Isn't there an AD/Cessna 210/61 to do with checking corrosion in the wing spar carry through every 6 years? Would evidence of fatigue cracking be discovered during these inspections? Or does it just suddenly occur ?

As I read the SL it states, and I am sure Cessna will not mind a quote here,

"The Carry-Thru spar inspection presented in this service letter applies to airplanes listed in the Effectivity Section. The compliance time is for unmodified airplanes, and represents the maximum allowable time. Airplanes that have been modified to alter the airplane design, gross weight, or airplane performance, may need to inspect sooner. Examples of common supplemental type certificates (STC’s) include vortex generators, wing cuffs, STOL kits, wing tips, and add-on wing fuel tanks. Contact the STC holder for revised inspection intervals.

MANDATORY. This service document must be accomplished within 10 flight hours from date of receipt as follows...>"

So I would say it is not strictly true that it "does not apply to modified aircraft." If you have any modifications it could well be that the ten hours to compliance will be less. And I'll bet that most of the STC holders have yet to do an analysis on the the stress loadings for their mod. If I had tip tanks on a high time C210 that has not been eddy current tested I would put it on the ground now and get it done.

In some countries this SB has been done at (I think) every 2,000 hours as a mandatory procedure for years.

Any idea how many carry through spars were found to be unserviceable in all those years? I would think approx none unless the aircraft had an unusual operational history like 6,000 hours of fast low flying in rough conditions.Does anyone yet if the aircraft had any previous accident history?

In most cases this "mandatory" Service Letter is not required to be carried out in Australia. (This will vary with some AWI's)

Different if it is a Service Bulletin.

Cessna have passed on responsibility of modified aircraft.

I have heard that 1 in 4 (25%) that have been inspected in the USA failed - I am not sure if that was due corrosion limits and/or cracking.

I expect an FAA AD will soon follow and booking your NDT now might be a smart move - An owner/operator saying "but SL's are not mandatory" should be looking for a new pilot.

One thing to think about is Cessna, and will they supply new Carry-Thru Spars?

It is not what the Name of the Document is - It is the actual content that needs attention or the required Maintenance etc. This SB and SL myth has been perpetuated for a long time. Look how McCauley have updated some of their older Prop. Manuals over the years, e.g. through the use of SLs.

Ya all take heed out there.

Not what CAsA say in most offices - just look at what SID's are now "mandatory" nothing short of a joke!

Digging a bit deeper into the SL and the documents that it references. I am working on the C177 figures but as far as we see the C210 is the same, as it has essentially the same carry through spar structure.

The documents for reference includes Temporary Revision number 8 dated 1 December 2011 (for the C177) which defined Severe usage and Severe environments. Severe Usage is defined below, and for Australia all the coastline and inland for about 500km is considered as severe environment. (It is difficult to see the scale on the corrosion maps but it looks about 500km to a non Oz)

The SL goes on to say that:

For airplanes flown in SEVERE Usage as defined in Section 2A-10-00:

• Airplanes that have more than 2,500 flight hours must accomplish this document within 10 flight hours.
  
• Airplanes that have less than 2,500 flight hours must accomplish this document within 10 flight hours upon reaching 2,500 flight hours.
  
• It is acceptable to accomplish this service document before 2,500 hours to align with a scheduled maintenance event or inspection.

2. Inspection Requirements

A. Two types of inspection requirements are available based on operating usage and two additional types of inspections are available based on operating environment.
(1) Operating Usage

1. (a) Severe Usage Environment
   1. 1 If the average flight length is less than 30 minutes, then you must use the SEVERE inspection time limits.
      
   2. 2 If the airplane has been engaged in operations at low altitudes such as pipeline patrol, fish or game spotting, aerial applications, police patrol, sightseeing, livestock management, etc. more than 30% of its life you must use the SEVERE inspection time limits.
      
2. (b) Typical Usage Environment
   1 If neither 2(A)(1)(a)(1) or 2(A)(1)(a)(2) above applies, the TYPICAL usage environment applies.

(2) Operating Environment

(a) Severe Corrosion Environment

1 If the airplane is operating more than 30% of the time in a zone shown as severe on the corrosion severity maps in Section 2A-30-01, then the SEVERE CORROSION environment time limits apply.

(b) Mild or Moderate Corrosion Environment

1 If 2(A)(2)(a)(1) does not apply, then the MILD/MODERATE CORROSION environment time limits apply.

So your low level aircraft around the coast of Australia come under the Severe Usage 2500 hour limit as above.

For airplanes flown in TYPICAL Usage as defined in Section 2A-10-00: (ie not Severe usage)

• Airplanes that have more than 15,000 flight hours must accomplish this document within 10 flight hours.
  
• Airplanes that have less than 15,000 flight hours must accomplish this document within 10 flight hours upon reaching 15,000 flight hours.
  
• It is acceptable to accomplish this service document before 15,000 flight hours to align with a scheduled maintenance event or inspection.
  

So if you have a 14,900 hour C210/C177 in the middle of Australia that has had an easy life, relax, you've got another 100 hours before you need to do the SL. :}

That is if CASA even follows the American SL/SB system, which as a non Oz I do not know.

FWIW

You will find almost every C210 and I expect C177 in Australia are classed as severe.

In Australia (depending on your CAsA personal) most systems of maintenance have required "Mandatory Service Bulletins" to be carried out, but not Service Letters.

That said - I do not recall seeing a Service Letter classed as mandatory before.

I see your point. If the 30% usage under stressful conditions is taken, then the examples given of "pipeline patrol, fish or game spotting, aerial applications, police patrol, sightseeing, livestock management, etc." would very likely make up 30% of a C210s life in Australia.

Putting it in "Severe use" and back to 2500 hrs inspection.

Less likely for a C177 which may well have been privately owned for recreational flying.

My apologies for duff gen.

The "Typical" hours of 15,000 came from the C177 letter. The C210 letter quotes

• For airplanes flown in TYPICAL Usage as defined in Section 2A-10-00:
  • Airplanes that have more than 4,000 flight hours must accomplish this document within 10 flight hours.
    
  • Airplanes that have less than 4,000 flight hours must accomplish this document within 10 flight hours upon reaching 4,000 flight hours.
    
  • It is acceptable to accomplish this service document before 4,000 flight hours to align with a scheduled maintenance event or inspection.
    
    So That brings a lot more C210s into the crosshairs, as Bend alot suggests.

It is the location that captures most of the aircraft both 210 &177 the rest will have some flight issues - but there are generally no clear records oh flight times, low level work other than modified for survey but again normally no clear records.

While not specifically related to the C210 or C177, this is another example of the Technical Maintenance content of a Service Letter which forms part of the Manufacturers Instructions for Continued Airworthiness.
Once again, it is the Technical content, and not what the Document is actually entitled.
Get that.
Em nau.

No, thermals build quick enough and their more sudden. The 210 is fairly slippery ,so after coming down a hill your moving along

Low level survey in hot countries would rate as the most severe, a lot more than other airwork ops. In fact I was told that 1 hour of mag survey amounts to 3 hours "normal flying".
I heard that this 210 failed at a different area of the fuse that is usually inspected.
MC

The spar would get checked every 100 Hrly

The crack was initially started from a pin piont on the lower side of the centre section. The crack went internal as could not be sighted from the outside.. the only way that the crack would have been visable would have been via a NDT inspection.
there was NO visable indiication under normal eye sight.
this accident was the first recorded failure of the centre section that is known.