The Weakest Link

Over at Freight Dog Tales, resident freight dog John is contemplating the possibility of engine failure in the single-engine Cessna Caravan he flies. It's a very serious scenario in any single-engine airplane, but the stakes go up in the Caravan: its size and landing speed make a survivable landing less probable than in, say, a C172.

The Caravan's engine, the Pratt & Whitney PT-6, is known for its relability and bulletproof design. Unfortunately, the Caravan's PT-6 installation has an Achilles' heel that has attracted FAA attention lately: fuel is supplied by a single high pressure engine driven fuel pump, with no backup. If the pump fails, the engine will quit from fuel starvation and the pilot has no alternative to attempting an emergency landing on whatever the most suitable terrain within gliding distance is.

I'm rather surprised that Cessna didn't put an auxilliary fuel pump on the Caravan. True, smaller Cessnas lack a backup pump, but in those cases gravity feed from the tanks in the high wings provides sufficient flow to the engine if the fuel pump fails. Low wing Pipers, on the hand, require a fuel pump to run, and every one of them has an electric standby pump. Seems like an oversight on Cessnas part; maybe they know something I don't.

That discussion reminded me of a similar weakness I found in the Piper Lance back when I was flying freight for AEX (that's the actual airplane I flew, N626JD). The Lance is powered by a Lycoming IO-540 that puts out 300 horsepower. Like most reciprocating aircraft engines, this engine uses magnetos to provide spark to the spark plugs. The usual arrangement is two independent magnetos with each magneto providing spark for one of the two spark plugs in each cylinder. The multiple spark plugs improve combustion within the cylinders, but the main purpose for having two magnetos is to improve the dependability of the engines.

One day I was preparing to depart for San Diego and had just called for taxi clearance when the engine abruptly quit. I figured that I just leaned it too far and tried to start it again. No good. I tried the hot and flooded start procedures. No dice. I let the starter cool for a while and tried again, and actually got the engine started. Kinda. It was firing about once every other revolution of the propeller, just enough to keep going for five or six seconds before quitting. By this time, a small army of mechanics had gathered nearby, pointing and laughing at the idiot pilot who couldn't even start the plane. I swallowed my pride and let one of them have a go at it, and then gloated when he couldn't start it either.

The mechanics pulled the cowling off, started tearing into the engine, and in short order found the culprit. Apparently, early Lances have both magnetos contained in a single component that bolts onto a single accessory pad. Both magnetos are driven by a single gear, which happens to be nylon. On my airplane, that gear had shattered into several pieces, rendering both magnetos inoperable. I still wonder how the engine fired even once. I shudder to think what would've happened if the gear failed even five minutes later, when I was in the air.

When I was in ground school at Ameriflight, I asked about the dual magneto assembly on their Lances. Apparently they had problems with it also, and had since installed a later model of the TIO-540 in all their Lances. It has a different accessory box with two truly independent magnetos. Some of their older Chieftains still had the dual magneto assembly, but at least it's a twin and a (properly managed) engine failure leaves you with some options.