Unobtanium and Unaffordium

un·ob·tain·i·um (ˌənəbˈtānēəm)—noun informal—a highly desirable material that is hypothetical, scientifically impossible, extremely rare, costly, or fictional, or has some of these properties in combination.

My 1979 Cessna T310R

My 1979 Cessna T310R

As I write this, I’m just in the final throes of completing the annual inspection on my Cessna Turbo 310. Usually I do this myself, and it takes me three or four weeks; that’s because my turbocharged piston twin has way too many moving parts, and because I’m the world’s slowest aircraft mechanic. This year, my travel schedule didn’t permit that much downtime, so I decided to do the annual at a maintenance shop owned by a colleague, and to enlist the help of a couple of his staff A&Ps to help with the opening, closing, repairs and preventive maintenance in order get everything done in just over one week. I performed and signed off the actual “inspection part of the inspection” myself.

Hartzell/C&D cabin heater

Hartzell/C&D cabin heater

I knew going in that this was going to be a costly ordeal. I got off easy during my 2015 and 2016 annuals, but the law of large numbers caught up with me this year. To begin with, the old Southwind (Stewart Warner) combustion cabin heater got hit with a costly AD that would have required me to spend more than $3,000 overhauling the heater, and thereafter to perform frequent pressure-decay tests. I’ve hated this heater for the 30 years I’ve owned this plane. It has always been a problem child, caused me lots of grief, and I just couldn’t see shoveling more money into it. So I ordered a brand new, far superior heater designed by C&D Associates (and now manufactured by Hartzell Engine Technologies). The new heater cost $6,000 (ouch!) but will hopefully be trouble-free and AD-free going forward.

Installation of the new heater turned out to be a lot quicker and easier than I expected. Other things didn’t.

Prop Sync

One item on my squawk sheet was that the propeller synchrophaser was inoperative. I’d done sufficient troubleshooting to determine that the sensors and servo were okay, and that the problem was in the ARC SP105-B control box. I called Cessna, and was told that all rights to ARC equipment (including my control box) had been acquired by Sigma-Tek. I contacted Sigma-Tek and learned that they had no replacement boxes and no bench repair capabilities, but would sell me a maintenance manual for the SP105-B for $400.

ARC SP105-B Prop Synchrophaser

ARC SP105-B Prop Synchrophaser

I contacted every major avionics shop I could think of with bench repair capabilities, and all responded with some variant of “we used to work on those back in the day, but we no longer have that capability.” Finally, I did an exhaustive search of salvage yards to see if I could find a serviceable SP105-B control box. My search led me to Preferred Airparts in Kidron, Ohio (the largest dismantler of twin Cessnas), who said they had 40 inoperative SP105-Bs in inventory, plus one that was in “as removed” condition and “probably” was in working condition.

I purchased that one for $500 and installed it in my airplane. When I powered it up, the prop sync annunciator illuminated and the slave governor servo centered, which was encouraging. But when I test flew the airplane, I found that turning on the prop sync switch yielded a rapidly flashing annunciator (meaning the control box was not happy about something) and no prop synchronization function. Drat! My next step is to create a test harness so I can measure the sensor pulses going into the prop sync box and its output voltages to the servo motor while airborne and try to figure out what’s going on.

Positronic GM-Series Connector

Positronic GM-Series Connector

To create that test harness, I’ll need a pair of connectors (one male and one female). After an hour of Googling, I determined that the connectors in question are Positronic GM26s. I couldn’t find these connectors listed in the catalogues of any of the electronics supply houses I usually rely upon (Newark, Mouser, Digikey), so I contacted Positronic and learned that the only place that has these connectors in stock is in Puerto Rico and was rendered incommunicado by Hurricane Maria. It may be weeks before I know whether the connectors I need are obtainable. Sadly, I placarded my prop sync “inoperative” and pulled the circuit breaker per FAR 91.213(d). The prop sync saga will continue.

Flap Preselect Cable

One of the items on my inspection checklist for this year was to check all the flight control cable tensions. I was particularly interested in checking the tension on the flap retract cables, because the maintenance manual calls for those to be tensioned to 280±20 pounds. Normal cable tensiometers used in piston GA maintenance only measure up to a maximum of 100 pounds, so my flap retract cable tension hadn’t been checked for years. My colleague said he had a high-range tensiometer that measured up to 300 pounds, so we pressed that into service. As I suspected, the flap retract cable tensions were low (about 225 pounds). That meant that during a high-speed descent, the flaps might not stay fully retracted.

Broken Flap Preselect Cable

Broken Flap Preselect Cable

One of the A&Ps who was assisting me volunteered to adjust the flap retract cables, and I gratefully accepted his offer. That turned out to be a mistake. After the A&P adjusted the turnbuckles to the specified tension, he performed a functional check and found, to his horror, that the flaps would not retract. Further investigation revealed that the flap preselect cable (which runs from the flap actuator under the floorboards to the flap preselect control on the instrument panel) was now severely kinked to the point of being unrepairable. This was clearly a MIF—a “maintenance-induced foul-up”—that had turned a minor issue (low cable tension) into a major one (inoperative flaps).

We checked Cessna’s parts inventory for a replacement flap preselect cable. I expected it to be “unaffordium” but found instead that it was “unobtanium”—Textron Aviation no longer had any, and didn’t plan to make any more, ever. A Google search of the part number uncovered a used-but-serviceable one at B.A.S. Parts & Sales LLC in Greely, Colorado. We ordered it and asked for expedited shipping. Installing the new cable and rigging the flap preselect control wound up being a full-day affair.

More Ouch

Tire and Tube

Tire and Tube

My inspection revealed more costly stuff that needed to be done. Two new main landing gear tires and tubes ($700), two new brake discs ($350), 12 new brake linings ($150), and a bunch of miscellany brought the tab for this year’s “annual ordeal” to nearly $15,000—and that doesn’t include my sweat equity. Aircraft ownership isn’t for sissies…and that goes double for twins. But I do have a feeling of accomplishment that a lot of important stuff got done this time around. With luck, the next year or two will be smooth sailing…

…except for the $6,000 I will soon need to spend installing ADS-B-out…sigh.

Mike Busch is arguably the best-known A&P/IA in general aviation, honored by the FAA in 2008 as National Aviation Maintenance Technician of the Year. Mike is a 8,000-hour pilot and CFI, an aircraft owner for 50 years, a prolific aviation author, co-founder of AVweb, and presently heads a team of world-class GA maintenance experts at Savvy Aviation. Mike writes a monthly Savvy Maintenance column in AOPA PILOT magazine, and his book Manifesto: A Revolutionary Approach to General Aviation Maintenance is available from in paperback and Kindle versions (112 pages). His second book titled Mike Busch on Engines was released on May 15, 2018, and is available from in paperback and Kindle versions. (508 pages).


  1. SP-105B



    Qty: 1

    Condition: Serviceable

    Updated :

    Mon, Sep 18, 2017

    Power Packs Plus

    Sparks Nevada USA

    (775) 235-4593

    Send a Message

    • I have used PowerPacks Plus for several items that have absolutely nothing to do with PowerPacks (as their name would imply). They are great people. Oh yeah, they did a powerpack for me too.

  2. I only have 850 hours in twins, but I never flew one that had a prop synchronizer other than the pilot. I’d stick with the “manual” method and save the $$. Why would the flaps not stay fully retracted during a high speed decent? I would think the only time the flaps would fail to stay retracted would be when there was no airspeed/flow pushing them up.

  3. The art of Cessna twin aircraft maintenance is to be deeply involved with a collective of parts resources. The survival of the Cessna twin is directly proportal to ones ability to adjust to a never ending learning curve. The kindest words one can hear, ” We have it in new old stock and the discount is 50% off of retail.”

  4. Decades ago, an article was written, “The Bottomless Baron.” I had a Baron; the author was correct. The article should have included the C-310.

  5. I can identify with these problems. My C310 ownership included extraordinary amounts of twiddling with electrical, Navomatic AP, vac/deice, & other systems. Parts searches were more like scavenger hunts. Just looking at the fuel system gave me a migraine.I had nightmares over rerigging of the gear, and upon every retraction swore I heard the unique sound of its bell crank breaking.It appeared that no amount of money/expertise/parts availability could ensure squak-free flight for longer than 6 months.
    Despite my conviction that the Wallace twins were superior-performing aircraft I became convinced that some rat holes can never be filled. I swapped for an A36, with its paralyzing parts costs,simpler systems, and cramped cabin, weeping as my twin flew off, leaving only a trail of bills.
    Carrying such rites of passage now feels like wearing the tattoo of an old girlfriend, stenciled upon my tender skin in the throes of a drunken euphoria.
    No wonder mechanics love these renaissance aircraft.

  6. Mike, as you say – “OUCH!”

  7. MIke,

    As the owner , can’t you start fabricating your own replacement part parts? My 340A has the same issue getting parts and I often use Preferred Parts, but many things are just not available so I fabricate. I would fabricate that prop sync control box using modern electronics and not waste time finding an old unit that might last a short time before it fails.


    • It wouldn’t take much to create a prop sync controller with a Strawberry Pi and a few A-to-D’s, and I’m sorely tempted. But it probably wouldn’t be legal without a field approval. I doubt it could be considered a minor alteration.

  8. Wow, Glad I have a Baron. Actually, I’ve had there of them and none were as expensive or as complicated annual as yours, Mike. I’ve rarely had unobtainum or unaffordium, but when I have there’s always been another option.

    But, Mike, I love your posts, comments and contributions to the maintenance of GA planes and will continue to follow, but probably won’t get a 310T…..

  9. I use to own a twin n,my c182rg is 25 knots lower but Burns half the fuel and can carry 800 pounds for 500 miles in just over three hours! All in my annuals average about $900 a year. Liability insurance this year with a million-dollar liability $406.

    • For sure, if I didn’t own this twin for 30 years and have a zillion dollars of sweat equity invested in it, I’d buy an SR22. I think CAPS offers more safety than a second engine.

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