Saturday, November 01, 2008

Skill Testing Question

I had the feeling my pilot skills would be tested today. Something unusual was going to happen. Maybe it's a premonition. Maybe it's just an alertness technique I've accidentally made up for myself, but I'm especially alert today.

Everything is normal in the run-up. All the temperatures and pressures sit where they should and the propeller governors respond to movement of the blue levers. The left propeller is slower to start to feather than the right and takes more travel of the lever before the rpm starts to come back when I exercise it slowly but both are within acceptable tolerances. I mentioned it to one of our mechanics a couple of inspections ago and he said it was easily explained by the fact that the right propeller was recently changed out. The pilot valves and oil lines for the recently replaced propeller are clear, where the older one has some arterial clogging, so the oil pressure in the hub doesn't drop as rapidly when I pull back the propeller lever. It will be fixed at the next propeller overhaul. There may also be a simple rigging difference. It's hard to rig the throttles, propeller levers an mixture all so that they are lined up with each other and the appropriate setting at each point in their travel. I know that at a 50 hour check they just check the endpoints, because I've been the one in the airplane wiggling the levers on command as the mechanic checks the results.

I complete the run up and the pre-takeoff checks then line up and roll. Keeping straight down the runway, watching airspeed come alive, a quick flick of my eyes to the engine gauges, most of my attention on the runway ahead. Rotation speed, lift the nose, the mains come up. If I lose an engine now I'm landing on the runway ... and now on the clearway ... and now --gear up-- it would be a single engine go around. The gear comes up fine. I double check the mirror. Nosewheel retracted, no red, handle neutral. Both engines continue to produce take off power. The first power reduction is the most common time for a failure. Through 400 feet, I make that reduction. No problems, the engines come smoothly back and the rpm come back as commanded when I move the propeller levers. The most exiting time for a failure, the biggest test of my skills is over.

Just before the altitude that I will be levelling off, I close the cowl flaps, and then lower the nose to cruise attitude. The airplane speeds up, and I gently reduce the power to cruise and adjust the rpm again. On a couple of previous flights I've seen some engine surging at this power reduction. Company thinks it could be fouled injectors and have promised to clean them at the 100 hour check, coming up soon. There's no problem today. The power comes back smoothly and I sync the propellers then lean the mixtures, then sync the propellers again. You know how it goes.

About four or five hours later I'm flying through a bit of turbulence and the right engine does surge. That's not an abnormal result of fuel sloshing in the outboard tanks. Some pilots won't run them below a quarter tank, because they can unport in turns, but these long flights require me to squeeze every drop out. The power doesn't come right back up to where it was, though. It's down by a couple of inches of manifold pressure. My eyes go to the gauges and I'm startled to see a huge drop in cylinder head temperature on that engine. It normally sits around 400 degrees but it's suddenly down to 150 degrees. There's a significant but not as extreme drop in exhaust gas temperature, too. The engine is still producing power and sounds okay though. I enrich the mixture a bit then bring the throttle up and match the manifold pressure. The split in the throttles isn't even really noticeable.

When a gauge does something wonky it's often the gauge that has a problem, but when the gauge does something odd at the same time as the engine, it's probably not a coincidence. And when two gauges show something odd, it's probably related to the same thing. I guess I was right about pilot skills being tested, but today they're not the adrenaline-fuelled "control-power-drag-identify-verify-cause-feather" reflex skills that are tested at a licence renewal. It's the thinking the situation through and taking appropriate action in due time skill that fate is testing today. Not the sort of thing that makes good YouTube videos, but a real pilot skill nevertheless.

Now, a CHT of 150 degrees on a working engine is nonsensical. The probe is probably not in the engine. And perhaps the wires on the CHT and EGT probe are fastened together in one wiring bundle so one pulled the other one loose. Or there's something terribly wrong with my right engine. Ignoring some places I might land if I had a fire, actual engine failure or other emergency, the nearest suitable diversion airport is my destination, anyway, so I continue on course.

There are no more strange surging events, and on the ground the temperatures read normal. Perhaps without cruise airflow, the probes are sitting back where they are supposed to be. Or perhaps I'm just crazy and hallucinating. You'd be amazed how often airplanes make you wonder if you're hallucinating. I call company and report the new symptoms. They agree that the two sensors are on the same wiring bundle, and that my observations are consistent with loose temperature probes. They still think the surging is dirty fuel injectors and have no problem with me continuing to fly the airplane. Although it's not me continuing to fly the airplane, it's my co-worker. I brief her on what is going on and leave her to make the decisions, exercising her own pilot skills. She will fly the airplane for the rest of the day and then take it south to an airport with airline service, where one of our maintenance folk will troubleshoot and perform the scheduled hundred hour check, plus the promised cleaning of the injectors.

As I write this up, having eventually found out what the problem was, it seems so blindingly obvious as to make me and my maintenance unit look stupid. I imagine half a dozen of you will tell me what has happened as soon as this is posted, but I'll post it anyway.


Anonymous said...

funny thing- I to had my skills tested today. I'm a very low time pilot in a rented plane. This morning I helped put the cowling back on the 172 after it's annual, then took off for a 70 mile jaunt to pick up some family and return home... basically a hop to add time. Anyhow, completed climb, leveled off and tried to lean the mixture. Brand new mixture cable was broken.... Nope not really a big deal but was still a new and funny thing.

Keep writing-- I found your blog quite a while back and love reading it... to the point of disappointed when a day is missed. I know your busy but I enjoy reading about your day


phil said...

fouled plugs?

Travis said...

Something happened to the cowl flap?

david said...

You've got me, Aviatrix. Even if the cylinder that has the CHT probe failed completely, there's no way the CHT would fall to 150 deg no matter how the cowl flaps were set -- at least not until a couple of hours after you shut off the engine on the ground.

I thought your initial explanation of a loose probe bundle was the best one, but I don't know what it would be different on the ground. I'm looking forward to hearing what it actually was.

Eh?Vee!Eh!Tor said...

Something caused the fire to go out or partially out in the cylinder monitored by CHT... and that would also explain the loss of "a couple of inches of MP..."

So - is it fuel/air or spark...

If the engine has surged before, maybe a sticking valve? Or a plug problem as someone already suggested?

I once retrieved a renter-abandoned C-177RG through the Rockies (he was trapped by bad weather and took the bus home). All seemed normal (I was lightly loaded). I was informed by the mechanic who did the now-due 100 hour inspection that one of the cylinders had almost no compression due to a burnt valve (renter over-leaning?). Live and learn ...

Wild Blue said...

I think the cowl flap did not close after you leveled off. I think that would explain the drop in temps in cruise, but normal temps on the ground. I'm guessing the fix was simple - reattach the cable to the cowl flap door.

dpierce said...

Your water bottle knocked the probe out?

Anonymous said...

Does your company think that its worth while to invest in an engine analyzer (EGT & CHT on each cylinder)? The price, versus repairing damage from running continuously with a subtle problem, is justified. I just put one into my single engine piston airplane and am quite pleased.

Anonymous said...

// I'm flying through a bit of turbulence and the right engine does surge//

Loose ignition cable on a spark plug?

A Squared said...

>>>>>>The first power reduction is the most common time for a failure.

Has anyone ever seen any kind of statistics which suggest that this is true?

I certainly haven't, and I can't fathom a mechanical reason this would be true, and while I've expereinced more engine failures than most modern pilots (you get that flying airplanes with 4 radial engines) neither I nor anyone I know has had a power failure while doing the initial power reduction.

Personally, I tend to think that this an old wives tale, made up by some instructor who was trying to break a student of reducing power immediately after liftoff, and it's just been repeated so many times that everyone "knows" it is "true". I'm certainly willing to modify that if somone can poiunt me to statistics which suggest that it true though.

A Squared said...

there's no way the CHT would fall to 150 deg no matter how the cowl flaps were set -- at least not until a couple of hours after you shut off the engine on the ground.

Not true, a dead cylinder, even on an otherwise running engine will cool very rapidly. I've seen it many times