Ok then.
Unfortunatley we didnt have enough sim time to go through various configuration and weight scenarios such as gear down and lighter (46T) gross weights. We therefore stuck to a gear-up landing as this would dictate the
minimum height for a return, which was your original question
WWW. Various heights were experimented. 1000 ft was not enough and 1500 ft was an overkill.
Please refer to my previous "Recap" for aircraft configuration and status before TO.
* Upon the double engine-failure the stick must be IMMEDIATLEY pushed forward from 21 deg NU to at least an initial 5deg down, while at the exact same time
violentley initiating (yes even though your picking up your into-turn flight spoilers at that point), then smoothley executing, a turn at 50 deg AOB while increasing speed to vref 5 (168kt). With 22% MAC the nose was quite agreeable to pitch down and increase speed at that bank angle which required only a .1 unit stab-trim input ND once at wings-level. In our exercise I overshot slightly to 172 kts IAS on the standby ASI but letting go the stick at that point was beneficial and 168 kts was quickly regained.
This manouver is the most costly of the exercise in terms of height. Rollout in intercept heading (about 30 deg) was at a height of 320 ft.
*APU start-attempt was tried, but as predicted we were alreadey on deck by the time the genny was avbl. So no AC electrics around throughout.
* Auto-activation of the standby hyd system valves had occured but the pump itself of course was dead.
* Around 2 deg ND (as read from the Standby AI) was necessary to maintain Vref 5. I was quite surprised at this as I thought 3-4 would be required.
* About 60 deg before intercept-heading rollout we were on manual reversion. This is why the initial manouver must be flown quickly and relativley violentley in order to utilise the flight controls before you lose all hydraulic sys A+B pressure. Of course from then on you fly rudder for bank and stab-trim for pitch where possible as you well know.
* Rollout on final was at 50ft, just in from the piano-keys. Flare and landing as per gear-up procedure in the Boeing FCTM.
Summary: If the crunch comes in real life Id recommend absolutley no lower than 1500 ft in height if your hard decision is to turn back. In our exercise no recognition time was allowed which of course is about 2 secs, and another 1 second to initiate the required action even assuming your wide-awake. Anything lower than 1500 ft Id aim straight ahead and pick a road or aim in between the houses or cows.
Notes: In our aircraft, standby pump only powered by xfer bus 2 (or 1 through the xfer sys) and FMC is not avbl when battery and standby inverter are only sources of power.