Friday, June 05, 2009

Refraction Distraction

I walk towards my airplane on the ramp and I can see that it is pissing fuel out of the left wing, a steady stream, not a drip but a spray like it's pressurized. I remember the wide-eyed horror I felt the first time I saw fuel streaming out of a parked airplane. Today my feeling is just, "Hmm, this had better not be something bad." I could write a long blog post on various reasons that fuel has dripped, flowed or sprayed out of my parked airplane. I duck under the wing to look at the source. It's coming out of a vent. The vent is recessed so that incident air doesn't pressurize it, but it still faces forward so the stream spurts out in front of the wing. There's no visible damage. To confirm my first suspicion about the cause, I open the fuel cap for that tank. The tank is completely full and some fuel flows out of the filler neck onto the top of the wing. Suspicion confirmed.

This airplane was fuelled last night while we were wearing sweaters and jackets on a rainy, overcast day. Now I'm wearing a t-shirt, and the angle of the sun over the hangar puts the left wing in bright sunlight. The fuel is expanding inside the wing and being forced out the vent. It's perfectly normal. That's one of the reasons the vent is there. (The other reason is so that as fuel is burned, air can get in to replace the volume so that the fuel will continue to flow).

The fuel stain on the ramp looks nasty, but the rest of the airplane is fine. I set up what gear I have, conscious that the bag in which I normally organize my spare batteries, extra flashlights, snacks, waterbottle, and the like is packed inside my checked luggage. I have enough equipment to be safe and legal, just not everything I usually have.

All is normal on the run up, and I talk to the flight services guy in the tower and then take off. I say good bye to flight services guy but continue to monitor his frequency, and tune 126.7 on the other radio for en route traffic. At eight-thirty in the evening up here the sun angle is like four in the afternoon down at the 49th. It's hot in the cockpit, but I need to run the heater for the comfort of the picky computer equipment in the back. I spend a while moving vent sliders around trying to make it comfortable for me, and cozy for the computers, too. The mission specialist goes off headset for a while in order to take off his fleece.

Not only have I not flown in three months, but they have changed out the type of equipment I am using, so I'm looking at a screen I haven't seen in about a year. I stare at it for a moment, trying to remember where it means I'm supposed to go, and then I go to work. I remember this. I know how to do it. I'm good at this. Somehow my temperament--and bladder--is suited for this type of flying. I settle back into my routine, burning an hour of fuel out of each set of tanks so that nothing is so full it's spilling out the vents, and then systematically work through the various tanks. I plan to land at 2:15 a.m.

There are a couple of pilots in Cessna 172s hopping around beneath me, position reporting between small places I saw on the map. They chat to each other about how fine the weather is tonight and where they are going. As they say their call letters I picture them painted on the side of the airplanes. I remember things better visually then as sounds. One of them is CXD, which trips a switch in my head because they are all letters used in Roman numerals: 110-500, not that that means anything. When there's a break in their conversation I call up CXD and ask them what they're doing, just trying to join the conversation. The answer is a fairly terse "flying from Worsely to Fairview." And then they stopped talking to each other. I guess they forgot they weren't alone up here.

Fifty gallons of avgas later I'm still in the sky and so is the sun. My hat is in my checked bag, so I have to depend on my skinny sunvisors and my sunglasses for protection from that ball of fire. I estimate it is still fifteen degrees above the horizon, maybe a little less. When will it set? When I'm flying I can't put my whole brain over to arithmetic, because I'm looking out the window, manipulating the controls, squinting at my screens, and paying attention to all the little things I don't even realize I'm looking at. So I chunk arithmetic up into easy bits. Three hundred sixty degrees in twenty-four hours, one hundred eighty in twelve, ninety in six, forty-five in three, so fifteen degrees an hour. But I know it will be in the sky for a lot longer than an hour. The sun doesn't go straight down here, it moves diagonally towards the horizon. At my latitude, three weeks before the solstice I suppose I could calculate its angle of descent and use trigonometry to determine when it will reach the horizon. But I just eyeball it and estimate that it will hit the horizon at about eleven. If I were a few degrees further north and it were a few weeks later it wouldn't go down at all. It would just slide around the edge of the horizon before angling back up.

I didn't see the exact moment it ducked behind the horizon, I just turned and saw that it was no longer trying to burn my eyeballs. It was about eleven. It's still light, but the light is more orangeish. I try to picture the refraction process that is making the light orange. I'm pretty sure it's the same reason that the sky is blue. But is it because the red light is refracted more by the atmosphere or less? I think the blue is refracted more, because the blue is on the inside of the rainbow, but I can't quite picture it. Now that I'm on the ground I could look it up, but I'll give you my airborne thought processes. I think that as the sun's rays hit the atmosphere at a low angle the blue is being refracted up away where I can't see it, but the red is refracted less, so it's the last colour I see. There's a moon, too, and its light is whiter. An hour after sunset it's not really dark yet. I can still see clouds in the distance and the shapes of lakes on the ground.

I call flight services for an altimeter setting. It hasn't changed in over four hours. 'Are you coming in to land now?" he asks. No, we have another couple of hours.

The mission specialist is tired and calls it done a little early. We touch down just before two a.m. I chat to the FSS guy on the way in, "when do you get to go home?" It turns out that he worked the day shift and then got called in to do the overnight, so he'll be there until 8:30 a.m. Nasty. I tell him he'll still be there when my flight partner comes on shift in a few hours. He asks her name so he can say hi. I forgot to ask his name.

I'm hungry enough to eat the seven hour old sandwich I bought before departure, and tired enough to go right to sleep.


Anonymous said...

Glad to have you back flying. Thanks for the post.
Bob in Minnesota

Geekzilla said...

Where's that suitcase?? :-(

Anonymous said...

You have achieved "Dave Zen" with your posts.

Very, very well done.

Your orange colors vice the Tyndall Effect are fully realized in a total lunar eclipse.

Rest ye well, Aviatrix. May suitcases and soft landings abound.

***off to finish making beer... mwahahahahaha...***

G said...

"Brrrrriiinnnnggg, Brrrrrriiinnnnnngggg, .... This is Greyhound calling - your suitcase has arrived..."

"Knock, knock - Housekeeping... Would you like us to make up this room yet? Are you checking out today..?"

"Knock, knock - Hello? Hotel staff, M'aam. Were you expecting a suitcase to be delivered today?"

Sleeeep! Must sleeeep!

(How'm I doin'? ... that's about how it usually goes - no?)

thanks for the fun blog!

Aviatrix said...

I googled why is the sunset orange? and am pleased to confirm that I got it pretty much right while looking out my cockpit windows and thinking about physics. This may not be the same way that a poet takes pleasure from a sunset, but it is mine.

My technical readers are welcome to add to this physics vignette; what else can we work out about the sunset without requiring one's full attention, diagrams, or computational tools?

Sarah said...

As the solstice approaches, will you find yourself in the land of the midnight sun? You're close. I think I'd like that cycle, but am not sure I could take the winter flip side where it is dark just as long.

I suspect your GPS has the sunrise/sunset times. But that would take all the fun out of it.

I don't really have a physics bent in appreciating physics ( being higher math challenged ), and even tend to enjoy them pre-linguistically. In other words, "dammit Jim, I'm an engineer, not a poet". I was flying yesterday afternoon and caught a glimpse of a high cumulus positioned just right to refract the sunlight into a rainbow. So it was an colorful wisp, which caught my eye even through my amber sunglasses. Quite pretty, like a grace note to a wonderful day (off).

Lynn said...

Regarding your tanks pissing fuel...

In one of my past lives, I used to work in the pits at Formula I car races. At the time, they were not allowed to refuel during the race, so they wanted to get absolutely the most amount of fuel into the car. They would refrigerate their drums of racing fuel, and bring them out in the morning wrapped in insulating blankets. They had a device they would hook to the filler port that was a pipe about 30 cm long and 6 cm diameter, with what looked like a large funnel on the end. As the cars sat on the grid, waiting for the start of the race, the fuel would warm up and expand, and go up into the funnel. And whatever fuel they burned while idling there was repleneshed from the funnel. Just before the start of the race, they would unhook the pipe/funnel assembly (it had a non-leak, quick-release fitting) and take it away. Thus, the car got the maximum possible amount of fuel.


dpierce said...

Lynn -- Tricks like that still happen in some events. Not sure if it really makes a difference, but as you may know, redneck racers, much like fishermen, are a superstitious bunch.

Aviatrix said...

G, yep, about like that.

Lynn, love the racecar story.

Anonymous, took me a while to realize what you meant by Dave Zen, but yes, it just goes to show that there's some pilot thing in staring out the window and thinking about why that does that.

Sarah, yes the GPS will tell me the sunset time if I ask it (until I get far enough north that it says "never"), but it won't give me the satisfaction of working it out from first principles, and it won't tell me why the sky is so lovely and orange as it happens.

A squared said...

If you're a bus ride from Grande Praire, I flew over you last night, with a truckload of Copper River Reds bound for the markets of Boston. Should have kept 126.7 in #2 and transmitted "Aviatrix, are you out there" every 5 minutes.

Anonymous said...

Indeed, Zen is achieved.

Ever hear "shooting the glory?" The Sr^2 pilot of my lineage said it time and again.

Glories are incredibly cool phenomena. What's whacked out is how they manifest themselves. Very, very interesting optical physics.

As a matter of fact, the beer I just made is a Pomegranate Dunkelweizen of my own design. I will call it Brocken Spectre Ale... in honor of your cool post.

zb said...

"The other reason is so that as fuel is burned, air can get in to replace the volume so that the fuel will continue to flow."

I'm glad you explained. No joke: The first thing my ill mind thought was this: "The other reason is so that as fuel is burned, ..." the wings don't collapse and look like shrink-wrap after the flight is done.

I consider returning my MSc degree.

zb said...

@ anon 1:21: How much Dunkelweizen does it take till one is Dunkelwei-Zen?

A Squared said...

I'm glad you explained. No joke: The first thing my ill mind thought was this: "The other reason is so that as fuel is burned, ..." the wings don't collapse and look like shrink-wrap after the flight is done.

Not entirely sure what your point was, but it is possible for a wet wing to get damaged if the vent is blocked. Doesn't take much negative pressure to damage something that wasn't designed for it.

zb said...

I am surprised this really is an issue. Is the fuel pump really strong enough to create so much (negative) pressure inside of the wing that it may become a problem?

Anonymous said...


When I make it? Much like the venerable owl and the Tootsie Roll Pop...

one, a-two, and a-three.

Three. Projected ABV is north of 8, because of the second yeast pitch and type.

But the real question is: How large are the beers? None of this namby-pamby 12 oz frittery. I keg, therefore I brew.

A Squared said...

Not sure how low a pressure a fuel pump could produce in an unvented tank, but I have heard of wings getting damaged that way. Bear in mind that when you have large areas, a fairly small pressure adds up to a great deal of force. And you have to have fairly rigid walls in a structure to sustain a lower internal pressure, and a wing is not designed for that. The airplane I fly will withstand a pressurization of 7 psi safely, but will only withstand 0.7 psi of negative pressure. Likewise a aluminum beverage can will contain a great deal of pressure without rupturing, but you can crush it quite easily

Anonymous said...

The sun can produce some amazing sights:
Check out the Archives too, button near the bottom.