To Err is Human, to Forgive is our Policy.

(As long as the mistake does not affect safety of flight . . .) 

      In a class such as ours two of the primary things we teach are precision and tolerance.  Not the tolerance of adolescent behavior but acceptable tolerance of error.  Do we need this part to be within an inch? A centimeter? A millimeter? A thousandth of an inch?  Students create parts and then check the precision with with they have made them.  Our usual tolerance is plus or minus one millimeter.  Students have to be able to measure their part, know what a millimeter is, and whether they have achieved acceptable accuracy.  Some parts may be off by more that one millimeter, others may have to be more precise than that.  

      Typically, a student will create his or her first part and bring it up for inspection.  Some need minor fit or finish corrections but a percentage simply do not meet the requirements.  Students are informed that we are not angry, upset, or disappointed, and the failure to produce an acceptable part will not affect their grade, but they don't fit, aren't safe, or otherwise aren't acceptable.  So they have to make them again.  

     Most students catch on to the concept that it is easier to take one's time and work carefully the first time, because they just have to do it again if they don't.  This is a valuable life lesson that goes well beyond the walls of our shop.  Many students have had to rebuild parts but most don't have to do it more than once or twice.  They catch on.  Pretty soon you hear them saying to the newer students, "You may as well take your time and check that again before you bend that or you'll be making it over."  Some can be told and some have to learn it for themselves.  The nice thing is that the lesson teaches itself.  

      If you're wondering. . .  The record is 11 times.  One student, two pieces (left and right), 11 attempts before acceptable parts were produced.  To be fair, the parts were difficult: thick stock, a tricky bend, and a precisely located hole.  After a while, though, he was able to check the accuracy himself, and knew when to start over and when he had achieved success.  This was not alarming to us.  All parts are checked and they are safe to fly or they are rejected.  

     Then we get to Misters Steed and Smith.  Instructors, yes?  Infallible?  Well, we thought so, but events proved otherwise.  We had to learn the lessons we were trying to teach the kids. 

Part I 

     Mr. Steed found himself in charge of installing the trim tab for the CH-701.  The original plans for the 701 did not include a pitch trim tab but many of the flying models had one so the design was easy enough.  Mr. Steed and a student drew out a tab about 3 inches deep and 24 inches wide to go on the port side of the elevator.  They removed the elevator from the airframe stored at Mr. Burnert's hangar and brought it down to the shop.  The intent was that the actuator would be installed inside the top skin of the elevator, driving a rod through the bottom skin, pushing against a horn, which would move the tab up and down 30 degrees.   The top and bottom of the elevator were not marked but one side was covered in dust and marked with bird droppings.  

      The locations were marked, the holes cut, the actuator installed, the opposite skin drilled and filed to run the rod, and the trim tab hinged and clecoed in place.  As the team proudly showed off it's handiwork, Mr. Smith raised an eyebrow.  He checked the plans, compared them to the elevator control horns, checked again and quietly asked, "Did you intend for the rod to come out of the top of the elevator?  And why put it on the starboard side?"  

     Mr. Steed replied smartly that he did not and he did not.  But he had.  Even though he had installed it as intended.  Whoever had reassembled the aircraft in the hangar last had set the elevator in upside down.  Since we didn't have the cables hooked up there was no need to examine which horn was on the top and which was on the bottom.  It was simply set in place.  Incorrectly.  And the dust and droppings accumulated on and indicated the wrong side as top.  We didn't know it could go in upside down, but apparently it can.  

      So what to do now?  We gathered a few students along with the mentors to have them watch the process.  The first question was:  Will this affect safety?  Then: Will this affect controllability?  Then: Will this affect airworthiness?  To the best of our accumulated knowledge it seemed that the biggest problem might be that rain could come in the slot in the top of the elevator that should have been in the bottom. The drag would be the same, the throw would be the same. the weight and balance would be the same . . . so for now this mistake was one we could live with.  Recognizing, though, that we don't know all there is to know about trim tabs and their upside-down installation, we will be asking a lot of people a lot of questions about this before the aircraft ever sees a runway.  

Part II

      The bending brake in the shop has a useful width of 36 inches.  The ailerons that must be bent are about 60 inches long.  Accordingly, a student will measure, mark, cut, trim, file and deburr the sheet, and Mr. Smith will take it to the District Maintenance Building and bend it on the large bending brake they have there.  

      Our second aileron was created by a team of students and given to Mr. Smith to bend.  He did, brought it back to the shop and the students primed and clecoed it together.  It was checked, found to have the proper twist, and then riveted.  Something, however, seemed wrong.  Measurements were taken and they were all correct, the twist was correct, the hinge overhang was correct . . . but something.  

     After consulting the plans and comparing every dimension to the aileron, Mr. Smith noticed the side profile.  The aileron was to have a 90 degree angle between the top and the front face and an acute angle at the bottom.  Instead it was the other way around.  The right angles had been bent into the wrong places.  

     So what to do now?  Again we gathered the team to make the decision but this one was easy.  With the bends in the wrong places, the aileron wouldn't fit.  It looked good, but it wouldn't fit, or if it did, it wouldn't work as intended.  This decision came quickly:  Make a new one.  And we did.  

      Which wasn't all bad.  The team got more practice but even so were able to mark, cut, and prepare the skin in very short order.  Mr. Smith got it bent quickly and the team was ready for it when he got back.  They had a new spar and ribs already marked and assembled and in half the time of the original we had a perfect starboard aileron. 

     If you would like to contact us please email Mr. Smith or  Mr. Steed   We would really like to hear from you.

Last Updated: 07/11/05 
 

Disclaimer: This page is the product of the Central Kitsap Junior High School Project SchoolFlight.