


This question can be answered rather quickly, with a couple references. But, being the detail minded engineer that I am, there is a lot more detail to explore.
First, the easy answer. The 747100B weighs 373,300 lb empty, which means no crew, no payload, and no fuel. In most aircraft, not all of the fuel in the tanks is actually "usable." Some of the fuel is inaccessible to the pumps, and is called "unusable fuel." This unusable fuel is usually included in the empty weight, and I didn't factor that into my calculations. The 747100B has four engines, and for this little exercise, we will assume it is fitted with Pratt & Whitney JT9D7A turbofans, which weigh in at 8,850 lb each, for a total of 35,400 lb. If we subtract the four engines from the empty weight, we get the weight of the 747100B without fuel or engines: 337,900 lb. This works out to about 46% of the maximum takeoff weight, which is 735,000 lb.
Now, as I mentioned before, the only reason I can see for calculating this number is for a display aircraft, where only the engines are removed prior to display. Since I am an engineer (not to be confused with "geek"), I decided to keep going. Anything beyond this point is a rough estimate (not that it matters). Let's consider for a moment that we actually were trying to construct a 747 glider. Since we no longer need engines or fuel, we don't need the fuel system, the engine controls, or the nacelles (the engine "pods"). Using an aircraft design textbook, the weights of these items can be estimated.
Fuel system: 1,440 lb
Engine controls: 115 lb
Nacelles: 11,120 lb
This gives us a new "clean" weight of 325,225 lb. Even though these numbers are rough estimates, there are still a
few abnormalities to consider before we build our 747 glider. We need the auxiliary power unit (APU) to power the
hydraulics and other avionics. While the weight of the APU is still included, we need to add some fuel to power
it. Pressurization will now become a bit of a problem without the added engine power, but I would assume we won't
need pressurization where we'll be gliding, so we could probably make some serious weight reductions by scaling
down the structure. There are other issues too, but I don't imagine too many of you have read this far, so I'll be
quiet now.
 answer by Doug Jackson
(who actually is a geek), 4 November 2001
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