The original premise:
"I must select a single speed (as fast or slow as I please) and altitude (as high or low as I please), head to it ASAP, and stay with it until flameout."
I'll venture just a similar hypothetical in the same vein - but with some vital differences peculiar to aircraft type. Disregarding cost index and fixed altitudes and speeds, can we look at a slightly different set of impertinent theoretical parameters?
In this scenario, my F/O has passed out after suddenly projectile vomiting. He's incapacitated and I'm starting to feel quite queasy myself. I'm not thinking straight and instantly find myself wondering whether it's hypoxia so I sweep my quick-donning mask on. I'm not ex-military and I've never done a hyperbaric or hypobaric chamber run, so I have no real idea what hypoxia is like at the onset. I'm feeling no better, my head is swimming and I'm losing focus so I decide to descend and so I disconnect the autopilot and lower the nose. Shortly thereafter, just before passing out, I suddenly realize that we both ate at the same mukkin cart outside Fatties just before heading to our beds last evening. We have a 772F freighter now not on autopilot and left to its own devices. The flight deck door is locked.... and access is not available.... not that there's anyone aft who can access the cockpit or likely to want or need to.
So having dispensed with its pilots (sorta like Arthur C. Clarke's computer HAL in "2001: A Space Odyssey" - but not in any extra-terrestrial context), what would my aerospaceship 772F now get up to? Once "left to its own devices", i.e. what exactly are those devices capable of?
Unlike an Airbus (an A320 like MS804 say), without its normal FBW system's protections, it won't enter a descending and tightening spiral . It has a completely different FBW system called the AFCS (an "Active' Flight Control System). Rather than different modes of degradation such as alternate 1 and 2 etc, Boeing's FBW design has quadruple redundancies and sports a multiplicity of fallback power sources and fault-tolerant workarounds - and it's not easily subjected to any degraded "laws". Its proclivities are to keep on aviating no matter what. Unlike the Airbus philosophy, my 772 won't disallow a pilot-selected overbank - but at anything above a pilot-selected 30 degs angle-of-bank, it
will disproportionately increase the yoke's roll axis feedback in order to remind me that I shouldn't be unnecessarily trying to aerobat an airliner. But if it isn't a pilot roll input, my 772's FBW will "actively" impose a restorative rolling moment back towards wings-level. In fact it's so good at this inherently "active" sub-routine that it can pick up a gust-induced "dropped" wing of a mere 5 degrees AoB much faster than the speed of vomit.... and promptly get us back on an even keel. So even if my unpiloted 772 should enter some nasty ITCZ induced turbulence and get a little "upset", as soon as it exits it will phugoid a little and quickly resume wings level flight - albeit upon a new heading. And it can keep on doing this all day (and night). But, believe it or not, I'm blithely and blissfully unaware of this 772's model-specific peculiar pecadillo, as I've rarely hand-flown this noble beast - and certainly never broken the law and done it up there at height, where RVSM rules the roost. It's not in any simulator syllabus and I've never read it anywhere. But the Boeing test pilots know all about it and it's not for publication. What operator needs its pilots to go prove or disprove it? It's just a natural and little known adjunct to the Boeing FBW design philosophy. Keep the bank vector somewhere near vertical and the crashworthiness is never tested.
So there I am, firmly ensconced in the messy subliminal soporifics of regurgitated exotic Asian food fanciers (and even though erupting unconsciously at both ends), it matters not a whit - my trusty steed is "taking care of business". The Man of La Mancha and his rusty sidekick Sancho Panza are both out of the picture and quite non-interventionist - however Don Quixote's trusty steed Rocinante knows what to do. It's in his genes.
But back to the postulated conundrum. Once spat out on a heading that's going to take my 772F southbound and clear of the ITCZ, what's the effect of a static cargo and trim-state as fuel burns off? I'll help you out with this. The 772 will gradually climb as fuel burns off. If its AFCS is always going to oppose any bank angle's lift vector that's other than vertical and it's "a climber" due to fuel burn-off, why should it do other than "proceed" on course (whatever that course might happen to be)? In fact why should it meander more than 3 to 5 degrees left or right of its final spat-out recovery heading in ever-smoothening upper air? What's its anmpp (air nautical miles per pound of fuel or aka specific air range) going to be? I'll help you out again here. Eventually the 772F will be up at around FL440 and its range will be optimized..... as good as it's ever going to get and around 103% of a fixed FL350 LRC cruise... and 105% of a stepped climb profile.
The only question remaining is Rocinante's conduct and technical decorum at:
a. first flame-out
b. second flame-out
c. APU start-up
d. APU flame-out (i.e. what can the AFCS now achieve RAT-wise?). i.e. Rather pointless having a RAT deploy as a lender of electrons of Last Resort - if it can't help the batteries provide the ergs required for the flight-control system's final earthbound functionalities.
e. I'm assuming that a RAT-powered Rocinante will just fly a 12 to 15 degrees nose-up wings level glide attitude to a nice optimal ditching. It may dig a wingtip into a swell and shed a flaperon (in its full aileron deflection response) and maybe some aileron trailing edge on the same side - but that's not a Boeing design deficiency.
What say you? I'd be interested in some knowledgeable researched input.
I awake from my reverie, reach for my lunch-box and ruminate upon my boring freight-dawg existence. These projectile vomiting spasms sure leave you peckish. You can sign me up for one of these pilotless projects any day. It's a freightening prospect.