Stab Nick

This blog covers the issues created by a nicking the elevator spar when installing the starboard fairing and how a simple task when bad.

Stuff ups do this

Overview
After completing drilling the port side fairing to secure the faring the next task was to repeat the work at the starboard side. All locations for the holes were transferred to the fairing with all holes then drilled with a 1.5 mm drill.  

It was late but it was decided to complete the task before stopping for the day. To secure the fairing tightly onto the fuselage side, it is necessary to use one hand to press it into position and using the other to drill holes. All went well until the hole nearest to the spar was drilled penetrating the skin striking something hard and causing the drill to slice forward. 

This is not good - high stress area
tension flange - a bad combination

Poking around with a length of brass wire a distinct click was felt and I then knew it was the spar that had been struck. 

To investigate fully what had happened the tail plane had to be removed which was achieved in under a hour - a quick removal of a few key rivets revealed a distinct gouge in the radius of the spar. 

With the skin de-riveted enough to allow access to the damage it was worse than expected with the side of the drill machining into the fillet of the spar.





After a lot of emails and a Skype call there were three alternatives available

• Repair damaged area - cheapest, untidy
• Replace damaged spar - complex, cost effective
• Replace stab - expensive, perfect

Comment

In the cold light there was about 2-3 mm difference all in the wrong direction and the damage was 60% depth. This was a result of believing all was correct, late night get there at all cost but there is no denying it was just dumb, stupid, blind decision - there are really no words to cover gross stupidity.  

If you make an error don't hide, investigate, speak to the factory or TC , they as interested in your safety as you are. The danger posed was not immediate structural failure but crack's developing creating an undetected, progressive failure over time. 

Doing it again mark all the locations and especially the position in the critical area of the spar for 25 mm either side of the rivet line and all the hole location line before installing the stab with the final position marked on that line in conjunction with the faring.

The critical area should be covered with 50 mm painters tape with a series of "DO NOT DRILL"  as all will be forgotten by the next week. 

The side fairing has a cut line marked by a groove in the faring - mark a new line that removes the minimum of material and sand it square. This should allow the installation of the rivet nuts OUTSIDE the line of the ribs and provide a flatter area to screw down as the curved area need a lot of pressure to get it tight and creates a awkward drilling scenario on the underside

Go to bed and drill all the holes next morning. 

Now that I have confessed, ordered a spar its time to return to the rock....!