Saturday, June 30, 2012

Mixtureless Travel

The ferry permit allows me to fly an aircraft without a valid certificate of airworthiness. As soon as I wrote in the logbook about the problem with the mixture, I voided the C of A. There's companies where you would not be well-regarded for doing that, and I always second guess myself if I over-reacted when I ground an airplane, but seeing as you're always supposed to be on the safe side of uncertain, and seeing the number of times something has been truly wrong when I have made the decision, this is my job. Every job has a not-as-fun part, and mine is every once in a while telling the boss that the airplane can't legally be flown.

A lot of people would have just said, "Hey, I can't really fly this properly, I'll just take it somewhere I can get it fixed," and a lot of companies would have expected that. There are probably plenty of people reading this thinking, "What a prig she is, putting her company to that trouble when she was willing to fly it the way it was, after talking to maintenance." Some are probably thinking, "Damn she's brave to snag something away from home like that." But I had given our maintenance unit plenty of opportunity to fix the problem when the plane was home and it had reached a point where it was my job to say no. And then to fly it while broken but legal.

I don't notice the power difference on takeoff. Perhaps all the fiddling has loosened it up and given it more travel. (Heh, I really wanted to put a silent t in "loosten" to make it match "fasten" even though I know perfectly well that fasten comes from fast, as in "make fast). Also I'm down the weight of a crewmember, his luggage, some fuel and that data unit. I can barely lift it. Maybe it's twenty kilograms. I don't have my detailed weight and balance with me to check. All the temps are in limits and the power is adequate to the job. I turn enroute and climb to a suitable altitude.

Hey pilots, remember set heading points? I haven't used one in years. All praise the thin pink line! (Non-pilots: when you're travelling visually from airport A to airport B, you can't take up a course directly from A to B, because you don't know while you're planning what runway you will take off from, exactly how the air traffic controllers will direct you, and what traffic you may have to avoid before you can turn on course. So you choose a point sufficiently far from the airport that you'll be out of the departure procedures by then, but sufficiently close that you won't get lost on the way there, and you calculate the exact heading from there to your destination. After takeoff you fly to your set heading point and turn on course. Except that thanks to the wonders of GPS, we don't have to do this anymore. Just take off, hit the direct-to button and follow the pink line on the display). That wasn't apropos of anything. I just remember turning on course without having to worry about where I was.

It's kind of scuddy--low clouds close to terrain--so I continue climb over them. There are a number of layers and eventually I want to go high enough that I have to call for clearance into class B airspace. It's like when you go out for a drink after work with a friend, and then you run into some more people you haven't seen in ages, so you have to have a drink with them, and then you might as well stay for dinner, and then you want dessert, but they know this place ... and before you know it you're out until three a.m. To tell the truth, that doesn't happen to me, but the other metaphor that came to mind was when you start to pick up something you dropped on the kitchen floor, but once you do that you see something else that needs cleaning and you end up having to move the refrigerator. I haven't ever cleaned behind my refrigerator. Who knows what's back there. If it gets too dirty, I'll just move. I hadn't really intended to go this high, but one thing led to another and here I am. It's cool. I have a clearance.

I level off. You piston pilots know that means that I close the cowl flaps, lower the nose to a cruise attitude, wait a bit for the airplane to speed up, slowly bring back the throttles to cruise power, adjust the propellers for the right "gear" for the power setting, and finally lean the mixture for efficient use of fuel. By habit I lean one at a time. That's because if I accidentally pulled both all the way back it would cut off fuel to both engines at once and that would be exciting. So I pull the left one, the good one back, seeing the fuel flow drop to the expected value as the EGT rises. And then I pull the right one, the bad one, back. I have to pull harder, so I have my hand kind of braced, so I can't pull it too far at once. It comes back. And so does the fuel flow. Nice.

The clouds are getting a little thicker beneath. It's still scattered. Or at least I'm calling it that, because in Canada the layers below need to be scattered or better for me to be VFR. But whatever the cloud coverage, they are too numerous for a smooth descent into destination, which is what ATC is offering me. I think they're already a little testy about my unfiled request for CVFR, and now they want to get rid of me. There's a big hole to my left, so I announce that I am descending in that direction to continue VFR below. And I do that, reducing power somewhat for the descent and then slowly enriching the mixture as the oxygen level in the air increases above what the turbochargers were providing when I first leaned the mixture.

Which is odd now that I stop to think about it, because I have great turbochargers. I can set a climb power manifold pressure with the throttles and it just sits there on the dot from 2000' up to FL190 without me having to touch anything. So why do I have to enrich the mixtures on the descent? I do. If I don't pay attention the EGT needles will creep up towards the red.

The turbochargers are compressing the ambient air such that the engine gets the same amount of oxygen it would at sea level. But on the way down I have to pull back the throttles to keep the manifold pressure the same, and I have to push the mixture up. The difference is that on the way up I have about ten inches more MP than on the way down. (Pilots have the weirdest units for power.)

I follow the valley under the clouds. The problem with being under the clouds is that its raining. Okay, this isn't a big big problem, but flying is more fun when you can see where you're going. It's really pouring. I fly to the VOR and then into the next valley where I can call ATC and announce inbound. They give me a VFR arrival to fly, and as I'm getting that sorted out they decide I don't have to fly that arrival after all, and should fly straight in, so I pull back the power more and dive into final, zooming to get gear and flaps sorted out, then land.

I taxi to the maintenance hangar and try to figure out where to park. It's kind of crowded outside and there are do-not-park-here lines in the places that don't have airplanes. I know they're going to work on it first thing, so I do the best I can and pull both mixture levers to idle. Both engines shut down. The cable is still in one piece.

And then it turns out that there's a guy there waiting for me at the AMO, even though it's after hours. He just wanted to make sure I got in safely. He says he'd offer me a ride, but he's on his bike. So I take a cab. That's right, I strung you along on a ferry in an unairworthy aircraft, with the mixture cable supposedly on the verge of breaking, and then nothing happened. But when I left I didn't know it wasn't going to break either, so it's only fair.


nec Timide said...

Interesting read even without the drama of an uncontrolled engine.

Also the first time I recall reading zoom in that context since "Stick and Rudder".

lsapilot said...

I'm not complaining - in a way I'm relieved everything went fine on that ferry flight.

john said...

Aha! There was a Mountie after all.

(On a Schwinn, not a horse.)

Michael5000 said...

Please don't feel like you need to apologize for a lack of plunging.

Sarah said...

As a neophyte power pilot, I always am interested in how to handle failures.

What would have you done if you were unable to richen the mixture on descent? Shut down the engine ( with the mags - assuming the cable were immobile or broken ) or accept the too-lean setting all the way to the gound?

Sarah said...

Forgot to say.. Happy Canada Day! Enjoy the fireworks, as we will in the US later this week.

Aviatrix said...

It wouldn't have made a huge difference, Sarah. I would have adjusted the mixture from the other end, brought the power setting on the affected engine down until the available fuel flow gave me a good mixture. And I would have advised ATC so they they didn't get too fancy with getting airplanes out in front of me and giving me a last moment landing clearance.

Mog said...

Do any modern aircraft piston engines have Engine Management Units, as in road vehicles? These would take care of the mixture automatically.

Sarah said...

Ah, got it. Makes sense, Aviatrix. Low power makes lean mixture a non-issue anyway.

Not really, Mog. The search keyword is "FADEC", Full Authority Digital Engine Control.

Some new/experimental piston aircraft have a "power lever". TCM/Continental, the engine mfgr, has pushed it a bit.

I think the only production a/c with FADEC are Diamonds ( Da40 ), Lancair and a few others... Corvalis/C400 ? Not even Cirrus offers it standard, though they may as an option.