Tuesday, June 26, 2012

Mixture Equation

Company is scrambling to fix the problem, both the problem that the mixture cable is mostly stuck, and the problem that we and the intensely valuable data we collect are stranded here. The non-pilot member of the crew books a commercial passenger flight home. I share a taxi with him over to the terminal, where he checks his overnight bag and does a really good job of nonchalantly pretending that the fifteen-plus kilogram unit containing our hard drives is a featherweight carry-on. He doesn't want to be separated from it or his computer.

We share a meal in a restaurant at the terminal and then he heads to his gate. He says he's taken the units through airport security many times without difficulty, and this proves no exception. It does have official looking stickers on it designating it data storage unit eleven or something, but it's not as if terrorists aren't going to try and make their lead-lined doomsday devices look like harmless technology.

I walk back to the airplane. As I'm walking along the service road, I'm slowly overtaking another pedestrian. I say hi as I pass and he comments that they could at least have sidewalks. It's true. Despite the fact that last time I was here the rental car lots weren't even paved (they are now), it's an airport large enough to have commercial service, a restaurant in the terminal, and rental car lots, but they can't manage to provide sidewalks? I tell him my airplane situation. He sympathizes. We've all been there, and then he goes off to fly his and I go off to keep mine company while company decides what is to be done with it.

I actually have terrific field support. I need a flight plan sent in, a hotel room booked, anything that can be done long distance, and it happens. I tell the PRM, "Hey at least I broke an airplane on a Monday morning." I usually seem to break them on the Friday afternoon before a holiday weekend. Later I discover that it IS a holiday Monday in the United States, severely limiting our options for getting parts shipped. Nevertheless company manages to find a supplier for the cable. It's cheap, only $250 plus shipping so they order two and ship one here and one to a different city. That may sound strange, but I'll get to the strategy later. Oh and someone asked last time for more info on the cable. It's essentially a heavy duty bicycle cable. Instead of a single cable inside a plastic sheath it's a multi-strand cable running inside a metal sheath.

Right now they are looking at the best options to find or send someone who can repair the problem. Or to get the airplane to someone who can. They're thinking of getting a ferry permit, that's permission to operate an officially non-airworthy airplane for a limited purpose, usually to get it to somewhere that it will be repaired or scrapped. A company engineer texts me a picture of the innards of a throttle quadrant and asks me to take one like it on my phone. It is inconclusive at best. I don't know how he can assess the soundness to fly based on that. It's not fair to ask the no-moonlighting guy to sign off on that, because if something happens and the airplane doesn't arrive, he gets looked at, and he doesn't know, me, the company or the airplane. Company asks me if I'm willing to ferry the airplane if maintenance and Transport Canada okays it. The runway here is long enough to do a reduced power takeoff empty, and there's lots of room to climb. I ask what happens if the cable breaks in flight, does it go full rich or stick where it is, or will it roll back to idle cut-off? Maintenance has to get back to me on that. The conclusion is that it will fail in place. I won't be able to enrich the mixture, but seeing as the "mixture" cable controls available fuel and the throttle controls the air, I can control the actual ratio with the throttle. It just means that I'll be limited to a maximum power of whatever throttle setting is appropriate to the max fuel flow I'm stuck with. It would mean that I would have to slowly decrease the throttle setting in descent when I otherwise would slowly increase the mixture, until I was ready to slow down.

Company tells me the wording to write in the journey log, underneath where I have written that the mixture is stuck. It's something like, "Unable to rectify. Aircraft is operated in accordance with ferry permit number ____." Transport Canada accepts the maintenance sign off and issues a ferry permit. I scrutinize my faxed copy carefully for the restrictions. I'm okay to fly over built up areas, good. I'm cleared home with any necessary technical stops enroute. No passengers. I read it over several times but I can't find the ferry permit number. I call company. They agree there isn't one. I sign it off as "in accordance with ferry permit issued at _____ dated ____."

I think this will be my first cross-country solo in this airplane. All the other solo flights have been local test flights, or recurrency. I check carefully the items that my co-worker usually manages, and then I call for clearance and taxi out.


J said...


Anoynmous said...

It's the kind of suspense one gets from episodic television. We already know the ultimate outcome: our heroine survives. It's still a very engaging narrative!

Crispy Kale said...

Thanks for the detail on the cable construction, Avatrix. If there's any analogy to bike repair, the usual cause of stickiness is corrosion on the braided cable itself causing the cable to not slide freely in the housing. Best fix is to replace both the cable and the housing. Mediocre fix is to dribble lube down the cable so it leaks through the housing, aided by working the cable back and forth, although for the length and (I'm guessing) horizontal-ness of this cable setup, that wouldn't work.

Aviatrix said...

Sorry for the initially horrible formatting of this post. Blogger has changed some default somewhere. Or I've forgotten how to blog.

Do let me know if something posts with no paragraph breaks or otherwise unreadable formatting.

Aviatrix said...

Crispy Kale, the mediocre fix you describe is exactly what my maintenance unit did the first couple of times I complained about it, and it worked about as well as you anticipated. The best fix is what I'm on my way to get done. The cable and the housing together are a single part for this airplane.

bjh21 said...

The bicycle cables I'm familiar with are also a multi-strand steel cable inside a helical steel sheath. The plastic coating is just there to make it look pretty and keep the rain out.

Chad said...

Am I right to assume this is a non-turbocharged aircraft? If it was normally aspirated you would indeed have to enrich the mixtures on descent.

If it was turbocharged (with an automatic wastegate), advancing the mixture levers on descent would be unnecessary because the turbos would keep the fuel mixture constant, even on descent.

john said...

Anonymous -

How do we know she survived? As we speak she could be stuck at some altitude, circling, fuel getting low, wings icing, sun setting, front moving in, having to go really bad.

All Canada is glued to their internet sets, waiting to be thrilled as our heroine appears out the sky and brings her steed to a safe touchdown.

(And will there be a Mountie in this story?)

Michael5000 said...

What a cliffhanger!

"Posted from my iPhone just before takeoff"

zb said...

Again, thanks for an entertaining blog post. Two details I really love about it:

(1) You learn how stuff works once it breaks - without this adventure of yours, you probably would have never told us about the tricks hidden in mixture vs. throttle/air. Without a ticket vending machine of Deutsche Bahn re-booting instead of selling tickets, I would never know it's a system running MS Windows.

(2) It always helps to have experience riding (and fixing) bikes and extend this knowledge to things like cars or planes: On a bike, what gear does the system default to when the cable breaks? (The fast gear for a Sachs/SRAM 3-speed hub gear, the smallest sprocket for a derailleur gear system, meaning slow for the one at the pedals and fast for the one at the back wheel.) On a plane, what happens for different failure modes of a mixture cable (fail open, fail stuck). How severe is the worst possible outcome? (short runway vs. long runway, surrounded by high/low terrain, in populated area vs. non-populated area, ...) Heck, this even is an FMEA-type blog post!

(Also, thanks again for the way-above-standard quality of your writing, encouraging me to learn while I read and making me look up words like dreailleur gear or sprocket.)