Good Drop, Effective Drop

When I started working on this article on the factors that make for an effective retardant drop, I was thinking about things like drop height (altitude), drop speed (aircraft speed) and winds. Then as sometimes happens, I set the draft aside for a few days. During this period I took some time to go through some files that I have accumulated over the last several months, along with some correspondence with a couple of my tanker pilot friends. The result, thanks to my reading (listed at the end of this article) and my correspondence with a couple of my tanker pilot friends is what you see here.

As I read through my files and reviewed some of my correspondence, what was driven home to me is that a good drop is when the crew completes the drop in a safe manner and returns home.

What makes for an effective drop then? Well, I have already talked about two factors: the tank system (some examples of tank systems currently in use may be found here, here, and here) and retardants . Other factors include the right altitude (height at which the retardant is dropped), correct airspeed, and calculating the drift of the retardant due to winds.

To set the stage, it is important to say what an ideal drop is, that is what is the tanker pilot aiming to do in making the drop? I understand from my reading as well as e-mails with a couple of tanker pilot friends that the goal in making an ideal drop is that retardant has no forward speed and rains down on the vegetation.

One of my tanker pilot friends tells me that one of the reasons that you want to eliminate forward speed of the retardant  is to (hopefully) eliminate shadowing. As I understand it, shadowing means that all the retardant is plastered against the side of the tree, bush, even grass facing the drop while the backside is left untouched by retardant. Eliminate forward speed of the retardant and shadowing of the retardant is eliminated.

In addition, retardant drops at forward speeds can be very dangerous to ground crews who have not moved away from the drop zone. Fortunately, Incident Command will usually be able to clear ground crews from the drop zone to avoid these situations.

My tanker pilot friend made a couple of more pertinent points on retardant drops. First, he told me that dropping retardant at too high an airspeed (aka drop speed) can shear the retardant load apart so that it disperses too much leaving gaps in the coverage. Two, dropping to high, especially in high winds means that pilots won't be able to predict where the retardant will land.

Finally, he went on to say that if it is safe to do so in these high wind conditions, pilots might be able to make a retardant drop from a lower altitude so that some retardant line can be built. There might be gaps in the line, and the pilots may not be able to predict the coverage level, but at least there will be some retardant line that will slow the fire until the ground crews can get there.

As I was doing my background reading for this article I was impressed by the problems of making retardant drops in high winds. That is, when making drops in high winds, not only does the pilot/crew risk not knowing where the drop will end up, they are flying in very extremely dangerous conditions.

References:

I am including links to some of the USFS documents that I read while doing background research for this article. Before you read the documents below, I should caution you that some of the numbers cited are under ideal conditions in the laboratory, so to speak. Aircraft performance in actual drop conditions on the fire line will be different.

Lavalette, Greg. Safe Drop Height for Fixed-Wing Airtankers. 0057-2317-MTDC. Aviation Tech Tips (March 2000). US Forest Service, Technology and Development Program, Missoula Technology and Development Center, Missoula, MT.

Suter, Ann. Aerial Delivery Systems User Information: Wind Speed and Drop Height. ug-7. Wildland Fire Chemical Systems - MTDC. February 2005. US Forest Service.

Suter, Ann. Aerial Delivery Systems User Information: Drop Speed. ug-13. Wildland Fire Chemical Systems - MTDC. February 2005. US Forest Service

Additional Aerial Delivery Systems User Information documents, Safe Drop Height for Fixed-Wing Airtankers, and other documents relating to testing aerial delivery systems may be found on this USFS webpage.