In his early thriller 'The Most Dangerous Game' the late Gavin Lyall has a Cessna floatplane sabotaged by 'fixing the flaps' (it's when the hero, the boozy pilot of knackered DH Beaver, realises he's up against a professional). There's a vivid description of the aeroplane flipping upside down and the pilot fighting to fly out inverted, with the trim dragging him down... Given the author's flying background I've often wondered how plausible the scenario was.

Not really relevant, but that section is not included in 14 CFR part 23 right now. The current version isn't very specific on these issues. Plenty of types will have been certificated to earlier versions of course.

Yes, welcome to the new certification world.
It appears to be a lot that is not included in CFR part 23, I think it would be a mistake to think that compliance will now be easier, or that the questions asked in previous FAR 23 amendments will not be asked in certification meetings.
It was comparatively easy to go through the certification standard line by line with the certification applicant, to reach an agreed certification plan. Not so sure that it will be so now.
I would not like to be an applicant trying to develop and present a consensus certification standard to the FAA.
I will bet that a flap interconnect failure will still have to be addressed if it is not shown to be extremely improbable.
Interesting days for both certification applicants and regulatory authorities.

"It's certification Jim, but not as we know it..." ;)

There are still several paragraphs that imply (or appear to imply) the need to avoid flap asymmetry, but not in those words. So yes, things have certainly got more interesting. I'm glad not to be involved in this area of the industry.

I find it hard to believe that a c172 could be flown with one flap at 40 degrees and one fully retracted; it flies like a wet brick with both at 40deg! But I am reassured that this is low down the list of things to worry about!!

I took a particularly close look at the flaps track system when I preflighted this morning, it looks very simple and very robust.

The old Part 23 required this:

My Boomerang aircraft were designed by Bill Whitney and have FAR 23 type certificate as well as an Australian type certificate. This about the asymmetric flap requirement is one of the things told to me by Bill Whitney himself when we were discussing the 10 year cable replacement. The aircraft has to be able to be controllable in all three axes following a total failure of one of the cables, and to be able to maintain control with one flap up and the other fully down.

Point is that many of the "thou shalt not"s taught (eg I was told NEVER to touch the mixture control below 5000 feet?!) are not backed up as limitations in the POH, some may be applied by lazy instructors as a "one size fits all" eg sideslipping with flaps down, use of carb heat in the approach, (Cessna says to put it cold AFTER landing) CSU power setting tables, use of electric fuel pumps in Cherokees, the list goes on. Question everything.

Thanks Pilot DAR for suggesting 'Turn off the Master Switch'... My first reaction would have been to pull the Flap Circuit Breaker, but I realise that I don't know where it is, and would have to look around for it.
maybe we should learn where all the CBs are, and the function of each half of the twin switch master switch.

Indeed, and make an effort to understand everything in you plane. For a Part 23 airplane there is no requirement to account for a control cable break from a handling/redundancy standpoint. Part 25 has differences in this regard. Very simply, as DB has remarked, very effective flaps, if asymmetric, could not be overcome with aileron control. In the context of a Cessna, this becomes even more extreme for those Cessnas modified with drooping ailerons connected to the flap system.

The rule about interconnection has it's roots in hydraulic flap systems. Before electric motors were common, and when the flap actuation forces were very high, a hydraulic cylinder was used to operate each of the flaps, which had the effect of making the flaps independent. Thus, a separate interconnection of the two flaps became desirable/necessary. So, in theory, that interconnection could fail, though both flaps still operate = problem. Most GA airplanes with more recent designs (all 100 series Cessnas) have flaps which are cable operated to move interconnected as both the actuation, and interconnection for both flaps. You can disconnect the Cessna flap motor, and the flaps will still move together. Thus, a failure to operate together is much more a risk than asymmetry. Indeed, due to the excellent aerodynamics of Cessna "paralift" flaps, their operating forces are happily low. Fly a manual flap Cessna, and you'll feel how nicely they move.

As I pick back through earlier versions of Part 23, I find:

So I do see where the reference to flight with flap asymmetry has a historical origin. Though an applicant for a modification to a presently certified plane could apply to use this standard, it is doubtful that it would be permitted. The FAA "Changed Product Rule" (CPR) would be invoked by the regulator, and drive the applicant to use a later standard. A new design would be required to use the most recent certification standard.

If an element of an aircraft design is found to be unsafe, a service bulletin (SB), and perhaps an airworthiness directive (AD) will be issued to correct it (SB sometimes optional, AD mandatory in all cases). Imperfect aircraft designs may remain in service as is in the absence of an SB/AD, even though we would not do it that way ever again. If that system becomes the subject of a modification, CPR will be considered to invoke a more recent certification standard if appropriate.

When I develop certification plans (for STCs I issue), one of my duties is to go through the CPR process, and either determine (if it's within my delegation) or recommend to the regulator (if is exceeds my delegation) which amendment of the design requirements should be applied to the modification. Often, I just use the most recent amendment of the standard for convenience. The only problem with doing that may be that other aspects of the older aircraft could not pass the newest standard (seats, for example), so you'd fail before you begin.

An applicant can always propose to make an aircraft much more compliant that the requirement states. It's usually expensive and heavy, but sometimes a good idea. The challenge with dual interconnections of control systems can be that the dual systems fight each other if misrigged, and create undesired friction, or very high forces within the interconnected circuit, and break something.

I have issued approvals for modification of flap systems, though the operating philosophy was not affected, so the original design standard was still appropriate. I have no concern about Cessna flap control systems which are correctly maintained, and used.

Scifi, pulling the breaker is excellent - except that most Cessnas have the cheap CB's which you cannot pull, so you're stuck with the master being the only control you have over that circuit. I replaced the CB's in my 150 to be the type you can pull - for that very reason!

Ah Ha! That explains it!!!!

I only see little flush buttons that I am assured will pop out if overloaded, but I could not see how to pull them! (On boats I'm used to cb's which are push make/push break or toggle switches with cb's incorporated.)

Lyall was known for getting his facts right in his books. Apparently there were several things that he tried out himself before writing about it, but I guess this wasn't one of them! ;)