Cargo Aircraft versus Passenger Aircraft

[CURAviation]

Hello All,

I have a question, I have heard and read several times before that cargo aircraft are heavier than passenger aircraft and that the EOW (Empty Operating Weight) and (E)ZFW ((Empty) Zero Fuel Weight) has something to do with this. I know cargo aircraft have a strengthened floor, strengthened landing gear and a strengthened wingbox if I'm not mistaken. I can imagine this leads in more detail on how cargo aircraft are heavier at landing and take-off compared to passenger aircraft?

Best regards,

CURAviation

[oceancrosser]

Not true, I have flown several types in both cargo and pax versions. The cargo version is always lighter, especially purpose built freighters. Converted aircraft are a little heavier. Example: A pax 757-200 OEW is around 60 tons, the freighter is around 50. Ferrying an empty freighter with little fuel is...interesting performance wise :ok:
However a freighter is more often loaded to the Max ZFW, passengers are comparatively light as opposed to some cargo items. Again using the 757, full of pax the payload will be in the region of 20 tons. On the freighter you can get close to 40 tons.

[CURAviation]

So what you're saying is that a cargo aircraft eventually is heavier then a passenger aircraft due to the Max ZFW being used and cargo being heavier compared to passengers?

CURAviation

[742]

Your question is more complicated than it appears.

First understand that there are different types of freight. Express shipments tend to be light, while large industrial shipments tend to be very heavy. An airplane full of overnight envelopes is a very different business model compared to a 747 full of drilling equipment, to take two extremes.

As for the airplane itself, let’s start with empty weight. The weight of the removed/missing interior is more than the additional weight of the structural strengthening and door, so a freighter will normally have a lighter empty weight than it would have as a passenger airplane.

Zero fuel weight is an important consideration. An operator that carries heavy freight will want a high maximum zero fuel weight limit, and purpose built freighters often feature higher maximum zero fuel weights than their passenger cousins. Of course the tradeoff here is shorter range when heavy, but freight generally does not mind a tech stop.

Maximum landing weights will also often be higher on a purpose built freighter, again to accommodate more payload.

The ability to increase these weight limits is limited when converting an airplane from passenger to cargo, and the higher weight limits of the purpose built freighters is a big benefit that is considered when an operator is choosing between new, purpose built freighters and conversions from existing passenger airplanes. And since passenger airplanes of the same type often are produced with various weight limits, it follows that some passenger airplanes are better candidates for conversion than others.

In practice an operator whose business model involves heavy freight (as opposed to express shipments), often are very close to the max zero fuel weights and max landing weights. It is in this context that cargo airplanes are “heavier”.

So consider two imaginary airplanes. A passenger 747 and a cargo 747 both leave HKG for Europe. Both may be close to their maximum takeoff weights. However the cargo airplane will have a much heavier payload and less fuel, and will make a tech stop. It will be near its maximum zero fuel weight, and near its maximum landing weight on both landings.

[CR2]

Am just curious to know what you mean by "Empty ZFW". ZFW is BOW (or DOW or OEW depending on company/national terminology) plus payload. I can't see what you mean by empty ZFW.

As an aside, EZFW usually refers to Extended ZFW (known also as "Variable" ZFW), a system of exceeding maximum normal ZFW by reducing the take-off weight. (B747F).

[Papa Sierra]

CUR I'm with CR2 on this one, I think you have assumed "Empty" for "Extended" in (E)ZFW. In a nutshell:

Zero Fuel means exactly that! Total weight of the aircraft without fuel.

The only other thing that I can think of is that without payload and fuel the DOW and the ZFW would be the same but effectively it would be an "Empty" aeroplane!!!

[exfreight]

On Freighters, BOW - means Basic Operating Weight - No freight, no fuel, only aircraft and crew.

ZFW - means Zero Fuel Weight - BOW (above) and freight loaded.

TOGW - means Take-Off Gross Weight - Total of BOW, Freight and Fuel weights.

Hopes this helps

Exfreight

[Papa Sierra]

ExFreight,

As CR2 pointed out, different terminology in different parts of the world and even then you can throw in the odd military terms to boot. For example in the RAF (I should think the RAAF is the same):

Basic Wt = Empty wt of a/c + flt crew + residual fuel
Variable load = Anything over and above the "basics" which are required
on the a/c to complete a task (role equipment, additional
crew members, crew bags etc)
The sum of Basic Wt + Variable load = Aircraft Prepared for Service or APS Weight.
APS (or DOW) + Payload = ZFW
ZFW + Fuel = Actual Take Off Weight (an allowance is made for fuel burnt during taxying so at T/O fuel on board should be sector fuel).

[CURAviation]

Thanks for the explanation 742, CR2 and Papa Sierra. As for CR2 and Papa Sierra, yes, you are right. What you said about EZFW is correct.

So, looking at the explanation of 742 fullfreighters with a maximum zero fuel weight is means the aircraft's weight is higher so it can carry more cargo. Because the weight of a empty purpose built full freighter is higher it translates to the aircraft having less range because it consumes more due to its higher weight if fully loaded (max zero fuel weight). This means that a technical stop is necessary is common in the air cargo industry.

Then there is another scenario which is take-offs from high altitude airports such as in Ecuador and Colombia. How does that fit in the full freighters not being able to make nonstop flights from Ecuador and Colombia to Europe even when they are only carrying flowers which should be lighter in terms of weight? The volume weight is more relevant in such a scenario. I assume a fully loaded 747 with flowers still reaches its max zero fuel weight and/or landing weight requiring it to make a fuel stop on the way to Europe?

CURAviation

[CR2]

No CURAviation, a purpose built freighter is lighter than a pax aircraft.

747-400 example

-No windows
-No underfloor pipe-work for all the toilets
-No tail-tank and associated fuel lines
-No stretched upper deck

A 744F empty weight is 159T-161T depending on engines. A pax 747 (I believe) is around 180-185T (does anyone have a number?)

Maximum ZFW-empty weight = maximum allowable payload. It follows that the lighter aircraft carries more cargo. Eg, 744F, max extended ZFW is 288T - 160T, gives a max payload of 128T.

The 747-400BCF (converted pax to -F) has a max payload of about 112T I believe.

As for the flowers ex Colombia/Ecuador, a tech stop is made. BGI is one that is used. MTOW for the -400F ex BOG is severely restricted (330T if I remember correctly). And flowers.... are very heavy :)

[CURAviation]

Okay thanks CR2. I see where my confusion was, the max zero fuel weight is the maximum weight of the cargo carried on board less the fuel and weight of the aircraft itself. Because a full freighter is lighter is can carry more load but it the expense of its range thereby necessitating a fuel stop along the way.

About high altitude airports, I've read a few times that aircraft can't take off fully loaded due to the high altitude nature of such an airport. I've read that the high density altitude environments at such airports is important but can anyone explain this in more detail?

CURAviation

[CR2]

To make it easier for yourself, remember that Zero Fuel Weight is exactly what is says... zero (no) fuel* (one exception). I see what you are trying to say, perhaps something is getting mixed up in translation.

Keep these main definitions in your head and you'll be ok.

OEW: (I call it) Operational Empty Weight. The empty aircraft, (includes crew & catering). Basic Operating or Dry Operating Weight is the same thing.
ZFW: OEW+cargo
Taxi or Ramp Weight: ZFW+fuel
Take Off Weight: Ramp Weight - taxi fuel.

* exception: fuel is sometimes used as ballast when ferrying an empty aircraft. In this case alone, the weight of this ballast fuel is incorporated into the ZFW as it is not burnable.

As for the high altitude take-offs, will let a pilot explain that in detail. In essence, the higher the airport elevation, the less efficient the engines are which makes for a longer take-off run (my quick and dirty explanation).

[MarkerInbound]

I'll try the pilot stuff. CUR, you live at sea level. In a cubic meter of air, there are X number of molecules. As a jet engine runs, we'll make up a number and say a cubic meter of air is drawn in per second, squeezed, mixed with fuel and lit off, and sent out the back of the engine. What gives us thrust is the weight or mass of the exhuast, Mr. Newton and his equal but opposite idea.

If we go up 5 or 6 thousand feet, we're still drawing a cubic meter of air in but since the density of the air decreases the mass leaving is less so less thrust.

Same thing with the wing. As we fly along at sea level, X number of molecules per second push against the bottom of the wing or speed up as they travel across the curved upper surface, whatever makes a plane fly. As we go higher, there are, again, less molecules for the wing to interact with. So it takes a higher speed during takeoff to generate the same amount of lift.

The one part of this density issue that is good for planes is as we go higher, there is less drag since we are not having to move as many air molecules out of the plane's path. So, within some limits, planes tend to cruise more efficiently at higher altitudes.

[offa]

I've read that the high density altitude environments at such airports is important but can anyone explain this in more detail?
Density altitude gives a better idea of the "real" altitude that affects the thrust and lift produced. Most high altitude airports have higher than standard temperatures as well so for example: MEX at 7300 feet and 25C ends up with a DENSITY altitude of 9700 feet (similarly JNB / NBO / Quito etc)