What is one major design feature of the Cessna Cardinal and most 210’s that set it apart from all other Cessna singles? If you said the elimination of the wing strut, you’d be right. Now we’re not going to get into all of the features and benefits of these airplanes as we’ve already covered them in detail in past episodes. However, in a nutshell, the absence of a wing strut means that the wing structure found in these airplanes must be much beefier than those found in say a Cessna 182. That’s because the wing spar itself must withstand a lot more aerodynamic load and stress that would otherwise be absorbed by the strut. This sort of wing design is known as a cantilever wing, where the carry-thru spar, ribs, and stringer take the stress.You may recall that certification standards for airplanes with a standard airworthiness certificate in the normal category are required to withstand loads of +3.8 g and -1.52g, meaning the wings must be able to withstand loads 3.8 times the weight of the airplane before structural failure occurs. Unfortunately, that wasn’t the case for a Cessna 210 that broke apart in flight back in May of this year. The cause of the accident was found to be fatigue cracking in the carry-thru spar.As a result, Textron Aviation issued a mandatory service bulletin SE-57-06 for the Cessna 210G thru T210R –the cantilever Cessna 210s, as well as SE-57-07 for all Cessna 177 Cardinals. According to Cessna, the reason for the mandatory service bulletin is because the carry-thru spar can develop corrosion and cracks. A crack in the carry-thru spar can develop to a point that the carry-thru spar can have structural failure that can result in the subsequent loss of the airplane and/or wing separation. Non-compliance with this service letter may result in undetected cracks. The service letter provides instructions to do a detailed visual inspection and an eddy-current inspection of the lower surface of the carry-thru spar.