No crash and burn because of ample thrust reserves. Captain Airclues provides some good hints and Rainboe dashes in to remind us (if ever we thought so) that rules of thumb are tests of reasonableness, written in putty, that have not hardened into tablets of stone. [Are there tablets of stone?]
Yes it is all in the AFM but drawing on Captain Airclues’ suggestion you can gain an insight into the broad brush picture of how figures get into the AFM in the first place. He has helped shed light into the dark cavities between my ears.
What Airclues did not say is that 300,000kgf is the weight at which the 747-400 can cruise at FL370 ISA conditions M0.85 and a lift coefficient of 0.5 where it is fat dumb happy and economical. But if you could manage to keep trimmed and driftdown with exactly the same L/D ratio it is a relatively simple task to draw up a graph and Airclues’ simple examples work beautifully.
CL is fairly easy to calculate. CD is more difficult but all you need to do in a paper exercise is to take a reasonable stab. You don’t have to be correct to test a theory just to be reasonable.
My value of CD is based on two visits to Seattle, Book No 5 in the Cambridge series on aeronautics and something to do besides drinking in hotel bars when away.
If an aircraft drifts down with constant L/D ratio each of the coefficients CL and CD maintain constant proportionality but density and speed vary. (Actually compressibility drag probably varies with the square of CL and Mach No – see Book No 5).
For constant weight and ISA conditions all the way down @ FL370 M0.85, FL330 M0.77, FL200 M0.58 the maths produces the same answers: CL = 0.5, CD = 0.0286 and L/D = 17.483 using my crib below.
I use
http://www.desktopaero.com/stdatm.html as my tame atmosphere computer and take CD = 0.015 + ((CL)^2)/20.73 + CDcc [which latter value I just stuck down 0.0015 for the sake of argument]. Just determine a rational basis for CD and stick to it to obtain a guesstimate of L/D and therefore the thrust the engines are required to supply.
Using a spreadsheet and the graph function you can then pick your own height and speed. Bingo they much resemble the AFM.
Now, bailing out? I don’t think so. Whatever you might think of Mr Boeing’s products I haven’t come remotely close but there was a lot of fuss a while back about driftdown on the B777 with Airbus alleging it would have to use route L888 to avoid the jaggy bits. That was when the GE "85" (bitch, bitch) couldn't pull the skin off a rice pudding and captains steered around suspect patches that might have been discarded bubblegum.
Rgds
E & KO Sally on tour, 346-9 as we speak!