[bookworm]

I guess I could post this anonymously to avoid embarrassment but you'll all guess right anyway

The Twin Comanche has three tanks feeding each engine -- a main tank (27 USG, + 3 unusable), an aux tank (15 USG), and a tip tank (15 USG). This makes fuel content assessment easier than having a single vast cavern, but needs careful management in flight, and you tend to get a dribble of fuel left in this tank and a drop in that one. The gauges are unreliable, and we tend to use the Shadin fuel flow meter to log when we change tanks and assess volumes in flight. Landing is always on main tanks, as the "unusable" element nominally allows their use in other than wings-level flight. Actually, it usually doesn't matter much...

It was my second flight to Switzerland. Mountains! I'm a flatland pilot and I'm very aware that I don't normally have to deal with terrain higher than about 300 ft. In a twin with a max-weight sea-level OEO climb rate of 200 fpm, it plays on your mind a bit when operating into a 1500 ft elev shortish runway.

I changed to the aux tanks somewhere around Dijon. I did make a mental note that the Shadin told me that the remaining fuel in these tanks would get me all the way to Grenchen, just.

I expected shortcuts. Maybe not from the French, but the Swiss surely clear you direct to the other side of their FIR or to your destination, just like the Belgians and Germans, don't they? Well they didn't. You fly SE into the Bern valley between the Jura and the Alps, and then fly NE up the valley. The approach into Grenchen runs from WIL to the SE, so you fly south of Grenchen, then turn NW, then finally make an approach to the SW. "Circuitous" is apt. It was a lovely day, but I thought I'd fly the approach for familiarisation -- I don't want to fly it for the first time in IMC at a later date.



The final approach is 17 miles long. It has to be really -- you start at 6000 ft and have to get down to 1500. So I delayed my pre-landing checks until final approach as I didn't really want the electric fuel pumps banging away at 6000 ft for 15 mins, and I certainly didn't want the gear down until a few miles out. At about 8 miles I did the pre-landing checks:

Brake pressure
Undercarriage ... delayed
Mixtures rich ... no we should leave those leaned a bit at this level
Props ... OK, nudge them forward to 2400
Fuel ... electric pumps go on
Altimeters ... QNH set and crosschecked
Landing lights ... flick them on
Security ... "Put the knitting away now please, darling..."

and enjoy the beautiful view all the way in. And we landed... with gear down, yes I did remember that.

And during shut down I looked down at the fuel selectors and saw they were still on the aux tanks. Oh well, no big deal, the stuff about landing on the main tanks is only because of that "unusable" element and ... hang on a minute, how much fuel is in the aux tanks? I dipped it. Half a gallon on one side, a gallon and a half on the other. Much bigger deal It struck me that had I needed to go around and fly for another 10 mins, I might have lost an engine, hopefully not both before I realised what was going on.

And how did this happen to me?

7 of the pre-landing check mnemonic letters are a single do or check. Just one, the F, has two distinct elements: fuel pumps on and fuel selectors to main. The fuel pumps part is almost always a do. The fuel selectors part is almost always a check, as I'll have set the fuel selectors to main before top of decent. This time, ToD was such a long way from landing I hadn't wanted leave an inconvenient 3 or 4 gallons in the aux tanks, so I delayed the switch, knowing I'd catch it later. I didn't. A combination with unfamiliarity with the airport, an unusually long approach, some extra thought required in the other checks and the complacency of being in VMC, disturbed my usual routine.

[Flyin'Dutch']

Welcome to the club.

[Timothy]

I have often run tanks dry in a twin, nearly always deliberately but occasionally accidentally. Don't worry, your hand moves to the fuel selector very quickly and the recovery is almost immediate. It is not so much an engine failure as a cough and a splutter.

And yes, I have had it happen on both engines simultaneously (feeding from one ferry tank). The feeling is unpleasant but still easily dealt with.

[bookworm]

Thanks Timothy. I've run a line (not tank, it was an ice blockage) dry on the ground and the engine stopped rapidly before I could get the selector over -- however, in the air, the prop will windmill, so I'm sure there's much longer.

One other gotcha that I'll share. Our twin com has tip tanks where the selection between aux and tips is based on electrically actuated solenoids, with the main selector in the same place. If the solenoid valve breaks, the light on the solenoid switch still says "tip", the fuel gauge still indicates the content of the tip tank, but the fuel flows from the aux tank. In an incident last year, the solenoid had failed and the pilot, thinking he was running on the tip ran the aux tank dry (with the gauge still showing almost full, of course). His instinct was to flip the solenoid switch rather than switch to the main tank. That did nothing, and by the time he tried the main tank the engine had already stopped. He diverted and landed SE without incident. But SOP for us now on engine failure is not just "switch tank" but "switch from main to other or vice versa".