Monday, January 30, 2006

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.

8 comments:

Lost Av8r said...

The PA31 I fly has dual Mags driven by a common gear. I don't care for the at much. The POH for my Chieftain says that; fully loaded on a 20 degree day, the single engine service ceiling is 7000'. The Lowest MOCA, once I leave the Fraser Valley, is 9000'. Needless to say, I'm doing my best to take real good care of those engines...

John said...

Wow! The devil is surely is in the details.

The Cirrus SR22 has an electrical system design that looks great on paper. A main bus and an essential bus, powered by separate batteries with separate alternators. There's even a diode between the two busses so that the essential bus will not attempt to power the main bus should the main battery and alternator fail.

The problem with the Cirrus is that all of these design "eggs" are crammed into one, physical "basket" called a Master Control Unit, or MCU. If something happens to the MCU, like oil mist from the engine getting on the circuit board, the stated behavior of the system may become ... how shall I put this ... undefined. In a plane with only electric attitude instruments, in IMC, this could be a bad deal indeed.

No system is foolproof, despite what the checklists and system descriptions may say. Still, some designs inspire more confidence than others.\

Excellent post!

Anonymous said...

The Caravan is knee deep in auxillary fuel pumps (well it has motive flow and backup electric pumps) for getting fuel from the tanks to the HIGH PRESSURE fuel pump.
When I went through ground school we were told there were no cases of the high pressure pump failing - but they did agree if it did then no suck, squeeze, bang would be happening.
I guess that record isn't standing up anymore.

John said...

Not to turn this into a Caravan discussion, but ...

The Caravan's motive pump is not auxillary in nature. The electric boost pump is meant as a backup for the motive pump should a bleed air problem develop. There is no backup for the high pressure pump, which is a shame.

Rumor has it the Caravan accident near Austin last autumn was an engine failure due to a high pressure pump going south. A few weeks after that accident, I witnessed a flurry of fuel pump inspections and some replacements. I actually got to handle a bad pump that had been replaced - it was ugly.

Frankly, I was surprised that AD has not been issued as some operators were apparently performing the pump inspection improperly, risking not detecting pumps that were on the verge of failure.

Sam said...

John, your discussion of the Cirrus' electrical system and its exposure to physical damage reminds me of an incident we had at my airline a few years back. A Megawhacker was circling to land at Medford, at night, when it hit a flock of large geese. They took out the captain's windscreen, seriously injuring him. They also punched holes in the radome and caused significant damage to the electrical components in the nose- which includes all three batteries, the transformer-rectifiers, and several busses. Virtually every peice of electrical equipment in the airplane went dark. Fortunately they were VMC at the time, so the first officer was able to take over and land the airplane. I'd hate to think of that happening in the clouds - guess it's a good thing that not many geese fly IMC!

clint said...

Sam-
I really appreciated the detail of your experience and research you included in your Flying Careers series. Extrmely insightful and a great reference for myself as I continue to move in that direction. Keep up the good work.

pops said...

hey Sam; good to see you writing again on a more favorable climate. You,re blogs are always informative and interesting. Keep it up!

airbusflyboy said...

Sam, I am looking at N626JD, 1975 Piper Lance ........ it is for sale on an auction site. I don't know what happened to this airplane, the engine and prop are gone, the plane has been sitting for a many many years, it appears that the aircraft had been used by a company that did charters out of Brackett, California and then moved to Chino, California. Cannot locate the last owners of record ........... Heidi Stobel, Ari David Lapin ........ it looks like the airplane was part of a business that Lapin owned and the business went belly up, I am thinking that the aircraft was repossessed by a bank and then Strobel ended up with the airplane in a bank auction sale, looks like she did not pay the taxes on the airplane ........ Amazing what kinds of people end up with airplanes. I might bid on the aircraft but not sure it is a good investment, looks like it will need a lot of work to get it flying again plus we do not have the logbooks, am sure one of the previous owners kept the logbooks.
Thanks Sam, I was surprised to find someone that had flown N626JD, I am sure it was a good flying airplane at one time.