Heavy Aircraft Class

Today we have ten operators of the early 747, both -100 and -200 series. We also have six operators of the DC-10. As we prepare to introduce the next operator of these aircraft (Seven Seas Air Cargo), let's do a little classwork to fully understand the limitations of these big beasts.

Both aircraft are excellent freight carrying aircraft. They do have their limitations though. If we do not pay attention to these limitations, then we are not being true to our hobby! With a heavy aircraft, typically the most limiting factor is the maximum landing weight (MLW). You can't just load it up to maximum takeoff weight and go fly. You will need to stay aloft long enough so that you burn enough fuel that allows you to land at or below your maximum landing weight. The manufacturer gives you the maximum takeoff weight for the aircraft. For the 747-200F it is 833,000 lbs. For the DC-10-30CF the weight is 555,000 lbs. This is a structural limitation. You must determine what your performance limitation is.

You cannot land at a weight greater than your maximum landing weight. This figure is a structural limitation too. Land too heavy and the landing gear may just pop right up through the wings! The 747-200F has a maximum landing weight of 630,000 lbs. Now while you are enroute to your destination, only one thing changes- the amount of fuel on board. Let's say you will burn off 100,000 lbs of fuel. Adding 100,000 to your max landing weight (100,000 + 630,000) gives you your performance limitation for maximum takeoff weight (730,000 lbs). This is well below your structural limit of 833,000 lbs. If you were to takeoff at 833,000 lbs and only burn 100,000 lbs of fuel, you would be landing at 733,000 lbs, or 103,000 lbs over your maximum landing weight.

The airplane needs to land at a speed 30% above stalling speed (1.3xVso). Vso is stalling speed in the landing configuration. 1.3x Vso is also known as reference speed, or Vref. As the weight of the airplane increases, so does the stalling speed. If you were to land 103,000 lbs overweight, you may be landing just a knot or two above Vso! The tires have a limitation as well as the aircraft structure. To land at 630,000 lbs, the tires must be this big and be inflated to this pressure. If you land faster than normal, you will most likely blow the tires. Also, heavier aircraft need more room to land and more braking to stop. You may overheat the brakes and this can cause the tires to overheat. Blam-o! Flat tires. So the maximum landing weight is a firgure we need to understand, respect and adhere to at all times.

For flight planning we can get very specific. There are plenty of old 747 and DC-10 pilots out there who have posted typical fuel burns for flight planning. Here is what I found on the web:

747-200F  fuel burn in lbs

DC-10-30F       fuel burn in lbs
TAXI OUT 2,000

TAXI OUT 1,500
CLIMB 33,000

CLIMB 25,000
CRUISE 28,000    1ST HOUR
CRUISE 20,000     1ST HOUR
CRUISE 21,000     2ND HOUR
CRUISE 15,000     2ND HOUR
CRUISE 21,000    3rd HOUR
CRUISE 15,000     3rd HOUR
CRUISE 21,000    4th HOUR
CRUISE 15,000     4th HOUR
CRUISE 21,000    5th HOUR
CRUISE 15,000     5th HOUR
CRUISE 21,000    6th HOUR
CRUISE 15,000     6th HOUR
CRUISE 21,000    7th HOUR
CRUISE 15,000     7th HOUR

TAXI IN 2,000

TAXI IN 1,500
RESERVE 40,000 1+30
RESERVE 30,000 1+30
Total 234,000

Total 170,000

For the 747-200F, the maximum fuel capacity is 362,240 lbs. The DC-10-30F is 245,935 lbs. So let's run the numbers for a trip from Miami, Florida to Lisbon, Portugal. This is a seven hour trip for either aircraft (Actually, the 747 cruises faster than a DC-10, but lets assume they are equal for this problem).

747 Maximum
DC-10 Maximum

LOAD 242,800 242,800
142,964 152,964
ZERO FUEL WEIGHT 590,000 590,000
381,000 391,000
FUEL (POUNDS) 234,000 362,240
170,000 245,935
TAKEOFF WEIGHT 824,000 833,000
551,000 555,000
FUEL BURN 194,000

LANDING WEIGHT 630,000 630,000
411,000 411,000

First, the 747. The Basic Operating Weight (BOW) is the empty weight plus other things, like crew, crew baggage, unusable fuel, oil, etc.

We will load it to the maximum load limit. This is typically the load limit of the floors, both main deck and lower compartments. So let's add 242,800 lbs of money-making freight. Added to the BOW gives us the Zero Fuel Weight. This is also a structural limitation. In our example it comes out exactly the ZFW. So far so good.

We have computed that we want to carry 234,000 pounds of fuel. We add that to the ZFW and we come up with the Takeoff weight. Actually it's the ramp weight. The airplane has a maximum ramp weight, but to keep things simple let's just look at maximum takeoff weight. Maximum ramp weight is MTOW plus taxi burn. In this example, our takeoff weight computes to 824,000 lbs, 9,000 lbs below maximum. Good!

Now we subtract the fuel burn, which we have computed to be 194,000 lbs. This gives us our landing weight of 630,000 lbs, right at maximum. So far so good!

To sum up the 747-200F, to carry maximum freight you must carry minimum fuel. Any more is called tankering, and if we tanker any fuel we'll be landing overweight.

The DC-10 requires more careful consideration. We could add the maximum load of 152,964 lbs and be right at maximum zero fuel weight, but when you add the fuel required (170,000 lbs) we would be over the maximum takeoff weight by 6,000 lbs. We can't carry any less fuel, so we must reduce the load. At first it seems we only need to reduce the load by 6,000 lbs. The problem is when you figure landing weight. The math shows we would be 4,000 lbs over maximum landing weight. So guess what? We need to reduce the load further, by another 4,000 lbs. So, if we reduce the maximum load by 10,000 lbs, we end up landing in Lisbon right at our maximum landing weight.

Reality: I used a stock HJG DC-10-30CF for this flight. The sim said I would need 5+50 hrs and 98,016 lbs of fuel (16,900 pph). Actual results were 8+03, with a burn of 203,129 lbs of fuel. That works out to 25,233 pph average. I landed with just over two hours of fuel which doesn't look like much on the gauges! Reality check: Real life fuel burns and flight sim fuel burns can vary widely. It all depends on how well the sim model was created.

So here's the bottom line: The 747 can carry 59% more cargo on this trip than the DC-10F can. The DC-10F however will burn 28% less fuel than the 747. If you were the business owner, you would chose the airplane carefully.

Freight comes in all shapes and sizes. Maybe you are carrying Cheese Doodles. The weight is much less than food parcels or machinery, so the 747F can carry more Cheese Doodles simply because it has more volume than the DC-10F. If your customer simply has 140,000 lbs to ship, and it will fit in either aircraft, then the DC-10F makes more sense.

Consider the DC8-63F on the same trip Miami to Lisbon.

TAXI OUT 1,000
CLIMB 10,000
CRUISE 15,000    1ST HOUR
CRUISE 12,000    2ND HOUR
CRUISE 12,000    3rd HOUR
CRUISE 12,000    4th HOUR
CRUISE 12,000    5th HOUR
CRUISE 12,000    6th HOUR
CRUISE 12,000    7th HOUR
TAXI IN 2,000
RESERVE 20,000       1+30
TOTAL 121,500

I flew this trip using a stock HJG DC8-63F. When I flight planned the trip, it said I would need 64,504 lbs of fuel to complete the trip, and it would take 6+46. That's a fuel load of 84,504 including reserves. In cruise I flew at FL370, with fuel flow set to 3200 pph/eng. This gave me a speed of M.82. To better study the actual fuel burn I topped off the tanks and loaded only enough freight to put us at MTOW. Arriving in Lisbon, I parked having used 108,400 lbs of fuel, with an enroute time of 7+38. According to FS2004, I would burn 9486 pph. My trip averaged 14,263 pph, which is closer to the figure shown above from real world DC8 flying. So for flight planning I would use 9500 pph with 20,000 lbs reserve. Using that information, I could carry 77,000 lbs of cargo, 83% of maximum. Not too bad!

We would burn about one third less fuel than the DC-10F, but carry half as much cargo. The DC8's were great freighters in their day, but the jumbos brought economy to the market. Of course, we are the classic virtual airline, so we will include the DC8-63F on our newest long haul markets. That does it for our Heavy Aircraft Operations class. Truth is, the same rules apply to your DC6 or 727. Don't exceed max takeoff weight, and don't land over max landing weight. Yes indeed, there is a lot of math involved in flying today!