I want to start out this column a little differently by sharing a true story I heard when I was about 14 years old. In June 1970, the British Aerobatic Champion Neil Williams was practicing for the upcoming World Aerobatic Championships in Hullavington, England.

Neil was flying his single-seat Zlin Akrobat in a shallow pull-up at less than 2,000 feet when he heard a loud bang. The lower left wing bolt had broken and the wing began to fold over towards him. He had on no chute that day, as a chute just added weight and would not have helped at that altitude anyway.

This man had the presence of mind to realize his predicament, and in a split second he rolled the Zlin to the right in a semi-outside snap roll and locked the wing back into position.

BANG!  It worked! He was still alive at less than 2,000 feet, but he was upside-down. He had to stay inverted to keep flying; the weight of the failed left wing was being held in place by the "top" wing bolt on that side. He then flew back to Hullavington inverted all the way.

On the approach, he flew a modified downwind, base and final inverted, ending up low and slow upside-down heading towards the runway, daring to un-weight the wing enough to descend more to get somewhere around 85 to 90 knots airspeed. Less than 10 feet off the ground, Neil did a half-outside snap roll and plopped the Zlin on her belly. He casually unlatched the canopy and walked away. Everyone who witnessed this aviation miracle was stunned.

 Figure 1: Neil Williams' Zlin Akrobat after his crash in June 1970.

Neil's quick decisions saved his life. After the bolt broke, he timed his actions perfectly. Most importantly, he didn't take anything for granted.

And when you deal with your fabricator, you shouldn't take anything for granted either. (Did you like my segue?) I have been on this soapbox before about the importance of informing your fabricator of your desired end-results if jobs are impedance controlled. I don't mean merely indicating a material type or specifying dielectrics. Now, when many jobs are going through turnkey environments, late communication about impedance issues takes valuable time out of the fabrication process and can delay the delivery of product, leaving the end-user and the turnkey assembler unhappy.

This can be easily avoided by sharing impedance information about the job up front on a drawing or read-me file. A fabricator really needs to know only four things:

1. What sizes are the impedance controlled traces?
2. On which layers do they reside?
3. What is the desired threshold?
4. What tolerance is associated with this threshold?

The material callout should express both the Tg and the Td desired. This allows the fabricator some alternatives in selecting materials to meet your needs. Avoid handing off conflicting materials information--conflicts require clarification and cause delays.