Let’s say I’m flying my Cessna 182 from Kansas City to Miami. The distance is 1300 nm. The aircraft’s range is 1000 nm. So is it better to fill up in Kansas City, fly a hair less than 1000 nm, then buy fuel for the remaining leg? Or is it best to split it up into two 650-nm trips? Or does it matter at all?
*This is purely an academic exercise, as I neither own my own plane nor live in Kansas City nor have any interest in going to Miami.
The actual range depends on winds aloft and a few other factors.
By the regulations you need to land with at least 30 or 45 minutes of fuel, depending on on whether you’re flying VFR or IFR. But generally, I don’t want to fly to the maximum range of the aircraft. That leaves little fuel to divert in the event of bad weather or emergency. So I’d break it up into two moderate distance flights.
When you get into jets, you’d rather not lift more fuel than necessary to altitude. So then you’re taking not the minimum fuel, but something closer to it. Though that factor applies in small piston-engine planes too, it’s not as important.
People frequently ask about maximum range of a prospective aircraft. I reply, “How long can you go without using the restroom?” That’s your effective range until you get into planes big enough to have a lavatory. And then you need a co-pilot.
Aside–what’s the smallest bizjet with a lavatory? Most private jets I see are way too small. Even the larger Gulfstreams seem like a lav would be very cramped. I.E.–if you can’t stand upright in the center aisle with a bit of headroom, how can it hav a lav?
Great answer by @Llama_Llogophile. Unsurprising.
In theory even in light airplanes you’ll consume more fuel while carrying more fuel. So ideally you’ll want to be as light as possible all the time. Which suggests putting just a little fuel in the tanks and making many stops would be optimal. But … each takeoff, landing, taxiing spends fuel. And since airports aren’t necessarily in a nice straight line between your starting and ending point, you’ll be detouring off to the side at least a little bit to make every stop. These latter effects will swamp the former effect.
As a practical matter, the weight vs fuel consumption effects in light airplanes are close to negligible. For a long trip like that it really doesn’t matter if you stop at the 1/3rd, 1/2, or 2/3rds point. The outcome will be within your measurement error.
In the real world, a typical lightplane doesn’t have a lot of weight capacity versus its seat count and fuel capacity. If you put 4 adult Americans in the 4 seats, and they each have an overnight bag, your plane with a nominal range of 1000 miles probably can only carry enough fuel for 250 miles flying. So you’ll be making 4 stops and 5 flights to get 1300 miles.
Not sure. The smallest jet I’ve flown was a CJ3 and it had a lav of the “honeypot” type. That is, it’s a tray beneath a toilet seat. To service it, you close it up, pull it out, empty and clean. I’ve seen that sort of setup in something as small as a Piper Cheyenne.
Then you get into larger jets which have externally serviced lavatories. After landing they hook up a suction machine which in turn replaces the fluid too (or it can be added by hand, which we call “over the top”). My current bizjet has that and it’s much better than the honeypot. Still cramped though. We say that you need to decide what you’re going to do before you go in, because there isn’t room to turn around.
When I regularly flew on a corporate jet, a Citation X, they started in the morning with 1600 gallons (80% capacity) which was way more than they needed for the whole day (four flights, two of which were ~300 miles each, the other two less than 100 miles). Even fully loaded the plane could fly 2000+ miles comfortably.
This was a decade ago and I’m going by memory from talking to the pilots and ground crew while we waited for the executives to show up.
The Challenger 300/350/3500 (all the same plane in terms of size) has a lavatory. It’s a decent size, I’d have no issues using it though men who refuse to be decent and sit down may have their problems with it. It spans the width of the fuselage, with the toilet in the right and vanity on the left. Passenger qty is 10, with the toilet (lid down) actually being a certified seat.
I haven’t been in smaller planes than that but the larger Learjets have lavatories too, and I think most also span the fuselage width.
Don’t fuel costs figure in to it? My understanding is fuel prices can vary a lot between different airports. Figure out where it is cheapest to buy fuel and you likely have a lot of the answer.
The only time I flew a business jet, the lavatory was the entrace. You come in the door just behind the cockpit, and there’s a wall and door on each side - one into the cockpit, one into a passenger compartment for about 7 people. To use the toilet - like above, under a 8th seat with cushion - close the cockpit door and the door to the passenger seating, lift the cushion piece, and there’s a toilet… so I was told. Nobody used it, and I assume in apinch that jet can take 8 passengers plus 2 crew.
As I understand, the thing with jets is they take a lot of fuel to climb to a higher operating altitude. So perhaps less weight in climb is the way to go, do 2 halves rahter than 3/4 and 1/4? I guess the math will likely say it doesn’t make a huge difference. As LSL points out, with a smaller prop plane you are probably loading it to the max with fuel, and fly wherever that allows you with decent safety allowance.
That’s a huge factor. In the airline biz we’re frequently tankering fuel from a place with low prices to a place with high prices. It’s not a lot, but as they say, the savings add up.
A major corporate flight department with a small fleet of bizjets might well be able to get a pretty good discount from the fuel seller at their home airport. Such that it pays to fill up in the morning and run all day with fuel bought cheaply at home.
It’s true that climb is the high fuel flow and hence expensive time of the flight. But however high you climb, that’s also how far back down you can coast at idle towards landing. So there’s largely offsetting effects there.
For flights that are a significant fraction of the aircraft’s range or while carrying a particularly heavy load, you get the outcome that the maximum altitude you can climb to becomes limited by your weight. And as a general matter, your cruise fuel mileage is better the higher you go. So carrying a heavy load you may be forced to cruise at a lower altitude and thereby burn more fuel in cruise. Such that carrying less fuel and stopping in the middle might let you climb higher enough to make some savings. In cruise.
But … it’s a near certainty that the inefficiencies of approach, landing, taxiing, takeoff, etc. will more than offset those small cruise savings.
Wind also gets a vote on this. Sometimes the winds up high are unfavorable and the winds down low are (more) favorable. In that case you’d be fuel ahead to stay low, which means the concerns about fuel weights limiting your climb disappear. When the winds are opposite, it can be real important to be able to climb high enough to get on the favorable free conveyor belt towards the destination. Even if that triggers a stop along the way.
Lotta moving parts in real-world fuel planning if you’re trying to eke out the last couple percent of savings.
Our fuel price at the GA airport we used were negotiated as a function of some wholesale index. I think this was true for all the corporate jets that flew semi-regularly routes (our plane flew the same route Monday-Wednesday-Friday, and a different route Tuesday-Thursday, 90% of the time).
The hobbyist flyers paid whatever the “pump” price was on the day at that FBO.
Again 10+ years ago.