One of the best things I get to do as a pilot is to fly an aerial refueling mission. It is extremely challenging and patience is a virtue that pays big dividends. At the same time, the few times I have been truly scared while flying has been during aerial refueling, especially when the other pilot is at the controls and I sit there watching a bad situation get worse.
One of the first things you hear as a pilot about aerial refueling is that it is inherently dangerous to place two airplanes in such proximity to each other. Numerous aerodynamic principles make it especially risky. However, the number of accidents are fortunately few and far between.
Aerial refueling is actually quite an old concept, but it did not take hold until after WWII. The first successful attempt took place in 1923, when fuel was transferred from one DH4 biplane to another. The practice did not become routine until 1948, when B-29 bombers were converted to tankers for operational use. Today, tankers use a manually controlled boom for Air Force aircraft and a drogue system for Navy and Marine Corps ones. In either case, an enlisted crew member, called a boom operator, sits at the rear of the tanker and directs the intricate aerial ballet between tanker and receiver. During refueling with an Air Force aircraft, the boom operator actually "flies" the boom down to the receiver's fuel receptacle to make the connection.
The whole operation starts while we are still on the ground, well before we even start engines. We program our flight computer with the route of flight, any headwinds or tailwinds, and the planned air refueling control time to rendezvous with the tanker. The computer will tell us when to takeoff and how much fuel we need to receive so that we can make it to our destination.
After we are airborne, we will continually monitor our flight's progress to ensure that we will arrive at the proper place at the proper time, down to the minute. When we are 30 minutes out, we check in with the tanker to make sure they will be there also. Hopefully, the tanker is holding in a special airspace called an air refueling track. These tracks exist all across the country and overseas as well. Once ATC approves the operation, we continue inbound towards the ARIP, or air refueling initial point, where our rendezvous will begin. This sample air refueling track may put it into perspective.
As we approach the ARIP, the tanker will be offset several miles left of track, 1000 feet above us, and heading outbound parallel to the inbound course. At the appropriate time, it will turn towards us, reversing its direction until it is heading inbound one mile ahead of us (and still 1000' above us).
At this time, we begin a gradual climb, with just enough speed to catch the tanker. We want to arrive at the ARCP, or air refueling control point (shown on the sample track) at the proper time. At this point we hope to be at the precontact position, which is approximately 50 feet aft and slightly below the tanker. We must stabilize our aircraft in precontact before we can continue towards the tanker - which means ZERO forward or aft movement.
Once we are stable, we are cleared to contact, which is approximately 20 feet behind the tanker. Here is where the aerodynamic forces come into play: the tanker's engines create a burble of agitated air around our tail and the nose of our aircraft creates a bow wave (like a boat) which can push around the tanker. A slow closure rate is key; one foot per second is ideal. Once we get into position, the boom operator will connect the boom and turn on the pumps.
Maintaining position is hard work, especially as more gas is onloaded, making us heavier. It'll take a little more power to hold us steady, but too much can lead to inadvertent closure. Power movements on the throttles require patience - add a little power and see if it works. Add too much and the boom operator will send us back to the penalty box.
If we get too close or the boom operator sees us coming in too fast, he or she will call for a breakaway. The tanker will accelerate as the boom is pulled clear; the receiver pilot will simultaneously pull the throttles to idle, fan the speed brakes, and push the stick down to attain positive separation, usually ending with us 1000 feet below the tanker.
Throw in a moonless night, some clouds, low visibility, and even some turbulence, and it gets very challenging. Daytime isn't always better either; it seems that half the time we are looking right into the sun. You can have good days and bad, but as long as it get the gas you need, it doesn't really matter how pretty it was.
Here's a video showing the climb to precontact and contact, but it's a little long.
No comments:
Post a Comment