Wednesday, August 6, 2008

Hot Rod Lincoln

Asleep at the Wheel do a version of Hot Rod Lincoln with a spirited introduction by Roy Benson. "The following story is true," he says, in his best Jack Webb voice, "only the oil has been changed to protect the pistons." The following story is true, but some details have been changed to protect the guilty. I heard it during my travels this summer. In honor of The Wheel, let's say that it happened in Texas.

Listen along while you read:



A charter company runs a small fleet of older airplanes. One of their pilots was doing a preflight (this is good), and noticed that the hydraulic fluid was low (this is also good). My first instructor, at about hour two of my career, pointed to the hydraulic reservoir in the Warrior and said "Check it, but if it's low let the mechanics fill it. They have some special way of doing it that keeps the air bubbles out." I left it at that.

I have a lot of time in King Airs, Senecas, and Centurions. The landing gear in these airplanes is hydraulic, and they are all different. The Seneca has a reversible electric pump. There were three versions of the Centurion hydraulic system, as far as I know. The original used an open-center (always circulating) engine-driven system with massive manually controlled valves. Next came an electric motor version, but the valves were still manual. These two systems had a large gear lever in the cockpit, I suppose so you could get enough mechanical advantage to move the valves. (The Fairchild F-27 reportedly came with a rubber mallet to help the crew move recalcitrant valves into position.) The final Centurion system was all-electric, with electric valves that failed in the safe position (no power means that hydraulic pressure opens the doors and pushes the wheels down). It had a little teeny gear switch, since all it had to move was electrons. These systems are complex, and I had a lot of students really struggle to understand them.

In more than two thousand hours of flying these three airplanes, I have never once added hydraulic fluid. I have lost fluid due to leaks (I once lost all of it in a Seneca, but following the checklist made this a non-event), but I have never added fluid. Never.

But this pilot thought otherwise, and went into the hangar and grabbed some hydraulic fluid, and poured it in the reservoir. This put a lot of air into the system, which is what saved him...

You see, this airplane has some hydraulic flight controls. No hydraulics means an airplane that is difficult or even impossible to control, sometimes called the "lawn dart" version of the airplane. The best outcome would be a belly landing, the worst would be...

This pilot ended up not flying that day, and the next day another pilot doing a preflight noticed that the hydraulic fluid level was high. Uh-oh. He called maintenance, and they, unaware of the added fluid, thought that there must be air in the system. They started to "bleed" the system to get the air out. Uh-oh again: they discovered that the pilot had added the wrong type of fluid.

I doubt that pilots know much about hydraulic fluid. I mean, my instructor told me to leave it alone. But at some point I thought that my aeronautical career would be advanced by taking the Flight Engineer written exam, so I spent a fair amount of time studying aircraft systems, including hydraulics. Hydraulics are pretty simple, in principle: in English units, psi in equals psi out, and more "si" (square inches) at the output end means more "p" (pounds) of force. So far so good. But there was a lot of material in these books about hydraulic fluids. Some of them are very nasty substances, indeed. One of these got into the airplane.

The damage? Seals, lines, and pump destroyed. Lots of down time. Angry customers. A pile-up in the maintenance shop, with retail customers angered because there was nobody available to work on their airplanes.

The bill? Right around 15% of the aircraft's value.

Fifteen percent of the aircraft's value, lost in one moment of failing to stop and think.

Let's remember to stop and think.

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