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Johnny L.A.
03-18-2003, 10:22 PM
I've heard of people ferrying General Aviation aircraft to Hawaii. I seem to remember reading about a single-engine Cessna (I don't recall which model) having to ditch just offshore when it ran out of fuel. IIRC, that airplane had temporary fuel tanks installed.

I am by no means a math wizard. I'm prone to make simple mistakes if I'm not careful. So could a pilot Doper check me on this? I've used data from a 1985 Cessna 172 POH for the calculations.

Fuel capacity: 68 gallons with integral tanks, 62 gallons usable. (Standard fuel, BTW, is 43 gallons.)
Weight of fuel: 408 pounds (68 X 6)
Useful load: 974 pounds
Payload w/full tanks: 566 pounds (974 - 408)
Weight of pilot: 190 pounds
Payload w/full 68 gallons of fuel and 190 pound pilot: 376 (62.67 gallons of extra fuel. 556 - 190)
Maximum Range: 875 nm
Economy: 14 nmpg (875 / 62)

62.27 gallons X 14 nmpg = 877.33 nm

Total range: 1,752.33 (875 + 877.33)

Distance to Hawaii: 2,080 nm.

So by my calculations a Cessna 172 will run out of fuel 328 miles from Hawaii. At 14 nmpg the aircraft would require an additional 23.43 gallons, or 140.57 pounds of fuel.

So how can a Cessna fly from California to Hawaii? Can 140 pounds of "stuff" be stripped from the aircraft, allowing the necessary fuel to be carried? Do they try to get heroin addicts (just kidding -- they're typically skinny) as pilots? Or do the pilots try for the most favourable winds aloft, and hope the forcasts are right?

(Note: I've done a little rounding here and there, and have not taken into account the weight of the extra temporary fuel tanks, nor where they would fit.)

Berkut
03-18-2003, 10:45 PM
You can apply for a ferry permit from the FAA that will let you load the plane over the gross weight for the trip.

Then, you turn it into a flying gas can.

I find the ferry pilot job strangely appealing...

BobT
03-18-2003, 10:48 PM
Maybe you can just refuel on those islands in between California and the Big Island.

You know the one.

Atlantis.

Johnny L.A.
03-18-2003, 10:50 PM
I find the ferry pilot job strangely appealing...
So do I, actually...

But not over vast stretches of water, with marginal fuel. ;)

Q.E.D.
03-18-2003, 10:51 PM
I'm not a pilot, but wouldn't there be a difference in fuel mileage depending on the wind? I would think flying into the wind would get you less than flying with it. So, would it make a difference if you were flying from Hawaii to Cali, rather than the other way?

Berkut
03-18-2003, 10:56 PM
Originally posted by Q.E.D.
I'm not a pilot, but wouldn't there be a difference in fuel mileage depending on the wind?Yep, wind has a huge effect on your range. I am sure a ferry pilot would take into account the winds aloft forecast for the trip, and allow a decent margin. You can try different altitudes, looking for more favorable winds.

When I was working on my commercial x-countries in a 172, I would see groundspeeds as low as 40 knots, all the way up to 140 knots. This was with a constant 105 knot airspeed.

That wind sure can cost you some money. :(

Lorenzo
03-18-2003, 11:04 PM
I think you have some circular references in your calculations. You calculate nmpg based on maximum range divided by gallons. Then you use this calculated nmpg multiplied by the available fuel to arrive at the new maximum range.

I would think that nmpg would fluctuate based on the conditions and that by multiplying an estimated nmpg upper and lower figure by the number of gallons would give you a more useful maximum range "range," if you will. For example, we'll say nmpg could be expected to average 14 nmpg but vary by 10% plus or minus. Then we would establish a minimum range of 1560 nmpg and a max of 1910 nmpg.

Given a 2080 nm distance to Hawaii and the availability of 124 gallons of fuel, the plane would have to achieve 16.8 nmpg to arrive with zero drops in the tank which would be cutting it a might close. 16.7 nmpg is almost 20% better mileage better than the stated 14 nmpg so it sounds unrealistic without some serious tail windage.

Gasoline, as noted, has weight so as the tank empties the plane weighs less which, all else equal, should improve mileage, but not by 20%

If the max fuel and typical mileage are as stated, this sounds like a longshot.

YMMV (could not resist)

Sam Stone
03-18-2003, 11:27 PM
When ferrying airplanes long distance, you work the problem backwards. You start with the distance you need to fly, plus adequate reserve, then calculate how much fuel you'll need to get there.

Then you figure out how to cram it into the airplane and get yourself off the ground without running off the end of the runway or entering a departure stall on the climbout.

For the purposes of a single ferry flight, you actually have a lot of margin over gross weight, and you can get a waiver for the flight that allows you to fly over gross.

Gross weight in an aircraft is limited in two different possible ways - some aircraft are limited by maximum climb rate - Gross weight is set by calculating the point at which you can no longer maintain a legal minimum climb rate. The other way is structural - gross weight is set by the maximum G loading of the airplane.

Both of these limits are determined to be safe across a wide variety of uses. For instance, the minimum climb rate has to be high enough so that pilots flying out of high altitude strips can still get into the air, or flying out of airports with FAA maximum height obstructions can still be done safely. In the case of gross weight limits, a utility category aircraft has to be able to withstand +4.4 g's at full gross. Also, the gear has to be able to withstand the landing with fuel left aboard.

In the case of an overwater ferry flight, you're probably leaving at sea level in the early morning when it is cool outside, on a day when there is little wind and turbulence, from a very long runway. And you won't be landing again until the fuel is mostly gone, so structural limits don't matter much.

"Private Pilot" magazine used to have a monthly column by a guy who made his living ferrying airplanes, and he used to describe the fuel setups before flights across the ocean. Typically, removable bladders were installed in the back seat and cargo area. I think he was usually limited mostly by Cg concerns.

Boxcar
03-19-2003, 12:34 AM
My first thought was that it couldn't be possible, you'd need about 20hrs of fuel to get there. However, after running some numbers, it looks like it might be possible. I would want a good autopilot tied into the GPS and a lot of caffeine to do it, though.

Current Skyhawk specs from Cessna (http://skyhawk.cessna.com/spec_perf.chtml)

On a new Skyhawk, usable fuel is 53 gallons. With 45minute reserves and allowances for takeoff, climb, taxi, Cessna claims you can fly for 6.6hrs, about 687nm, at 10,000ft and 60%cruise and recommended lean.

If I have the numbers right, that translates to about 104kts airspeed and a fuel burn between 8gph on the high end and maybe as low as 6.8gph (not sure how to adjust for the allowances Cessna uses to get the right fuel burn)

Based on a 2080nm trip, at 104kts, the trip would take about 20hours. Using a fuel burn for the trip of 6.8gph, you would have to have 139 gallons of fuel on board to make it. (20hours of flight time at 6.8gph fuel burn + 3 gallons unusable in the wing tanks).

139gallons = 834 pounds of fuel, about 80gallons (or 540lbs) over and above what the aircraft is designed to carry.

Weight Specs (http://skyhawk.cessna.com/spec_gen.chtml)
Cessna says maximum useful load of a C172 is 837lbs (which I think, reading their numbers) is for fuel, people, and baggage.
Max takeoff weight is 2450lbs
Max Ramp Weight is 2457lbs
Standard Empty weight is 1620lbs

So, Standard empty weight + 139 gallons of fuel ( 56 (53 usable) in the wings plus another 83 in ferry tanks somewhere) + pilot and fuel tank weights = 1620+834+220 = 2674lbs at takeoff. About 224lbs overweight. That is only 9% over the recommended weights.

I think it could fly as long as you have the CG established properly and the winds aren't too much against you.

However, if a real pilot would check these, I would appreciate it.

Looking things over again, I think I might have left us about 10gallons short (I burned everything up on the trip and left out the allowances for climb, taxi, 45minute reserve) blast, that's another 60lbs.

Sorry man, I just ran us out of gas within sight of the islands. How far can we glide from 10,000ft?

Berkut
03-19-2003, 12:48 AM
Originally posted by Boxcar
Sorry man, I just ran us out of gas within sight of the islands. How far can we glide from 10,000ft? 9:1 glide ratio, so about 17 miles or so.

Sam Stone
03-19-2003, 01:32 AM
For the purpose of a ferry flight, you can do an awful lot better than 224 pounds over gross. Transport Canada will allow you to get a ferry permit up to 25% over gross in many airplanes. For a Cessna 172, that's about 600 lbs. But like I said, because there are a limited number of places to put the fuel, cg concerns will probably be the limiting factor.

For long overwater ferry flights, you need a lot of reserve. 3 hours minimum reserve is required for the transatlantic ferry route, I believe.

KeithT
03-19-2003, 03:35 AM
According to the performance charts for a 172P (carbureted O-320 engine), 55% power and 6000' MSL are recommended for maximum range. With a leaned mixture and standard temperature, this translates into 6.4 GPH fuel flow at 101 KTAS.

Since factory figures are never perfect, add about 1 GPH and subtract about 4 knots to be safe. This gives a fuel flow of 7.4 GPH at 97 KTAS. Assuming no wind, it's a 21 hour trip. Add three hours' reserve, and you'll need 180 gallons, or about 1100 pounds of 100LL. Adding in 230 pounds for the pilot, bags, life raft, and other various accessories brings the gross weight up to about 2700. If the rear seat is removed and the auxiliary tank put in its place (75" arm), the CG will be at about 46", within the specified limits for the normal category.

So, you'd be at most 300 pounds overweight, and only for the first six hours of the flight. I wouldn't be itching to make this flight, but it's doable.

jacksen9
03-19-2003, 05:51 AM
Sam Stone
In the case of an overwater ferry flight, you're probably leaving at sea level in the early morning when it is cool outside, on a day when there is little wind and turbulence, from a very long runway. And you won't be landing again until the fuel is mostly gone, so structural limits don't matter much.

Obviously I am not a pilot. Are there not circumstances in which a pilot would need to land earlier than expected? For some reason I would not want to take off if I knew that I would not be able to land until the fuel was almost gone. Just curious.

Johnny L.A.
03-19-2003, 08:44 AM
I think you have some circular references in your calculations. You calculate nmpg based on maximum range divided by gallons. Then you use this calculated nmpg multiplied by the available fuel to arrive at the new maximum range.
The POH gives maximum range and maximum fuel. Miles divided by gallons = miles per gallon. Multiplying a greater amount of fuel by miles per gallon gives a new flying range. So it's not circular.

But you are correct that I did not include flying conditions in the calculations. I was trying to keep it simple.
Gasoline, as noted, has weight so as the tank empties the plane weighs less which, all else equal, should improve mileage
This is true. The figure of 14 nmpg was obtained from the maximum range with long-range tanks, so it's an average. Mileage would be less at the beginning of the flight and greater at the end. While I did consider that mileage with a bunch of extra fuel would be less and the average nmpg would be lower, I didn't figure it into the OP because I don't know how much less it would be and thus could not come up with a closer average nmpg.
quote]In the case of an overwater ferry flight, you're probably leaving at sea level in the early morning when it is cool outside, on a day when there is little wind and turbulence, from a very long runway. And you won't be landing again until the fuel is mostly gone, so structural limits don't matter much.[/quote]
When I was flying fixed-wing, the field altitude was 2,347 feet and the temperature was usually 35C to 40C. The runway was 6,000 feet long. Of course if I were flying to Hawaii I'd find the closest departure airport, which would be near sea-level.
Obviously I am not a pilot. Are there not circumstances in which a pilot would need to land earlier than expected? For some reason I would not want to take off if I knew that I would not be able to land until the fuel was almost gone. Just curious.
Yes, there are such circumstances. While flying is a very safe form of travel, it does have some risks. You need to weigh those risks against your goal to see if you are willing to accept them. Pilots are taught to be ever vigilant for a place to land in case of an emergency. A properly maintained aircraft is probably not going to let you down, but it's something you have to think about.

Real-life example: A helicopter has a "height/velocity" envelope, colloquially called the "dead man's curve". Operating the aircraft outside of the envelope would mean that you would not be able to autorotate in case of an engine failure. But some helicopter pilots routinely fly outside the envelope when taking off or landing in confined areas. It's part of the helicopter training. I mentioned this to my instructor when we were doing confined area work, and he said that I was right; but the likelihood of an engine failure is remote, and you have to consider the risk when making such a maneuver and return to the envelope as soon as possible. I'll do it for training purposes, but except for that I'll just look for a more favourable spot to get in and out of (which is usually pretty easy in a fling-wing. ;)

Thanks for the answers, everyone. Flying to Hawaii is one of those things I imagine from time to time, but it's something that I'll never do.

racekarl
03-19-2003, 09:34 AM
Johnny, I agree with Lorenzo. Even after your explanation I still don't see how you got to a range of ~1700 nm.

Isn't the stated maximum range of 875 nm exactly that? That is, Cessna is saying that 875 nm is as far as this baby can go on a full tank?

To the layman (me - I've taken a few lessons and my dad was a pilot, but that's it) it looks like you took the stated maximum range, divided by the max fuel load to get how many miles per gallon, then multiplied that back against fuel load to get the same number you started with, then inexplicably (to me) added that number back to the range you started with.

I don't get it, what am I missing?

ski
03-19-2003, 10:02 AM
1. You could lean very aggressively (even on the lean side of peak EGT) to decrease fuel consumption. A columnist on AvWeb (avweb.com) loves to talk about getting amazingly low fuel burn rates and long ranges.

2. The FAA will allow a takeoff weight of 30% over max gross with special authorization, at least according to an article in this month's Plane and Pilot magazine (talk about good timing).

racekarl
03-19-2003, 10:23 AM
Oh I see it now: in parantheses after you list max payload you imply that you are going to fill that payload with an extra 62 gallons of fuel.

It's very hard to see that the way you've got the OP arranged and worded....

Johnny L.A.
03-19-2003, 11:22 AM
Oh I see it now: in parantheses after you list max payload you imply that you are going to fill that payload with an extra 62 gallons of fuel.

It's very hard to see that the way you've got the OP arranged and worded....
Yes. I should have put that on a separate line.

Berkut
03-19-2003, 12:22 PM
RE: Ferry pilot jobOriginally posted by Johnny L.A.
So do I, actually...

But not over vast stretches of water, with marginal fuel. ;) Doesn't your location field say that you like to drink water? :)

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