684  Wheels            Time: 14:51   

Weather: Temperature: 16˚C, Relative Humidity: 54%, Wind: SW10 km/h

The load was held up by the wind, and had some drift to the east. The coverage was good on the ground. Retardant was very evident on the south side of the boles of the trees. This was unusual as one would expect the shadowing to be on the north side of the trees in a North to South drop. This may have been the result of the aircraft pitching up when it released the load or the wind conditions at the time of the drop.

Drop Length: 90 metres        Drop Width: 32 metres.  

Drop # 4:  Tanker 682 Amphibious Floats                            Time:14:58

Weather Temp: 16˚C, Relative Humidity: 54%, Wind: SW10 k/h

This drop reached terminal velocity[1] and came straight down through the canopy. There was no evidence of any retardant on the boles of the trees. Coverage on the ground fuels was excellent and very even throughout the drop.

Drop Length: 60 metres   Drop Width: 30 metres

Figure 3 shows the patterns of retardant for drops 3 and 4.

Figure 2. Retardant patterns: Drops 1 and 2.

Figure 3. Retardant patterns: Drops 3 and 4.


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At the end of the test, the evaluation team met, reviewed the evaluation sheets, and discussed what was observed on the ground. 

Differences in the drop patterns from the two aircraft were apparent. Drops from the AT 802 on amphibious floats were consistently shorter and narrower than those from the AT 802 on wheels. Recognition of this characteristic may allow manipulation of coverage levels out of the amphibious AT 802 aircraft.

Follow-up and recommendations

After further discussion on the test results, SRD Fire Protection Division and FERIC agreed further testing should be done on the two AT 802 configurations. Testing should include:

1. Conducting comparisons between the two aircraft at selected coverage levels 2 and 3 to determine if there are differences in the actual retardant coverage levels on the ground.

2. Conducting comparisons between the AT 802 on wheels dropping at coverage level 4 and the AT 802 on amphibious floats dropping at coverage level 3 to determine if the actual retardant coverage on the ground may be the same.

3. Contacting Air Tractor, Inc. and Conair Group Inc. engineering personnel to determine if any information is available on the effect the floats may have on airflow under the aircraft, or if it is feasible to instrument the AT 802 on amphibious floats to determine what, if any, effect the floats have on the air flow both in flight and during retardant drops. Knowing where the amphibious floats affect the drop (i.e., immediately as the load exits the tank or later as the load passes between the amphibious floats), may allow the AT 802s on wheels to be modified to improve their drop patterns.

If you want more information about this study, please call Wally McCulloch at 250 573 3834.

ã Copyright 2006, Forest Engineering Research Institute of Canada.

[1] The retardant has no forward motion.

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