I have been dealing with a large number of rescues, some on a regular basis and some only once. I find that pilots and aviation are alien to rescues, probably like the skills and arena that rescues work in are alien to most pilots.
We pilots have a lingo that we don't even realize we are using, and aviation itself seems to inspire awe. I think this is the place that rescues and pilots should ask and answer questions. I know from a pilot's perspective some of our decisions about transports and flights don't make sense. Add to that seemingly inconsistent choice of when to fly the variations between pilots and their planes and it appears rescues can never predict when a transport will be easy to fill and when it will be impossible.
A good starting point is the pilot. Some pilots are limited to flight when weather conditions are what we call VFR, or can be flown under visual flight rules. To do that the visibility and the ceiling (below a cloud layer) must meet certain minimums, and it is perfectly acceptable for the pilot to establish his or her own minimums that might be higher than the FAA minimums.
Some pilots can fly when weather is below visual minimums. Depending on the airport and the type of instrument approaches a pilot with an IFR (instrument flight rules) rating could fly in weather and land when the ceiling is only 200 feet and the visibility is as little as 1/2 mile. But not all pilots will fly to those minimums and not all airports allow instrument guided approaches to those minimums.
Decisions about the weather and conditions (such as winds) are solely the discretion of the pilot who should know his or her personal limits. Each pilot will be different from another pilot and that may have nothing to do with skills, but perhaps everything to do with how the plane is equipped. So you can see we already have variations in pilots and aircraft and weather to affect decisions regarding a transport.
Another consideration is the plane. The plan not only influences the weather a pilot may be able to fly in, but it also may dictate the size of the animal to be carried, the number of animals, and the distance they are carried. After a year of transports, and a wide variety of animals and sizes I have gotten fairly good at knowing my limitations on the size and number of animals and so have some rescues I routinely deal with. But I am still trying different methods to accomodate more animals if necessary so my capability is still being determined. The same goes of other pilots and planes and a typical pilot will not commit to a passenger load, especially if he is part of a relay unless he has specific knowledge he can fit his animals. The range of the plane or the number of miles traveled is going to vary widely depending on the plane, its fuel capacity, what power setting the pilot uses and most importantly on the winds aloft. If the winds at altitude are on the nose, the plane is going to go slow and use a lot more fuel between two points than if the winds are calm or even a tailwind. We pilots typically will not push our limits.
Finally, there are factors a pilot cannot control and often cannot be anticipated. I have had solid gold weather forecasts that were completely wrong. On one occasion I expected weather at my destination to be above visual approach minimums at my time of landing. Instead the weather which was holding pretty well to the forecast during my entire trip disintegrated prior to my arrival and on my first approach I was unable to land due to conditions being too low for even an instrument approach. Another surprise a pilot may encounter on the day of a planned flight is to have his airport within an area in which there are temporary flight restrictions. These are relatively new (since 9/11) and if the President is visiting a certain city the TSA or Homeland Security folks may shut down an airport, and all airspace around that airport for quite a few miles. If the pilot is based within the area flight will not be allowed. Sometimes there is considerable notice, and other times such as during an unexpected event like a natural disaster the restrictions can be imposed without notice.
In summary, remember that each pilot and plane has certain capabilities and limitations. What may work for one, may not for another. We make decisions based on our own skills, ratings, experience and aircraft. Some of the aircraft I have seen in the photos under success stories are very capable and some are less capable for certain conditions. If you ever wonder why we do what we do as pilots ask. We will be happy to answer. If we don't know the answer we will make one up.
(I was just kidding about the last remark.)