Wednesday, October 24, 2007

More Cessna 206 Technical talk.

As promised two posts ago, here is some additional technical information that I have been learning here at JAARS during my technical orientation. Let me first step back and say that I have appreciated the professional attitude and approach that JAARS takes in their orientation process. The old image of a dashing bush pilot who flies over gr0ss weight in bad weather on a wing and a prayer is far from the professional pilot that missionary aviation mandates today.

When I first started learning about performance in terms of flying, it was always in terms of looking at the numbers published in the Pilot Operating Handbook (POH) for a particular aircraft. Since much of mission flying is done from "non-standard" airstrips that are not 3000 feet long, we have developed specific takeoff and landing calculations for the aircraft we fly. Since the takeoff and landing distance along with climb out performance will change with weight, airstrip slope, temperature, altitude, and wind conditions, all this needs to be take into account to maintain safety.

Aside from takeoff and landing distance calculations, climb and approach angles also need to be factored in. It would not do any good to get off the runway but not be able to climb over the trees at the end of the clearing. Thankfully all these numbers and factors have been tested so that pilots can calculate all the needed data before taking off or landing. Lets look at a sample problem.
Starting with our hard facts I know that the C-206 in a no
wind condition will fly a 5 degree approach angle. Thus from my touch down position there can not be any trees or obstructions that stick up more than 5 degrees from my touchdown point.
Second, I know that below 3000' density altitude my 206 will climb out at a 4 degree angle. However we factor in a 2 degree margin for safety. Thus from the point of rotation on takeoff I need to have a clear climb out path above 2 degrees. All of these angles can be measures with a tool called an inclinometer which is actually a foresty tool that works well for our needs. See the picture below.

Now that we have hard numbers on our approach and climbs angles, lets look at calculating takeoff and landing distance. Starting with a given weight, lets say 3400 lbs. the takeoff distance is 650 feet. I now have to correct that distance for runways conditions, density altitude, and wind. Then lets say the runway has short grass on it that is wet, I take the 650 feet and multiply by a modifier. For short grass that is wet, this happens to be an increase of 15% in takeoff roll. I multiply 650 by 1.15. The new number is 748 feet. Density altitude is 2000 feet so I adjust for an additional 10% or 1.10 and get 822 feet. The runways is sloped down 1 degree so that will reduce my takeoff roll. The adjustment for 1 degree down slope is 7 percent (0.93) for a total of 765 feet. A 5 knot headwind further decreases ground roll by 5 percent (0.95) giving a total takeoff distance of 727 feet. While I am on the ground at the runway, I can walk down 727 feet and whip out the inclinometer, measure the angle to the obstacle, and if it is less than 2 degrees, I can safely takeoff with my margin.

I hope this gives a little insight into how performance data is calculated, how safety margins are factored in, and how the modern day missionary pilots are going on far more than a wing and a prayer.

Sunday, October 21, 2007

Two of a kind?

This past weekend I had the chance to give a few planes rides to some of my friends. A special treat was giving a "first flight" to a special 1 year old passenger. Alex came along with his mother (who I knew from my years in Suriname) and enjoyed the view from 2000 feet on a clear fall day. While Alex still can't talk and more than likely will not remember the flight, his mother told me that she could tell Alex was looking around very intently at everything and even made an attempt to grab the control stick...

The pictures I took made me remember a similar picture that my father took of me when I was about 2 years old. Are we going to have a second pilot on our hands just because of a 15 minute airplane ride? Only time will tell if these children are two of a kind.

Sunday, October 14, 2007

Flying Videos!

This is pretty neat! I just found out that Blogger, the host of my blog, has added a new feature. It now allows direct up loads of videos! I am testing it out here and thought what better thing than a couple aviation videos. These videos are from my training here at the JAARS center. We have begun flying into some local grass strips in preparation for what I will see in Peru.
This particular strip called Edgemoore is about 10 minutes away and is 1650 long and 70 feet wide. In my next post I will go into greater detail about climb angles, slope, and calculating takeoff and landing performance, but here just note on the landing video how there is an audible "whistling" sound. These are the air vents and conveniently enough they start "coming on" at 58kts and change pitch all the way down to 48 kts. Our typical STOL (Short TakeOff and Landing) approach speed is 52-55 kts with a 350-450 feet per minute descent.
Also note the LARGE trees just off the left wingtip. This is why centerline control is very important. Those trees are level with out wings when we pass them and there is no more than 5 feet of clearance! Yup I am having fun!

Friday, October 5, 2007

Flight traning begins!

Today was my first day of flight training here at JAARS. After finishing up a very thorough maintenance refresher class on the Cessna 206 (pronounced; two oh six), it was off to ground school and then today, two flight lessons. The first few flights were just for getting used to the airplane and setting up the various configurations that we will be using through the rest of training here.

After being in Peru for 4 months, I was curious to see how I would do back in the cockpit. Two weeks ago I went to a local airport and got checked out for rental in an American Champion Explorer (7GCBC). That went well and I had a great weekend as I flew up to Asheville, NC to see one of my former college roommates. There is just something about tandem seating, great visibility on all sides and up through the the Plexiglases ceiling, control stick, 180 HP engine and little wheel in the right place. Needless to say, I had a great time. There are no airplanes to rent in Peru so I have to take advantage of these chances while I can!

Anyways, my flights in the C-206 went well. I was a little behind the airplane and it was a challenge to remember all of the "V speeds" for different configurations. "V" stands for velocity, for example; Vr is the speed for rotation (48 kts) Vy is climb speed (61 kts) and we have different speeds and flap setting for different points in the traffic pattern.

The 206 is a great airplane and while it may have the appearance of a truck, the handling is very stable. Our first periods will be at 3200 lbs and moving up to 3600 lbs (max gross weight) for the rest of the training. In Peru our airplanes have wingtip fuel tanks which add another 3 feet to the total wingspan and give us another 200 lbs of gross weight for a total of 3800 lbs.