Monday, February 25, 2013

Time in the Tanks

I'm coming in to land from another remote location, once again balancing greater true airspeed at altitude against expected decrease in headwind below, and rationing my gravitational potential energy as I descend. I've been operating under controlled VFR, required for flights between 12,500' and 18,000'. The controller gave me leave to descend unrestricted and I cancelled the CVFR through twelve five, leaving me plain old VFR. On my own and and far from anything, including airports. The GPS shows me forty-five minutes from landing, but that's because of the increased speed on descent. It should be about an hour to landing and I have an hour and thirty minutes of fuel on board.

As I level off, the airspeed drops and the GPS estimated time en route climbs. The groundspeed is a little higher than it was at altitude, but not as much as I had expected. The groundspeed settles down and I've really made no progress with the descent. I'm going to be eating reserve fuel.

I know that's what reserve fuel is for but I'd rather it be in the tank when I land. I reduce power a little. It's always hard when you're eager to get somewhere before the fuel runs out, and you have been trained to think of fuel as time, to reduce power and get to the destination slower, but just as you'd sprint to catch a bus, but slow to a jog or walk if you missed it and thus had to go ten kilometres on foot, the airplane will go further on the same amount of gas at a lower power setting. The problem of how slowly to go when you are also fighting a headwind is another fun curve to play with, but again I'm missing information. I have forecase wind and altitude information from two stations, one to the west and one to the south. I don't know right now whether the wind is stronger than forecast for this altitude or just more from the south than forecast. I'm trying to draw a picture in my head of the high and low pressure spots and figure out how the wind is moving, so as to guess where the wind will let up, or how to use knowledge to beat the system.

Never let the fuel gauge become the primary instrument in your scan. That is don't put yourself in a position where fuel is such a concern that you look at the gauge every five seconds. I'm going to make the field with most of my half hour reserve on board. I still think the headwind will drop as I get further south, but I still need to allow for an airplane crashing on the destination runway just before I get there. But really up here it wouldn't be a hard decision what to do. I'd ask the FSS if they had a preference between me landing on the taxiway and me landing on the runway over or short of the existing wreckage. I don't even recall if that airport had a long taxiway. It may have just had an apron. There are places where I would land beside the runway, but those are flatter places with better groomed airports.

There is no traffic at the airport, crashed or otherwise and I shut down after landing plus a fairly long post-landing engine run with twenty minutes of fuel on board. I take on another load of fuel and then head south again, two hours to an airport most people still consider north. The other crew member (a non-pilot, with no duties on this repositioning flight) sleeps most of the way. I was going to see if I could keep him asleep right to engine shutdown, but turbulence wakes him up in the descent.

Unrelated, B787s are grounded all over the world while Boeing comes up with a solution to the battery problem. Here's a list of where they are.


david said...

A bit over 8 years ago, I put up a blog post about power settings into a headwind. It turns out that in a Cessna 172 at 8,000 ft DA, 55% power will continue give you the best range until the headwind hits 40kt:

The cutoff headwind is probably much higher for what you're flying (I'd be interested to see the same table made up from your POH).

Anonymous said...

So glad you're blogging again, Aviatrix!

Landing with 20-30 minutes of fuel left seems like cutting it a bit close ...

Ben Read

Aviatrix said...

As a commercial pilot my job is to optimize use of the airplane, so every excess kilogram of fuel I land with is a kilogram of cargo that could have been carried, or a unit of time we could have been in the air working. If I did my job perfectly we'd land day VFR with exactly 30 minutes in the tank every time. I land with a few minutes more 99% of the time. This was a one percent flight.

Anonymous said...

I had an anxious 2 hour flight from Parry Sound to Sudbury in a J3-C65. The fuel gauge is a little metal rod on a floating cork which sticks out of the 12gal fuel tank in front of the plane before the engine.

The plane is supposed to burn 4GPH, but, you don't really know until you land. I kept my leg to 2 hours, but this was pushing that and I spent the whole time white knuckling it staring the fuel gauge at the GPS ground speed and ETA. Luckily there was no headwind, just a little turbulence.

When you said "Never let the fuel gauge become the primary instrument in your scan. That is don't put yourself in a position where fuel is such a concern that you look at the gauge every five seconds." I thought of this adventure. Problem was there was no closer airport en route in the CFS or on my chart.

I made it safe and sound, and swore I'd never do it again.

david said...

I never thought of Parry Sound to Sudbury as a flight where you'd worry about fuel.

It's interesting looking at flying from the perspective of different aircraft. I always think of my Warrior as a bottom-end, underpowered plane (the Volkswagen Beetle of the sky), but Sault-Ste-Marie to Toronto still leaves 2-3 hours in my tanks, and I can do Ottawa-Charlottetown non-stop VFR with an OK tailwind. I imagine the J3 is a lot more fun, though.

Aviatrix said...

Thunder Bay to Kenora is another long slow stretch in a tiny airplane, especially as you very often have to divert for lines of thunderstorms.

Canada = big

david said...

Isn't there an old ad from the 1940s showing a J3 at a gas station? Since the J3 burns mogas and has a back-yard-sized takeoff roll, it must be tempting just to land beside the TransCan near a gas station and walk over with a jerry can.